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<pre>Internet Engineering Task Force (IETF) P. Wouters, Ed.
Request for Comments: 7250 Red Hat
Category: Standards Track H. Tschofenig, Ed.
ISSN: 2070-1721 ARM Ltd.
J. Gilmore
Electronic Frontier Foundation
S. Weiler
Parsons
T. Kivinen
INSIDE Secure
June 2014
<span class="h1">Using Raw Public Keys in Transport Layer Security (TLS)</span>
<span class="h1">and Datagram Transport Layer Security (DTLS)</span>
Abstract
This document specifies a new certificate type and two TLS extensions
for exchanging raw public keys in Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS). The new certificate type
allows raw public keys to be used for authentication.
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="./rfc5741#section-2">Section 2 of RFC 5741</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/rfc7250">http://www.rfc-editor.org/info/rfc7250</a>.
<span class="grey">Wouters, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
Copyright Notice
Copyright (c) 2014 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.
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3">3</a>. Structure of the Raw Public Key Extension . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-4">4</a>. TLS Client and Server Handshake Behavior . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.1">4.1</a>. Client Hello . . . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.2">4.2</a>. Server Hello . . . . . . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-4.3">4.3</a>. Client Authentication . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-4.4">4.4</a>. Server Authentication . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5">5</a>. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.1">5.1</a>. TLS Server Uses a Raw Public Key . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.2">5.2</a>. TLS Client and Server Use Raw Public Keys . . . . . . . . <a href="#page-11">11</a>
5.3. Combined Usage of Raw Public Keys and X.509 Certificates 12
<a href="#section-6">6</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-7">7</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-8">8</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-9">9</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-9.1">9.1</a>. Normative References . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-9.2">9.2</a>. Informative References . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#appendix-A">Appendix A</a>. Example Encoding . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
<span class="grey">Wouters, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
Traditionally, TLS client and server public keys are obtained in PKIX
containers in-band as part of the TLS handshake procedure and are
validated using trust anchors based on a [<a href="#ref-PKIX" title=""Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"">PKIX</a>] certification
authority (CA). This method can add a complicated trust relationship
that is difficult to validate. Examples of such complexity can be
seen in [<a href="#ref-Defeating-SSL">Defeating-SSL</a>]. TLS is, however, also commonly used with
self-signed certificates in smaller deployments where the self-signed
certificates are distributed to all involved protocol endpoints out-
of-band. This practice does, however, still require the overhead of
the certificate generation even though none of the information found
in the certificate is actually used.
Alternative methods are available that allow a TLS client/server to
obtain the TLS server/client public key:
o The TLS client can obtain the TLS server public key from a DNSSEC-
secured resource record using DNS-Based Authentication of Named
Entities (DANE) [<a href="./rfc6698" title=""The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA"">RFC6698</a>].
o The TLS client or server public key is obtained from a [<a href="#ref-PKIX" title=""Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"">PKIX</a>]
certificate chain from a Lightweight Directory Access Protocol
[<a href="#ref-LDAP" title=""Lightweight Directory Access Protocol (LDAP): The Protocol"">LDAP</a>] server or web page.
o The TLS client and server public key is provisioned into the
operating system firmware image and updated via software updates.
For example:
Some smart objects use the UDP-based Constrained Application
Protocol [<a href="#ref-CoAP" title=""The Constrained Application Protocol (CoAP)"">CoAP</a>] to interact with a Web server to upload sensor
data at regular intervals, such as temperature readings. CoAP can
utilize DTLS for securing the client-to-server communication. As
part of the manufacturing process, the embedded device may be
configured with the address and the public key of a dedicated CoAP
server, as well as a public/private key pair for the client
itself.
This document introduces the use of raw public keys in TLS/DTLS.
With raw public keys, only a subset of the information found in
typical certificates is utilized: namely, the SubjectPublicKeyInfo
structure of a PKIX certificate that carries the parameters necessary
to describe the public key. Other parameters found in PKIX
certificates are omitted. By omitting various certificate-related
structures, the resulting raw public key is kept fairly small in
comparison to the original certificate, and the code to process the
keys can be simpler. Only a minimalistic ASN.1 parser is needed;
code for certificate path validation and other PKIX-related
<span class="grey">Wouters, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
processing is not required. Note, however, the SubjectPublicKeyInfo
structure is still in an ASN.1 format. To further reduce the size of
the exchanged information, this specification can be combined with
the TLS Cached Info extension [<a href="#ref-CACHED-INFO">CACHED-INFO</a>], which enables TLS peers
to exchange just fingerprints of their public keys.
