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<pre>Internet Engineering Task Force (IETF)                         J. Schaad
Request for Comments: 8152                                August Cellars
Category: Standards Track                                      July 2017
ISSN: 2070-1721


               <span class="h1">CBOR Object Signing and Encryption (COSE)</span>

Abstract

   Concise Binary Object Representation (CBOR) is a data format designed
   for small code size and small message size.  There is a need for the
   ability to have basic security services defined for this data format.
   This document defines the CBOR Object Signing and Encryption (COSE)
   protocol.  This specification describes how to create and process
   signatures, message authentication codes, and encryption using CBOR
   for serialization.  This specification additionally describes how to
   represent cryptographic keys using CBOR.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc8152">http://www.rfc-editor.org/info/rfc8152</a>.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-4">4</a>
      <a href="#section-1.1">1.1</a>. Design Changes from JOSE ...................................<a href="#page-5">5</a>
      <a href="#section-1.2">1.2</a>. Requirements Terminology ...................................<a href="#page-6">6</a>
      <a href="#section-1.3">1.3</a>. CBOR Grammar ...............................................<a href="#page-6">6</a>
      <a href="#section-1.4">1.4</a>. CBOR-Related Terminology ...................................<a href="#page-7">7</a>
      <a href="#section-1.5">1.5</a>. Document Terminology .......................................<a href="#page-8">8</a>
   <a href="#section-2">2</a>. Basic COSE Structure ............................................<a href="#page-8">8</a>
   <a href="#section-3">3</a>. Header Parameters ..............................................<a href="#page-10">10</a>
      <a href="#section-3.1">3.1</a>. Common COSE Headers Parameters ............................<a href="#page-12">12</a>
   <a href="#section-4">4</a>. Signing Objects ................................................<a href="#page-16">16</a>
      <a href="#section-4.1">4.1</a>. Signing with One or More Signers ..........................<a href="#page-16">16</a>
      <a href="#section-4.2">4.2</a>. Signing with One Signer ...................................<a href="#page-18">18</a>
      <a href="#section-4.3">4.3</a>. Externally Supplied Data ..................................<a href="#page-19">19</a>
      <a href="#section-4.4">4.4</a>. Signing and Verification Process ..........................<a href="#page-20">20</a>
      <a href="#section-4.5">4.5</a>. Computing Counter Signatures ..............................<a href="#page-22">22</a>
   <a href="#section-5">5</a>. Encryption Objects .............................................<a href="#page-22">22</a>
      <a href="#section-5.1">5.1</a>. Enveloped COSE Structure ..................................<a href="#page-23">23</a>
           <a href="#section-5.1.1">5.1.1</a>. Content Key Distribution Methods ...................<a href="#page-24">24</a>
      <a href="#section-5.2">5.2</a>. Single Recipient Encrypted ................................<a href="#page-25">25</a>
      <a href="#section-5.3">5.3</a>. How to Encrypt and Decrypt for AEAD Algorithms ............<a href="#page-26">26</a>
      <a href="#section-5.4">5.4</a>. How to Encrypt and Decrypt for AE Algorithms ..............<a href="#page-28">28</a>
   <a href="#section-6">6</a>. MAC Objects ....................................................<a href="#page-29">29</a>
      <a href="#section-6.1">6.1</a>. MACed Message with Recipients .............................<a href="#page-30">30</a>
      <a href="#section-6.2">6.2</a>. MACed Messages with Implicit Key ..........................<a href="#page-31">31</a>
      <a href="#section-6.3">6.3</a>. How to Compute and Verify a MAC ...........................<a href="#page-32">32</a>
   <a href="#section-7">7</a>. Key Objects ....................................................<a href="#page-33">33</a>
      <a href="#section-7.1">7.1</a>. COSE Key Common Parameters ................................<a href="#page-34">34</a>
   <a href="#section-8">8</a>. Signature Algorithms ...........................................<a href="#page-37">37</a>
      <a href="#section-8.1">8.1</a>. ECDSA .....................................................<a href="#page-38">38</a>
           <a href="#section-8.1.1">8.1.1</a>. Security Considerations ............................<a href="#page-40">40</a>
      <a href="#section-8.2">8.2</a>. Edwards-Curve Digital Signature Algorithms (EdDSAs) .......<a href="#page-40">40</a>
           <a href="#section-8.2.1">8.2.1</a>. Security Considerations ............................<a href="#page-41">41</a>
   <a href="#section-9">9</a>. Message Authentication Code (MAC) Algorithms ...................<a href="#page-42">42</a>
      <a href="#section-9.1">9.1</a>. Hash-Based Message Authentication Codes (HMACs) ...........<a href="#page-42">42</a>
           <a href="#section-9.1.1">9.1.1</a>. Security Considerations ............................<a href="#page-44">44</a>
      <a href="#section-9.2">9.2</a>. AES Message Authentication Code (AES-CBC-MAC) .............<a href="#page-44">44</a>
           <a href="#section-9.2.1">9.2.1</a>. Security Considerations ............................<a href="#page-45">45</a>
   <a href="#section-10">10</a>. Content Encryption Algorithms .................................<a href="#page-45">45</a>
      <a href="#section-10.1">10.1</a>. AES GCM ..................................................<a href="#page-46">46</a>
           <a href="#section-10.1.1">10.1.1</a>. Security Considerations ...........................<a href="#page-47">47</a>
      <a href="#section-10.2">10.2</a>. AES CCM ..................................................<a href="#page-47">47</a>
           <a href="#section-10.2.1">10.2.1</a>. Security Considerations ...........................<a href="#page-50">50</a>
      <a href="#section-10.3">10.3</a>. ChaCha20 and Poly1305 ....................................<a href="#page-50">50</a>
           <a href="#section-10.3.1">10.3.1</a>. Security Considerations ...........................<a href="#page-51">51</a>
   <a href="#section-11">11</a>. Key Derivation Functions (KDFs) ...............................<a href="#page-51">51</a>




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      11.1. HMAC-Based Extract-and-Expand Key Derivation
            Function (HKDF) ..........................................<a href="#page-52">52</a>
      <a href="#section-11.2">11.2</a>. Context Information Structure ............................<a href="#page-54">54</a>
   <a href="#section-12">12</a>. Content Key Distribution Methods ..............................<a href="#page-60">60</a>
      <a href="#section-12.1">12.1</a>. Direct Encryption ........................................<a href="#page-60">60</a>
           <a href="#section-12.1.1">12.1.1</a>. Direct Key ........................................<a href="#page-61">61</a>
           <a href="#section-12.1.2">12.1.2</a>. Direct Key with KDF ...............................<a href="#page-61">61</a>
       <a href="#section-12.2">12.2</a>. Key Wrap ................................................<a href="#page-63">63</a>
           <a href="#section-12.2.1">12.2.1</a>. AES Key Wrap ......................................<a href="#page-64">64</a>
       <a href="#section-12.3">12.3</a>. Key Transport ...........................................<a href="#page-65">65</a>
       <a href="#section-12.4">12.4</a>. Direct Key Agreement ....................................<a href="#page-65">65</a>
           <a href="#section-12.4.1">12.4.1</a>. ECDH ..............................................<a href="#page-66">66</a>
           <a href="#section-12.4.2">12.4.2</a>. Security Considerations ...........................<a href="#page-69">69</a>
      <a href="#section-12.5">12.5</a>. Key Agreement with Key Wrap ..............................<a href="#page-69">69</a>
           <a href="#section-12.5.1">12.5.1</a>. ECDH ..............................................<a href="#page-70">70</a>
   <a href="#section-13">13</a>. Key Object Parameters .........................................<a href="#page-72">72</a>
      <a href="#section-13.1">13.1</a>. Elliptic Curve Keys ......................................<a href="#page-73">73</a>
           <a href="#section-13.1.1">13.1.1</a>. Double Coordinate Curves ..........................<a href="#page-73">73</a>
      <a href="#section-13.2">13.2</a>. Octet Key Pair ...........................................<a href="#page-74">74</a>
      <a href="#section-13.3">13.3</a>. Symmetric Keys ...........................................<a href="#page-75">75</a>
   <a href="#section-14">14</a>. CBOR Encoder Restrictions .....................................<a href="#page-76">76</a>
   <a href="#section-15">15</a>. Application Profiling Considerations ..........................<a href="#page-76">76</a>
   <a href="#section-16">16</a>. IANA Considerations ...........................................<a href="#page-78">78</a>
      <a href="#section-16.1">16.1</a>. CBOR Tag Assignment ......................................<a href="#page-78">78</a>
      <a href="#section-16.2">16.2</a>. COSE Header Parameters Registry ..........................<a href="#page-78">78</a>
      <a href="#section-16.3">16.3</a>. COSE Header Algorithm Parameters Registry ................<a href="#page-79">79</a>
      <a href="#section-16.4">16.4</a>. COSE Algorithms Registry .................................<a href="#page-79">79</a>
      <a href="#section-16.5">16.5</a>. COSE Key Common Parameters Registry ......................<a href="#page-81">81</a>
      <a href="#section-16.6">16.6</a>. COSE Key Type Parameters Registry ........................<a href="#page-81">81</a>
      <a href="#section-16.7">16.7</a>. COSE Key Types Registry ..................................<a href="#page-82">82</a>
      <a href="#section-16.8">16.8</a>. COSE Elliptic Curves Registry ............................<a href="#page-83">83</a>
      <a href="#section-16.9">16.9</a>. Media Type Registrations .................................<a href="#page-84">84</a>
           <a href="#section-16.9.1">16.9.1</a>. COSE Security Message .............................<a href="#page-84">84</a>
           <a href="#section-16.9.2">16.9.2</a>. COSE Key Media Type ...............................<a href="#page-85">85</a>
      <a href="#section-16.10">16.10</a>. CoAP Content-Formats Registry ...........................<a href="#page-87">87</a>
      <a href="#section-16.11">16.11</a>. Expert Review Instructions ..............................<a href="#page-87">87</a>
   <a href="#section-17">17</a>. Security Considerations .......................................<a href="#page-88">88</a>
   <a href="#section-18">18</a>. References ....................................................<a href="#page-90">90</a>
      <a href="#section-18.1">18.1</a>. Normative References .....................................<a href="#page-90">90</a>
      <a href="#section-18.2">18.2</a>. Informative References ...................................<a href="#page-92">92</a>
   <a href="#appendix-A">Appendix A</a>. Guidelines for External Data Authentication of
               Algorithms ............................................<a href="#page-96">96</a>
      <a href="#appendix-A.1">A.1</a>. Algorithm Identification ..................................<a href="#page-96">96</a>
      <a href="#appendix-A.2">A.2</a>. Counter Signature without Headers .........................<a href="#page-99">99</a>
   <a href="#appendix-B">Appendix B</a>. Two Layers of Recipient Information ..................<a href="#page-100">100</a>
   <a href="#appendix-C">Appendix C</a>. Examples .............................................<a href="#page-102">102</a>
      <a href="#appendix-C.1">C.1</a>. Examples of Signed Messages ..............................<a href="#page-103">103</a>
           <a href="#appendix-C.1.1">C.1.1</a>. Single Signature ..................................<a href="#page-103">103</a>



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           <a href="#appendix-C.1.2">C.1.2</a>. Multiple Signers ..................................<a href="#page-103">103</a>
           <a href="#appendix-C.1.3">C.1.3</a>. Counter Signature .................................<a href="#page-104">104</a>
           <a href="#appendix-C.1.4">C.1.4</a>. Signature with Criticality ........................<a href="#page-105">105</a>
      <a href="#appendix-C.2">C.2</a>. Single Signer Examples ...................................<a href="#page-106">106</a>
           <a href="#appendix-C.2.1">C.2.1</a>. Single ECDSA Signature  ...........................<a href="#page-106">106</a>
      <a href="#appendix-C.3">C.3</a>. Examples of Enveloped Messages ...........................<a href="#page-107">107</a>
           <a href="#appendix-C.3.1">C.3.1</a>. Direct ECDH .......................................<a href="#page-107">107</a>
           <a href="#appendix-C.3.2">C.3.2</a>. Direct Plus Key Derivation ........................<a href="#page-108">108</a>
           <a href="#appendix-C.3.3">C.3.3</a>. Counter Signature on Encrypted Content ............<a href="#page-109">109</a>
           <a href="#appendix-C.3.4">C.3.4</a>. Encrypted Content with External Data ..............<a href="#page-111">111</a>
      <a href="#appendix-C.4">C.4</a>. Examples of Encrypted Messages ...........................<a href="#page-111">111</a>
           <a href="#appendix-C.4.1">C.4.1</a>. Simple Encrypted Message ..........................<a href="#page-111">111</a>
           <a href="#appendix-C.4.2">C.4.2</a>. Encrypted Message with a Partial IV ...............<a href="#page-112">112</a>
      <a href="#appendix-C.5">C.5</a>. Examples of MACed Messages ...............................<a href="#page-112">112</a>
           <a href="#appendix-C.5.1">C.5.1</a>. Shared Secret Direct MAC ..........................<a href="#page-112">112</a>
           <a href="#appendix-C.5.2">C.5.2</a>. ECDH Direct MAC ...................................<a href="#page-113">113</a>
           <a href="#appendix-C.5.3">C.5.3</a>. Wrapped MAC .......................................<a href="#page-114">114</a>
           <a href="#appendix-C.5.4">C.5.4</a>. Multi-Recipient MACed Message .....................<a href="#page-115">115</a>
      <a href="#appendix-C.6">C.6</a>. Examples of MAC0 Messages ................................<a href="#page-117">117</a>
           <a href="#appendix-C.6.1">C.6.1</a>. Shared Secret Direct MAC ..........................<a href="#page-117">117</a>
      <a href="#appendix-C.7">C.7</a>. COSE Keys ................................................<a href="#page-117">117</a>
           <a href="#appendix-C.7.1">C.7.1</a>. Public Keys .......................................<a href="#page-117">117</a>
           <a href="#appendix-C.7.2">C.7.2</a>. Private Keys ......................................<a href="#page-119">119</a>
   Acknowledgments ..................................................<a href="#page-121">121</a>
   Author's Address .................................................<a href="#page-121">121</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   There has been an increased focus on small, constrained devices that
   make up the Internet of Things (IoT).  One of the standards that has
   come out of this process is "Concise Binary Object Representation
   (CBOR)" [<a href="./rfc7049" title="&quot;Concise Binary Object Representation (CBOR)&quot;">RFC7049</a>].  CBOR extended the data model of the JavaScript
   Object Notation (JSON) [<a href="./rfc7159" title="&quot;The JavaScript Object Notation (JSON) Data Interchange Format&quot;">RFC7159</a>] by allowing for binary data, among
   other changes.  CBOR is being adopted by several of the IETF working
   groups dealing with the IoT world as their encoding of data
   structures.  CBOR was designed specifically to be both small in terms
   of messages transport and implementation size and be a schema-free
   decoder.  A need exists to provide message security services for IoT,
   and using CBOR as the message-encoding format makes sense.

   The JOSE working group produced a set of documents [<a href="./rfc7515" title="&quot;JSON Web Signature (JWS)&quot;">RFC7515</a>]
   [<a href="./rfc7516" title="&quot;JSON Web Encryption (JWE)&quot;">RFC7516</a>] [<a href="./rfc7517" title="&quot;JSON Web Key (JWK)&quot;">RFC7517</a>] [<a href="./rfc7518" title="&quot;JSON Web Algorithms (JWA)&quot;">RFC7518</a>] using JSON that specified how to
   process encryption, signatures, and Message Authentication Code (MAC)
   operations and how to encode keys using JSON.  This document defines
   the CBOR Object Signing and Encryption (COSE) standard, which does
   the same thing for the CBOR encoding format.  While there is a strong





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   attempt to keep the flavor of the original JSON Object Signing and
   Encryption (JOSE) documents, two considerations are taken into
   account:

   o  CBOR has capabilities that are not present in JSON and are
      appropriate to use.  One example of this is the fact that CBOR has
      a method of encoding binary directly without first converting it
      into a base64-encoded string.

   o  COSE is not a direct copy of the JOSE specification.  In the
      process of creating COSE, decisions that were made for JOSE were
      re-examined.  In many cases, different results were decided on as
      the criteria were not always the same.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Design Changes from JOSE</span>

   o  Define a single top message structure so that encrypted, signed,
      and MACed messages can easily be identified and still have a
      consistent view.

   o  Signed messages distinguish between the protected and unprotected
      parameters that relate to the content from those that relate to
      the signature.

   o  MACed messages are separated from signed messages.

   o  MACed messages have the ability to use the same set of recipient
      algorithms as enveloped messages for obtaining the MAC
      authentication key.

   o  Use binary encodings for binary data rather than base64url
      encodings.

   o  Combine the authentication tag for encryption algorithms with the
      ciphertext.

   o  The set of cryptographic algorithms has been expanded in some
      directions and trimmed in others.













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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Requirements Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in <a href="https://www.rfc-editor.org/bcp/bcp14">BCP</a>
   <a href="https://www.rfc-editor.org/bcp/bcp14">14</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] [<a href="./rfc8174" title="&quot;Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words&quot;">RFC8174</a>] when, and only when, they appear in all
   capitals, as shown here.

   When the words appear in lowercase, this interpretation does not
   apply.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  CBOR Grammar</span>

   There is currently no standard CBOR grammar available for use by
   specifications.  The CBOR structures are therefore described in
   prose.

   The document was developed by first working on the grammar and then
   developing the prose to go with it.  An artifact of this is that the
   prose was written using the primitive type strings defined by CBOR
   Data Definition Language (CDDL) [<a href="#ref-CDDL" title="&quot;CBOR data definition language (CDDL): a notational convention to express CBOR data structures&quot;">CDDL</a>].  In this specification, the
   following primitive types are used:

      any -- non-specific value that permits all CBOR values to be
      placed here.

      bool -- a boolean value (true: major type 7, value 21; false:
      major type 7, value 20).

      bstr -- byte string (major type 2).

      int -- an unsigned integer or a negative integer.

      nil -- a null value (major type 7, value 22).

      nint -- a negative integer (major type 1).

      tstr -- a UTF-8 text string (major type 3).

      uint -- an unsigned integer (major type 0).

   Two syntaxes from CDDL appear in this document as shorthand.  These
   are:

      FOO / BAR -- indicates that either FOO or BAR can appear here.

      [+ FOO] -- indicates that the type FOO appears one or more times
      in an array.



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   As well as the prose description, a version of a CBOR grammar is
   presented in CDDL.  Since CDDL has not been published in an RFC, this
   grammar may not work with the final version of CDDL.  The CDDL
   grammar is informational; the prose description is normative.

   The collected CDDL can be extracted from the XML version of this
   document via the following XPath expression below.  (Depending on the
   XPath evaluator one is using, it may be necessary to deal with &amp;gt;
   as an entity.)

   //artwork[@type='CDDL']/text()

   CDDL expects the initial non-terminal symbol to be the first symbol
   in the file.  For this reason, the first fragment of CDDL is
   presented here.

   start = COSE_Messages / COSE_Key / COSE_KeySet / Internal_Types

   ; This is defined to make the tool quieter:
   Internal_Types = Sig_structure / Enc_structure / MAC_structure /
           COSE_KDF_Context

   The non-terminal Internal_Types is defined for dealing with the
   automated validation tools used during the writing of this document.
   It references those non-terminals that are used for security
   computations but are not emitted for transport.

<span class="h3"><a class="selflink" id="section-1.4" href="#section-1.4">1.4</a>.  CBOR-Related Terminology</span>

   In JSON, maps are called objects and only have one kind of map key: a
   string.  In COSE, we use strings, negative integers, and unsigned
   integers as map keys.  The integers are used for compactness of
   encoding and easy comparison.  The inclusion of strings allows for an
   additional range of short encoded values to be used as well.  Since
   the word "key" is mainly used in its other meaning, as a
   cryptographic key, we use the term "label" for this usage as a map
   key.

   The presence of a label in a COSE map that is not a string or an
   integer is an error.  Applications can either fail processing or
   process messages with incorrect labels; however, they MUST NOT create
   messages with incorrect labels.

   A CDDL grammar fragment defines the non-terminal 'label', as in the
   previous paragraph, and 'values', which permits any value to be used.

   label = int / tstr
   values = any



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<span class="h3"><a class="selflink" id="section-1.5" href="#section-1.5">1.5</a>.  Document Terminology</span>

   In this document, we use the following terminology:

   Byte is a synonym for octet.

   Constrained Application Protocol (CoAP) is a specialized web transfer
   protocol for use in constrained systems.  It is defined in [<a href="./rfc7252" title="&quot;The Constrained Application Protocol (CoAP)&quot;">RFC7252</a>].

   Authenticated Encryption (AE) [<a href="./rfc5116" title="&quot;An Interface and Algorithms for Authenticated Encryption&quot;">RFC5116</a>] algorithms are those
   encryption algorithms that provide an authentication check of the
   contents algorithm with the encryption service.

   Authenticated Encryption with Authenticated Data (AEAD) [<a href="./rfc5116" title="&quot;An Interface and Algorithms for Authenticated Encryption&quot;">RFC5116</a>]
   algorithms provide the same content authentication service as AE
   algorithms, but they additionally provide for authentication of non-
   encrypted data as well.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Basic COSE Structure</span>

   The COSE object structure is designed so that there can be a large
   amount of common code when parsing and processing the different types
   of security messages.  All of the message structures are built on the
   CBOR array type.  The first three elements of the array always
   contain the same information:

   1.  The set of protected header parameters wrapped in a bstr.

   2.  The set of unprotected header parameters as a map.

   3.  The content of the message.  The content is either the plaintext
       or the ciphertext as appropriate.  The content may be detached,
       but the location is still used.  The content is wrapped in a bstr
       when present and is a nil value when detached.

   Elements after this point are dependent on the specific message type.

   COSE messages are also built using the concept of layers to separate
   different types of cryptographic concepts.  As an example of how this
   works, consider the COSE_Encrypt message (<a href="#section-5.1">Section 5.1</a>).  This message
   type is broken into two layers: the content layer and the recipient
   layer.  In the content layer, the plaintext is encrypted and
   information about the encrypted message is placed.  In the recipient
   layer, the content encryption key (CEK) is encrypted and information
   about how it is encrypted for each recipient is placed.  A single
   layer version of the encryption message COSE_Encrypt0 (<a href="#section-5.2">Section 5.2</a>)
   is provided for cases where the CEK is pre-shared.




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   Identification of which type of message has been presented is done by
   the following methods:

   1.  The specific message type is known from the context.  This may be
       defined by a marker in the containing structure or by
       restrictions specified by the application protocol.

   2.  The message type is identified by a CBOR tag.  Messages with a
       CBOR tag are known in this specification as tagged messages,
       while those without the CBOR tag are known as untagged messages.
       This document defines a CBOR tag for each of the message
       structures.  These tags can be found in Table 1.

   3.  When a COSE object is carried in a media type of 'application/
       cose', the optional parameter 'cose-type' can be used to identify
       the embedded object.  The parameter is OPTIONAL if the tagged
       version of the structure is used.  The parameter is REQUIRED if
       the untagged version of the structure is used.  The value to use
       with the parameter for each of the structures can be found in
       Table 1.

   4.  When a COSE object is carried as a CoAP payload, the CoAP
       Content-Format Option can be used to identify the message
       content.  The CoAP Content-Format values can be found in
       Table 26.  The CBOR tag for the message structure is not required
       as each security message is uniquely identified.

   +-------+---------------+---------------+---------------------------+
   | CBOR  | cose-type     | Data Item     | Semantics                 |
   | Tag   |               |               |                           |
   +-------+---------------+---------------+---------------------------+
   | 98    | cose-sign     | COSE_Sign     | COSE Signed Data Object   |
   | 18    | cose-sign1    | COSE_Sign1    | COSE Single Signer Data   |
   |       |               |               | Object                    |
   | 96    | cose-encrypt  | COSE_Encrypt  | COSE Encrypted Data       |
   |       |               |               | Object                    |
   | 16    | cose-encrypt0 | COSE_Encrypt0 | COSE Single Recipient     |
   |       |               |               | Encrypted Data Object     |
   | 97    | cose-mac      | COSE_Mac      | COSE MACed Data Object    |
   | 17    | cose-mac0     | COSE_Mac0     | COSE Mac w/o Recipients   |
   |       |               |               | Object                    |
   +-------+---------------+---------------+---------------------------+

                   Table 1: COSE Message Identification







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   The following CDDL fragment identifies all of the top messages
   defined in this document.  Separate non-terminals are defined for the
   tagged and the untagged versions of the messages.

   COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message

   COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
       COSE_Encrypt / COSE_Encrypt0 /
       COSE_Mac / COSE_Mac0

   COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
       COSE_Encrypt_Tagged / COSE_Encrypt0_Tagged /
       COSE_Mac_Tagged / COSE_Mac0_Tagged

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Header Parameters</span>

   The structure of COSE has been designed to have two buckets of
   information that are not considered to be part of the payload itself,
   but are used for holding information about content, algorithms, keys,
   or evaluation hints for the processing of the layer.  These two
   buckets are available for use in all of the structures except for
   keys.  While these buckets are present, they may not all be usable in
   all instances.  For example, while the protected bucket is defined as
   part of the recipient structure, some of the algorithms used for
   recipient structures do not provide for authenticated data.  If this
   is the case, the protected bucket is left empty.