The mechanism defined herein only provides authentication when an
out-of-band mechanism is also used to bind the public key to the
entity presenting the key.
<a href="#section-3">Section 3</a> defines the structure of the two new TLS extensions,
client_certificate_type and server_certificate_type, which can be
used as part of an extended TLS handshake when raw public keys are to
be used. <a href="#section-4">Section 4</a> defines the behavior of the TLS client and the
TLS server. Example exchanges are described in <a href="#section-5">Section 5</a>. <a href="#section-6">Section 6</a>
describes security considerations with this approach. Finally, in
<a href="#section-7">Section 7</a> this document registers a new value to the IANA "TLS
Certificate Types" subregistry for the support of raw public keys.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology</span>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
We use the terms "TLS server" and "server" as well as "TLS client"
and "client" interchangeably.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Structure of the Raw Public Key Extension</span>
This section defines the two TLS extensions client_certificate_type
and server_certificate_type, which can be used as part of an extended
TLS handshake when raw public keys are used. <a href="#section-4">Section 4</a> defines the
behavior of the TLS client and the TLS server using these extensions.
This specification uses raw public keys whereby the already available
encoding used in a PKIX certificate in the form of a
SubjectPublicKeyInfo structure is reused. To carry the raw public
key within the TLS handshake, the Certificate payload is used as a
container, as shown in Figure 1. The shown Certificate structure is
an adaptation of its original form [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>].
<span class="grey">Wouters, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
opaque ASN.1Cert<1..2^24-1>;
struct {
select(certificate_type){
// certificate type defined in this document.
case RawPublicKey:
opaque ASN.1_subjectPublicKeyInfo<1..2^24-1>;
// X.509 certificate defined in <a href="./rfc5246">RFC 5246</a>
case X.509:
ASN.1Cert certificate_list<0..2^24-1>;
// Additional certificate type based on
// "TLS Certificate Types" subregistry
};
} Certificate;
Figure 1: Certificate Payload as a Container for the Raw Public Key
The SubjectPublicKeyInfo structure is defined in Section 4.1 of <a href="./rfc5280">RFC</a>
<a href="./rfc5280">5280</a> [<a href="#ref-PKIX" title=""Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"">PKIX</a>] and not only contains the raw keys, such as the public
exponent and the modulus of an RSA public key, but also an algorithm
identifier. The algorithm identifier can also include parameters.
The SubjectPublicKeyInfo value in the Certificate payload MUST
contain the DER encoding [<a href="#ref-X.690" title=""Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)"">X.690</a>] of the SubjectPublicKeyInfo. The
structure, as shown in Figure 2, therefore also contains length
information. An example is provided in <a href="#appendix-A">Appendix A</a>.
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
Figure 2: SubjectPublicKeyInfo ASN.1 Structure
The algorithm identifiers are Object Identifiers (OIDs). <a href="./rfc3279">RFC 3279</a>
[<a href="./rfc3279" title=""Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"">RFC3279</a>] and <a href="./rfc5480">RFC 5480</a> [<a href="./rfc5480" title=""Elliptic Curve Cryptography Subject Public Key Information"">RFC5480</a>], for example, define the OIDs shown
in Figure 3. Note that this list is not exhaustive, and more OIDs
may be defined in future RFCs.
<span class="grey">Wouters, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
Key Type | Document | OID
--------------------+----------------------------+-------------------
RSA | <a href="./rfc3279#section-2.3.1">Section 2.3.1 of RFC 3279</a> | 1.2.840.113549.1.1
....................|............................|...................
Digital Signature | |
Algorithm (DSA) | <a href="./rfc3279#section-2.3.2">Section 2.3.2 of RFC 3279</a> | 1.2.840.10040.4.1
....................|............................|...................