   Both buckets are implemented as CBOR maps.  The map key is a 'label'
   (<a href="#section-1.4">Section 1.4</a>).  The value portion is dependent on the definition for
   the label.  Both maps use the same set of label/value pairs.  The
   integer and string values for labels have been divided into several
   sections including a standard range, a private range, and a range
   that is dependent on the algorithm selected.  The defined labels can
   be found in the "COSE Header Parameters" IANA registry
   (<a href="#section-16.2">Section 16.2</a>).

   Two buckets are provided for each layer:

   protected:  Contains parameters about the current layer that are to
      be cryptographically protected.  This bucket MUST be empty if it
      is not going to be included in a cryptographic computation.  This
      bucket is encoded in the message as a binary object.  This value
      is obtained by CBOR encoding the protected map and wrapping it in
      a bstr object.  Senders SHOULD encode a zero-length map as a zero-
      length string rather than as a zero-length map (encoded as h'a0').
      The zero-length binary encoding is preferred because it is both
      shorter and the version used in the serialization structures for
      cryptographic computation.  After encoding the map, the value is



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      wrapped in the binary object.  Recipients MUST accept both a zero-
      length binary value and a zero-length map encoded in the binary
      value.  The wrapping allows for the encoding of the protected map
      to be transported with a greater chance that it will not be
      altered in transit.  (Badly behaved intermediates could decode and
      re-encode, but this will result in a failure to verify unless the
      re-encoded byte string is identical to the decoded byte string.)
      This avoids the problem of all parties needing to be able to do a
      common canonical encoding.

   unprotected:  Contains parameters about the current layer that are
      not cryptographically protected.

   Only parameters that deal with the current layer are to be placed at
   that layer.  As an example of this, the parameter 'content type'
   describes the content of the message being carried in the message.
   As such, this parameter is placed only in the content layer and is
   not placed in the recipient or signature layers.  In principle, one
   should be able to process any given layer without reference to any
   other layer.  With the exception of the COSE_Sign structure, the only
   data that needs to cross layers is the cryptographic key.

   The buckets are present in all of the security objects defined in
   this document.  The fields in order are the 'protected' bucket (as a
   CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map'
   type).  The presence of both buckets is required.  The parameters
   that go into the buckets come from the IANA "COSE Header Parameters"
   registry (<a href="#section-16.2">Section 16.2</a>).  Some common parameters are defined in the
   next section, but a number of parameters are defined throughout this
   document.

   Labels in each of the maps MUST be unique.  When processing messages,
   if a label appears multiple times, the message MUST be rejected as
   malformed.  Applications SHOULD verify that the same label does not
   occur in both the protected and unprotected headers.  If the message
   is not rejected as malformed, attributes MUST be obtained from the
   protected bucket before they are obtained from the unprotected
   bucket.













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   The following CDDL fragment represents the two header buckets.  A
   group "Headers" is defined in CDDL that represents the two buckets in
   which attributes are placed.  This group is used to provide these two
   fields consistently in all locations.  A type is also defined that
   represents the map of common headers.

   Headers = (
       protected : empty_or_serialized_map,
       unprotected : header_map
   )

   header_map = {
       Generic_Headers,
       * label =&gt; values
   }

   empty_or_serialized_map = bstr .cbor header_map / bstr .size 0


<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Common COSE Headers Parameters</span>

   This section defines a set of common header parameters.  A summary of
   these parameters can be found in Table 2.  This table should be
   consulted to determine the value of label and the type of the value.

   The set of header parameters defined in this section are:

   alg:  This parameter is used to indicate the algorithm used for the
      security processing.  This parameter MUST be authenticated where
      the ability to do so exists.  This support is provided by AEAD
      algorithms or construction (COSE_Sign, COSE_Sign0, COSE_Mac, and
      COSE_Mac0).  This authentication can be done either by placing the
      header in the protected header bucket or as part of the externally
      supplied data.  The value is taken from the "COSE Algorithms"
      registry (see <a href="#section-16.4">Section 16.4</a>).

   crit:  The parameter is used to indicate which protected header
      labels an application that is processing a message is required to
      understand.  Parameters defined in this document do not need to be
      included as they should be understood by all implementations.
      When present, this parameter MUST be placed in the protected
      header bucket.  The array MUST have at least one value in it.
      Not all labels need to be included in the 'crit' parameter.  The
      rules for deciding which header labels are placed in the array
      are:

      *  Integer labels in the range of 0 to 8 SHOULD be omitted.




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      *  Integer labels in the range -1 to -128 can be omitted as they
         are algorithm dependent.  If an application can correctly
         process an algorithm, it can be assumed that it will correctly
         process all of the common parameters associated with that
         algorithm.  Integer labels in the range -129 to -65536 SHOULD
         be included as these would be less common parameters that might
         not be generally supported.

      *  Labels for parameters required for an application MAY be
         omitted.  Applications should have a statement if the label can
         be omitted.

      The header parameter values indicated by 'crit' can be processed
      by either the security library code or an application using a
      security library; the only requirement is that the parameter is
      processed.  If the 'crit' value list includes a value for which
      the parameter is not in the protected bucket, this is a fatal
      error in processing the message.

   content type:  This parameter is used to indicate the content type of
      the data in the payload or ciphertext fields.  Integers are from
      the "CoAP Content-Formats" IANA registry table [<a href="#ref-COAP.Formats">COAP.Formats</a>].
      Text values following the syntax of "&lt;type-name&gt;/&lt;subtype-name&gt;"
      where &lt;type-name&gt; and &lt;subtype-name&gt; are defined in <a href="./rfc6838#section-4.2">Section&nbsp;4.2 of
      [RFC6838]</a>.  Leading and trailing whitespace is also omitted.
      Textual content values along with parameters and subparameters can
      be located using the IANA "Media Types" registry.  Applications
      SHOULD provide this parameter if the content structure is
      potentially ambiguous.

   kid:  This parameter identifies one piece of data that can be used as
      input to find the needed cryptographic key.  The value of this
      parameter can be matched against the 'kid' member in a COSE_Key
      structure.  Other methods of key distribution can define an
      equivalent field to be matched.  Applications MUST NOT assume that
      'kid' values are unique.  There may be more than one key with the
      same 'kid' value, so all of the keys associated with this 'kid'
      may need to be checked.  The internal structure of 'kid' values is
      not defined and cannot be relied on by applications.  Key
      identifier values are hints about which key to use.  This is not a
      security-critical field.  For this reason, it can be placed in the
      unprotected headers bucket.

   IV:  This parameter holds the Initialization Vector (IV) value.  For
      some symmetric encryption algorithms, this may be referred to as a
      nonce.  The IV can be placed in the unprotected header as
      modifying the IV will cause the decryption to yield plaintext that
      is readily detectable as garbled.



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   Partial IV:  This parameter holds a part of the IV value.  When using
      the COSE_Encrypt0 structure, a portion of the IV can be part of
      the context associated with the key.  This field is used to carry
      a value that causes the IV to be changed for each message.  The IV
      can be placed in the unprotected header as modifying the IV will
      cause the decryption to yield plaintext that is readily detectable
      as garbled.  The 'Initialization Vector' and 'Partial
      Initialization Vector' parameters MUST NOT both be present in the
      same security layer.

      The message IV is generated by the following steps:

      1.  Left-pad the Partial IV with zeros to the length of IV.

      2.  XOR the padded Partial IV with the context IV.

   counter signature:  This parameter holds one or more counter
      signature values.  Counter signatures provide a method of having a
      second party sign some data.  The counter signature parameter can
      occur as an unprotected attribute in any of the following
      structures: COSE_Sign1, COSE_Signature, COSE_Encrypt,
      COSE_recipient, COSE_Encrypt0, COSE_Mac, and COSE_Mac0.  These
      structures all have the same beginning elements, so that a
      consistent calculation of the counter signature can be computed.
      Details on computing counter signatures are found in <a href="#section-4.5">Section 4.5</a>.


























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   +-----------+-------+----------------+-------------+----------------+
   | Name      | Label | Value Type     | Value       | Description    |
   |           |       |                | Registry    |                |
   +-----------+-------+----------------+-------------+----------------+
   | alg       | 1     | int / tstr     | COSE        | Cryptographic  |
   |           |       |                | Algorithms  | algorithm to   |
   |           |       |                | registry    | use            |
   | crit      | 2     | [+ label]      | COSE Header | Critical       |
   |           |       |                | Parameters  | headers to be  |
   |           |       |                | registry    | understood     |
   | content   | 3     | tstr / uint    | CoAP        | Content type   |
   | type      |       |                | Content-    | of the payload |
   |           |       |                | Formats or  |                |
   |           |       |                | Media Types |                |
   |           |       |                | registries  |                |
   | kid       | 4     | bstr           |             | Key identifier |
   | IV        | 5     | bstr           |             | Full           |
   |           |       |                |             | Initialization |
   |           |       |                |             | Vector         |
   | Partial   | 6     | bstr           |             | Partial        |
   | IV        |       |                |             | Initialization |
   |           |       |                |             | Vector         |
   | counter   | 7     | COSE_Signature |             | CBOR-encoded   |
   | signature |       | / [+           |             | signature      |
   |           |       | COSE_Signature |             | structure      |
   |           |       | ]              |             |                |
   +-----------+-------+----------------+-------------+----------------+

                     Table 2: Common Header Parameters

   The CDDL fragment that represents the set of headers defined in this
   section is given below.  Each of the headers is tagged as optional
   because they do not need to be in every map; headers required in
   specific maps are discussed above.

   Generic_Headers = (
       ? 1 =&gt; int / tstr,  ; algorithm identifier
       ? 2 =&gt; [+label],    ; criticality
       ? 3 =&gt; tstr / int,  ; content type
       ? 4 =&gt; bstr,        ; key identifier
       ? 5 =&gt; bstr,        ; IV
       ? 6 =&gt; bstr,        ; Partial IV
       ? 7 =&gt; COSE_Signature / [+COSE_Signature] ; Counter signature
   )







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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Signing Objects</span>

   COSE supports two different signature structures.  COSE_Sign allows
   for one or more signatures to be applied to the same content.
   COSE_Sign1 is restricted to a single signer.  The structures cannot
   be converted between each other; as the signature computation
   includes a parameter identifying which structure is being used, the
   converted structure will fail signature validation.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Signing with One or More Signers</span>

   The COSE_Sign structure allows for one or more signatures to be
   applied to a message payload.  Parameters relating to the content and
   parameters relating to the signature are carried along with the
   signature itself.  These parameters may be authenticated by the
   signature, or just present.  An example of a parameter about the
   content is the content type.  Examples of parameters about the
   signature would be the algorithm and key used to create the signature
   and counter signatures.

   <a href="./rfc5652">RFC 5652</a> indicates that:

      When more than one signature is present, the successful validation
      of one signature associated with a given signer is usually treated
      as a successful signature by that signer.  However, there are some
      application environments where other rules are needed.  An
      application that employs a rule other than one valid signature for
      each signer must specify those rules.  Also, where simple matching
      of the signer identifier is not sufficient to determine whether
      the signatures were generated by the same signer, the application
      specification must describe how to determine which signatures were
      generated by the same signer.  Support for different communities
      of recipients is the primary reason that signers choose to include
      more than one signature.

   For example, the COSE_Sign structure might include signatures
   generated with the Edwards-curve Digital Signature Algorithm (EdDSA)
   [<a href="./rfc8032" title="&quot;Edwards-Curve Digital Signature Algorithm (EdDSA)&quot;">RFC8032</a>] and with the Elliptic Curve Digital Signature Algorithm
   (ECDSA) [<a href="#ref-DSS" title="&quot;Digital Signature Standard (DSS)&quot;">DSS</a>].  This allows recipients to verify the signature
   associated with one algorithm or the other.  More-detailed
   information on multiple signature evaluations can be found in
   [<a href="./rfc5752" title="&quot;Multiple Signatures in Cryptographic Message Syntax (CMS)&quot;">RFC5752</a>].









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   The signature structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Sign
   structure is identified by the CBOR tag 98.  The CDDL fragment that
   represents this is:

   COSE_Sign_Tagged = #6.98(COSE_Sign)

   A COSE Signed Message is defined in two parts.  The CBOR object that
   carries the body and information about the body is called the
   COSE_Sign structure.  The CBOR object that carries the signature and
   information about the signature is called the COSE_Signature
   structure.  Examples of COSE Signed Messages can be found in
   <a href="#appendix-C.1">Appendix C.1</a>.

   The COSE_Sign structure is a CBOR array.  The fields of the array in
   order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   payload:  This field contains the serialized content to be signed.
      If the payload is not present in the message, the application is
      required to supply the payload separately.  The payload is wrapped
      in a bstr to ensure that it is transported without changes.  If
      the payload is transported separately ("detached content"), then a
      nil CBOR object is placed in this location, and it is the
      responsibility of the application to ensure that it will be
      transported without changes.

      Note: When a signature with a message recovery algorithm is used
      (<a href="#section-8">Section 8</a>), the maximum number of bytes that can be recovered is
      the length of the payload.  The size of the payload is reduced by
      the number of bytes that will be recovered.  If all of the bytes
      of the payload are consumed, then the payload is encoded as a
      zero-length binary string rather than as being absent.

   signatures:  This field is an array of signatures.  Each signature is
      represented as a COSE_Signature structure.












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   The CDDL fragment that represents the above text for COSE_Sign
   follows.

   COSE_Sign = [
       Headers,
       payload : bstr / nil,
       signatures : [+ COSE_Signature]
   ]

   The COSE_Signature structure is a CBOR array.  The fields of the
   array in order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   signature:  This field contains the computed signature value.  The
      type of the field is a bstr.  Algorithms MUST specify padding if
      the signature value is not a multiple of 8 bits.

   The CDDL fragment that represents the above text for COSE_Signature
   follows.

   COSE_Signature =  [
       Headers,
       signature : bstr
   ]

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Signing with One Signer</span>

   The COSE_Sign1 signature structure is used when only one signature is
   going to be placed on a message.  The parameters dealing with the
   content and the signature are placed in the same pair of buckets
   rather than having the separation of COSE_Sign.

   The structure can be encoded as either tagged or untagged depending
   on the context it will be used in.  A tagged COSE_Sign1 structure is
   identified by the CBOR tag 18.  The CDDL fragment that represents
   this is:

   COSE_Sign1_Tagged = #6.18(COSE_Sign1)

   The CBOR object that carries the body, the signature, and the
   information about the body and signature is called the COSE_Sign1
   structure.  Examples of COSE_Sign1 messages can be found in
   <a href="#appendix-C.2">Appendix C.2</a>.





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   The COSE_Sign1 structure is a CBOR array.  The fields of the array in
   order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   payload:  This is as described in <a href="#section-4.1">Section 4.1</a>.

   signature:  This field contains the computed signature value.  The
      type of the field is a bstr.

   The CDDL fragment that represents the above text for COSE_Sign1
   follows.

   COSE_Sign1 = [
       Headers,
       payload : bstr / nil,
       signature : bstr
   ]

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Externally Supplied Data</span>

   One of the features offered in the COSE document is the ability for
   applications to provide additional data to be authenticated, but that
   is not carried as part of the COSE object.  The primary reason for
   supporting this can be seen by looking at the CoAP message structure
   [<a href="./rfc7252" title="&quot;The Constrained Application Protocol (CoAP)&quot;">RFC7252</a>], where the facility exists for options to be carried before
   the payload.  Examples of data that can be placed in this location
   would be the CoAP code or CoAP options.  If the data is in the header
   section, then it is available for proxies to help in performing its
   operations.  For example, the Accept Option can be used by a proxy to
   determine if an appropriate value is in the proxy's cache.  But the
   sender can prevent a proxy from changing the set of values that it
   will accept by including that value in the resulting authentication
   tag.  However, it may also be desired to protect these values so that
   if they are modified in transit, it can be detected.

   This document describes the process for using a byte array of
   externally supplied authenticated data; however, the method of
   constructing the byte array is a function of the application.
   Applications that use this feature need to define how the externally
   supplied authenticated data is to be constructed.  Such a
   construction needs to take into account the following issues:

   o  If multiple items are included, applications need to ensure that
      the same byte string is not produced if there are different
      inputs.  This could occur by appending the strings 'AB' and 'CDE'



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      or by appending the strings 'ABC' and 'DE'.  This is usually
      addressed by making fields a fixed width and/or encoding the
      length of the field as part of the output.  Using options from
      CoAP [<a href="./rfc7252" title="&quot;The Constrained Application Protocol (CoAP)&quot;">RFC7252</a>] as an example, these fields use a TLV structure so
      they can be concatenated without any problems.

   o  If multiple items are included, an order for the items needs to be
      defined.  Using options from CoAP as an example, an application
      could state that the fields are to be ordered by the option
      number.

   o  Applications need to ensure that the byte stream is going to be
      the same on both sides.  Using options from CoAP might give a
      problem if the same relative numbering is kept.  An intermediate
      node could insert or remove an option, changing how the relative
      number is done.  An application would need to specify that the
      relative number must be re-encoded to be relative only to the
      options that are in the external data.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Signing and Verification Process</span>

   In order to create a signature, a well-defined byte stream is needed.
   The Sig_structure is used to create the canonical form.  This signing
   and verification process takes in the body information (COSE_Sign or
   COSE_Sign1), the signer information (COSE_Signature), and the
   application data (external source).  A Sig_structure is a CBOR array.
   The fields of the Sig_structure in order are:

   1.  A text string identifying the context of the signature.  The
       context string is:

          "Signature" for signatures using the COSE_Signature structure.

          "Signature1" for signatures using the COSE_Sign1 structure.

          "CounterSignature" for signatures used as counter signature
          attributes.

   2.  The protected attributes from the body structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.

   3.  The protected attributes from the signer structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.  This field is omitted for the COSE_Sign1
       signature structure.





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   4.  The protected attributes from the application encoded in a bstr
       type.  If this field is not supplied, it defaults to a zero-
       length binary string.  (See <a href="#section-4.3">Section 4.3</a> for application guidance
       on constructing this field.)

   5.  The payload to be signed encoded in a bstr type.  The payload is
       placed here independent of how it is transported.

   The CDDL fragment that describes the above text is:

   Sig_structure = [
       context : "Signature" / "Signature1" / "CounterSignature",
       body_protected : empty_or_serialized_map,
       ? sign_protected : empty_or_serialized_map,
       external_aad : bstr,
       payload : bstr
   ]

   How to compute a signature:

   1.  Create a Sig_structure and populate it with the appropriate
       fields.

   2.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string, using the encoding described in <a href="#section-14">Section 14</a>.

   3.  Call the signature creation algorithm passing in K (the key to
       sign with), alg (the algorithm to sign with), and ToBeSigned (the
       value to sign).

   4.  Place the resulting signature value in the 'signature' field of
       the array.

   The steps for verifying a signature are:

   1.  Create a Sig_structure object and populate it with the
       appropriate fields.

   2.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string, using the encoding described in <a href="#section-14">Section 14</a>.

   3.  Call the signature verification algorithm passing in K (the key
       to verify with), alg (the algorithm used sign with), ToBeSigned
       (the value to sign), and sig (the signature to be verified).







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   In addition to performing the signature verification, the application
   may also perform the appropriate checks to ensure that the key is
   correctly paired with the signing identity and that the signing
   identity is authorized before performing actions.

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Computing Counter Signatures</span>

   Counter signatures provide a method of associating a different
   signature generated by different signers with some piece of content.
   This is normally used to provide a signature on a signature allowing
   for a proof that a signature existed at a given time (i.e., a
   Timestamp).  In this document, we allow for counter signatures to
   exist in a greater number of environments.  As an example, it is
   possible to place a counter signature in the unprotected attributes
   of a COSE_Encrypt object.  This would allow for an intermediary to
   either verify that the encrypted byte stream has not been modified,
   without being able to decrypt it, or assert that an encrypted byte
   stream either existed at a given time or passed through it in terms
   of routing (i.e., a proxy signature).

   An example of a counter signature on a signature can be found in
   <a href="#appendix-C.1.3">Appendix C.1.3</a>.  An example of a counter signature in an encryption
   object can be found in <a href="#appendix-C.3.3">Appendix C.3.3</a>.

   The creation and validation of counter signatures over the different
   items relies on the fact that the objects have the same structure.
   The elements are a set of protected attributes, a set of unprotected
   attributes, and a body, in that order.  This means that the
   Sig_structure can be used in a uniform manner to get the byte stream
   for processing a signature.  If the counter signature is going to be
   computed over a COSE_Encrypt structure, the body_protected and
   payload items can be mapped into the Sig_structure in the same manner
   as from the COSE_Sign structure.

   It should be noted that only a signature algorithm with appendix (see
   <a href="#section-8">Section 8</a>) can be used for counter signatures.  This is because the
   body should be able to be processed without having to evaluate the
   counter signature, and this is not possible for signature schemes
   with message recovery.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Encryption Objects</span>

   COSE supports two different encryption structures.  COSE_Encrypt0 is
   used when a recipient structure is not needed because the key to be
   used is known implicitly.  COSE_Encrypt is used the rest of the time.
   This includes cases where there are multiple recipients or a
   recipient algorithm other than direct is used.




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<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Enveloped COSE Structure</span>

   The enveloped structure allows for one or more recipients of a
   message.  There are provisions for parameters about the content and
   parameters about the recipient information to be carried in the
   message.  The protected parameters associated with the content are
   authenticated by the content encryption algorithm.  The protected
   parameters associated with the recipient are authenticated by the
   recipient algorithm (when the algorithm supports it).  Examples of
   parameters about the content are the type of the content and the
   content encryption algorithm.  Examples of parameters about the
   recipient are the recipient's key identifier and the recipient's
   encryption algorithm.

   The same techniques and structures are used for encrypting both the
   plaintext and the keys.  This is different from the approach used by
   both "Cryptographic Message Syntax (CMS)" [<a href="./rfc5652" title="&quot;Cryptographic Message Syntax (CMS)&quot;">RFC5652</a>] and "JSON Web
   Encryption (JWE)" [<a href="./rfc7516" title="&quot;JSON Web Encryption (JWE)&quot;">RFC7516</a>] where different structures are used for
   the content layer and for the recipient layer.  Two structures are
   defined: COSE_Encrypt to hold the encrypted content and
   COSE_recipient to hold the encrypted keys for recipients.  Examples
   of encrypted messages can be found in <a href="#appendix-C.3">Appendix C.3</a>.

   The COSE_Encrypt structure can be encoded as either tagged or
   untagged depending on the context it will be used in.  A tagged
   COSE_Encrypt structure is identified by the CBOR tag 96.  The CDDL
   fragment that represents this is:

   COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)

   The COSE_Encrypt structure is a CBOR array.  The fields of the array
   in order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   ciphertext:  This field contains the ciphertext encoded as a bstr.
      If the ciphertext is to be transported independently of the
      control information about the encryption process (i.e., detached
      content), then the field is encoded as a nil value.

   recipients:  This field contains an array of recipient information
      structures.  The type for the recipient information structure is a
      COSE_recipient.






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   The CDDL fragment that corresponds to the above text is:

   COSE_Encrypt = [
       Headers,
       ciphertext : bstr / nil,
       recipients : [+COSE_recipient]
   ]

   The COSE_recipient structure is a CBOR array.  The fields of the
   array in order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   ciphertext:  This field contains the encrypted key encoded as a bstr.
      All encoded keys are symmetric keys; the binary value of the key
      is the content.  If there is not an encrypted key, then this field
      is encoded as a nil value.

   recipients:  This field contains an array of recipient information
      structures.  The type for the recipient information structure is a
      COSE_recipient (an example of this can be found in <a href="#appendix-B">Appendix B</a>).
      If there are no recipient information structures, this element is
      absent.

   The CDDL fragment that corresponds to the above text for
   COSE_recipient is:

   COSE_recipient = [
       Headers,
       ciphertext : bstr / nil,
       ? recipients : [+COSE_recipient]
   ]

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Content Key Distribution Methods</span>

   An encrypted message consists of an encrypted content and an
   encrypted CEK for one or more recipients.  The CEK is encrypted for
   each recipient, using a key specific to that recipient.  The details
   of this encryption depend on which class the recipient algorithm
   falls into.  Specific details on each of the classes can be found in
   <a href="#section-12">Section 12</a>.  A short summary of the five content key distribution
   methods is:

   direct:  The CEK is the same as the identified previously distributed
      symmetric key or is derived from a previously distributed secret.
      No CEK is transported in the message.



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   symmetric key-encryption keys (KEK):  The CEK is encrypted using a
      previously distributed symmetric KEK.  Also known as key wrap.

   key agreement:  The recipient's public key and a sender's private key
      are used to generate a pairwise secret, a Key Derivation Function
      (KDF) is applied to derive a key, and then the CEK is either the
      derived key or encrypted by the derived key.

   key transport:  The CEK is encrypted with the recipient's public key.
      No key transport algorithms are defined in this document.

   passwords:  The CEK is encrypted in a KEK that is derived from a
      password.  No password algorithms are defined in this document.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Single Recipient Encrypted</span>

   The COSE_Encrypt0 encrypted structure does not have the ability to
   specify recipients of the message.  The structure assumes that the
   recipient of the object will already know the identity of the key to
   be used in order to decrypt the message.  If a key needs to be
   identified to the recipient, the enveloped structure ought to be
   used.

   Examples of encrypted messages can be found in <a href="#appendix-C.3">Appendix C.3</a>.

   The COSE_Encrypt0 structure can be encoded as either tagged or
   untagged depending on the context it will be used in.  A tagged
   COSE_Encrypt0 structure is identified by the CBOR tag 16.  The CDDL
   fragment that represents this is:

   COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)

   The COSE_Encrypt0 structure is a CBOR array.  The fields of the array
   in order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   ciphertext:  This is as described in <a href="#section-5.1">Section 5.1</a>.

   The CDDL fragment for COSE_Encrypt0 that corresponds to the above
   text is:

   COSE_Encrypt0 = [
       Headers,
       ciphertext : bstr / nil,
   ]



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<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  How to Encrypt and Decrypt for AEAD Algorithms</span>

   The encryption algorithm for AEAD algorithms is fairly simple.  The
   first step is to create a consistent byte stream for the
   authenticated data structure.  For this purpose, we use an
   Enc_structure.  The Enc_structure is a CBOR array.  The fields of the
   Enc_structure in order are:

   1.  A text string identifying the context of the authenticated data
       structure.  The context string is:

          "Encrypt0" for the content encryption of a COSE_Encrypt0 data
          structure.

          "Encrypt" for the first layer of a COSE_Encrypt data structure
          (i.e., for content encryption).

          "Enc_Recipient" for a recipient encoding to be placed in an
          COSE_Encrypt data structure.

          "Mac_Recipient" for a recipient encoding to be placed in a
          MACed message structure.

          "Rec_Recipient" for a recipient encoding to be placed in a
          recipient structure.

   2.  The protected attributes from the body structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.

   3.  The protected attributes from the application encoded in a bstr
       type.  If this field is not supplied, it defaults to a zero-
       length bstr.  (See <a href="#section-4.3">Section 4.3</a> for application guidance on
       constructing this field.)

   The CDDL fragment that describes the above text is:

   Enc_structure = [
       context : "Encrypt" / "Encrypt0" / "Enc_Recipient" /
           "Mac_Recipient" / "Rec_Recipient",
       protected : empty_or_serialized_map,
       external_aad : bstr
   ]

   How to encrypt a message:

   1.  Create an Enc_structure and populate it with the appropriate
       fields.



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   2.  Encode the Enc_structure to a byte stream (Additional
       Authenticated Data (AAD)), using the encoding described in
       <a href="#section-14">Section 14</a>.

   3.  Determine the encryption key (K).  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current layer.  Examples are key transport keys
          (<a href="#section-12.3">Section 12.3</a>), key wrap keys (<a href="#section-12.2.1">Section 12.2.1</a>), or pre-shared
          secrets.

       Direct Encryption and Direct Key Agreement:  The key is
          determined by the key and algorithm in the recipient
          structure.  The encryption algorithm and size of the key to be
          used are inputs into the KDF used for the recipient.  (For
          direct, the KDF can be thought of as the identity operation.)
          Examples of these algorithms are found in Sections <a href="#section-12.1.2">12.1.2</a> and
          12.4.1.

       Other:  The key is randomly or pseudorandomly generated.

   4.  Call the encryption algorithm with K (the encryption key), P (the
       plaintext), and AAD.  Place the returned ciphertext into the
       'ciphertext' field of the structure.

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient, using K (the encryption key) as the
       plaintext.

   How to decrypt a message:

   1.  Create an Enc_structure and populate it with the appropriate
       fields.

   2.  Encode the Enc_structure to a byte stream (AAD), using the
       encoding described in <a href="#section-14">Section 14</a>.

   3.  Determine the decryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current layer.  Examples are key transport keys
          (<a href="#section-12.3">Section 12.3</a>), key wrap keys (<a href="#section-12.2.1">Section 12.2.1</a>), or pre-shared
          secrets.






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       Direct Encryption and Direct Key Agreement:  The key is
          determined by the key and algorithm in the recipient
          structure.  The encryption algorithm and size of the key to be
          used are inputs into the KDF used for the recipient.  (For
          direct, the KDF can be thought of as the identity operation.)
          Examples of these algorithms are found in Sections <a href="#section-12.1.2">12.1.2</a> and
          12.4.1.

       Other:  The key is determined by decoding and decrypting one of
          the recipient structures.

   4.  Call the decryption algorithm with K (the decryption key to use),
       C (the ciphertext), and AAD.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  How to Encrypt and Decrypt for AE Algorithms</span>

   How to encrypt a message:

   1.  Verify that the 'protected' field is empty.

   2.  Verify that there was no external additional authenticated data
       supplied for this operation.

   3.  Determine the encryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current layer.  Examples are key transport keys
          (<a href="#section-12.3">Section 12.3</a>), key wrap keys (<a href="#section-12.2.1">Section 12.2.1</a>), or pre-shared
          secrets.

       Direct Encryption and Direct Key Agreement:  The key is
          determined by the key and algorithm in the recipient
          structure.  The encryption algorithm and size of the key to be
          used are inputs into the KDF used for the recipient.  (For
          direct, the KDF can be thought of as the identity operation.)
          Examples of these algorithms are found in Sections <a href="#section-12.1.2">12.1.2</a> and
          12.4.1.

       Other:  The key is randomly generated.

   4.  Call the encryption algorithm with K (the encryption key to use)
       and P (the plaintext).  Place the returned ciphertext into the
       'ciphertext' field of the structure.

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient, using K (the encryption key) as the
       plaintext.



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   How to decrypt a message:

   1.  Verify that the 'protected' field is empty.

   2.  Verify that there was no external additional authenticated data
       supplied for this operation.

   3.  Determine the decryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current layer.  Examples are key transport keys
          (<a href="#section-12.3">Section 12.3</a>), key wrap keys (<a href="#section-12.2.1">Section 12.2.1</a>), or pre-shared
          secrets.

       Direct Encryption and Direct Key Agreement:  The key is
          determined by the key and algorithm in the recipient
          structure.  The encryption algorithm and size of the key to be
          used are inputs into the KDF used for the recipient.  (For
          direct, the KDF can be thought of as the identity operation.)
          Examples of these algorithms are found in Sections <a href="#section-12.1.2">12.1.2</a> and
          12.4.1.

       Other:  The key is determined by decoding and decrypting one of
          the recipient structures.

   4.  Call the decryption algorithm with K (the decryption key to use)
       and C (the ciphertext).

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  MAC Objects</span>

   COSE supports two different MAC structures.  COSE_MAC0 is used when a
   recipient structure is not needed because the key to be used is
   implicitly known.  COSE_MAC is used for all other cases.  These
   include a requirement for multiple recipients, the key being unknown,
   and a recipient algorithm of other than direct.

   In this section, we describe the structure and methods to be used
   when doing MAC authentication in COSE.  This document allows for the
   use of all of the same classes of recipient algorithms as are allowed
   for encryption.

   When using MAC operations, there are two modes in which they can be
   used.  The first is just a check that the content has not been
   changed since the MAC was computed.  Any class of recipient algorithm
   can be used for this purpose.  The second mode is to both check that
   the content has not been changed since the MAC was computed and to
   use the recipient algorithm to verify who sent it.  The classes of



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   recipient algorithms that support this are those that use a pre-
   shared secret or do static-static (SS) key agreement (without the key
   wrap step).  In both of these cases, the entity that created and sent
   the message MAC can be validated.  (This knowledge of the sender
   assumes that there are only two parties involved and that you did not
   send the message to yourself.)  The origination property can be
   obtained with both of the MAC message structures.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  MACed Message with Recipients</span>

   The multiple recipient MACed message uses two structures: the
   COSE_Mac structure defined in this section for carrying the body and
   the COSE_recipient structure (<a href="#section-5.1">Section 5.1</a>) to hold the key used for
   the MAC computation.  Examples of MACed messages can be found in
   <a href="#appendix-C.5">Appendix C.5</a>.

   The MAC structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Mac
   structure is identified by the CBOR tag 97.  The CDDL fragment that
   represents this is:

   COSE_Mac_Tagged = #6.97(COSE_Mac)

   The COSE_Mac structure is a CBOR array.  The fields of the array in
   order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   payload:  This field contains the serialized content to be MACed.  If
      the payload is not present in the message, the application is
      required to supply the payload separately.  The payload is wrapped
      in a bstr to ensure that it is transported without changes.  If
      the payload is transported separately (i.e., detached content),
      then a nil CBOR value is placed in this location, and it is the
      responsibility of the application to ensure that it will be
      transported without changes.

   tag:  This field contains the MAC value.

   recipients:  This is as described in <a href="#section-5.1">Section 5.1</a>.









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   The CDDL fragment that represents the above text for COSE_Mac
   follows.

   COSE_Mac = [
      Headers,
      payload : bstr / nil,
      tag : bstr,
      recipients :[+COSE_recipient]
   ]

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  MACed Messages with Implicit Key</span>

   In this section, we describe the structure and methods to be used
   when doing MAC authentication for those cases where the recipient is
   implicitly known.

   The MACed message uses the COSE_Mac0 structure defined in this
   section for carrying the body.  Examples of MACed messages with an
   implicit key can be found in <a href="#appendix-C.6">Appendix C.6</a>.

   The MAC structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Mac0
   structure is identified by the CBOR tag 17.  The CDDL fragment that
   represents this is:


   COSE_Mac0_Tagged = #6.17(COSE_Mac0)

   The COSE_Mac0 structure is a CBOR array.  The fields of the array in
   order are:

   protected:  This is as described in <a href="#section-3">Section 3</a>.

   unprotected:  This is as described in <a href="#section-3">Section 3</a>.

   payload:  This is as described in <a href="#section-6.1">Section 6.1</a>.

   tag:  This field contains the MAC value.

   The CDDL fragment that corresponds to the above text is:

   COSE_Mac0 = [
      Headers,
      payload : bstr / nil,
      tag : bstr,
   ]





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<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  How to Compute and Verify a MAC</span>

   In order to get a consistent encoding of the data to be
   authenticated, the MAC_structure is used to have a canonical form.
   The MAC_structure is a CBOR array.  The fields of the MAC_structure
   in order are:

   1.  A text string that identifies the structure that is being
       encoded.  This string is "MAC" for the COSE_Mac structure.  This
       string is "MAC0" for the COSE_Mac0 structure.

   2.  The protected attributes from the COSE_MAC structure.  If there
       are no protected attributes, a zero-length bstr is used.

   3.  The protected attributes from the application encoded as a bstr
       type.  If this field is not supplied, it defaults to a zero-
       length binary string.  (See <a href="#section-4.3">Section 4.3</a> for application guidance
       on constructing this field.)

   4.  The payload to be MACed encoded in a bstr type.  The payload is
       placed here independent of how it is transported.

   The CDDL fragment that corresponds to the above text is:

   MAC_structure = [
        context : "MAC" / "MAC0",
        protected : empty_or_serialized_map,
        external_aad : bstr,
        payload : bstr
   ]

   The steps to compute a MAC are:

   1.  Create a MAC_structure and populate it with the appropriate
       fields.

   2.  Create the value ToBeMaced by encoding the MAC_structure to a
       byte stream, using the encoding described in <a href="#section-14">Section 14</a>.

   3.  Call the MAC creation algorithm passing in K (the key to use),
       alg (the algorithm to MAC with), and ToBeMaced (the value to
       compute the MAC on).

   4.  Place the resulting MAC in the 'tag' field of the COSE_Mac or
       COSE_Mac0 structure.

   5.  Encrypt and encode the MAC key for each recipient of the message.




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   The steps to verify a MAC are:

   1.  Create a MAC_structure object and populate it with the
       appropriate fields.

   2.  Create the value ToBeMaced by encoding the MAC_structure to a
       byte stream, using the encoding described in <a href="#section-14">Section 14</a>.

   3.  Obtain the cryptographic key from one of the recipients of the
       message.

   4.  Call the MAC creation algorithm passing in K (the key to use),
       alg (the algorithm to MAC with), and ToBeMaced (the value to
       compute the MAC on).

   5.  Compare the MAC value to the 'tag' field of the COSE_Mac or
       COSE_Mac0 structure.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Key Objects</span>

   A COSE Key structure is built on a CBOR map object.  The set of
   common parameters that can appear in a COSE Key can be found in the
   IANA "COSE Key Common Parameters" registry (<a href="#section-16.5">Section 16.5</a>).
   Additional parameters defined for specific key types can be found in
   the IANA "COSE Key Type Parameters" registry (<a href="#section-16.6">Section 16.6</a>).

   A COSE Key Set uses a CBOR array object as its underlying type.  The
   values of the array elements are COSE Keys.  A COSE Key Set MUST have
   at least one element in the array.  Examples of COSE Key Sets can be
   found in <a href="#appendix-C.7">Appendix C.7</a>.

   Each element in a COSE Key Set MUST be processed independently.  If
   one element in a COSE Key Set is either malformed or uses a key that
   is not understood by an application, that key is ignored and the
   other keys are processed normally.

   The element "kty" is a required element in a COSE_Key map.














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   The CDDL grammar describing COSE_Key and COSE_KeySet is:

   COSE_Key = {
       1 =&gt; tstr / int,          ; kty
       ? 2 =&gt; bstr,              ; kid
       ? 3 =&gt; tstr / int,        ; alg
       ? 4 =&gt; [+ (tstr / int) ], ; key_ops
       ? 5 =&gt; bstr,              ; Base IV
       * label =&gt; values
   }

   COSE_KeySet = [+COSE_Key]

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  COSE Key Common Parameters</span>

   This document defines a set of common parameters for a COSE Key
   object.  Table 3 provides a summary of the parameters defined in this
   section.  There are also parameters that are defined for specific key
   types.  Key-type-specific parameters can be found in <a href="#section-13">Section 13</a>.
































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   +---------+-------+----------------+------------+-------------------+
   | Name    | Label | CBOR Type      | Value      | Description       |
   |         |       |                | Registry   |                   |
   +---------+-------+----------------+------------+-------------------+
   | kty     | 1     | tstr / int     | COSE Key   | Identification of |
   |         |       |                | Common     | the key type      |
   |         |       |                | Parameters |                   |
   |         |       |                |            |                   |
   | kid     | 2     | bstr           |            | Key               |
   |         |       |                |            | identification    |
   |         |       |                |            | value -- match to |
   |         |       |                |            | kid in message    |
   |         |       |                |            |                   |
   | alg     | 3     | tstr / int     | COSE       | Key usage         |
   |         |       |                | Algorithms | restriction to    |
   |         |       |                |            | this algorithm    |
   |         |       |                |            |                   |
   | key_ops | 4     | [+ (tstr/int)] |            | Restrict set of   |
   |         |       |                |            | permissible       |
   |         |       |                |            | operations        |
   |         |       |                |            |                   |
   | Base IV | 5     | bstr           |            | Base IV to be     |
   |         |       |                |            | xor-ed with       |
   |         |       |                |            | Partial IVs       |
   +---------+-------+----------------+------------+-------------------+

                          Table 3: Key Map Labels

   kty:  This parameter is used to identify the family of keys for this
      structure and, thus, the set of key-type-specific parameters to be
      found.  The set of values defined in this document can be found in
      Table 21.  This parameter MUST be present in a key object.
      Implementations MUST verify that the key type is appropriate for
      the algorithm being processed.  The key type MUST be included as
      part of the trust decision process.

   alg:  This parameter is used to restrict the algorithm that is used
      with the key.  If this parameter is present in the key structure,
      the application MUST verify that this algorithm matches the
      algorithm for which the key is being used.  If the algorithms do
      not match, then this key object MUST NOT be used to perform the
      cryptographic operation.  Note that the same key can be in a
      different key structure with a different or no algorithm
      specified; however, this is considered to be a poor security
      practice.






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   kid:  This parameter is used to give an identifier for a key.  The
      identifier is not structured and can be anything from a user-
      provided string to a value computed on the public portion of the
      key.  This field is intended for matching against a 'kid'
      parameter in a message in order to filter down the set of keys
      that need to be checked.

   key_ops:  This parameter is defined to restrict the set of operations
      that a key is to be used for.  The value of the field is an array
      of values from Table 4.  Algorithms define the values of key ops
      that are permitted to appear and are required for specific
      operations.  The set of values matches that in [<a href="./rfc7517" title="&quot;JSON Web Key (JWK)&quot;">RFC7517</a>] and
      [<a href="#ref-W3C.WebCrypto">W3C.WebCrypto</a>].

   Base IV:  This parameter is defined to carry the base portion of an
      IV.  It is designed to be used with the Partial IV header
      parameter defined in <a href="#section-3.1">Section 3.1</a>.  This field provides the ability
      to associate a Partial IV with a key that is then modified on a
      per message basis with the Partial IV.

      Extreme care needs to be taken when using a Base IV in an
      application.  Many encryption algorithms lose security if the same
      IV is used twice.

      If different keys are derived for each sender, using the same Base
      IV with Partial IVs starting at zero is likely to ensure that the
      IV would not be used twice for a single key.  If different keys
      are derived for each sender, starting at the same Base IV is
      likely to satisfy this condition.  If the same key is used for
      multiple senders, then the application needs to provide for a
      method of dividing the IV space up between the senders.  This
      could be done by providing a different base point to start from or
      a different Partial IV to start with and restricting the number of
      messages to be sent before rekeying.

















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   +---------+-------+-------------------------------------------------+
   | Name    | Value | Description                                     |
   +---------+-------+-------------------------------------------------+
   | sign    | 1     | The key is used to create signatures.  Requires |
   |         |       | private key fields.                             |
   | verify  | 2     | The key is used for verification of signatures. |
   | encrypt | 3     | The key is used for key transport encryption.   |
   | decrypt | 4     | The key is used for key transport decryption.   |
   |         |       | Requires private key fields.                    |
   | wrap    | 5     | The key is used for key wrap encryption.        |
   | key     |       |                                                 |
   | unwrap  | 6     | The key is used for key wrap decryption.        |
   | key     |       | Requires private key fields.                    |
   | derive  | 7     | The key is used for deriving keys.  Requires    |
   | key     |       | private key fields.                             |
   | derive  | 8     | The key is used for deriving bits not to be     |
   | bits    |       | used as a key.  Requires private key fields.    |
   | MAC     | 9     | The key is used for creating MACs.              |
   | create  |       |                                                 |
   | MAC     | 10    | The key is used for validating MACs.            |
   | verify  |       |                                                 |
   +---------+-------+-------------------------------------------------+

                       Table 4: Key Operation Values

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Signature Algorithms</span>

   There are two signature algorithm schemes.  The first is signature
   with appendix.  In this scheme, the message content is processed and
   a signature is produced; the signature is called the appendix.  This
   is the scheme used by algorithms such as ECDSA and the RSA
   Probabilistic Signature Scheme (RSASSA-PSS).  (In fact, the SSA in
   RSASSA-PSS stands for Signature Scheme with Appendix.)

   The signature functions for this scheme are:

      signature = Sign(message content, key)

      valid = Verification(message content, key, signature)

   The second scheme is signature with message recovery (an example of
   such an algorithm is [<a href="#ref-PVSig" title="&quot;Formal Security Proofs for a Signature Scheme with Partial Message Recovery&quot;">PVSig</a>]).  In this scheme, the message content
   is processed, but part of it is included in the signature.  Moving
   bytes of the message content into the signature allows for smaller
   signatures; the signature size is still potentially large, but the
   message content has shrunk.  This has implications for systems
   implementing these algorithms and for applications that use them.
   The first is that the message content is not fully available until



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   after a signature has been validated.  Until that point, the part of
   the message contained inside of the signature is unrecoverable.  The
   second is that the security analysis of the strength of the signature
   is very much based on the structure of the message content.  Messages
   that are highly predictable require additional randomness to be
   supplied as part of the signature process.  In the worst case, it
   becomes the same as doing a signature with appendix.  Finally, in the
   event that multiple signatures are applied to a message, all of the
   signature algorithms are going to be required to consume the same
   number of bytes of message content.  This means that the mixing of
   the different schemes in a single message is not supported, and if a
   recovery signature scheme is used, then the same amount of content
   needs to be consumed by all of the signatures.

   The signature functions for this scheme are:

    signature, message sent = Sign(message content, key)

    valid, message content = Verification(message sent, key, signature)

   Signature algorithms are used with the COSE_Signature and COSE_Sign1
   structures.  At this time, only signatures with appendixes are
   defined for use with COSE; however, considerable interest has been
   expressed in using a signature with message recovery algorithm due to
   the effective size reduction that is possible.  Implementations will
   need to keep this in mind for later possible integration.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  ECDSA</span>

   ECDSA [<a href="#ref-DSS" title="&quot;Digital Signature Standard (DSS)&quot;">DSS</a>] defines a signature algorithm using ECC.  Implementations
   SHOULD use a deterministic version of ECDSA such as the one defined
   in [<a href="./rfc6979" title="&quot;Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)&quot;">RFC6979</a>].  The use of a deterministic signature algorithm allows
   for systems to avoid relying on random number generators in order to
   avoid generating the same value of 'k' (the per-message random
   value).  Biased generation of the value 'k' can be attacked, and
   collisions of this value leads to leaked keys.  It additionally
   allows for doing deterministic tests for the signature algorithm.
   The use of deterministic ECDSA does not lessen the need to have good
   random number generation when creating the private key.

   The ECDSA signature algorithm is parameterized with a hash function
   (h).  In the event that the length of the hash function output is
   greater than the group of the key, the leftmost bytes of the hash
   output are used.







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   The algorithms defined in this document can be found in Table 5.

              +-------+-------+---------+------------------+
              | Name  | Value | Hash    | Description      |
              +-------+-------+---------+------------------+
              | ES256 | -7    | SHA-256 | ECDSA w/ SHA-256 |
              | ES384 | -35   | SHA-384 | ECDSA w/ SHA-384 |
              | ES512 | -36   | SHA-512 | ECDSA w/ SHA-512 |
              +-------+-------+---------+------------------+

                      Table 5: ECDSA Algorithm Values

   This document defines ECDSA to work only with the curves P-256,
   P-384, and P-521.  This document requires that the curves be encoded
   using the 'EC2' (2 coordinate elliptic curve) key type.
   Implementations need to check that the key type and curve are correct
   when creating and verifying a signature.  Other documents can define
   it to work with other curves and points in the future.

   In order to promote interoperability, it is suggested that SHA-256 be
   used only with curve P-256, SHA-384 be used only with curve P-384,
   and SHA-512 be used with curve P-521.  This is aligned with the
   recommendation in <a href="./rfc5480#section-4">Section&nbsp;4 of [RFC5480]</a>.

   The signature algorithm results in a pair of integers (R, S).  These
   integers will be the same length as the length of the key used for
   the signature process.  The signature is encoded by converting the
   integers into byte strings of the same length as the key size.  The
   length is rounded up to the nearest byte and is left padded with zero
   bits to get to the correct length.  The two integers are then
   concatenated together to form a byte string that is the resulting
   signature.

   Using the function defined in [<a href="./rfc8017" title="&quot;PKCS #1: RSA Cryptography Specifications Version 2.2&quot;">RFC8017</a>], the signature is:
   Signature = I2OSP(R, n) | I2OSP(S, n)
   where n = ceiling(key_length / 8)

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'EC2'.

   o  If the 'alg' field is present, it MUST match the ECDSA signature
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'sign' when
      creating an ECDSA signature.




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   o  If the 'key_ops' field is present, it MUST include 'verify' when
      verifying an ECDSA signature.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  Security Considerations</span>

   The security strength of the signature is no greater than the minimum
   of the security strength associated with the bit length of the key
   and the security strength of the hash function.

   Note: Use of this technique is a good idea even when good random
   number generation exists.  Doing so both reduces the possibility of
   having the same value of 'k' in two signature operations and allows
   for reproducible signature values, which helps testing.