Elliptic Curve | |
Digital Signature | |
Algorithm (ECDSA) | <a href="./rfc5480#section-2">Section 2 of RFC 5480</a> | 1.2.840.10045.2.1
--------------------+----------------------------+-------------------
Figure 3: Example Algorithm Object Identifiers
The extension format for extended client and server hellos, which
uses the "extension_data" field, is used to carry the
ClientCertTypeExtension and the ServerCertTypeExtension structures.
These two structures are shown in Figure 4. The CertificateType
structure is an enum with values taken from the "TLS Certificate
Types" subregistry of the "Transport Layer Security (TLS) Extensions"
registry [<a href="#ref-TLS-Ext-Registry">TLS-Ext-Registry</a>].
struct {
select(ClientOrServerExtension) {
case client:
CertificateType client_certificate_types<1..2^8-1>;
case server:
CertificateType client_certificate_type;
}
} ClientCertTypeExtension;
struct {
select(ClientOrServerExtension) {
case client:
CertificateType server_certificate_types<1..2^8-1>;
case server:
CertificateType server_certificate_type;
}
} ServerCertTypeExtension;
Figure 4: CertTypeExtension Structure
<span class="grey">Wouters, et al. Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. TLS Client and Server Handshake Behavior</span>
This specification extends the ClientHello and the ServerHello
messages, according to the extension procedures defined in [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>].
It does not extend or modify any other TLS message.
Note: No new cipher suites are required to use raw public keys. All
existing cipher suites that support a key exchange method compatible
with the defined extension can be used.
The high-level message exchange in Figure 5 shows the
client_certificate_type and server_certificate_type extensions added
to the client and server hello messages.
client_hello,
client_certificate_type,
server_certificate_type ->
<- server_hello,
client_certificate_type,
server_certificate_type,
certificate,
server_key_exchange,
certificate_request,
server_hello_done
certificate,
client_key_exchange,
certificate_verify,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 5: Basic Raw Public Key TLS Exchange
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Client Hello</span>
In order to indicate the support of raw public keys, clients include
the client_certificate_type and/or the server_certificate_type
extensions in an extended client hello message. The hello extension
mechanism is described in <a href="#section-7.4.1.4">Section 7.4.1.4</a> of TLS 1.2 [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>].
The client_certificate_type extension in the client hello indicates
the certificate types the client is able to provide to the server,
when requested using a certificate_request message.
<span class="grey">Wouters, et al. Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
The server_certificate_type extension in the client hello indicates
the types of certificates the client is able to process when provided
by the server in a subsequent certificate payload.
The client_certificate_type and server_certificate_type extensions
sent in the client hello each carry a list of supported certificate
types, sorted by client preference. When the client supports only
one certificate type, it is a list containing a single element.
The TLS client MUST omit certificate types from the
client_certificate_type extension in the client hello if it does not
possess the corresponding raw public key or certificate that it can
provide to the server when requested using a certificate_request
message, or if it is not configured to use one with the given TLS
server. If the client has no remaining certificate types to send in
the client hello, other than the default X.509 type, it MUST omit the
client_certificate_type extension in the client hello.
The TLS client MUST omit certificate types from the
server_certificate_type extension in the client hello if it is unable
to process the corresponding raw public key or other certificate
type. If the client has no remaining certificate types to send in
the client hello, other than the default X.509 certificate type, it
MUST omit the entire server_certificate_type extension from the
client hello.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Server Hello</span>
If the server receives a client hello that contains the
client_certificate_type extension and/or the server_certificate_type
extension, then three outcomes are possible:
1. The server does not support the extension defined in this
document. In this case, the server returns the server hello
without the extensions defined in this document.
2. The server supports the extension defined in this document, but
it does not have any certificate type in common with the client.
Then, the server terminates the session with a fatal alert of
type "unsupported_certificate".
3. The server supports the extensions defined in this document and
has at least one certificate type in common with the client. In
this case, the processing rules described below are followed.