   There are two substitution attacks that can theoretically be mounted
   against the ECDSA signature algorithm.

   o  Changing the curve used to validate the signature: If one changes
      the curve used to validate the signature, then potentially one
      could have two messages with the same signature, each computed
      under a different curve.  The only requirement on the new curve is
      that its order be the same as the old one and it be acceptable to
      the client.  An example would be to change from using the curve
      secp256r1 (aka P-256) to using secp256k1.  (Both are 256-bit
      curves.)  We currently do not have any way to deal with this
      version of the attack except to restrict the overall set of curves
      that can be used.

   o  Change the hash function used to validate the signature: If one
      either has two different hash functions of the same length or can
      truncate a hash function down, then one could potentially find
      collisions between the hash functions rather than within a single
      hash function (for example, truncating SHA-512 to 256 bits might
      collide with a SHA-256 bit hash value).  As the hash algorithm is
      part of the signature algorithm identifier, this attack is
      mitigated by including a signature algorithm identifier in the
      protected header.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Edwards-Curve Digital Signature Algorithms (EdDSAs)</span>

   [<a id="ref-RFC8032">RFC8032</a>] describes the elliptic curve signature scheme Edwards-curve
   Digital Signature Algorithm (EdDSA).  In that document, the signature
   algorithm is instantiated using parameters for edwards25519 and
   edwards448 curves.  The document additionally describes two variants
   of the EdDSA algorithm: Pure EdDSA, where no hash function is applied
   to the content before signing, and HashEdDSA, where a hash function
   is applied to the content before signing and the result of that hash
   function is signed.  For EdDSA, the content to be signed (either the



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   message or the pre-hash value) is processed twice inside of the
   signature algorithm.  For use with COSE, only the pure EdDSA version
   is used.  This is because it is not expected that extremely large
   contents are going to be needed and, based on the arrangement of the
   message structure, the entire message is going to need to be held in
   memory in order to create or verify a signature.  This means that
   there does not appear to be a need to be able to do block updates of
   the hash, followed by eliminating the message from memory.
   Applications can provide the same features by defining the content of
   the message as a hash value and transporting the COSE object (with
   the hash value) and the content as separate items.

   The algorithms defined in this document can be found in Table 6.  A
   single signature algorithm is defined, which can be used for multiple
   curves.

                      +-------+-------+-------------+
                      | Name  | Value | Description |
                      +-------+-------+-------------+
                      | EdDSA | -8    | EdDSA       |
                      +-------+-------+-------------+

                      Table 6: EdDSA Algorithm Values

   [<a id="ref-RFC8032">RFC8032</a>] describes the method of encoding the signature value.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'OKP' (Octet Key
      Pair).

   o  The 'crv' field MUST be present, and it MUST be a curve defined
      for this signature algorithm.

   o  If the 'alg' field is present, it MUST match 'EdDSA'.

   o  If the 'key_ops' field is present, it MUST include 'sign' when
      creating an EdDSA signature.

   o  If the 'key_ops' field is present, it MUST include 'verify' when
      verifying an EdDSA signature.

<span class="h4"><a class="selflink" id="section-8.2.1" href="#section-8.2.1">8.2.1</a>.  Security Considerations</span>

   How public values are computed is not the same when looking at EdDSA
   and Elliptic Curve Diffie-Hellman (ECDH); for this reason, they
   should not be used with the other algorithm.



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   If batch signature verification is performed, a well-seeded
   cryptographic random number generator is REQUIRED.  Signing and non-
   batch signature verification are deterministic operations and do not
   need random numbers of any kind.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Message Authentication Code (MAC) Algorithms</span>

   Message Authentication Codes (MACs) provide data authentication and
   integrity protection.  They provide either no or very limited data
   origination.  A MAC, for example, can be used to prove the identity
   of the sender to a third party.

   MACs use the same scheme as signature with appendix algorithms.  The
   message content is processed and an authentication code is produced.
   The authentication code is frequently called a tag.

   The MAC functions are:

   tag = MAC_Create(message content, key)

   valid = MAC_Verify(message content, key, tag)

   MAC algorithms can be based on either a block cipher algorithm (i.e.,
   AES-MAC) or a hash algorithm (i.e., a Hash-based Message
   Authentication Code (HMAC)).  This document defines a MAC algorithm
   using each of these constructions.

   MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Hash-Based Message Authentication Codes (HMACs)</span>

   HMAC [<a href="./rfc2104" title="&quot;HMAC: Keyed- Hashing for Message Authentication&quot;">RFC2104</a>] [<a href="./rfc4231" title="&quot;Identifiers and Test Vectors for HMAC-SHA- 224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512&quot;">RFC4231</a>] was designed to deal with length extension
   attacks.  The algorithm was also designed to allow for new hash
   algorithms to be directly plugged in without changes to the hash
   function.  The HMAC design process has been shown as solid since,
   while the security of hash algorithms such as MD5 has decreased over
   time; the security of HMAC combined with MD5 has not yet been shown
   to be compromised [<a href="./rfc6151" title="&quot;Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms&quot;">RFC6151</a>].

   The HMAC algorithm is parameterized by an inner and outer padding, a
   hash function (h), and an authentication tag value length.  For this
   specification, the inner and outer padding are fixed to the values
   set in [<a href="./rfc2104" title="&quot;HMAC: Keyed- Hashing for Message Authentication&quot;">RFC2104</a>].  The length of the authentication tag corresponds
   to the difficulty of producing a forgery.  For use in constrained
   environments, we define a set of HMAC algorithms that are truncated.






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   There are currently no known issues with truncation; however, the
   security strength of the message tag is correspondingly reduced in
   strength.  When truncating, the leftmost tag length bits are kept and
   transmitted.

   The algorithms defined in this document can be found in Table 7.

   +-----------+-------+---------+----------+--------------------------+
   | Name      | Value | Hash    | Tag      | Description              |
   |           |       |         | Length   |                          |
   +-----------+-------+---------+----------+--------------------------+
   | HMAC      | 4     | SHA-256 | 64       | HMAC w/ SHA-256          |
   | 256/64    |       |         |          | truncated to 64 bits     |
   | HMAC      | 5     | SHA-256 | 256      | HMAC w/ SHA-256          |
   | 256/256   |       |         |          |                          |
   | HMAC      | 6     | SHA-384 | 384      | HMAC w/ SHA-384          |
   | 384/384   |       |         |          |                          |
   | HMAC      | 7     | SHA-512 | 512      | HMAC w/ SHA-512          |
   | 512/512   |       |         |          |                          |
   +-----------+-------+---------+----------+--------------------------+

                      Table 7: HMAC Algorithm Values

   Some recipient algorithms carry the key while others derive a key
   from secret data.  For those algorithms that carry the key (such as
   AES Key Wrap), the size of the HMAC key SHOULD be the same size as
   the underlying hash function.  For those algorithms that derive the
   key (such as ECDH), the derived key MUST be the same size as the
   underlying hash function.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the HMAC algorithm
      being used.

   o  If the 'key_ops' field is present, it MUST include 'MAC create'
      when creating an HMAC authentication tag.

   o  If the 'key_ops' field is present, it MUST include 'MAC verify'
      when verifying an HMAC authentication tag.

   Implementations creating and validating MAC values MUST validate that
   the key type, key length, and algorithm are correct and appropriate
   for the entities involved.




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<span class="h4"><a class="selflink" id="section-9.1.1" href="#section-9.1.1">9.1.1</a>.  Security Considerations</span>

   HMAC has proved to be resistant to attack even when used with
   weakened hash algorithms.  The current best known attack is to brute
   force the key.  This means that key size is going to be directly
   related to the security of an HMAC operation.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  AES Message Authentication Code (AES-CBC-MAC)</span>

   AES-CBC-MAC is defined in [<a href="#ref-MAC" title="&quot;Computer Data Authentication&quot;">MAC</a>].  (Note that this is not the same
   algorithm as AES Cipher-Based Message Authentication Code (AES-CMAC)
   [<a href="./rfc4493" title="&quot;The AES-CMAC Algorithm&quot;">RFC4493</a>].)

   AES-CBC-MAC is parameterized by the key length, the authentication
   tag length, and the IV used.  For all of these algorithms, the IV is
   fixed to all zeros.  We provide an array of algorithms for various
   key lengths and tag lengths.  The algorithms defined in this document
   are found in Table 8.

   +-------------+-------+----------+----------+-----------------------+
   | Name        | Value | Key      | Tag      | Description           |
   |             |       | Length   | Length   |                       |
   +-------------+-------+----------+----------+-----------------------+
   | AES-MAC     | 14    | 128      | 64       | AES-MAC 128-bit key,  |
   | 128/64      |       |          |          | 64-bit tag            |
   | AES-MAC     | 15    | 256      | 64       | AES-MAC 256-bit key,  |
   | 256/64      |       |          |          | 64-bit tag            |
   | AES-MAC     | 25    | 128      | 128      | AES-MAC 128-bit key,  |
   | 128/128     |       |          |          | 128-bit tag           |
   | AES-MAC     | 26    | 256      | 128      | AES-MAC 256-bit key,  |
   | 256/128     |       |          |          | 128-bit tag           |
   +-------------+-------+----------+----------+-----------------------+

                     Table 8: AES-MAC Algorithm Values

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations creating and validating MAC values MUST
   validate that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the AES-MAC algorithm
      being used.




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   o  If the 'key_ops' field is present, it MUST include 'MAC create'
      when creating an AES-MAC authentication tag.

   o  If the 'key_ops' field is present, it MUST include 'MAC verify'
      when verifying an AES-MAC authentication tag.

<span class="h4"><a class="selflink" id="section-9.2.1" href="#section-9.2.1">9.2.1</a>.  Security Considerations</span>

   A number of attacks exist against Cipher Block Chaining Message
   Authentication Code (CBC-MAC) that need to be considered.

   o  A single key must only be used for messages of a fixed and known
      length.  If this is not the case, an attacker will be able to
      generate a message with a valid tag given two message and tag
      pairs.  This can be addressed by using different keys for messages
      of different lengths.  The current structure mitigates this
      problem, as a specific encoding structure that includes lengths is
      built and signed.  (CMAC also addresses this issue.)

   o  Cipher Block Chaining (CBC) mode, if the same key is used for both
      encryption and authentication operations, an attacker can produce
      messages with a valid authentication code.

   o  If the IV can be modified, then messages can be forged.  This is
      addressed by fixing the IV to all zeros.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Content Encryption Algorithms</span>

   Content encryption algorithms provide data confidentiality for
   potentially large blocks of data using a symmetric key.  They provide
   integrity on the data that was encrypted; however, they provide
   either no or very limited data origination.  (One cannot, for
   example, be used to prove the identity of the sender to a third
   party.)  The ability to provide data origination is linked to how the
   CEK is obtained.

   COSE restricts the set of legal content encryption algorithms to
   those that support authentication both of the content and additional
   data.  The encryption process will generate some type of
   authentication value, but that value may be either explicit or
   implicit in terms of the algorithm definition.  For simplicity's
   sake, the authentication code will normally be defined as being
   appended to the ciphertext stream.  The encryption functions are:

   ciphertext = Encrypt(message content, key, additional data)

   valid, message content = Decrypt(cipher text, key, additional data)




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   Most AEAD algorithms are logically defined as returning the message
   content only if the decryption is valid.  Many but not all

   implementations will follow this convention.  The message content
   MUST NOT be used if the decryption does not validate.

   These algorithms are used in COSE_Encrypt and COSE_Encrypt0.

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>.  AES GCM</span>

   The Galois/Counter Mode (GCM) mode is a generic authenticated
   encryption block cipher mode defined in [<a href="#ref-AES-GCM" title="&quot;Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC&quot;">AES-GCM</a>].  The GCM mode is
   combined with the AES block encryption algorithm to define an AEAD
   cipher.

   The GCM mode is parameterized by the size of the authentication tag
   and the size of the nonce.  This document fixes the size of the nonce
   at 96 bits.  The size of the authentication tag is limited to a small
   set of values.  For this document however, the size of the
   authentication tag is fixed at 128 bits.

   The set of algorithms defined in this document are in Table 9.

      +---------+-------+------------------------------------------+
      | Name    | Value | Description                              |
      +---------+-------+------------------------------------------+
      | A128GCM | 1     | AES-GCM mode w/ 128-bit key, 128-bit tag |
      | A192GCM | 2     | AES-GCM mode w/ 192-bit key, 128-bit tag |
      | A256GCM | 3     | AES-GCM mode w/ 256-bit key, 128-bit tag |
      +---------+-------+------------------------------------------+

                   Table 9: Algorithm Value for AES-GCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the AES-GCM algorithm
      being used.

   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'wrap key' when encrypting.



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   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'unwrap key' when decrypting.

<span class="h4"><a class="selflink" id="section-10.1.1" href="#section-10.1.1">10.1.1</a>.  Security Considerations</span>

   When using AES-GCM, the following restrictions MUST be enforced:

   o  The key and nonce pair MUST be unique for every message encrypted.

   o  The total amount of data encrypted for a single key MUST NOT
      exceed 2^39 - 256 bits.  An explicit check is required only in
      environments where it is expected that it might be exceeded.

   Consideration was given to supporting smaller tag values; the
   constrained community would desire tag sizes in the 64-bit range.
   Doing so drastically changes both the maximum messages size
   (generally not an issue) and the number of times that a key can be
   used.  Given that Counter with CBC-MAC (CCM) is the usual mode for
   constrained environments, restricted modes are not supported.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>.  AES CCM</span>

   CCM is a generic authentication encryption block cipher mode defined
   in [<a href="./rfc3610" title="&quot;Counter with CBC-MAC (CCM)&quot;">RFC3610</a>].  The CCM mode is combined with the AES block encryption
   algorithm to define a commonly used content encryption algorithm used
   in constrained devices.

   The CCM mode has two parameter choices.  The first choice is M, the
   size of the authentication field.  The choice of the value for M
   involves a trade-off between message growth (from the tag) and the
   probability that an attacker can undetectably modify a message.  The
   second choice is L, the size of the length field.  This value
   requires a trade-off between the maximum message size and the size of
   the Nonce.

   It is unfortunate that the specification for CCM specified L and M as
   a count of bytes rather than a count of bits.  This leads to possible
   misunderstandings where AES-CCM-8 is frequently used to refer to a
   version of CCM mode where the size of the authentication is 64 bits
   and not 8 bits.  These values have traditionally been specified as
   bit counts rather than byte counts.  This document will follow the
   convention of using bit counts so that it is easier to compare the
   different algorithms presented in this document.

   We define a matrix of algorithms in this document over the values of
   L and M.  Constrained devices are usually operating in situations
   where they use short messages and want to avoid doing recipient-
   specific cryptographic operations.  This favors smaller values of



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   both L and M.  Less-constrained devices will want to be able to use
   larger messages and are more willing to generate new keys for every
   operation.  This favors larger values of L and M.

   The following values are used for L:

   16 bits (2):  This limits messages to 2^16 bytes (64 KiB) in length.
      This is sufficiently long for messages in the constrained world.
      The nonce length is 13 bytes allowing for 2^(13*8) possible values
      of the nonce without repeating.

   64 bits (8):  This limits messages to 2^64 bytes in length.  The
      nonce length is 7 bytes allowing for 2^56 possible values of the
      nonce without repeating.

   The following values are used for M:

   64 bits (8):  This produces a 64-bit authentication tag.  This
      implies that there is a 1 in 2^64 chance that a modified message
      will authenticate.

   128 bits (16):  This produces a 128-bit authentication tag.  This
      implies that there is a 1 in 2^128 chance that a modified message
      will authenticate.



























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   +--------------------+-------+----+-----+-----+---------------------+
   | Name               | Value | L  | M   | k   | Description         |
   +--------------------+-------+----+-----+-----+---------------------+
   | AES-CCM-16-64-128  | 10    | 16 | 64  | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key, 64-bit |
   |                    |       |    |     |     | tag, 13-byte nonce  |
   | AES-CCM-16-64-256  | 11    | 16 | 64  | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key, 64-bit |
   |                    |       |    |     |     | tag, 13-byte nonce  |
   | AES-CCM-64-64-128  | 12    | 64 | 64  | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key, 64-bit |
   |                    |       |    |     |     | tag, 7-byte nonce   |
   | AES-CCM-64-64-256  | 13    | 64 | 64  | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key, 64-bit |
   |                    |       |    |     |     | tag, 7-byte nonce   |
   | AES-CCM-16-128-128 | 30    | 16 | 128 | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key,        |
   |                    |       |    |     |     | 128-bit tag,        |
   |                    |       |    |     |     | 13-byte nonce       |
   | AES-CCM-16-128-256 | 31    | 16 | 128 | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key,        |
   |                    |       |    |     |     | 128-bit tag,        |
   |                    |       |    |     |     | 13-byte nonce       |
   | AES-CCM-64-128-128 | 32    | 64 | 128 | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key,        |
   |                    |       |    |     |     | 128-bit tag, 7-byte |
   |                    |       |    |     |     | nonce               |
   | AES-CCM-64-128-256 | 33    | 64 | 128 | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key,        |
   |                    |       |    |     |     | 128-bit tag, 7-byte |
   |                    |       |    |     |     | nonce               |
   +--------------------+-------+----+-----+-----+---------------------+

                  Table 10: Algorithm Values for AES-CCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the AES-CCM algorithm
      being used.




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   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'wrap key' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'unwrap key' when decrypting.

<span class="h4"><a class="selflink" id="section-10.2.1" href="#section-10.2.1">10.2.1</a>.  Security Considerations</span>

   When using AES-CCM, the following restrictions MUST be enforced:

   o  The key and nonce pair MUST be unique for every message encrypted.
      Note that the value of L influences the number of unique nonces.

   o  The total number of times the AES block cipher is used MUST NOT
      exceed 2^61 operations.  This limitation is the sum of times the
      block cipher is used in computing the MAC value and in performing
      stream encryption operations.  An explicit check is required only
      in environments where it is expected that it might be exceeded.

   [<a id="ref-RFC3610">RFC3610</a>] additionally calls out one other consideration of note.  It
   is possible to do a pre-computation attack against the algorithm in
   cases where portions of the plaintext are highly predictable.  This
   reduces the security of the key size by half.  Ways to deal with this
   attack include adding a random portion to the nonce value and/or
   increasing the key size used.  Using a portion of the nonce for a
   random value will decrease the number of messages that a single key
   can be used for.  Increasing the key size may require more resources
   in the constrained device.  See Sections <a href="#section-5">5</a> and <a href="#section-10">10</a> of [<a href="./rfc3610" title="&quot;Counter with CBC-MAC (CCM)&quot;">RFC3610</a>] for
   more information.

<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a>.  ChaCha20 and Poly1305</span>

   ChaCha20 and Poly1305 combined together is an AEAD mode that is
   defined in [<a href="./rfc7539" title="&quot;ChaCha20 and Poly1305 for IETF Protocols&quot;">RFC7539</a>].  This is an algorithm defined to be a cipher
   that is not AES and thus would not suffer from any future weaknesses
   found in AES.  These cryptographic functions are designed to be fast
   in software-only implementations.

   The ChaCha20/Poly1305 AEAD construction defined in [<a href="./rfc7539" title="&quot;ChaCha20 and Poly1305 for IETF Protocols&quot;">RFC7539</a>] has no
   parameterization.  It takes a 256-bit key and a 96-bit nonce, as well
   as the plaintext and additional data as inputs and produces the
   ciphertext as an option.  We define one algorithm identifier for this
   algorithm in Table 11.








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   +-------------------+-------+---------------------------------------+
   | Name              | Value | Description                           |
   +-------------------+-------+---------------------------------------+
   | ChaCha20/Poly1305 | 24    | ChaCha20/Poly1305 w/ 256-bit key,     |
   |                   |       | 128-bit tag                           |
   +-------------------+-------+---------------------------------------+

                   Table 11: Algorithm Value for AES-GCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the ChaCha20/Poly1305
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'wrap key' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'unwrap key' when decrypting.

<span class="h4"><a class="selflink" id="section-10.3.1" href="#section-10.3.1">10.3.1</a>.  Security Considerations</span>

   The key and nonce values MUST be a unique pair for every invocation
   of the algorithm.  Nonce counters are considered to be an acceptable
   way of ensuring that they are unique.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Key Derivation Functions (KDFs)</span>

   KDFs are used to take some secret value and generate a different one.
   The secret value comes in three flavors:

   o  Secrets that are uniformly random: This is the type of secret that
      is created by a good random number generator.

   o  Secrets that are not uniformly random: This is type of secret that
      is created by operations like key agreement.

   o  Secrets that are not random: This is the type of secret that
      people generate for things like passwords.




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   General KDFs work well with the first type of secret, can do
   reasonably well with the second type of secret, and generally do
   poorly with the last type of secret.  None of the KDFs in this
   section are designed to deal with the type of secrets that are used
   for passwords.  Functions like PBES2 [<a href="./rfc8018" title="&quot;PKCS #5: Password-Based Cryptography Specification Version 2.1&quot;">RFC8018</a>] need to be used for
   that type of secret.

   The same KDF can be set up to deal with the first two types of
   secrets in a different way.  The KDF defined in <a href="#section-11.1">Section 11.1</a> is such
   a function.  This is reflected in the set of algorithms defined for
   the HMAC-based Extract-and-Expand Key Derivation Function (HKDF).

   When using KDFs, one component that is included is context
   information.  Context information is used to allow for different
   keying information to be derived from the same secret.  The use of
   context-based keying material is considered to be a good security
   practice.

   This document defines a single context structure and a single KDF.
   These elements are used for all of the recipient algorithms defined
   in this document that require a KDF process.  These algorithms are
   defined in Sections <a href="#section-12.1.2">12.1.2</a>, <a href="#section-12.4.1">12.4.1</a>, and <a href="#section-12.5.1">12.5.1</a>.

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  HMAC-Based Extract-and-Expand Key Derivation Function (HKDF)</span>

   The HKDF key derivation algorithm is defined in [<a href="./rfc5869" title="&quot;HMAC-based Extract-and-Expand Key Derivation Function (HKDF)&quot;">RFC5869</a>].

   The HKDF algorithm takes these inputs:

      secret -- a shared value that is secret.  Secrets may be either
      previously shared or derived from operations like a Diffie-Hellman
      (DH) key agreement.

      salt -- an optional value that is used to change the generation
      process.  The salt value can be either public or private.  If the
      salt is public and carried in the message, then the 'salt'
      algorithm header parameter defined in Table 13 is used.  While
      [<a href="./rfc5869" title="&quot;HMAC-based Extract-and-Expand Key Derivation Function (HKDF)&quot;">RFC5869</a>] suggests that the length of the salt be the same as the
      length of the underlying hash value, any amount of salt will
      improve the security as different key values will be generated.
      This parameter is protected by being included in the key
      computation and does not need to be separately authenticated.  The
      salt value does not need to be unique for every message sent.

      length -- the number of bytes of output that need to be generated.






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      context information -- Information that describes the context in
      which the resulting value will be used.  Making this information
      specific to the context in which the material is going to be used
      ensures that the resulting material will always be tied to that
      usage.  The context structure defined in <a href="#section-11.2">Section 11.2</a> is used by
      the KDFs in this document.

      PRF -- The underlying pseudorandom function to be used in the HKDF
      algorithm.  The PRF is encoded into the HKDF algorithm selection.

   HKDF is defined to use HMAC as the underlying PRF.  However, it is
   possible to use other functions in the same construct to provide a
   different KDF that is more appropriate in the constrained world.
   Specifically, one can use AES-CBC-MAC as the PRF for the expand step,
   but not for the extract step.  When using a good random shared secret
   of the correct length, the extract step can be skipped.  For the AES
   algorithm versions, the extract step is always skipped.

   The extract step cannot be skipped if the secret is not uniformly
   random, for example, if it is the result of an ECDH key agreement
   step.  This implies that the AES HKDF version cannot be used with
   ECDH.  If the extract step is skipped, the 'salt' value is not used
   as part of the HKDF functionality.

   The algorithms defined in this document are found in Table 12.

   +---------------+-----------------+---------------------------------+
   | Name          | PRF             | Description                     |
   +---------------+-----------------+---------------------------------+
   | HKDF SHA-256  | HMAC with       | HKDF using HMAC SHA-256 as the  |
   |               | SHA-256         | PRF                             |
   | HKDF SHA-512  | HMAC with       | HKDF using HMAC SHA-512 as the  |
   |               | SHA-512         | PRF                             |
   | HKDF AES-     | AES-CBC-MAC-128 | HKDF using AES-MAC as the PRF   |
   | MAC-128       |                 | w/ 128-bit key                  |
   | HKDF AES-     | AES-CBC-MAC-256 | HKDF using AES-MAC as the PRF   |
   | MAC-256       |                 | w/ 256-bit key                  |
   +---------------+-----------------+---------------------------------+

                         Table 12: HKDF Algorithms











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   +------+-------+------+-------------------------------+-------------+
   | Name | Label | Type | Algorithm                     | Description |
   +------+-------+------+-------------------------------+-------------+
   | salt | -20   | bstr | direct+HKDF-SHA-256, direct   | Random salt |
   |      |       |      | +HKDF-SHA-512, direct+HKDF-   |             |
   |      |       |      | AES-128, direct+HKDF-AES-256, |             |
   |      |       |      | ECDH-ES+HKDF-256, ECDH-       |             |
   |      |       |      | ES+HKDF-512, ECDH-            |             |
   |      |       |      | SS+HKDF-256, ECDH-            |             |
   |      |       |      | SS+HKDF-512, ECDH-ES+A128KW,  |             |
   |      |       |      | ECDH-ES+A192KW, ECDH-         |             |
   |      |       |      | ES+A256KW, ECDH-SS+A128KW,    |             |
   |      |       |      | ECDH-SS+A192KW, ECDH-         |             |
   |      |       |      | SS+A256KW                     |             |
   +------+-------+------+-------------------------------+-------------+

                    Table 13: HKDF Algorithm Parameters

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Context Information Structure</span>

   The context information structure is used to ensure that the derived
   keying material is "bound" to the context of the transaction.  The
   context information structure used here is based on that defined in
   [<a href="#ref-SP800-56A">SP800-56A</a>].  By using CBOR for the encoding of the context
   information structure, we automatically get the same type and length
   separation of fields that is obtained by the use of ASN.1.  This
   means that there is no need to encode the lengths for the base
   elements, as it is done by the encoding used in JOSE (<a href="./rfc7518#section-4.6.2">Section&nbsp;4.6.2
   of [RFC7518]</a>).

   The context information structure refers to PartyU and PartyV as the
   two parties that are doing the key derivation.  Unless the
   application protocol defines differently, we assign PartyU to the
   entity that is creating the message and PartyV to the entity that is
   receiving the message.  By doing this association, different keys
   will be derived for each direction as the context information is
   different in each direction.

   The context structure is built from information that is known to both
   entities.  This information can be obtained from a variety of
   sources:

   o  Fields can be defined by the application.  This is commonly used
      to assign fixed names to parties, but it can be used for other
      items such as nonces.

   o  Fields can be defined by usage of the output.  Examples of this
      are the algorithm and key size that are being generated.



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   o  Fields can be defined by parameters from the message.  We define a
      set of parameters in Table 14 that can be used to carry the values
      associated with the context structure.  Examples of this are
      identities and nonce values.  These parameters are designed to be
      placed in the unprotected bucket of the recipient structure; they
      do not need to be in the protected bucket since they already are
      included in the cryptographic computation by virtue of being
      included in the context structure.











































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   +----------+-------+------+---------------------------+-------------+
   | Name     | Label | Type | Algorithm                 | Description |
   +----------+-------+------+---------------------------+-------------+
   | PartyU   | -21   | bstr | direct+HKDF-SHA-256,      | Party U     |
   | identity |       |      | direct+HKDF-SHA-512,      | identity    |
   |          |       |      | direct+HKDF-AES-128,      | information |
   |          |       |      | direct+HKDF-AES-256,      |             |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |
   |          |       |      |                           |             |
   | PartyU   | -22   | bstr | direct+HKDF-SHA-256,      | Party U     |
   | nonce    |       | /    | direct+HKDF-SHA-512,      | provided    |
   |          |       | int  | direct+HKDF-AES-128,      | nonce       |
   |          |       |      | direct+HKDF-AES-256,      |             |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |
   |          |       |      |                           |             |
   | PartyU   | -23   | bstr | direct+HKDF-SHA-256,      | Party U     |
   | other    |       |      | direct+HKDF-SHA-512,      | other       |
   |          |       |      | direct+HKDF-AES-128,      | provided    |
   |          |       |      | direct+HKDF-AES-256,      | information |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |







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   | PartyV   | -24   | bstr | direct+HKDF-SHA-256,      | Party V     |
   | identity |       |      | direct+HKDF-SHA-512,      | identity    |
   |          |       |      | direct+HKDF-AES-128,      | information |
   |          |       |      | direct+HKDF-AES-256,      |             |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |
   |          |       |      |                           |             |
   | PartyV   | -25   | bstr | direct+HKDF-SHA-256,      | Party V     |
   | nonce    |       | /    | direct+HKDF-SHA-512,      | provided    |
   |          |       | int  | direct+HKDF-AES-128,      | nonce       |
   |          |       |      | direct+HKDF-AES-256,      |             |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |
   |          |       |      |                           |             |
   | PartyV   | -26   | bstr | direct+HKDF-SHA-256,      | Party V     |
   | other    |       |      | direct+HKDF-SHA-512,      | other       |
   |          |       |      | direct+HKDF-AES-128,      | provided    |
   |          |       |      | direct+HKDF-AES-256,      | information |
   |          |       |      | ECDH-ES+HKDF-256, ECDH-   |             |
   |          |       |      | ES+HKDF-512, ECDH-        |             |
   |          |       |      | SS+HKDF-256, ECDH-        |             |
   |          |       |      | SS+HKDF-512, ECDH-        |             |
   |          |       |      | ES+A128KW, ECDH-          |             |
   |          |       |      | ES+A192KW, ECDH-          |             |
   |          |       |      | ES+A256KW, ECDH-          |             |
   |          |       |      | SS+A128KW, ECDH-          |             |
   |          |       |      | SS+A192KW, ECDH-SS+A256KW |             |
   +----------+-------+------+---------------------------+-------------+

                  Table 14: Context Algorithm Parameters







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   We define a CBOR object to hold the context information.  This object
   is referred to as COSE_KDF_Context.  The object is based on a CBOR
   array type.  The fields in the array are:

   AlgorithmID:  This field indicates the algorithm for which the key
      material will be used.  This normally is either a key wrap
      algorithm identifier or a content encryption algorithm identifier.
      The values are from the "COSE Algorithms" registry.  This field is
      required to be present.  The field exists in the context
      information so that if the same environment is used for different
      algorithms, then completely different keys will be generated for
      each of those algorithms.  This practice means if algorithm A is
      broken and thus is easier to find, the key derived for algorithm B
      will not be the same as the key derived for algorithm A.

   PartyUInfo:  This field holds information about party U.  The
      PartyUInfo is encoded as a CBOR array.  The elements of PartyUInfo
      are encoded in the order presented.  The elements of the
      PartyUInfo array are:

      identity:  This contains the identity information for party U.
         The identities can be assigned in one of two manners.  First, a
         protocol can assign identities based on roles.  For example,
         the roles of "client" and "server" may be assigned to different
         entities in the protocol.  Each entity would then use the
         correct label for the data they send or receive.  The second
         way for a protocol to assign identities is to use a name based
         on a naming system (i.e., DNS, X.509 names).

         We define an algorithm parameter 'PartyU identity' that can be
         used to carry identity information in the message.  However,
         identity information is often known as part of the protocol and
         can thus be inferred rather than made explicit.  If identity
         information is carried in the message, applications SHOULD have
         a way of validating the supplied identity information.  The
         identity information does not need to be specified and is set
         to nil in that case.

      nonce:  This contains a nonce value.  The nonce can either be
         implicit from the protocol or be carried as a value in the
         unprotected headers.

         We define an algorithm parameter 'PartyU nonce' that can be
         used to carry this value in the message; however, the nonce
         value could be determined by the application and the value
         determined from elsewhere.





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         This option does not need to be specified and is set to nil in
         that case.

      other:  This contains other information that is defined by the
         protocol.  This option does not need to be specified and is set
         to nil in that case.

   PartyVInfo:  This field holds information about party V.  The content
      of the structure is the same as for the PartyUInfo but for party
      V.

   SuppPubInfo:  This field contains public information that is mutually
      known to both parties.

      keyDataLength:  This is set to the number of bits of the desired
         output value.  This practice means if algorithm A can use two
         different key lengths, the key derived for longer key size will
         not contain the key for shorter key size as a prefix.

      protected:  This field contains the protected parameter field.  If
         there are no elements in the protected field, then use a zero-
         length bstr.

      other:  This field is for free form data defined by the
         application.  An example is that an application could define
         two different strings to be placed here to generate different
         keys for a data stream versus a control stream.  This field is
         optional and will only be present if the application defines a
         structure for this information.  Applications that define this
         SHOULD use CBOR to encode the data so that types and lengths
         are correctly included.

   SuppPrivInfo:  This field contains private information that is
      mutually known private information.  An example of this
      information would be a preexisting shared secret.  (This could,
      for example, be used in combination with an ECDH key agreement to
      provide a secondary proof of identity.)  The field is optional and
      will only be present if the application defines a structure for
      this information.  Applications that define this SHOULD use CBOR
      to encode the data so that types and lengths are correctly
      included.










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   The following CDDL fragment corresponds to the text above.

   PartyInfo = (
       identity : bstr / nil,
       nonce : bstr / int / nil,
       other : bstr / nil
   )

   COSE_KDF_Context = [
       AlgorithmID : int / tstr,
       PartyUInfo : [ PartyInfo ],
       PartyVInfo : [ PartyInfo ],
       SuppPubInfo : [
           keyDataLength : uint,
           protected : empty_or_serialized_map,
           ? other : bstr
       ],
       ? SuppPrivInfo : bstr
   ]

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Content Key Distribution Methods</span>

   Content key distribution methods (recipient algorithms) can be
   defined into a number of different classes.  COSE has the ability to
   support many classes of recipient algorithms.  In this section, a
   number of classes are listed, and then a set of algorithms are
   specified for each of the classes.  The names of the recipient
   algorithm classes used here are the same as those defined in
   [<a href="./rfc7516" title="&quot;JSON Web Encryption (JWE)&quot;">RFC7516</a>].  Other specifications use different terms for the
   recipient algorithm classes or do not support some of the recipient
   algorithm classes.

<span class="h3"><a class="selflink" id="section-12.1" href="#section-12.1">12.1</a>.  Direct Encryption</span>

   The direct encryption class algorithms share a secret between the
   sender and the recipient that is used either directly or after
   manipulation as the CEK.  When direct encryption mode is used, it
   MUST be the only mode used on the message.

   The COSE_Recipient structure for the recipient is organized as
   follows:

   o  The 'protected' field MUST be a zero-length item unless it is used
      in the computation of the content key.

   o  The 'alg' parameter MUST be present.

   o  A parameter identifying the shared secret SHOULD be present.



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   o  The 'ciphertext' field MUST be a zero-length item.

   o  The 'recipients' field MUST be absent.

<span class="h4"><a class="selflink" id="section-12.1.1" href="#section-12.1.1">12.1.1</a>.  Direct Key</span>

   This recipient algorithm is the simplest; the identified key is
   directly used as the key for the next layer down in the message.
   There are no algorithm parameters defined for this algorithm.  The
   algorithm identifier value is assigned in Table 15.

   When this algorithm is used, the protected field MUST be zero length.
   The key type MUST be 'Symmetric'.

                  +--------+-------+-------------------+
                  | Name   | Value | Description       |
                  +--------+-------+-------------------+
                  | direct | -6    | Direct use of CEK |
                  +--------+-------+-------------------+

                           Table 15: Direct Key

<span class="h5"><a class="selflink" id="section-12.1.1.1" href="#section-12.1.1.1">12.1.1.1</a>.  Security Considerations</span>

   This recipient algorithm has several potential problems that need to
   be considered:

   o  These keys need to have some method to be regularly updated over
      time.  All of the content encryption algorithms specified in this
      document have limits on how many times a key can be used without
      significant loss of security.

   o  These keys need to be dedicated to a single algorithm.  There have
      been a number of attacks developed over time when a single key is
      used for multiple different algorithms.  One example of this is
      the use of a single key for both the CBC encryption mode and the
      CBC-MAC authentication mode.

   o  Breaking one message means all messages are broken.  If an
      adversary succeeds in determining the key for a single message,
      then the key for all messages is also determined.

<span class="h4"><a class="selflink" id="section-12.1.2" href="#section-12.1.2">12.1.2</a>.  Direct Key with KDF</span>

   These recipient algorithms take a common shared secret between the
   two parties and applies the HKDF function (<a href="#section-11.1">Section 11.1</a>), using the
   context structure defined in <a href="#section-11.2">Section 11.2</a> to transform the shared




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   secret into the CEK.  The 'protected' field can be of non-zero
   length.  Either the 'salt' parameter of HKDF or the 'PartyU nonce'
   parameter of the context structure MUST be present.  The salt/nonce
   parameter can be generated either randomly or deterministically.  The
   requirement is that it be a unique value for the shared secret in
   question.

   If the salt/nonce value is generated randomly, then it is suggested
   that the length of the random value be the same length as the hash
   function underlying HKDF.  While there is no way to guarantee that it
   will be unique, there is a high probability that it will be unique.
   If the salt/nonce value is generated deterministically, it can be
   guaranteed to be unique, and thus there is no length requirement.

   A new IV must be used for each message if the same key is used.  The
   IV can be modified in a predictable manner, a random manner, or an
   unpredictable manner (i.e., encrypting a counter).

   The IV used for a key can also be generated from the same HKDF
   functionality as the key is generated.  If HKDF is used for
   generating the IV, the algorithm identifier is set to "IV-
   GENERATION".

   When these algorithms are used, the key type MUST be 'symmetric'.

   The set of algorithms defined in this document can be found in
   Table 16.

   +---------------------+-------+-------------+-----------------------+
   | Name                | Value | KDF         | Description           |
   +---------------------+-------+-------------+-----------------------+
   | direct+HKDF-SHA-256 | -10   | HKDF        | Shared secret w/ HKDF |
   |                     |       | SHA-256     | and SHA-256           |
   | direct+HKDF-SHA-512 | -11   | HKDF        | Shared secret w/ HKDF |
   |                     |       | SHA-512     | and SHA-512           |
   | direct+HKDF-AES-128 | -12   | HKDF AES-   | Shared secret w/ AES- |
   |                     |       | MAC-128     | MAC 128-bit key       |
   | direct+HKDF-AES-256 | -13   | HKDF AES-   | Shared secret w/ AES- |
   |                     |       | MAC-256     | MAC 256-bit key       |
   +---------------------+-------+-------------+-----------------------+

                       Table 16: Direct Key with KDF









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   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the algorithm being
      used.

   o  If the 'key_ops' field is present, it MUST include 'deriveKey' or
      'deriveBits'.

<span class="h5"><a class="selflink" id="section-12.1.2.1" href="#section-12.1.2.1">12.1.2.1</a>.  Security Considerations</span>

   The shared secret needs to have some method to be regularly updated
   over time.  The shared secret forms the basis of trust.  Although not
   used directly, it should still be subject to scheduled rotation.

   While these methods do not provide for perfect forward secrecy, as
   the same shared secret is used for all of the keys generated, if the
   key for any single message is discovered, only the message (or series
   of messages) using that derived key are compromised.  A new key
   derivation step will generate a new key that requires the same amount
   of work to get the key.

<span class="h3"><a class="selflink" id="section-12.2" href="#section-12.2">12.2</a>.  Key Wrap</span>

   In key wrap mode, the CEK is randomly generated and that key is then
   encrypted by a shared secret between the sender and the recipient.
   All of the currently defined key wrap algorithms for COSE are AE
   algorithms.  Key wrap mode is considered to be superior to direct
   encryption if the system has any capability for doing random key
   generation.  This is because the shared key is used to wrap random
   data rather than data that has some degree of organization and may in
   fact be repeating the same content.  The use of key wrap loses the
   weak data origination that is provided by the direct encryption
   algorithms.

   The COSE_Encrypt structure for the recipient is organized as follows:

   o  The 'protected' field MUST be absent if the key wrap algorithm is
      an AE algorithm.

   o  The 'recipients' field is normally absent, but can be used.
      Applications MUST deal with a recipient field being present, not
      being able to decrypt that recipient is an acceptable way of
      dealing with it.  Failing to process the message is not an
      acceptable way of dealing with it.




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   o  The plaintext to be encrypted is the key from next layer down
      (usually the content layer).

   o  At a minimum, the 'unprotected' field MUST contain the 'alg'
      parameter and SHOULD contain a parameter identifying the shared
      secret.

<span class="h4"><a class="selflink" id="section-12.2.1" href="#section-12.2.1">12.2.1</a>.  AES Key Wrap</span>

   The AES Key Wrap algorithm is defined in [<a href="./rfc3394" title="&quot;Advanced Encryption Standard (AES) Key Wrap Algorithm&quot;">RFC3394</a>].  This algorithm
   uses an AES key to wrap a value that is a multiple of 64 bits.  As
   such, it can be used to wrap a key for any of the content encryption
   algorithms defined in this document.  The algorithm requires a single
   fixed parameter, the initial value.  This is fixed to the value
   specified in <a href="./rfc3394#section-2.2.3.1">Section&nbsp;2.2.3.1 of [RFC3394]</a>.  There are no public
   parameters that vary on a per-invocation basis.  The protected header
   field MUST be empty.

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   o  If the 'alg' field is present, it MUST match the AES Key Wrap
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'wrap key' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'unwrap key' when decrypting.

        +--------+-------+----------+-----------------------------+
        | Name   | Value | Key Size | Description                 |
        +--------+-------+----------+-----------------------------+
        | A128KW | -3    | 128      | AES Key Wrap w/ 128-bit key |
        | A192KW | -4    | 192      | AES Key Wrap w/ 192-bit key |
        | A256KW | -5    | 256      | AES Key Wrap w/ 256-bit key |
        +--------+-------+----------+-----------------------------+

                  Table 17: AES Key Wrap Algorithm Values





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<span class="h5"><a class="selflink" id="section-12.2.1.1" href="#section-12.2.1.1">12.2.1.1</a>.  Security Considerations for AES-KW</span>

   The shared secret needs to have some method to be regularly updated
   over time.  The shared secret is the basis of trust.

<span class="h3"><a class="selflink" id="section-12.3" href="#section-12.3">12.3</a>.  Key Transport</span>

   Key transport mode is also called key encryption mode in some
   standards.  Key transport mode differs from key wrap mode in that it
   uses an asymmetric encryption algorithm rather than a symmetric
   encryption algorithm to protect the key.  This document does not
   define any key transport mode algorithms.

   When using a key transport algorithm, the COSE_Encrypt structure for
   the recipient is organized as follows:

   o  The 'protected' field MUST be absent.

   o  The plaintext to be encrypted is the key from the next layer down
      (usually the content layer).

   o  At a minimum, the 'unprotected' field MUST contain the 'alg'
      parameter and SHOULD contain a parameter identifying the
      asymmetric key.

<span class="h3"><a class="selflink" id="section-12.4" href="#section-12.4">12.4</a>.  Direct Key Agreement</span>

   The 'direct key agreement' class of recipient algorithms uses a key
   agreement method to create a shared secret.  A KDF is then applied to
   the shared secret to derive a key to be used in protecting the data.
   This key is normally used as a CEK or MAC key, but could be used for
   other purposes if more than two layers are in use (see <a href="#appendix-B">Appendix B</a>).

   The most commonly used key agreement algorithm is Diffie-Hellman, but
   other variants exist.  Since COSE is designed for a store and forward
   environment rather than an online environment, many of the DH
   variants cannot be used as the receiver of the message cannot provide
   any dynamic key material.  One side effect of this is that perfect
   forward secrecy (see [<a href="./rfc4949" title="&quot;Internet Security Glossary, Version 2&quot;">RFC4949</a>]) is not achievable.  A static key will
   always be used for the receiver of the COSE object.

   Two variants of DH that are supported are:

      Ephemeral-Static (ES) DH: where the sender of the message creates
      a one-time DH key and uses a static key for the recipient.  The
      use of the ephemeral sender key means that no additional random
      input is needed as this is randomly generated for each message.




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      Static-Static DH: where a static key is used for both the sender
      and the recipient.  The use of static keys allows for the
      recipient to get a weak version of data origination for the
      message.  When static-static key agreement is used, then some
      piece of unique data for the KDF is required to ensure that a
      different key is created for each message.

   When direct key agreement mode is used, there MUST be only one
   recipient in the message.  This method creates the key directly, and
   that makes it difficult to mix with additional recipients.  If
   multiple recipients are needed, then the version with key wrap needs
   to be used.

   The COSE_Encrypt structure for the recipient is organized as follows:

   o  At a minimum, headers MUST contain the 'alg' parameter and SHOULD
      contain a parameter identifying the recipient's asymmetric key.

   o  The headers SHOULD identify the sender's key for the static-static
      versions and MUST contain the sender's ephemeral key for the
      ephemeral-static versions.

<span class="h4"><a class="selflink" id="section-12.4.1" href="#section-12.4.1">12.4.1</a>.  ECDH</span>

   The mathematics for ECDH can be found in [<a href="./rfc6090" title="&quot;Fundamental Elliptic Curve Cryptography Algorithms&quot;">RFC6090</a>].  In this
   document, the algorithm is extended to be used with the two curves
   defined in [<a href="./rfc7748" title="&quot;Elliptic Curves for Security&quot;">RFC7748</a>].

   ECDH is parameterized by the following:

   o  Curve Type/Curve: The curve selected controls not only the size of
      the shared secret, but the mathematics for computing the shared
      secret.  The curve selected also controls how a point in the curve
      is represented and what happens for the identity points on the
      curve.  In this specification, we allow for a number of different
      curves to be used.  A set of curves are defined in Table 22.
      The math used to obtain the computed secret is based on the curve
      selected and not on the ECDH algorithm.  For this reason, a new
      algorithm does not need to be defined for each of the curves.

   o  Computed Secret to Shared Secret: Once the computed secret is
      known, the resulting value needs to be converted to a byte string
      to run the KDF.  The x-coordinate is used for all of the curves
      defined in this document.  For curves X25519 and X448, the
      resulting value is used directly as it is a byte string of a known
      length.  For the P-256, P-384, and P-521 curves, the x-coordinate
      is run through the I2OSP function defined in [<a href="./rfc8017" title="&quot;PKCS #1: RSA Cryptography Specifications Version 2.2&quot;">RFC8017</a>], using the
      same computation for n as is defined in <a href="#section-8.1">Section 8.1</a>.



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   o  Ephemeral-Static or Static-Static: The key agreement process may
      be done using either a static or an ephemeral key for the sender's
      side.  When using ephemeral keys, the sender MUST generate a new
      ephemeral key for every key agreement operation.  The ephemeral
      key is placed in the 'ephemeral key' parameter and MUST be present
      for all algorithm identifiers that use ephemeral keys.  When using
      static keys, the sender MUST either generate a new random value or
      create a unique value.  For the KDFs used, this means either the
      'salt' parameter for HKDF (Table 13) or the 'PartyU nonce'
      parameter for the context structure (Table 14) MUST be present
      (both can be present if desired).  The value in the parameter MUST
      be unique for the pair of keys being used.  It is acceptable to
      use a global counter that is incremented for every static-static
      operation and use the resulting value.  When using static keys,
      the static key should be identified to the recipient.  The static
      key can be identified either by providing the key ('static key')
      or by providing a key identifier for the static key ('static key
      id').  Both of these parameters are defined in Table 19.

   o  Key Derivation Algorithm: The result of an ECDH key agreement
      process does not provide a uniformly random secret.  As such, it
      needs to be run through a KDF in order to produce a usable key.
      Processing the secret through a KDF also allows for the
      introduction of context material: how the key is going to be used
      and one-time material for static-static key agreement.  All of the
      algorithms defined in this document use one of the HKDF algorithms
      defined in <a href="#section-11.1">Section 11.1</a> with the context structure defined in
      <a href="#section-11.2">Section 11.2</a>.

   o  Key Wrap Algorithm: No key wrap algorithm is used.  This is
      represented in Table 18 as 'none'.  The key size for the context
      structure is the content layer encryption algorithm size.

   The set of direct ECDH algorithms defined in this document are found
   in Table 18.
















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   +-----------+-------+---------+------------+--------+---------------+
   | Name      | Value | KDF     | Ephemeral- | Key    | Description   |
   |           |       |         | Static     | Wrap   |               |
   +-----------+-------+---------+------------+--------+---------------+
   | ECDH-ES + | -25   | HKDF -  | yes        | none   | ECDH ES w/    |
   | HKDF-256  |       | SHA-256 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   | ECDH-ES + | -26   | HKDF -  | yes        | none   | ECDH ES w/    |
   | HKDF-512  |       | SHA-512 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   | ECDH-SS + | -27   | HKDF -  | no         | none   | ECDH SS w/    |
   | HKDF-256  |       | SHA-256 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   | ECDH-SS + | -28   | HKDF -  | no         | none   | ECDH SS w/    |
   | HKDF-512  |       | SHA-512 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   +-----------+-------+---------+------------+--------+---------------+

                      Table 18: ECDH Algorithm Values

   +-----------+-------+----------+---------------------+--------------+
   | Name      | Label | Type     | Algorithm           | Description  |
   +-----------+-------+----------+---------------------+--------------+
   | ephemeral | -1    | COSE_Key | ECDH-ES+HKDF-256,   | Ephemeral    |
   | key       |       |          | ECDH-ES+HKDF-512,   | public key   |
   |           |       |          | ECDH-ES+A128KW,     | for the      |
   |           |       |          | ECDH-ES+A192KW,     | sender       |
   |           |       |          | ECDH-ES+A256KW      |              |
   | static    | -2    | COSE_Key | ECDH-SS+HKDF-256,   | Static       |
   | key       |       |          | ECDH-SS+HKDF-512,   | public key   |
   |           |       |          | ECDH-SS+A128KW,     | for the      |
   |           |       |          | ECDH-SS+A192KW,     | sender       |
   |           |       |          | ECDH-SS+A256KW      |              |
   | static    | -3    | bstr     | ECDH-SS+HKDF-256,   | Static       |
   | key id    |       |          | ECDH-SS+HKDF-512,   | public key   |
   |           |       |          | ECDH-SS+A128KW,     | identifier   |
   |           |       |          | ECDH-SS+A192KW,     | for the      |
   |           |       |          | ECDH-SS+A256KW      | sender       |
   +-----------+-------+----------+---------------------+--------------+

                    Table 19: ECDH Algorithm Parameters






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   This document defines these algorithms to be used with the curves
   P-256, P-384, P-521, X25519, and X448.  Implementations MUST verify
   that the key type and curve are correct.  Different curves are
   restricted to different key types.  Implementations MUST verify that
   the curve and algorithm are appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'EC2' or 'OKP'.

   o  If the 'alg' field is present, it MUST match the key agreement
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'derive key' or
      'derive bits' for the private key.

   o  If the 'key_ops' field is present, it MUST be empty for the public
      key.