The client_certificate_type extension in the client hello indicates
the certificate types the client is able to provide to the server,
when requested using a certificate_request message. If the TLS
<span class="grey">Wouters, et al. Standards Track [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
server wants to request a certificate from the client (via the
certificate_request message), it MUST include the
client_certificate_type extension in the server hello. This
client_certificate_type extension in the server hello then indicates
the type of certificates the client is requested to provide in a
subsequent certificate payload. The value conveyed in the
client_certificate_type extension MUST be selected from one of the
values provided in the client_certificate_type extension sent in the
client hello. The server MUST also include a certificate_request
payload in the server hello message.
If the server does not send a certificate_request payload (for
example, because client authentication happens at the application
layer or no client authentication is required) or none of the
certificates supported by the client (as indicated in the
client_certificate_type extension in the client hello) match the
server-supported certificate types, then the client_certificate_type
payload in the server hello MUST be omitted.
The server_certificate_type extension in the client hello indicates
the types of certificates the client is able to process when provided
by the server in a subsequent certificate payload. If the client
hello indicates support of raw public keys in the
server_certificate_type extension and the server chooses to use raw
public keys, then the TLS server MUST place the SubjectPublicKeyInfo
structure into the Certificate payload. With the
server_certificate_type extension in the server hello, the TLS server
indicates the certificate type carried in the Certificate payload.
This additional indication enables avoiding parsing ambiguities since
the Certificate payload may contain either the X.509 certificate or a
SubjectPublicKeyInfo structure. Note that only a single value is
permitted in the server_certificate_type extension when carried in
the server hello.
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Client Authentication</span>
When the TLS server has specified RawPublicKey as the
client_certificate_type, authentication of the TLS client to the TLS
server is supported only through authentication of the received
client SubjectPublicKeyInfo via an out-of-band method.
<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. Server Authentication</span>
When the TLS server has specified RawPublicKey as the
server_certificate_type, authentication of the TLS server to the TLS
client is supported only through authentication of the received
client SubjectPublicKeyInfo via an out-of-band method.
<span class="grey">Wouters, et al. Standards Track [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Examples</span>
Figures 6, 7, and 8 illustrate example exchanges. Note that TLS
ciphersuites using a Diffie-Hellman exchange offering forward secrecy
can be used with a raw public key, although this document does not
show the information exchange at that level with the subsequent
message flows.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. TLS Server Uses a Raw Public Key</span>
This section shows an example where the TLS client indicates its
ability to receive and validate a raw public key from the server. In
this example, the client is quite restricted since it is unable to
process other certificate types sent by the server. It also does not
have credentials at the TLS layer it could send to the server and
therefore omits the client_certificate_type extension. Hence, the
client only populates the server_certificate_type extension with the
raw public key type, as shown in (1).
When the TLS server receives the client hello, it processes the
extension. Since it has a raw public key, it indicates in (2) that
it had chosen to place the SubjectPublicKeyInfo structure into the
Certificate payload (3).
The client uses this raw public key in the TLS handshake together
with an out-of-band validation technique, such as DANE, to verify it.
client_hello,
server_certificate_type=(RawPublicKey) // (1)
->
<- server_hello,
server_certificate_type=RawPublicKey, // (2)
certificate, // (3)
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 6: Example with Raw Public Key Provided by the TLS Server
<span class="grey">Wouters, et al. Standards Track [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. TLS Client and Server Use Raw Public Keys</span>
This section shows an example where the TLS client as well as the TLS
server use raw public keys. This is one of the use cases envisioned
for smart object networking. The TLS client in this case is an
embedded device that is configured with a raw public key for use with
TLS and is also able to process a raw public key sent by the server.
Therefore, it indicates these capabilities in (1). As in the
previously shown example, the server fulfills the client's request,
indicates this via the RawPublicKey value in the
server_certificate_type payload (2), and provides a raw public key in
the Certificate payload back to the client (see (3)). The TLS server
demands client authentication, and therefore includes a
certificate_request (4). The client_certificate_type payload in (5)
indicates that the TLS server accepts a raw public key. The TLS
client, which has a raw public key pre-provisioned, returns it in the
Certificate payload (6) to the server.
client_hello,
client_certificate_type=(RawPublicKey) // (1)
server_certificate_type=(RawPublicKey) // (1)
->
<- server_hello,
server_certificate_type=RawPublicKey // (2)
certificate, // (3)
client_certificate_type=RawPublicKey // (5)
certificate_request, // (4)
server_key_exchange,
server_hello_done
certificate, // (6)
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 7: Example with Raw Public Key provided by the TLS Server and
the Client
<span class="grey">Wouters, et al. Standards Track [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Combined Usage of Raw Public Keys and X.509 Certificates</span>
This section shows an example combining a raw public key and an X.509
certificate. The client uses a raw public key for client
authentication, and the server provides an X.509 certificate. This
exchange starts with the client indicating its ability to process an
X.509 certificate, OpenPGP certificate, or a raw public key, if
provided by the server. It prefers a raw public key, since the
RawPublicKey value precedes the other values in the
server_certificate_type vector. Additionally, the client indicates
that it has a raw public key for client-side authentication (see
(1)). The server chooses to provide its X.509 certificate in (3) and
indicates that choice in (2). For client authentication, the server
indicates in (4) that it has selected the raw public key format and
requests a certificate from the client in (5). The TLS client
provides a raw public key in (6) after receiving and processing the
TLS server hello message.
client_hello,
server_certificate_type=(RawPublicKey, X.509, OpenPGP)
client_certificate_type=(RawPublicKey) // (1)
->
<- server_hello,
server_certificate_type=X.509 // (2)
certificate, // (3)
client_certificate_type=RawPublicKey // (4)
certificate_request, // (5)
server_key_exchange,
server_hello_done
certificate, // (6)
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 8: Hybrid Certificate Example
<span class="grey">Wouters, et al. Standards Track [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
The transmission of raw public keys, as described in this document,
provides benefits by lowering the over-the-air transmission overhead
since raw public keys are naturally smaller than an entire
certificate. There are also advantages from a code-size point of
view for parsing and processing these keys. The cryptographic
procedures for associating the public key with the possession of a
private key also follows standard procedures.
However, the main security challenge is how to associate the public
key with a specific entity. Without a secure binding between
identifier and key, the protocol will be vulnerable to man-in-the-
middle attacks. This document assumes that such binding can be made
out-of-band, and we list a few examples in <a href="#section-1">Section 1</a>. DANE [<a href="./rfc6698" title=""The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA"">RFC6698</a>]
offers one such approach. In order to address these vulnerabilities,
specifications that make use of the extension need to specify how the
identifier and public key are bound. In addition to ensuring the
binding is done out-of-band, an implementation also needs to check
the status of that binding.
If public keys are obtained using DANE, these public keys are
authenticated via DNSSEC. Using pre-configured keys is another out-
of-band method for authenticating raw public keys. While pre-
configured keys are not suitable for a generic Web-based e-commerce
environment, such keys are a reasonable approach for many smart
object deployments where there is a close relationship between the
software running on the device and the server-side communication
endpoint. Regardless of the chosen mechanism for out-of-band public
key validation, an assessment of the most suitable approach has to be
made prior to the start of a deployment to ensure the security of the
system.
An attacker might try to influence the handshake exchange to make the
parties select different certificate types than they would normally
choose.
For this attack, an attacker must actively change one or more
handshake messages. If this occurs, the client and server will
compute different values for the handshake message hashes. As a
result, the parties will not accept each others' Finished messages.
Without the master_secret, the attacker cannot repair the Finished
messages, so the attack will be discovered.
<span class="grey">Wouters, et al. Standards Track [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
IANA has registered a new value in the "TLS Certificate Types"
subregistry of the "Transport Layer Security (TLS) Extensions"
registry [<a href="#ref-TLS-Ext-Registry">TLS-Ext-Registry</a>], as follows:
Value: 2
Description: Raw Public Key
Reference: <a href="./rfc7250">RFC 7250</a>
IANA has allocated two new TLS extensions, client_certificate_type
and server_certificate_type, from the "TLS ExtensionType Values"
subregistry defined in [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>]. These extensions are used in both
the client hello message and the server hello message. The new
extension types are used for certificate type negotiation. The
values carried in these extensions are taken from the "TLS
Certificate Types" subregistry of the "Transport Layer Security (TLS)
Extensions" registry [<a href="#ref-TLS-Ext-Registry">TLS-Ext-Registry</a>].