<span class="h4"><a class="selflink" id="section-12.4.2" href="#section-12.4.2">12.4.2</a>.  Security Considerations</span>

   There is a method of checking that points provided from external
   entities are valid.  For the 'EC2' key format, this can be done by
   checking that the x and y values form a point on the curve.  For the
   'OKP' format, there is no simple way to do point validation.

   Consideration was given to requiring that the public keys of both
   entities be provided as part of the key derivation process (as
   recommended in <a href="./rfc7748#section-6.1">Section&nbsp;6.1 of [RFC7748]</a>).  This was not done as COSE
   is used in a store and forward format rather than in online key
   exchange.  In order for this to be a problem, either the receiver
   public key has to be chosen maliciously or the sender has to be
   malicious.  In either case, all security evaporates anyway.

   A proof of possession of the private key associated with the public
   key is recommended when a key is moved from untrusted to trusted
   (either by the end user or by the entity that is responsible for
   making trust statements on keys).

<span class="h3"><a class="selflink" id="section-12.5" href="#section-12.5">12.5</a>.  Key Agreement with Key Wrap</span>

   Key Agreement with Key Wrap uses a randomly generated CEK.  The CEK
   is then encrypted using a key wrap algorithm and a key derived from
   the shared secret computed by the key agreement algorithm.  The
   function for this would be:

   encryptedKey = KeyWrap(KDF(DH-Shared, context), CEK)



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   The COSE_Encrypt structure for the recipient is organized as follows:

   o  The 'protected' field is fed into the KDF context structure.

   o  The plaintext to be encrypted is the key from the next layer down
      (usually the content layer).

   o  The 'alg' parameter MUST be present in the layer.

   o  A parameter identifying the recipient's key SHOULD be present.  A
      parameter identifying the sender's key SHOULD be present.

<span class="h4"><a class="selflink" id="section-12.5.1" href="#section-12.5.1">12.5.1</a>.  ECDH</span>

   These algorithms are defined in Table 20.

   ECDH with Key Agreement is parameterized by the same parameters as
   for ECDH; see <a href="#section-12.4.1">Section 12.4.1</a>, with the following modifications:

   o  Key Wrap Algorithm: Any of the key wrap algorithms defined in
      <a href="#section-12.2.1">Section 12.2.1</a> are supported.  The size of the key used for the
      key wrap algorithm is fed into the KDF.  The set of identifiers
      are found in Table 20.




























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   +-----------+-------+---------+------------+--------+---------------+
   | Name      | Value | KDF     | Ephemeral- | Key    | Description   |
   |           |       |         | Static     | Wrap   |               |
   +-----------+-------+---------+------------+--------+---------------+
   | ECDH-ES + | -29   | HKDF -  | yes        | A128KW | ECDH ES w/    |
   | A128KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 128-bit key   |
   |           |       |         |            |        |               |
   | ECDH-ES + | -30   | HKDF -  | yes        | A192KW | ECDH ES w/    |
   | A192KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 192-bit key   |
   |           |       |         |            |        |               |
   | ECDH-ES + | -31   | HKDF -  | yes        | A256KW | ECDH ES w/    |
   | A256KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 256-bit key   |
   |           |       |         |            |        |               |
   | ECDH-SS + | -32   | HKDF -  | no         | A128KW | ECDH SS w/    |
   | A128KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 128-bit key   |
   |           |       |         |            |        |               |
   | ECDH-SS + | -33   | HKDF -  | no         | A192KW | ECDH SS w/    |
   | A192KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 192-bit key   |
   |           |       |         |            |        |               |
   | ECDH-SS + | -34   | HKDF -  | no         | A256KW | ECDH SS w/    |
   | A256KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 256-bit key   |
   +-----------+-------+---------+------------+--------+---------------+

               Table 20: ECDH Algorithm Values with Key Wrap









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   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present, and it MUST be 'EC2' or 'OKP'.

   o  If the 'alg' field is present, it MUST match the key agreement
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'derive key' or
      'derive bits' for the private key.

   o  If the 'key_ops' field is present, it MUST be empty for the public
      key.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Key Object Parameters</span>

   The COSE_Key object defines a way to hold a single key object.  It is
   still required that the members of individual key types be defined.
   This section of the document is where we define an initial set of
   members for specific key types.

   For each of the key types, we define both public and private members.
   The public members are what is transmitted to others for their usage.
   Private members allow for the archival of keys by individuals.
   However, there are some circumstances in which private keys may be
   distributed to entities in a protocol.  Examples include: entities
   that have poor random number generation, centralized key creation for
   multi-cast type operations, and protocols in which a shared secret is
   used as a bearer token for authorization purposes.

   Key types are identified by the 'kty' member of the COSE_Key object.
   In this document, we define four values for the member:

   +-----------+-------+-----------------------------------------------+
   | Name      | Value | Description                                   |
   +-----------+-------+-----------------------------------------------+
   | OKP       | 1     | Octet Key Pair                                |
   | EC2       | 2     | Elliptic Curve Keys w/ x- and y-coordinate    |
   |           |       | pair                                          |
   | Symmetric | 4     | Symmetric Keys                                |
   | Reserved  | 0     | This value is reserved                        |
   +-----------+-------+-----------------------------------------------+

                         Table 21: Key Type Values







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<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>.  Elliptic Curve Keys</span>

   Two different key structures are defined for elliptic curve keys.
   One version uses both an x-coordinate and a y-coordinate, potentially
   with point compression ('EC2').  This is the traditional EC point
   representation that is used in [<a href="./rfc5480" title="&quot;Elliptic Curve Cryptography Subject Public Key Information&quot;">RFC5480</a>].  The other version uses
   only the x-coordinate as the y-coordinate is either to be recomputed
   or not needed for the key agreement operation ('OKP').

   Applications MUST check that the curve and the key type are
   consistent and reject a key if they are not.

    +---------+-------+----------+------------------------------------+
    | Name    | Value | Key Type | Description                        |
    +---------+-------+----------+------------------------------------+
    | P-256   | 1     | EC2      | NIST P-256 also known as secp256r1 |
    | P-384   | 2     | EC2      | NIST P-384 also known as secp384r1 |
    | P-521   | 3     | EC2      | NIST P-521 also known as secp521r1 |
    | X25519  | 4     | OKP      | X25519 for use w/ ECDH only        |
    | X448    | 5     | OKP      | X448 for use w/ ECDH only          |
    | Ed25519 | 6     | OKP      | Ed25519 for use w/ EdDSA only      |
    | Ed448   | 7     | OKP      | Ed448 for use w/ EdDSA only        |
    +---------+-------+----------+------------------------------------+

                         Table 22: Elliptic Curves

<span class="h4"><a class="selflink" id="section-13.1.1" href="#section-13.1.1">13.1.1</a>.  Double Coordinate Curves</span>

   The traditional way of sending ECs has been to send either both the
   x-coordinate and y-coordinate or the x-coordinate and a sign bit for
   the y-coordinate.  The latter encoding has not been recommended in
   the IETF due to potential IPR issues.  However, for operations in
   constrained environments, the ability to shrink a message by not
   sending the y-coordinate is potentially useful.

   For EC keys with both coordinates, the 'kty' member is set to 2
   (EC2).  The key parameters defined in this section are summarized in
   Table 23.  The members that are defined for this key type are:

   crv: This contains an identifier of the curve to be used with the
        key.  The curves defined in this document for this key type can
        be found in Table 22.  Other curves may be registered in the
        future, and private curves can be used as well.

   x:   This contains the x-coordinate for the EC point.  The integer is
        converted to an octet string as defined in [<a href="#ref-SEC1" title="&quot;SEC 1: Elliptic Curve Cryptography&quot;">SEC1</a>].  Leading zero
        octets MUST be preserved.




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   y:   This contains either the sign bit or the value of the
        y-coordinate for the EC point.  When encoding the value y, the
        integer is converted to an octet string (as defined in [<a href="#ref-SEC1" title="&quot;SEC 1: Elliptic Curve Cryptography&quot;">SEC1</a>])
        and encoded as a CBOR bstr.  Leading zero octets MUST be
        preserved.  The compressed point encoding is also supported.
        Compute the sign bit as laid out in the Elliptic-Curve-Point-to-
        Octet-String Conversion function of [<a href="#ref-SEC1" title="&quot;SEC 1: Elliptic Curve Cryptography&quot;">SEC1</a>].  If the sign bit is
        zero, then encode y as a CBOR false value; otherwise, encode y
        as a CBOR true value.  The encoding of the infinity point is not
        supported.

   d:   This contains the private key.

   For public keys, it is REQUIRED that 'crv', 'x', and 'y' be present
   in the structure.  For private keys, it is REQUIRED that 'crv' and
   'd' be present in the structure.  For private keys, it is RECOMMENDED
   that 'x' and 'y' also be present, but they can be recomputed from the
   required elements and omitting them saves on space.

   +-------+------+-------+--------+-----------------------------------+
   | Key   | Name | Label | CBOR   | Description                       |
   | Type  |      |       | Type   |                                   |
   +-------+------+-------+--------+-----------------------------------+
   | 2     | crv  | -1    | int /  | EC identifier - Taken from the    |
   |       |      |       | tstr   | "COSE Elliptic Curves" registry   |
   | 2     | x    | -2    | bstr   | x-coordinate                      |
   | 2     | y    | -3    | bstr / | y-coordinate                      |
   |       |      |       | bool   |                                   |
   | 2     | d    | -4    | bstr   | Private key                       |
   +-------+------+-------+--------+-----------------------------------+

                        Table 23: EC Key Parameters

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>.  Octet Key Pair</span>

   A new key type is defined for Octet Key Pairs (OKP).  Do not assume
   that keys using this type are elliptic curves.  This key type could
   be used for other curve types (for example, mathematics based on
   hyper-elliptic surfaces).

   The key parameters defined in this section are summarized in
   Table 24.  The members that are defined for this key type are:

   crv: This contains an identifier of the curve to be used with the
        key.  The curves defined in this document for this key type can
        be found in Table 22.  Other curves may be registered in the
        future and private curves can be used as well.




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   x:   This contains the x-coordinate for the EC point.  The octet
        string represents a little-endian encoding of x.

   d:   This contains the private key.

   For public keys, it is REQUIRED that 'crv' and 'x' be present in the
   structure.  For private keys, it is REQUIRED that 'crv' and 'd' be
   present in the structure.  For private keys, it is RECOMMENDED that
   'x' also be present, but it can be recomputed from the required
   elements and omitting it saves on space.

   +------+-------+-------+--------+-----------------------------------+
   | Name | Key   | Label | Type   | Description                       |
   |      | Type  |       |        |                                   |
   +------+-------+-------+--------+-----------------------------------+
   | crv  | 1     | -1    | int /  | EC identifier - Taken from the    |
   |      |       |       | tstr   | "COSE Key Common Parameters"      |
   |      |       |       |        | registry                          |
   | x    | 1     | -2    | bstr   | x-coordinate                      |
   | d    | 1     | -4    | bstr   | Private key                       |
   +------+-------+-------+--------+-----------------------------------+

                    Table 24: Octet Key Pair Parameters

<span class="h3"><a class="selflink" id="section-13.3" href="#section-13.3">13.3</a>.  Symmetric Keys</span>

   Occasionally it is required that a symmetric key be transported
   between entities.  This key structure allows for that to happen.

   For symmetric keys, the 'kty' member is set to 4 ('Symmetric').  The
   member that is defined for this key type is:

   k: This contains the value of the key.

   This key structure does not have a form that contains only public
   members.  As it is expected that this key structure is going to be
   transmitted, care must be taken that it is never transmitted
   accidentally or insecurely.  For symmetric keys, it is REQUIRED that
   'k' be present in the structure.

             +------+----------+-------+------+-------------+
             | Name | Key Type | Label | Type | Description |
             +------+----------+-------+------+-------------+
             | k    | 4        | -1    | bstr | Key Value   |
             +------+----------+-------+------+-------------+

                    Table 25: Symmetric Key Parameters




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<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  CBOR Encoder Restrictions</span>

   There has been an attempt to limit the number of places where the
   document needs to impose restrictions on how the CBOR Encoder needs
   to work.  We have managed to narrow it down to the following
   restrictions:

   o  The restriction applies to the encoding of the Sig_structure, the
      Enc_structure, and the MAC_structure.

   o  The rules for "Canonical CBOR" (<a href="./rfc7049#section-3.9">Section&nbsp;3.9 of RFC 7049</a>) MUST be
      used in these locations.  The main rule that needs to be enforced
      is that all lengths in these structures MUST be encoded such that
      they are using definite lengths, and the minimum length encoding
      is used.

   o  Applications MUST NOT generate messages with the same label used
      twice as a key in a single map.  Applications MUST NOT parse and
      process messages with the same label used twice as a key in a
      single map.  Applications can enforce the parse and process
      requirement by using parsers that will fail the parse step or by
      using parsers that will pass all keys to the application, and the
      application can perform the check for duplicate keys.

<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  Application Profiling Considerations</span>

   This document is designed to provide a set of security services, but
   not implementation requirements for specific usage.  The
   interoperability requirements are provided for how each of the
   individual services are used and how the algorithms are to be used
   for interoperability.  The requirements about which algorithms and
   which services are needed are deferred to each application.

   An example of a profile can be found in [<a href="#ref-OSCOAP" title="&quot;Object Security of CoAP (OSCOAP)&quot;">OSCOAP</a>] where two profiles
   are being developed.  One is for carrying content by itself, and the
   other is for carrying content in combination with CoAP headers.

   It is intended that a profile of this document be created that
   defines the interoperability requirements for that specific
   application.  This section provides a set of guidelines and topics
   that need to be considered when profiling this document.

   o  Applications need to determine the set of messages defined in this
      document that they will be using.  The set of messages corresponds
      fairly directly to the set of security services that are needed
      and to the security levels needed.





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   o  Applications may define new header parameters for a specific
      purpose.  Applications will often times select specific header
      parameters to use or not to use.  For example, an application
      would normally state a preference for using either the IV or the
      Partial IV parameter.  If the Partial IV parameter is specified,
      then the application would also need to define how the fixed
      portion of the IV would be determined.

   o  When applications use externally defined authenticated data, they
      need to define how that data is encoded.  This document assumes
      that the data will be provided as a byte stream.  More information
      can be found in <a href="#section-4.3">Section 4.3</a>.

   o  Applications need to determine the set of security algorithms that
      are to be used.  When selecting the algorithms to be used as the
      mandatory-to-implement set, consideration should be given to
      choosing different types of algorithms when two are chosen for a
      specific purpose.  An example of this would be choosing HMAC-
      SHA512 and AES-CMAC as different MAC algorithms; the construction
      is vastly different between these two algorithms.  This means that
      a weakening of one algorithm would be unlikely to lead to a
      weakening of the other algorithms.  Of course, these algorithms do
      not provide the same level of security and thus may not be
      comparable for the desired security functionality.

   o  Applications may need to provide some type of negotiation or
      discovery method if multiple algorithms or message structures are
      permitted.  The method can be as simple as requiring
      preconfiguration of the set of algorithms to providing a discovery
      method built into the protocol.  S/MIME provided a number of
      different ways to approach the problem that applications could
      follow:

      *  Advertising in the message (S/MIME capabilities) [<a href="./rfc5751" title="&quot;Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification&quot;">RFC5751</a>].

      *  Advertising in the certificate (capabilities extension)
         [<a href="./rfc4262" title="&quot;X.509 Certificate Extension for Secure/ Multipurpose Internet Mail Extensions (S/MIME) Capabilities&quot;">RFC4262</a>].

      *  Minimum requirements for the S/MIME, which have been updated
         over time [<a href="./rfc2633" title="&quot;S/MIME Version 3 Message Specification&quot;">RFC2633</a>] [<a href="./rfc5751" title="&quot;Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification&quot;">RFC5751</a>] (note that [<a href="./rfc2633" title="&quot;S/MIME Version 3 Message Specification&quot;">RFC2633</a>] has been
         obsoleted by [<a href="./rfc5751" title="&quot;Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification&quot;">RFC5751</a>]).










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<span class="h2"><a class="selflink" id="section-16" href="#section-16">16</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-16.1" href="#section-16.1">16.1</a>.  CBOR Tag Assignment</span>

   IANA has assigned the following tags from the "CBOR Tags" registry.
   The tags for COSE_Sign1, COSE_Encrypt0, and COSE_Mac0 were assigned
   in the 1 to 23 value range (one byte long when encoded).  The tags
   for COSE_Sign, COSE_Encrypt, and COSE_Mac were assigned in the 24 to
   255 value range (two bytes long when encoded).

   The tags assigned are in Table 1.

<span class="h3"><a class="selflink" id="section-16.2" href="#section-16.2">16.2</a>.  COSE Header Parameters Registry</span>

   IANA has created a new registry titled "COSE Header Parameters".  The
   registry has been created to use the "Expert Review Required"
   registration procedure [<a href="./rfc8126" title="">RFC8126</a>].  Guidelines for the experts are
   provided in <a href="#section-16.11">Section 16.11</a>.  It should be noted that, in addition to
   the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of the registry are:

   Name:  The name is present to make it easier to refer to and discuss
      the registration entry.  The value is not used in the protocol.
      Names are to be unique in the table.

   Label:  This is the value used for the label.  The label can be
      either an integer or a string.  Registration in the table is based
      on the value of the label requested.  Integer values between 1 and
      255 and strings of length 1 are designated as "Standards Action".
      Integer values from 256 to 65535 and strings of length 2 are
      designated as "Specification Required".  Integer values of greater
      than 65535 and strings of length greater than 2 are designated as
      "Expert Review".  Integer values in the range -1 to -65536 are
      "delegated to the COSE Header Algorithm Parameters registry".
      Integer values less than -65536 are marked as private use.

   Value Type:  This contains the CBOR type for the value portion of the
      label.

   Value Registry:  This contains a pointer to the registry used to
      contain values where the set is limited.

   Description:  This contains a brief description of the header field.





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   Reference:  This contains a pointer to the specification defining the
      header field (where public).

   The initial contents of the registry can be found in Tables 2 and 27.
   All of the entries in the "References" column of this registry point
   to this document.

   Additionally, the label of 0 is to be marked as 'Reserved'.

<span class="h3"><a class="selflink" id="section-16.3" href="#section-16.3">16.3</a>.  COSE Header Algorithm Parameters Registry</span>

   IANA has created a new registry titled "COSE Header Algorithm
   Parameters".  The registry uses the "Expert Review Required"
   registration procedure.  Expert review guidelines are provided in
   <a href="#section-16.11">Section 16.11</a>.

   The columns of the registry are:

   Name:  The name is present to make it easier to refer to and discuss
      the registration entry.  The value is not used in the protocol.

   Algorithm:  The algorithm(s) that this registry entry is used for.
      This value is taken from the "COSE Algorithms" registry.  Multiple
      algorithms can be specified in this entry.  For the table, the
      algorithm/label pair MUST be unique.

   Label:  This is the value used for the label.  The label is an
      integer in the range of -1 to -65536.

   Type:  This contains the CBOR type for the value portion of the
      label.

   Description:  This contains a brief description of the header field.

   Reference:  This contains a pointer to the specification defining the
      header field (where public).

   The initial contents of the registry can be found in Tables 13, 14,
   and 19.  All of the entries in the "References" column of this
   registry point to this document.

<span class="h3"><a class="selflink" id="section-16.4" href="#section-16.4">16.4</a>.  COSE Algorithms Registry</span>

   IANA has created a new registry titled "COSE Algorithms".  The
   registry has been created to use the "Expert Review Required"
   registration procedure.  Guidelines for the experts are provided in





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   <a href="#section-16.11">Section 16.11</a>.  It should be noted that, in addition to the expert
   review, some portions of the registry require a specification,
   potentially a Standards Track RFC, be supplied as well.

   The columns of the registry are:

   Name:  A value that can be used to identify an algorithm in documents
      for easier comprehension.  The name SHOULD be unique.  However,
      the 'Value' field is what is used to identify the algorithm, not
      the 'name' field.

   Value:  The value to be used to identify this algorithm.  Algorithm
      values MUST be unique.  The value can be a positive integer, a
      negative integer, or a string.  Integer values between -256 and
      255 and strings of length 1 are designated as "Standards Action".
      Integer values from -65536 to 65535 and strings of length 2 are
      designated as "Specification Required".  Integer values greater
      than 65535 and strings of length greater than 2 are designated as
      "Expert Review".  Integer values less than -65536 are marked as
      private use.

   Description:  A short description of the algorithm.

   Reference:  A document where the algorithm is defined (if publicly
      available).

   Recommended:  Does the IETF have a consensus recommendation to use
      the algorithm?  The legal values are 'Yes', 'No', and
      'Deprecated'.

   The initial contents of the registry can be found in Tables 5, 6, 7,
   8, 9, 10, 11, 15, 16, 17, 18, and 20.  All of the entries in the
   "References" column of this registry point to this document.  All of
   the entries in the "Recommended" column are set to "Yes".

   Additionally, the label of 0 is to be marked as 'Reserved'.

   NOTE: The assignment of algorithm identifiers in this document was
   done so that positive numbers were used for the first layer objects
   (COSE_Sign, COSE_Sign1, COSE_Encrypt, COSE_Encrypt0, COSE_Mac, and
   COSE_Mac0).  Negative numbers were used for second layer objects
   (COSE_Signature and COSE_recipient).  Expert reviewers should
   consider this practice, but are not expected to be restricted by this
   precedent.







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<span class="h3"><a class="selflink" id="section-16.5" href="#section-16.5">16.5</a>.  COSE Key Common Parameters Registry</span>

   IANA has created a new registry titled "COSE Key Common Parameters".
   The registry has been created to use the "Expert Review Required"
   registration procedure.  Guidelines for the experts are provided in
   <a href="#section-16.11">Section 16.11</a>.  It should be noted that, in addition to the expert
   review, some portions of the registry require a specification,
   potentially a Standards Track RFC, be supplied as well.

   The columns of the registry are:

   Name:  This is a descriptive name that enables easier reference to
      the item.  It is not used in the encoding.

   Label:  The value to be used to identify this algorithm.  Key map
      labels MUST be unique.  The label can be a positive integer, a
      negative integer, or a string.  Integer values between 0 and 255
      and strings of length 1 are designated as "Standards Action".
      Integer values from 256 to 65535 and strings of length 2 are
      designated as "Specification Required".  Integer values of greater
      than 65535 and strings of length greater than 2 are designated as
      "Expert Review".  Integer values in the range -65536 to -1 are
      "used for key parameters specific to a single algorithm delegated
      to the COSE Key Type Parameters registry".  Integer values less
      than -65536 are marked as private use.

   CBOR Type:  This field contains the CBOR type for the field.

   Value Registry:  This field denotes the registry that values come
      from, if one exists.

   Description:  This field contains a brief description for the field.

   Reference:  This contains a pointer to the public specification for
      the field if one exists.

   This registry has been initially populated by the values in Table 3.
   All of the entries in the "References" column of this registry point
   to this document.

<span class="h3"><a class="selflink" id="section-16.6" href="#section-16.6">16.6</a>.  COSE Key Type Parameters Registry</span>

   IANA has created a new registry titled "COSE Key Type Parameters".
   The registry has been created to use the "Expert Review Required"
   registration procedure.  Expert review guidelines are provided in
   <a href="#section-16.11">Section 16.11</a>.





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   The columns of the table are:

   Key Type:  This field contains a descriptive string of a key type.
      This should be a value that is in the "COSE Key Common Parameters"
      registry and is placed in the 'kty' field of a COSE Key structure.

   Name:  This is a descriptive name that enables easier reference to
      the item.  It is not used in the encoding.

   Label:  The label is to be unique for every value of key type.  The
      range of values is from -65536 to -1.  Labels are expected to be
      reused for different keys.

   CBOR Type:  This field contains the CBOR type for the field.

   Description:  This field contains a brief description for the field.

   Reference:  This contains a pointer to the public specification for
      the field if one exists.

   This registry has been initially populated by the values in Tables
   23, 24, and 25.  All of the entries in the "References" column of
   this registry point to this document.

<span class="h3"><a class="selflink" id="section-16.7" href="#section-16.7">16.7</a>.  COSE Key Types Registry</span>

   IANA has created a new registry titled "COSE Key Types".  The
   registry has been created to use the "Expert Review Required"
   registration procedure.  Expert review guidelines are provided in
   <a href="#section-16.11">Section 16.11</a>.