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgements</span>
The feedback from the TLS working group meeting at IETF 81 has
substantially shaped the document, and we would like to thank the
meeting participants for their input. The support for hashes of
public keys has been moved to [<a href="#ref-CACHED-INFO">CACHED-INFO</a>] after the discussions at
the IETF 82 meeting.
We would like to thank the following persons for their review
comments: Martin Rex, Bill Frantz, Zach Shelby, Carsten Bormann,
Cullen Jennings, Rene Struik, Alper Yegin, Jim Schaad, Barry Leiba,
Paul Hoffman, Robert Cragie, Nikos Mavrogiannopoulos, Phil Hunt, John
Bradley, Klaus Hartke, Stefan Jucker, Kovatsch Matthias, Daniel Kahn
Gillmor, Peter Sylvester, Hauke Mehrtens, Alexey Melnikov, Stephen
Farrell, Richard Barnes, and James Manger. Nikos Mavrogiannopoulos
contributed the design for reusing the certificate type registry.
Barry Leiba contributed guidance for the IANA Considerations text.
Stefan Jucker, Kovatsch Matthias, and Klaus Hartke provided
implementation feedback regarding the SubjectPublicKeyInfo structure.
Christer Holmberg provided the General Area (Gen-Art) review, Yaron
Sheffer provided the Security Directorate (SecDir) review, Bert
Greevenbosch provided the Applications Area Directorate review, and
Linda Dunbar provided the Operations Directorate review.
We would like to thank our TLS working group chairs, Eric Rescorla
and Joe Salowey, for their guidance and support. Finally, we would
like to thank Sean Turner, who is the responsible Security Area
Director for this work, for his review comments and suggestions.
<span class="grey">Wouters, et al. Standards Track [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-PKIX">PKIX</a>] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", <a href="./rfc5280">RFC 5280</a>, May 2008.
[<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>, March 1997.
[<a id="ref-RFC3279">RFC3279</a>] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", <a href="./rfc3279">RFC 3279</a>, April 2002.
[<a id="ref-RFC5246">RFC5246</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, August 2008.
[<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>, March 2009.
[<a id="ref-TLS-Ext-Registry">TLS-Ext-Registry</a>]
IANA, "Transport Layer Security (TLS) Extensions",
<<a href="http://www.iana.org/assignments/tls-extensiontype-values">http://www.iana.org/assignments/</a>
<a href="http://www.iana.org/assignments/tls-extensiontype-values">tls-extensiontype-values</a>>.
[<a id="ref-X.690">X.690</a>] ITU-T, "Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2002,
2002.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-ASN.1-Dump">ASN.1-Dump</a>]
Gutmann, P., "ASN.1 Object Dump Program", February 2013,
<<a href="http://www.cs.auckland.ac.nz/~pgut001/">http://www.cs.auckland.ac.nz/~pgut001/</a>>.
[<a id="ref-CACHED-INFO">CACHED-INFO</a>]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", Work in Progress,
February 2014.
[<a id="ref-CoAP">CoAP</a>] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", <a href="./rfc7252">RFC 7252</a>, June 2014.
<span class="grey">Wouters, et al. Standards Track [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
[<a id="ref-Defeating-SSL">Defeating-SSL</a>]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", February 2009, <<a href="http://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf">http://www.blackhat.com/</a>
<a href="http://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf">presentations/bh-dc-09/Marlinspike/</a>
<a href="http://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf">BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf</a>>.
[<a id="ref-LDAP">LDAP</a>] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", <a href="./rfc4511">RFC 4511</a>, June 2006.
[<a id="ref-RFC6698">RFC6698</a>] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", <a href="./rfc6698">RFC 6698</a>, August 2012.
<span class="grey">Wouters, et al. Standards Track [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Example Encoding</span>
For example, the hex sequence shown in Figure 9 describes a
SubjectPublicKeyInfo structure inside the certificate payload.