   The columns of this table are:

   Name:  This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   Value:  This is the value used to identify the curve.  These values
      MUST be unique.  The value can be a positive integer, a negative
      integer, or a string.

   Description:  This field contains a brief description of the curve.

   References:  This contains a pointer to the public specification for
      the curve if one exists.






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   This registry has been initially populated by the values in Table 21.
   The specification column for all of these entries will be this
   document.

<span class="h3"><a class="selflink" id="section-16.8" href="#section-16.8">16.8</a>.  COSE Elliptic Curves Registry</span>

   IANA has created a new registry titled "COSE Elliptic Curves".  The
   registry has been created to use the "Expert Review Required"
   registration procedure.  Guidelines for the experts are provided in
   <a href="#section-16.11">Section 16.11</a>.  It should be noted that, in addition to the expert
   review, some portions of the registry require a specification,
   potentially a Standards Track RFC, be supplied as well.

   The columns of the table are:

   Name:  This is a descriptive name that enables easier reference to
      the item.  It is not used in the encoding.

   Value:  This is the value used to identify the curve.  These values
      MUST be unique.  The integer values from -256 to 255 are
      designated as "Standards Action".  The integer values from 256 to
      65535 and -65536 to -257 are designated as "Specification
      Required".  Integer values over 65535 are designated as "Expert
      Review".  Integer values less than -65536 are marked as private
      use.

   Key Type:  This designates the key type(s) that can be used with this
      curve.

   Description:  This field contains a brief description of the curve.

   Reference:  This contains a pointer to the public specification for
      the curve if one exists.

   Recommended:  Does the IETF have a consensus recommendation to use
      the algorithm?  The legal values are 'Yes', 'No', and
      'Deprecated'.

   This registry has been initially populated by the values in Table 22.
   All of the entries in the "References" column of this registry point
   to this document.  All of the entries in the "Recommended" column are
   set to "Yes".









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<span class="h3"><a class="selflink" id="section-16.9" href="#section-16.9">16.9</a>.  Media Type Registrations</span>

<span class="h4"><a class="selflink" id="section-16.9.1" href="#section-16.9.1">16.9.1</a>.  COSE Security Message</span>

   This section registers the 'application/cose' media type in the
   "Media Types" registry.  These media types are used to indicate that
   the content is a COSE message.

      Type name: application

      Subtype name: cose

      Required parameters: N/A

      Optional parameters: cose-type

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of <a href="./rfc8152">RFC 8152</a>.

      Interoperability considerations: N/A

      Published specification: <a href="./rfc8152">RFC 8152</a>

      Applications that use this media type: IoT applications sending
      security content over HTTP(S) transports.

      Fragment identifier considerations: N/A

      Additional information:

      *  Deprecated alias names for this type: N/A

      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A

      Person &amp; email address to contact for further information:
      iesg@ietf.org

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com



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      Change Controller: IESG

      Provisional registration?  No

<span class="h4"><a class="selflink" id="section-16.9.2" href="#section-16.9.2">16.9.2</a>.  COSE Key Media Type</span>

   This section registers the 'application/cose-key' and 'application/
   cose-key-set' media types in the "Media Types" registry.  These media
   types are used to indicate, respectively, that content is a COSE_Key
   or COSE_KeySet object.

   The template for registering 'application/cose-key' is:

      Type name: application

      Subtype name: cose-key

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of <a href="./rfc8152">RFC 8152</a>.

      Interoperability considerations: N/A

      Published specification: <a href="./rfc8152">RFC 8152</a>

      Applications that use this media type: Distribution of COSE based
      keys for IoT applications.

      Fragment identifier considerations: N/A

      Additional information:

      *  Deprecated alias names for this type: N/A

      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A

      Person &amp; email address to contact for further information:
      iesg@ietf.org




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      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

   The template for registering 'application/cose-key-set' is:

      Type name: application

      Subtype name: cose-key-set

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of <a href="./rfc8152">RFC 8152</a>.

      Interoperability considerations: N/A

      Published specification: <a href="./rfc8152">RFC 8152</a>

      Applications that use this media type: Distribution of COSE based
      keys for IoT applications.

      Fragment identifier considerations: N/A

      Additional information:

      *  Deprecated alias names for this type: N/A

      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A

      Person &amp; email address to contact for further information:
      iesg@ietf.org

      Intended usage: COMMON



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      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

<span class="h3"><a class="selflink" id="section-16.10" href="#section-16.10">16.10</a>.  CoAP Content-Formats Registry</span>

   IANA has added the following entries to the "CoAP Content-Formats"
   registry.

   +--------------------------------------+----------+-----+-----------+
   | Media Type                           | Encoding | ID  | Reference |
   +--------------------------------------+----------+-----+-----------+
   | application/cose; cose-type="cose-   |          | 98  | [<a href="./rfc8152">RFC8152</a>] |
   | sign"                                |          |     |           |
   | application/cose; cose-type="cose-   |          | 18  | [<a href="./rfc8152">RFC8152</a>] |
   | sign1"                               |          |     |           |
   | application/cose; cose-type="cose-   |          | 96  | [<a href="./rfc8152">RFC8152</a>] |
   | encrypt"                             |          |     |           |
   | application/cose; cose-type="cose-   |          | 16  | [<a href="./rfc8152">RFC8152</a>] |
   | encrypt0"                            |          |     |           |
   | application/cose; cose-type="cose-   |          | 97  | [<a href="./rfc8152">RFC8152</a>] |
   | mac"                                 |          |     |           |
   | application/cose; cose-type="cose-   |          | 17  | [<a href="./rfc8152">RFC8152</a>] |
   | mac0"                                |          |     |           |
   | application/cose-key                 |          | 101 | [<a href="./rfc8152">RFC8152</a>] |
   | application/cose-key-set             |          | 102 | [<a href="./rfc8152">RFC8152</a>] |
   +--------------------------------------+----------+-----+-----------+

                  Table 26: CoAP Content-Formats for COSE

<span class="h3"><a class="selflink" id="section-16.11" href="#section-16.11">16.11</a>.  Expert Review Instructions</span>

   All of the IANA registries established in this document are defined
   as expert review.  This section gives some general guidelines for
   what the experts should be looking for, but they are being designated
   as experts for a reason, so they should be given substantial
   latitude.

   Expert reviewers should take into consideration the following points:

   o  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered, and that the point is likely to be used in



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      deployments.  The zones tagged as private use are intended for
      testing purposes and closed environments; code points in other
      ranges should not be assigned for testing.

   o  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are
      needed for the first-come, first-serve range if they are expected
      to be used outside of closed environments in an interoperable way.
      When specifications are not provided, the description provided
      needs to have sufficient information to identify what the point is
      being used for.

   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

   o  When algorithms are registered, vanity registrations should be
      discouraged.  One way to do this is to require registrations to
      provide additional documentation on security analysis of the
      algorithm.  Another thing that should be considered is requesting
      an opinion on the algorithm from the Crypto Forum Research Group
      (CFRG).  Algorithms that do not meet the security requirements of
      the community and the messages structures should not be
      registered.

<span class="h2"><a class="selflink" id="section-17" href="#section-17">17</a>.  Security Considerations</span>

   There are a number of security considerations that need to be taken
   into account by implementers of this specification.  The security
   considerations that are specific to an individual algorithm are
   placed next to the description of the algorithm.  While some
   considerations have been highlighted here, additional considerations
   may be found in the documents listed in the references.

   Implementations need to protect the private key material for any
   individuals.  There are some cases in this document that need to be
   highlighted on this issue.

   o  Using the same key for two different algorithms can leak
      information about the key.  It is therefore recommended that keys
      be restricted to a single algorithm.



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   o  Use of 'direct' as a recipient algorithm combined with a second
      recipient algorithm exposes the direct key to the second
      recipient.

   o  Several of the algorithms in this document have limits on the
      number of times that a key can be used without leaking information
      about the key.

   The use of ECDH and direct plus KDF (with no key wrap) will not
   directly lead to the private key being leaked; the one way function
   of the KDF will prevent that.  There is, however, a different issue
   that needs to be addressed.  Having two recipients requires that the
   CEK be shared between two recipients.  The second recipient therefore
   has a CEK that was derived from material that can be used for the
   weak proof of origin.  The second recipient could create a message
   using the same CEK and send it to the first recipient; the first
   recipient would, for either static-static ECDH or direct plus KDF,
   make an assumption that the CEK could be used for proof of origin
   even though it is from the wrong entity.  If the key wrap step is
   added, then no proof of origin is implied and this is not an issue.

   Although it has been mentioned before, the use of a single key for
   multiple algorithms has been demonstrated in some cases to leak
   information about a key, provide the opportunity for attackers to
   forge integrity tags, or gain information about encrypted content.
   Binding a key to a single algorithm prevents these problems.  Key
   creators and key consumers are strongly encouraged not only to create
   new keys for each different algorithm, but to include that selection
   of algorithm in any distribution of key material and strictly enforce
   the matching of algorithms in the key structure to algorithms in the
   message structure.  In addition to checking that algorithms are
   correct, the key form needs to be checked as well.  Do not use an
   'EC2' key where an 'OKP' key is expected.

   Before using a key for transmission, or before acting on information
   received, a trust decision on a key needs to be made.  Is the data or
   action something that the entity associated with the key has a right
   to see or a right to request?  A number of factors are associated
   with this trust decision.  Some of the ones that are highlighted here
   are:

   o  What are the permissions associated with the key owner?

   o  Is the cryptographic algorithm acceptable in the current context?

   o  Have the restrictions associated with the key, such as algorithm
      or freshness, been checked and are they correct?




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   o  Is the request something that is reasonable, given the current
      state of the application?

   o  Have any security considerations that are part of the message been
      enforced (as specified by the application or 'crit' parameter)?

   There are a large number of algorithms presented in this document
   that use nonce values.  For all of the nonces defined in this
   document, there is some type of restriction on the nonce being a
   unique value either for a key or for some other conditions.  In all
   of these cases, there is no known requirement on the nonce being both
   unique and unpredictable; under these circumstances, it's reasonable
   to use a counter for creation of the nonce.  In cases where one wants
   the pattern of the nonce to be unpredictable as well as unique, one
   can use a key created for that purpose and encrypt the counter to
   produce the nonce value.

   One area that has been starting to get exposure is doing traffic
   analysis of encrypted messages based on the length of the message.
   This specification does not provide for a uniform method of providing
   padding as part of the message structure.  An observer can
   distinguish between two different strings (for example, 'YES' and
   'NO') based on the length for all of the content encryption
   algorithms that are defined in this document.  This means that it is
   up to the applications to document how content padding is to be done
   in order to prevent or discourage such analysis.  (For example, the
   strings could be defined as 'YES' and 'NO '.)

<span class="h2"><a class="selflink" id="section-18" href="#section-18">18</a>.  References</span>

<span class="h3"><a class="selflink" id="section-18.1" href="#section-18.1">18.1</a>.  Normative References</span>

   [<a id="ref-AES-GCM">AES-GCM</a>]  National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC", NIST Special
              Publication 800-38D, DOI 10.6028/NIST.SP.800-38D, November
              2007, &lt;<a href="https://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf">https://csrc.nist.gov/publications/nistpubs/800-</a>
              <a href="https://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf">38D/SP-800-38D.pdf</a>&gt;.

   [<a id="ref-COAP.Formats">COAP.Formats</a>]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              &lt;<a href="http://www.iana.org/assignments/core-parameters/">http://www.iana.org/assignments/core-parameters/</a>&gt;.

   [<a id="ref-DSS">DSS</a>]      National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,
              &lt;<a href="http://nvlpubs.nist.gov/nistpubs/FIPS/">http://nvlpubs.nist.gov/nistpubs/FIPS/</a>



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              NIST.FIPS.186-4.pdf&gt;.

   [<a id="ref-MAC">MAC</a>]      National Institute of Standards and Technology, "Computer
              Data Authentication", FIPS PUB 113, May 1985,
              &lt;<a href="http://csrc.nist.gov/publications/fips/fips113/fips113.html">http://csrc.nist.gov/publications/fips/fips113/</a>
              <a href="http://csrc.nist.gov/publications/fips/fips113/fips113.html">fips113.html</a>&gt;.

   [<a id="ref-RFC2104">RFC2104</a>]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", <a href="./rfc2104">RFC 2104</a>,
              DOI 10.17487/RFC2104, February 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2104">http://www.rfc-editor.org/info/rfc2104</a>&gt;.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3394">RFC3394</a>]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", <a href="./rfc3394">RFC 3394</a>, DOI 10.17487/RFC3394,
              September 2002, &lt;<a href="http://www.rfc-editor.org/info/rfc3394">http://www.rfc-editor.org/info/rfc3394</a>&gt;.

   [<a id="ref-RFC3610">RFC3610</a>]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", <a href="./rfc3610">RFC 3610</a>, DOI 10.17487/RFC3610, September
              2003, &lt;<a href="http://www.rfc-editor.org/info/rfc3610">http://www.rfc-editor.org/info/rfc3610</a>&gt;.

   [<a id="ref-RFC5869">RFC5869</a>]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", <a href="./rfc5869">RFC 5869</a>,
              DOI 10.17487/RFC5869, May 2010,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5869">http://www.rfc-editor.org/info/rfc5869</a>&gt;.

   [<a id="ref-RFC6090">RFC6090</a>]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", <a href="./rfc6090">RFC 6090</a>,
              DOI 10.17487/RFC6090, February 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6090">http://www.rfc-editor.org/info/rfc6090</a>&gt;.

   [<a id="ref-RFC6979">RFC6979</a>]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", <a href="./rfc6979">RFC 6979</a>, DOI 10.17487/RFC6979, August
              2013, &lt;<a href="http://www.rfc-editor.org/info/rfc6979">http://www.rfc-editor.org/info/rfc6979</a>&gt;.

   [<a id="ref-RFC7049">RFC7049</a>]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", <a href="./rfc7049">RFC 7049</a>, DOI 10.17487/RFC7049,
              October 2013, &lt;<a href="http://www.rfc-editor.org/info/rfc7049">http://www.rfc-editor.org/info/rfc7049</a>&gt;.

   [<a id="ref-RFC7539">RFC7539</a>]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", <a href="./rfc7539">RFC 7539</a>, DOI 10.17487/RFC7539, May 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7539">http://www.rfc-editor.org/info/rfc7539</a>&gt;.




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   [<a id="ref-RFC7748">RFC7748</a>]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", <a href="./rfc7748">RFC 7748</a>, DOI 10.17487/RFC7748, January
              2016, &lt;<a href="http://www.rfc-editor.org/info/rfc7748">http://www.rfc-editor.org/info/rfc7748</a>&gt;.

   [<a id="ref-RFC8032">RFC8032</a>]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", <a href="./rfc8032">RFC 8032</a>,
              DOI 10.17487/RFC8032, January 2017,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8032">http://www.rfc-editor.org/info/rfc8032</a>&gt;.

   [<a id="ref-RFC8174">RFC8174</a>]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
              <a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
              May 2017, &lt;<a href="http://www.rfc-editor.org/info/rfc8174">http://www.rfc-editor.org/info/rfc8174</a>&gt;.

   [<a id="ref-SEC1">SEC1</a>]     Certicom Research, "SEC 1: Elliptic Curve Cryptography",
              Standards for Efficient Cryptography, Version 2.0, May
              2009, &lt;<a href="http://www.secg.org/sec1-v2.pdf">http://www.secg.org/sec1-v2.pdf</a>&gt;.

<span class="h3"><a class="selflink" id="section-18.2" href="#section-18.2">18.2</a>.  Informative References</span>

   [<a id="ref-CDDL">CDDL</a>]     Vigano, C. and H. Birkholz, "CBOR data definition language
              (CDDL): a notational convention to express CBOR data
              structures", Work in Progress, <a href="./draft-greevenbosch-appsawg-cbor-cddl-09">draft-greevenbosch-appsawg-</a>
              <a href="./draft-greevenbosch-appsawg-cbor-cddl-09">cbor-cddl-09</a>, March 2017.

   [<a id="ref-OSCOAP">OSCOAP</a>]   Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", Work in Progress,
              <a href="./draft-ietf-core-object-security-03">draft-ietf-core-object-security-03</a>, May 2017.

   [<a id="ref-PVSig">PVSig</a>]    Brown, D. and D. Johnson, "Formal Security Proofs for a
              Signature Scheme with Partial Message Recovery",
              DOI 10.1007/3-540-45353-9_11, LNCS Volume 2020, June 2000.

   [<a id="ref-RFC2633">RFC2633</a>]  Ramsdell, B., Ed., "S/MIME Version 3 Message
              Specification", <a href="./rfc2633">RFC 2633</a>, DOI 10.17487/RFC2633, June 1999,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2633">http://www.rfc-editor.org/info/rfc2633</a>&gt;.

   [<a id="ref-RFC4231">RFC4231</a>]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              <a href="./rfc4231">RFC 4231</a>, DOI 10.17487/RFC4231, December 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4231">http://www.rfc-editor.org/info/rfc4231</a>&gt;.

   [<a id="ref-RFC4262">RFC4262</a>]  Santesson, S., "X.509 Certificate Extension for Secure/
              Multipurpose Internet Mail Extensions (S/MIME)
              Capabilities", <a href="./rfc4262">RFC 4262</a>, DOI 10.17487/RFC4262, December
              2005, &lt;<a href="http://www.rfc-editor.org/info/rfc4262">http://www.rfc-editor.org/info/rfc4262</a>&gt;.






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   [<a id="ref-RFC4493">RFC4493</a>]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", <a href="./rfc4493">RFC 4493</a>, DOI 10.17487/RFC4493, June
              2006, &lt;<a href="http://www.rfc-editor.org/info/rfc4493">http://www.rfc-editor.org/info/rfc4493</a>&gt;.

   [<a id="ref-RFC4949">RFC4949</a>]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, <a href="./rfc4949">RFC 4949</a>, DOI 10.17487/RFC4949, August 2007,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4949">http://www.rfc-editor.org/info/rfc4949</a>&gt;.

   [<a id="ref-RFC5116">RFC5116</a>]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", <a href="./rfc5116">RFC 5116</a>, DOI 10.17487/RFC5116, January 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5116">http://www.rfc-editor.org/info/rfc5116</a>&gt;.

   [<a id="ref-RFC5480">RFC5480</a>]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", <a href="./rfc5480">RFC 5480</a>, DOI 10.17487/RFC5480, March 2009,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5480">http://www.rfc-editor.org/info/rfc5480</a>&gt;.

   [<a id="ref-RFC5652">RFC5652</a>]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              <a href="./rfc5652">RFC 5652</a>, DOI 10.17487/RFC5652, September 2009,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5652">http://www.rfc-editor.org/info/rfc5652</a>&gt;.

   [<a id="ref-RFC5751">RFC5751</a>]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", <a href="./rfc5751">RFC 5751</a>, DOI 10.17487/RFC5751, January
              2010, &lt;<a href="http://www.rfc-editor.org/info/rfc5751">http://www.rfc-editor.org/info/rfc5751</a>&gt;.

   [<a id="ref-RFC5752">RFC5752</a>]  Turner, S. and J. Schaad, "Multiple Signatures in
              Cryptographic Message Syntax (CMS)", <a href="./rfc5752">RFC 5752</a>,
              DOI 10.17487/RFC5752, January 2010,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5752">http://www.rfc-editor.org/info/rfc5752</a>&gt;.

   [<a id="ref-RFC5990">RFC5990</a>]  Randall, J., Kaliski, B., Brainard, J., and S. Turner,
              "Use of the RSA-KEM Key Transport Algorithm in the
              Cryptographic Message Syntax (CMS)", <a href="./rfc5990">RFC 5990</a>,
              DOI 10.17487/RFC5990, September 2010,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5990">http://www.rfc-editor.org/info/rfc5990</a>&gt;.

   [<a id="ref-RFC6151">RFC6151</a>]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              <a href="./rfc6151">RFC 6151</a>, DOI 10.17487/RFC6151, March 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6151">http://www.rfc-editor.org/info/rfc6151</a>&gt;.

   [<a id="ref-RFC6838">RFC6838</a>]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", <a href="https://www.rfc-editor.org/bcp/bcp13">BCP 13</a>,
              <a href="./rfc6838">RFC 6838</a>, DOI 10.17487/RFC6838, January 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6838">http://www.rfc-editor.org/info/rfc6838</a>&gt;.





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   [<a id="ref-RFC7159">RFC7159</a>]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", <a href="./rfc7159">RFC 7159</a>, DOI 10.17487/RFC7159, March
              2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7159">http://www.rfc-editor.org/info/rfc7159</a>&gt;.

   [<a id="ref-RFC7252">RFC7252</a>]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", <a href="./rfc7252">RFC 7252</a>,
              DOI 10.17487/RFC7252, June 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7252">http://www.rfc-editor.org/info/rfc7252</a>&gt;.

   [<a id="ref-RFC7515">RFC7515</a>]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", <a href="./rfc7515">RFC 7515</a>, DOI 10.17487/RFC7515, May
              2015, &lt;<a href="http://www.rfc-editor.org/info/rfc7515">http://www.rfc-editor.org/info/rfc7515</a>&gt;.

   [<a id="ref-RFC7516">RFC7516</a>]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              <a href="./rfc7516">RFC 7516</a>, DOI 10.17487/RFC7516, May 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7516">http://www.rfc-editor.org/info/rfc7516</a>&gt;.

   [<a id="ref-RFC7517">RFC7517</a>]  Jones, M., "JSON Web Key (JWK)", <a href="./rfc7517">RFC 7517</a>,
              DOI 10.17487/RFC7517, May 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7517">http://www.rfc-editor.org/info/rfc7517</a>&gt;.

   [<a id="ref-RFC7518">RFC7518</a>]  Jones, M., "JSON Web Algorithms (JWA)", <a href="./rfc7518">RFC 7518</a>,
              DOI 10.17487/RFC7518, May 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7518">http://www.rfc-editor.org/info/rfc7518</a>&gt;.

   [<a id="ref-RFC8017">RFC8017</a>]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              <a href="./rfc8017">RFC 8017</a>, DOI 10.17487/RFC8017, November 2016,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8017">http://www.rfc-editor.org/info/rfc8017</a>&gt;.

   [<a id="ref-RFC8018">RFC8018</a>]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              <a href="./rfc8018">RFC 8018</a>, DOI 10.17487/RFC8018, January 2017,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8018">http://www.rfc-editor.org/info/rfc8018</a>&gt;.

   [<a id="ref-RFC8126">RFC8126</a>]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>,
              <a href="./rfc8126">RFC 8126</a>, DOI 10.17487/RFC8126, June 2017,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8126">http://www.rfc-editor.org/info/rfc8126</a>&gt;.

   [<a id="ref-SP800-56A">SP800-56A</a>]
              Barker, E., Chen, L., Roginsky, A., and M. Smid,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography", NIST Special
              Publication 800-56A, Revision 2,
              DOI 10.6028/NIST.SP.800-56Ar2, May 2013,
              &lt;<a href="http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf">http://nvlpubs.nist.gov/nistpubs/SpecialPublications/</a>
              <a href="http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf">NIST.SP.800-56Ar2.pdf</a>&gt;.



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   [<a id="ref-W3C.WebCrypto">W3C.WebCrypto</a>]
              Watson, M., "Web Cryptography API", W3C Recommendation,
              January 2017, &lt;<a href="https://www.w3.org/TR/WebCryptoAPI/">https://www.w3.org/TR/WebCryptoAPI/</a>&gt;.
















































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Guidelines for External Data Authentication of Algorithms</span>

   A portion of the working group has expressed a strong desire to relax
   the rule that the algorithm identifier be required to appear in each
   level of a COSE object.  There are two basic reasons that have been
   advanced to support this position.  First, the resulting message will
   be smaller if the algorithm identifier is omitted from the most
   common messages in a CoAP environment.  Second, there is a potential
   bug that will arise if full checking is not done correctly between
   the different places that an algorithm identifier could be placed
   (the message itself, an application statement, the key structure that
   the sender possesses, and the key structure the recipient possesses).

   This appendix lays out how such a change can be made and the details
   that an application needs to specify in order to use this option.
   Two different sets of details are specified: those needed to omit an
   algorithm identifier and those needed to use a variant on the counter
   signature attribute that contains no attributes about itself.

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  Algorithm Identification</span>

   In this section, three sets of recommendations are laid out.  The
   first set of recommendations apply to having an implicit algorithm
   identified for a single layer of a COSE object.  The second set of
   recommendations apply to having multiple implicit algorithms
   identified for multiple layers of a COSE object.  The third set of
   recommendations apply to having implicit algorithms for multiple COSE
   object constructs.

   The key words from <a href="./rfc2119">RFC 2119</a> are deliberately not used here.  This
   specification can provide recommendations, but it cannot enforce
   them.