0 1 2 3 4 5 6 7 8 9
+------+-----+-----+-----+-----+-----+-----+-----+-----+-----
1 | 0x30, 0x81, 0x9f, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48,
2 | 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00, 0x03, 0x81,
3 | 0x8d, 0x00, 0x30, 0x81, 0x89, 0x02, 0x81, 0x81, 0x00, 0xcd,
4 | 0xfd, 0x89, 0x48, 0xbe, 0x36, 0xb9, 0x95, 0x76, 0xd4, 0x13,
5 | 0x30, 0x0e, 0xbf, 0xb2, 0xed, 0x67, 0x0a, 0xc0, 0x16, 0x3f,
6 | 0x51, 0x09, 0x9d, 0x29, 0x2f, 0xb2, 0x6d, 0x3f, 0x3e, 0x6c,
7 | 0x2f, 0x90, 0x80, 0xa1, 0x71, 0xdf, 0xbe, 0x38, 0xc5, 0xcb,
8 | 0xa9, 0x9a, 0x40, 0x14, 0x90, 0x0a, 0xf9, 0xb7, 0x07, 0x0b,
9 | 0xe1, 0xda, 0xe7, 0x09, 0xbf, 0x0d, 0x57, 0x41, 0x86, 0x60,
10 | 0xa1, 0xc1, 0x27, 0x91, 0x5b, 0x0a, 0x98, 0x46, 0x1b, 0xf6,
11 | 0xa2, 0x84, 0xf8, 0x65, 0xc7, 0xce, 0x2d, 0x96, 0x17, 0xaa,
12 | 0x91, 0xf8, 0x61, 0x04, 0x50, 0x70, 0xeb, 0xb4, 0x43, 0xb7,
13 | 0xdc, 0x9a, 0xcc, 0x31, 0x01, 0x14, 0xd4, 0xcd, 0xcc, 0xc2,
14 | 0x37, 0x6d, 0x69, 0x82, 0xd6, 0xc6, 0xc4, 0xbe, 0xf2, 0x34,
15 | 0xa5, 0xc9, 0xa6, 0x19, 0x53, 0x32, 0x7a, 0x86, 0x0e, 0x91,
16 | 0x82, 0x0f, 0xa1, 0x42, 0x54, 0xaa, 0x01, 0x02, 0x03, 0x01,
17 | 0x00, 0x01
Figure 9: Example SubjectPublicKeyInfo Structure Byte Sequence
The decoded byte sequence shown in Figure 9 (for example, using Peter
Gutmann's ASN.1 decoder [<a href="#ref-ASN.1-Dump">ASN.1-Dump</a>]) illustrates the structure, as
shown in Figure 10.
Offset Length Description
-------------------------------------------------------------------
0 3+159: SEQUENCE {
3 2+13: SEQUENCE {
5 2+9: OBJECT IDENTIFIER Value (1 2 840 113549 1 1 1)
: PKCS #1, rsaEncryption
16 2+0: NULL
: }
18 3+141: BIT STRING, encapsulates {
22 3+137: SEQUENCE {
25 3+129: INTEGER Value (1024 bit)
157 2+3: INTEGER Value (65537)
: }
: }
: }
Figure 10: Decoding of Example SubjectPublicKeyInfo Structure
<span class="grey">Wouters, et al. Standards Track [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc7250">RFC 7250</a> Using Raw Public Keys in TLS/DTLS June 2014</span>
Authors' Addresses
Paul Wouters (editor)
Red Hat
EMail: pwouters@redhat.com
Hannes Tschofenig (editor)
ARM Ltd.
6060 Hall in Tirol
Austria
EMail: Hannes.tschofenig@gmx.net
URI: <a href="http://www.tschofenig.priv.at">http://www.tschofenig.priv.at</a>
John Gilmore
Electronic Frontier Foundation
PO Box 170608
San Francisco, California 94117
USA
Phone: +1 415 221 6524
EMail: gnu@toad.com
URI: <a href="https://www.toad.com/">https://www.toad.com/</a>
Samuel Weiler
Parsons
7110 Samuel Morse Drive
Columbia, Maryland 21046
US
EMail: weiler@tislabs.com
Tero Kivinen
INSIDE Secure
Eerikinkatu 28
Helsinki FI-00180
FI
EMail: kivinen@iki.fi
Wouters, et al. Standards Track [Page 18]
</pre>
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