   This set of recommendations applies to the case where an application
   is distributing a fixed algorithm along with the key information for
   use in a single COSE object.  This normally applies to the smallest
   of the COSE objects, specifically COSE_Sign1, COSE_Mac0, and
   COSE_Encrypt0, but could apply to the other structures as well.

   The following items should be taken into account:

   o  Applications need to list the set of COSE structures that implicit
      algorithms are to be used in.  Applications need to require that
      the receipt of an explicit algorithm identifier in one of these
      structures will lead to the message being rejected.  This
      requirement is stated so that there will never be a case where
      there is any ambiguity about the question of which algorithm
      should be used, the implicit or the explicit one.  This applies



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      even if the transported algorithm identifier is a protected
      attribute.  This applies even if the transported algorithm is the
      same as the implicit algorithm.

   o  Applications need to define the set of information that is to be
      considered to be part of a context when omitting algorithm
      identifiers.  At a minimum, this would be the key identifier (if
      needed), the key, the algorithm, and the COSE structure it is used
      with.  Applications should restrict the use of a single key to a
      single algorithm.  As noted for some of the algorithms in this
      document, the use of the same key in different related algorithms
      can lead to leakage of information about the key, leakage about
      the data or the ability to perform forgeries.

   o  In many cases, applications that make the algorithm identifier
      implicit will also want to make the context identifier implicit
      for the same reason.  That is, omitting the context identifier
      will decrease the message size (potentially significantly
      depending on the length of the identifier).  Applications that do
      this will need to describe the circumstances where the context
      identifier is to be omitted and how the context identifier is to
      be inferred in these cases.  (An exhaustive search over all of the
      keys would normally not be considered to be acceptable.)  An
      example of how this can be done is to tie the context to a
      transaction identifier.  Both would be sent on the original
      message, but only the transaction identifier would need to be sent
      after that point as the context is tied into the transaction
      identifier.  Another way would be to associate a context with a
      network address.  All messages coming from a single network
      address can be assumed to be associated with a specific context.
      (In this case, the address would normally be distributed as part
      of the context.)

   o  Applications cannot rely on key identifiers being unique unless
      they take significant efforts to ensure that they are computed in
      such a way as to create this guarantee.  Even when an application
      does this, the uniqueness might be violated if the application is
      run in different contexts (i.e., with a different context
      provider) or if the system combines the security contexts from
      different applications together into a single store.

   o  Applications should continue the practice of protecting the
      algorithm identifier.  Since this is not done by placing it in the
      protected attributes field, applications should define an
      application-specific external data structure that includes this
      value.  This external data field can be used as such for content
      encryption, MAC, and signature algorithms.  It can be used in the
      SuppPrivInfo field for those algorithms that use a KDF to derive a



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      key value.  Applications may also want to protect other
      information that is part of the context structure as well.  It
      should be noted that those fields, such as the key or a Base IV,
      are protected by virtue of being used in the cryptographic
      computation and do not need to be included in the external data
      field.

   The second case is having multiple implicit algorithm identifiers
   specified for a multiple layer COSE object.  An example of how this
   would work is the encryption context that an application specifies,
   which contains a content encryption algorithm, a key wrap algorithm,
   a key identifier, and a shared secret.  The sender omits sending the
   algorithm identifier for both the content layer and the recipient
   layer leaving only the key identifier.  The receiver then uses the
   key identifier to get the implicit algorithm identifiers.

   The following additional items need to be taken into consideration:

   o  Applications that want to support this will need to define a
      structure that allows for, and clearly identifies, both the COSE
      structure to be used with a given key and the structure and
      algorithm to be used for the secondary layer.  The key for the
      secondary layer is computed as normal from the recipient layer.

   The third case is having multiple implicit algorithm identifiers, but
   targeted at potentially unrelated layers or different COSE objects.
   There are a number of different scenarios where this might be
   applicable.  Some of these scenarios are:

   o  Two contexts are distributed as a pair.  Each of the contexts is
      for use with a COSE_Encrypt message.  Each context will consist of
      distinct secret keys and IVs and potentially even different
      algorithms.  One context is for sending messages from party A to
      party B, and the second context is for sending messages from party
      B to party A.  This means that there is no chance for a reflection
      attack to occur as each party uses different secret keys to send
      its messages; a message that is reflected back to it would fail to
      decrypt.

   o  Two contexts are distributed as a pair.  The first context is used
      for encryption of the message, and the second context is used to
      place a counter signature on the message.  The intention is that
      the second context can be distributed to other entities
      independently of the first context.  This allows these entities to
      validate that the message came from an individual without being
      able to decrypt the message and see the content.





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   o  Two contexts are distributed as a pair.  The first context
      contains a key for dealing with MACed messages, and the second
      context contains a key for dealing with encrypted messages.  This
      allows for a unified distribution of keys to participants for
      different types of messages that have different keys, but where
      the keys may be used in a coordinated manner.

   For these cases, the following additional items need to be
   considered:

   o  Applications need to ensure that the multiple contexts stay
      associated.  If one of the contexts is invalidated for any reason,
      all of the contexts associated with it should also be invalidated.

<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  Counter Signature without Headers</span>

   There is a group of people who want to have a counter signature
   parameter that is directly tied to the value being signed, and thus
   the authenticated and unauthenticated buckets can be removed from the
   message being sent.  The focus on this is an even smaller size, as
   all of the information on the process of creating the counter
   signature is implicit rather than being explicitly carried in the
   message.  This includes not only the algorithm identifier as
   presented above, but also items such as the key identification, which
   is always external to the signature structure.  This means that the
   entities that are doing the validation of the counter signature are
   required to infer which key is to be used from context rather than
   being explicit.  One way of doing this would be to presume that all
   data coming from a specific port (or to a specific URL) is to be
   validated by a specific key.  (Note that this does not require that
   the key identifier be part of the value signed as it does not serve a
   cryptographic purpose.  If the key validates the counter signature,
   then it should be presumed that the entity associated with that key
   produced the signature.)

   When computing the signature for the bare counter signature header,
   the same Sig_structure defined in <a href="#section-4.4">Section 4.4</a> is used.  The
   sign_protected field is omitted, as there is no protected header
   field in this counter signature header.  The value of
   "CounterSignature0" is placed in the context field of the
   Sig_stucture.










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   +-------------------+-------+-------+-------+-----------------------+
   | Name              | Label | Value | Value | Description           |
   |                   |       | Type  |       |                       |
   +-------------------+-------+-------+-------+-----------------------+
   | CounterSignature0 | 9     | bstr  |       | Counter signature     |
   |                   |       |       |       | with implied signer   |
   |                   |       |       |       | and headers           |
   +-------------------+-------+-------+-------+-----------------------+

             Table 27: Header Parameter for CounterSignature0

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Two Layers of Recipient Information</span>

   All of the currently defined recipient algorithm classes only use two
   layers of the COSE_Encrypt structure.  The first layer is the message
   content, and the second layer is the content key encryption.
   However, if one uses a recipient algorithm such as the RSA Key
   Encapsulation Mechanism (RSA-KEM) (see <a href="#appendix-A">Appendix A</a> of RSA-KEM
   [<a href="./rfc5990" title="&quot;Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)&quot;">RFC5990</a>]), then it makes sense to have three layers of the
   COSE_Encrypt structure.

   These layers would be:

   o  Layer 0: The content encryption layer.  This layer contains the
      payload of the message.

   o  Layer 1: The encryption of the CEK by a KEK.

   o  Layer 2: The encryption of a long random secret using an RSA key
      and a key derivation function to convert that secret into the KEK.

   This is an example of what a triple layer message would look like.
   The message has the following layers:

   o  Layer 0: Has a content encrypted with AES-GCM using a 128-bit key.

   o  Layer 1: Uses the AES Key Wrap algorithm with a 128-bit key.

   o  Layer 2: Uses ECDH Ephemeral-Static direct to generate the layer 1
      key.











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   In effect, this example is a decomposed version of using the
   ECDH-ES+A128KW algorithm.

   Size of binary file is 183 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852948658f0
   811139868826e89218a75715b',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-3 / A128KW /
           },
           / ciphertext / h'dbd43c4e9d719c27c6275c67d628d493f090593db82
   18f11',
           / recipients / [
             [
               / protected / h'a1013818' / {
                   \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
                 } / ,
               / unprotected / {
                 / ephemeral / -1:{
                   / kty / 1:2,
                   / crv / -1:1,
                   / x / -2:h'b2add44368ea6d641f9ca9af308b4079aeb519f11
   e9b8a55a600b21233e86e68',
                   / y / -3:false
                 },
                 / kid / 4:'meriadoc.brandybuck@buckland.example'
               },
               / ciphertext / h''
             ]
           ]
         ]
       ]
     ]
   )






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<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Examples</span>

   This appendix includes a set of examples that show the different
   features and message types that have been defined in this document.
   To make the examples easier to read, they are presented using the
   extended CBOR diagnostic notation (defined in [<a href="#ref-CDDL" title="&quot;CBOR data definition language (CDDL): a notational convention to express CBOR data structures&quot;">CDDL</a>]) rather than as
   a binary dump.

   A GitHub project has been created at &lt;<a href="https://github.com/cose-wg/Examples">https://github.com/cose-wg/</a>
   <a href="https://github.com/cose-wg/Examples">Examples</a>&gt; that contains not only the examples presented in this
   document, but a more complete set of testing examples as well.  Each
   example is found in a JSON file that contains the inputs used to
   create the example, some of the intermediate values that can be used
   in debugging the example and the output of the example presented in
   both a hex and a CBOR diagnostic notation format.  Some of the
   examples at the site are designed failure testing cases; these are
   clearly marked as such in the JSON file.  If errors in the examples
   in this document are found, the examples on GitHub will be updated,
   and a note to that effect will be placed in the JSON file.

   As noted, the examples are presented using the CBOR's diagnostic
   notation.  A Ruby-based tool exists that can convert between the
   diagnostic notation and binary.  This tool can be installed with the
   command line:

   gem install cbor-diag

   The diagnostic notation can be converted into binary files using the
   following command line:

   diag2cbor.rb &lt; inputfile &gt; outputfile

   The examples can be extracted from the XML version of this document
   via an XPath expression as all of the artwork is tagged with the
   attribute type='CBORdiag'.  (Depending on the XPath evaluator one is
   using, it may be necessary to deal with &amp;gt; as an entity.)

   //artwork[@type='CDDL']/text()













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<span class="h3"><a class="selflink" id="appendix-C.1" href="#appendix-C.1">C.1</a>.  Examples of Signed Messages</span>

<span class="h4"><a class="selflink" id="appendix-C.1.1" href="#appendix-C.1.1">C.1.1</a>.  Single Signature</span>

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256

   Size of binary file is 103 bytes

   98(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.1.2" href="#appendix-C.1.2">C.1.2</a>.  Multiple Signers</span>

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256

   o  Signature Algorithm: ECDSA w/ SHA-512, Curve P-521













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   Size of binary file is 277 bytes

   98(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ],
         [
           / protected / h'a1013823' / {
               \ alg \ 1:-36
             } / ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
   de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
   7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
   c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
   83ab87bb4f7a0297'
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.1.3" href="#appendix-C.1.3">C.1.3</a>.  Counter Signature</span>

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256

   o  The same parameters are used for both the signature and the
      counter signature.







<span class="grey">Schaad                       Standards Track                  [Page 104]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-105" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 180 bytes

   98(
     [
       / protected / h'',
       / unprotected / {
         / countersign / 7:[
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'5ac05e289d5d0e1b0a7f048a5d2b643813ded50bc9e4
   9220f4f7278f85f19d4a77d655c9d3b51e805a74b099e1e085aacd97fc29d72f887e
   8802bb6650cceb2c'
         ]
       },
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.1.4" href="#appendix-C.1.4">C.1.4</a>.  Signature with Criticality</span>

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256

   o  There is a criticality marker on the "reserved" header parameter









<span class="grey">Schaad                       Standards Track                  [Page 105]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-106" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 125 bytes

   98(
     [
       / protected / h'a2687265736572766564f40281687265736572766564' /
   {
           "reserved":false,
           \ crit \ 2:[
             "reserved"
           ]
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
   69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
   18aba9d1fad1bd9c'
         ]
       ]
     ]
   )

<span class="h3"><a class="selflink" id="appendix-C.2" href="#appendix-C.2">C.2</a>.  Single Signer Examples</span>

<span class="h4"><a class="selflink" id="appendix-C.2.1" href="#appendix-C.2.1">C.2.1</a>.  Single ECDSA Signature</span>

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256















<span class="grey">Schaad                       Standards Track                  [Page 106]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-107" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 98 bytes

   18(
     [
       / protected / h'a10126' / {
           \ alg \ 1:-7 \ ECDSA 256 \
         } / ,
       / unprotected / {
         / kid / 4:'11'
       },
       / payload / 'This is the content.',
       / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
   d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
   a4c345cacb36'
     ]
   )

<span class="h3"><a class="selflink" id="appendix-C.3" href="#appendix-C.3">C.3</a>.  Examples of Enveloped Messages</span>

<span class="h4"><a class="selflink" id="appendix-C.3.1" href="#appendix-C.3.1">C.3.1</a>.  Direct ECDH</span>

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH Ephemeral-Static, Curve P-256

























<span class="grey">Schaad                       Standards Track                  [Page 107]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-108" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 151 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413'
       },
       / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
   c52a357da7a644b8070a151b0',
       / recipients / [
         [
           / protected / h'a1013818' / {
               \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:1,
               / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
   bf054e1c7b4d91d6280',
               / y / -3:true
             },
             / kid / 4:'meriadoc.brandybuck@buckland.example'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.3.2" href="#appendix-C.3.2">C.3.2</a>.  Direct Plus Key Derivation</span>

   This example uses the following:

   o  CEK: AES-CCM w/ 128-bit key, truncate the tag to 64 bits

   o  Recipient class: Use HKDF on a shared secret with the following
      implicit fields as part of the context.

      *  salt: "aabbccddeeffgghh"

      *  PartyU identity: "lighting-client"

      *  PartyV identity: "lighting-server"




<span class="grey">Schaad                       Standards Track                  [Page 108]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-109" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


      *  Supplementary Public Other: "Encryption Example 02"

   Size of binary file is 91 bytes

   96(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a76c'
       },
       / ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
   1b687b847',
       / recipients / [
         [
           / protected / h'a10129' / {
               \ alg \ 1:-10
             } / ,
           / unprotected / {
             / salt / -20:'aabbccddeeffgghh',
             / kid / 4:'our-secret'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.3.3" href="#appendix-C.3.3">C.3.3</a>.  Counter Signature on Encrypted Content</span>

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH Ephemeral-Static, Curve P-256















<span class="grey">Schaad                       Standards Track                  [Page 109]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-110" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 326 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413',
         / countersign / 7:[
           / protected / h'a1013823' / {
               \ alg \ 1:-36
             } / ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'00929663c8789bb28177ae28467e66377da12302d7f9
   594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
   cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
   3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
   c3430c9d65e7ddff'
         ]
       },
       / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
   c52a357da7a644b8070a151b0',
       / recipients / [
         [
           / protected / h'a1013818' / {
               \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:1,
               / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
   bf054e1c7b4d91d6280',
               / y / -3:true
             },
             / kid / 4:'meriadoc.brandybuck@buckland.example'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )






<span class="grey">Schaad                       Standards Track                  [Page 110]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-111" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


<span class="h4"><a class="selflink" id="appendix-C.3.4" href="#appendix-C.3.4">C.3.4</a>.  Encrypted Content with External Data</span>

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH static-Static, Curve P-256 with AES Key Wrap

   o  Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077'

   Size of binary file is 173 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
   e5f0165eee976b4a5f6c6f09d',
       / recipients / [
         [
           / protected / h'a101381f' / {
               \ alg \ 1:-32 \ ECHD-SS+A128KW \
             } / ,
           / unprotected / {
             / static kid / -3:'peregrin.took@tuckborough.example',
             / kid / 4:'meriadoc.brandybuck@buckland.example',
             / U nonce / -22:h'0101'
           },
           / ciphertext / h'41e0d76f579dbd0d936a662d54d8582037de2e366fd
   e1c62'
         ]
       ]
     ]
   )

<span class="h3"><a class="selflink" id="appendix-C.4" href="#appendix-C.4">C.4</a>.  Examples of Encrypted Messages</span>

<span class="h4"><a class="selflink" id="appendix-C.4.1" href="#appendix-C.4.1">C.4.1</a>.  Simple Encrypted Message</span>

   This example uses the following:

   o  CEK: AES-CCM w/ 128-bit key and a 64-bit tag





<span class="grey">Schaad                       Standards Track                  [Page 111]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-112" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 52 bytes

   16(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a78c'
       },
       / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce71cb45ce
   460ffb569'
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.4.2" href="#appendix-C.4.2">C.4.2</a>.  Encrypted Message with a Partial IV</span>

   This example uses the following:

   o  CEK: AES-CCM w/ 128-bit key and a 64-bit tag

   o  Prefix for IV is 89F52F65A1C580933B52

   Size of binary file is 41 bytes

   16(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / partial iv / 6:h'61a7'
       },
       / ciphertext / h'252a8911d465c125b6764739700f0141ed09192de139e05
   3bd09abca'
     ]
   )

<span class="h3"><a class="selflink" id="appendix-C.5" href="#appendix-C.5">C.5</a>.  Examples of MACed Messages</span>

<span class="h4"><a class="selflink" id="appendix-C.5.1" href="#appendix-C.5.1">C.5.1</a>.  Shared Secret Direct MAC</span>

   This example uses the following:

   o  MAC: AES-CMAC, 256-bit key, truncated to 64 bits

   o  Recipient class: direct shared secret




<span class="grey">Schaad                       Standards Track                  [Page 112]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-113" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 57 bytes

   97(
     [
       / protected / h'a1010f' / {
           \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'9e1226ba1f81b848',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-6 / direct /,
             / kid / 4:'our-secret'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.5.2" href="#appendix-C.5.2">C.5.2</a>.  ECDH Direct MAC</span>

   This example uses the following:

   o  MAC: HMAC w/SHA-256, 256-bit key

   o  Recipient class: ECDH key agreement, two static keys, HKDF w/
      context structure




















<span class="grey">Schaad                       Standards Track                  [Page 113]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-114" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 214 bytes

   97(
     [
       / protected / h'a10105' / {
           \ alg \ 1:5 \ HMAC 256//256 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
   4bc3f16a41',
       / recipients / [
         [
           / protected / h'a101381a' / {
               \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
             } / ,
           / unprotected / {
             / static kid / -3:'peregrin.took@tuckborough.example',
             / kid / 4:'meriadoc.brandybuck@buckland.example',
             / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
   19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
   68b017e7f2a9e5ce4db5'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.5.3" href="#appendix-C.5.3">C.5.3</a>.  Wrapped MAC</span>

   This example uses the following:

   o  MAC: AES-MAC, 128-bit key, truncated to 64 bits

   o  Recipient class: AES Key Wrap w/ a pre-shared 256-bit key















<span class="grey">Schaad                       Standards Track                  [Page 114]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-115" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 109 bytes

   97(
     [
       / protected / h'a1010e' / {
           \ alg \ 1:14 \ AES-CBC-MAC-128//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'36f5afaf0bab5d43',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-5 / A256KW /,
             / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
           },
           / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
   b6eb0'
         ]
       ]
     ]
   )

<span class="h4"><a class="selflink" id="appendix-C.5.4" href="#appendix-C.5.4">C.5.4</a>.  Multi-Recipient MACed Message</span>

   This example uses the following:

   o  MAC: HMAC w/ SHA-256, 128-bit key

   o  Recipient class: Uses three different methods

      1.  ECDH Ephemeral-Static, Curve P-521, AES Key Wrap w/ 128-bit
          key

      2.  AES Key Wrap w/ 256-bit key















<span class="grey">Schaad                       Standards Track                  [Page 115]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-116" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 309 bytes

   97(
     [
       / protected / h'a10105' / {
           \ alg \ 1:5 \ HMAC 256//256 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
   1e49e9323e',
       / recipients / [
         [
           / protected / h'a101381c' / {
               \ alg \ 1:-29 \ ECHD-ES+A128KW \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:3,
               / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
   71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
   d613574e7dc242f79c3',
               / y / -3:true
             },
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / ciphertext / h'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
   a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
         ],
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-5 / A256KW /,
             / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
           },
           / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
   518e7736549e998370695e6d6a83b4ae507bb'
         ]
       ]
     ]
   )









<span class="grey">Schaad                       Standards Track                  [Page 116]</span></pre>
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<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


<span class="h3"><a class="selflink" id="appendix-C.6" href="#appendix-C.6">C.6</a>.  Examples of MAC0 Messages</span>

<span class="h4"><a class="selflink" id="appendix-C.6.1" href="#appendix-C.6.1">C.6.1</a>.  Shared Secret Direct MAC</span>

   This example uses the following:

   o  MAC: AES-CMAC, 256-bit key, truncated to 64 bits

   o  Recipient class: direct shared secret

   Size of binary file is 37 bytes

   17(
     [
       / protected / h'a1010f' / {
           \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'726043745027214f'
     ]
   )

   Note that this example uses the same inputs as <a href="#appendix-C.5.1">Appendix C.5.1</a>.

<span class="h3"><a class="selflink" id="appendix-C.7" href="#appendix-C.7">C.7</a>.  COSE Keys</span>

<span class="h4"><a class="selflink" id="appendix-C.7.1" href="#appendix-C.7.1">C.7.1</a>.  Public Keys</span>

   This is an example of a COSE Key Set.  This example includes the
   public keys for all of the previous examples.

   In order the keys are:

   o  An EC key with a kid of "meriadoc.brandybuck@buckland.example"

   o  An EC key with a kid of "peregrin.took@tuckborough.example"

   o  An EC key with a kid of "bilbo.baggins@hobbiton.example"

   o  An EC key with a kid of "11"










<span class="grey">Schaad                       Standards Track                  [Page 117]</span></pre>
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<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


   Size of binary file is 481 bytes

   [
     {
       -1:1,
       -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
   8551d',
       -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
   4d19c',
       1:2,
       2:'meriadoc.brandybuck@buckland.example'
     },
     {
       -1:1,
       -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
   09eff',
       -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
   c117e',
       1:2,
       2:'11'
     },
     {
       -1:3,
       -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
   7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
   f42ad',
       -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
   60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
   d9475',
       1:2,
       2:'bilbo.baggins@hobbiton.example'
     },
     {
       -1:1,
       -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
   d6280',
       -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
   822bb',
       1:2,
       2:'peregrin.took@tuckborough.example'
     }
   ]









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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-119" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


<span class="h4"><a class="selflink" id="appendix-C.7.2" href="#appendix-C.7.2">C.7.2</a>.  Private Keys</span>

   This is an example of a COSE Key Set.  This example includes the
   private keys for all of the previous examples.

   In order the keys are:

   o  An EC key with a kid of "meriadoc.brandybuck@buckland.example"

   o  A shared-secret key with a kid of "our-secret"

   o  An EC key with a kid of "peregrin.took@tuckborough.example"

   o  A shared-secret key with a kid of "018c0ae5-4d9b-471b-
      bfd6-eef314bc7037"

   o  An EC key with a kid of "bilbo.baggins@hobbiton.example"

   o  An EC key with a kid of "11"

   Size of binary file is 816 bytes

   [
     {
       1:2,
       2:'meriadoc.brandybuck@buckland.example',
       -1:1,
       -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
   8551d',
       -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
   4d19c',
       -4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa
   208cf'
     },
     {
       1:2,
       2:'11',
       -1:1,
       -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
   09eff',
       -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
   c117e',
       -4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
   7b4d3'
     },
     {
       1:2,
       2:'bilbo.baggins@hobbiton.example',



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-120" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


       -1:3,
       -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
   7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
   f42ad',
       -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
   60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
   d9475',
       -4:h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476680b
   55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609fdf177f
   eb26d'
     },
     {
       1:4,
       2:'our-secret',
       -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
   27188'
     },
     {
       1:2,
       -1:1,
       2:'peregrin.took@tuckborough.example',
       -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
   d6280',
       -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
   822bb',
       -4:h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522a848
   df1c3'
     },
     {
       1:4,
       2:'our-secret2',
       -1:h'849b5786457c1491be3a76dcea6c4271'
     },
     {
       1:4,
       2:'018c0ae5-4d9b-471b-bfd6-eef314bc7037',
       -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
   27188'
     }
   ]











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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-121" ></span>
<span class="grey"><a href="./rfc8152">RFC 8152</a>        CBOR Object Signing and Encryption (COSE)      July 2017</span>


Acknowledgments

   This document is a product of the COSE working group of the IETF.

   The following individuals are to blame for getting me started on this
   project in the first place: Richard Barnes, Matt Miller, and Martin
   Thomson.

   The initial version of the specification was based to some degree on
   the outputs of the JOSE and S/MIME working groups.

   The following individuals provided input into the final form of the
   document: Carsten Bormann, John Bradley, Brain Campbell, Michael B.
   Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and Goran
   Selander.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com





























Schaad                       Standards Track                  [Page 121]
</pre>