<pre>Network Working Group                                       F. Andreasen
Request for Comments: 4568                                    M. Baugher
Category: Standards Track                                        D. Wing
                                                           Cisco Systems
                                                               July 2006


                   <span class="h1">Session Description Protocol (SDP)</span>
                <span class="h1">Security Descriptions for Media Streams</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines a Session Description Protocol (SDP)
   cryptographic attribute for unicast media streams.  The attribute
   describes a cryptographic key and other parameters that serve to
   configure security for a unicast media stream in either a single
   message or a roundtrip exchange.  The attribute can be used with a
   variety of SDP media transports, and this document defines how to use
   it for the Secure Real-time Transport Protocol (SRTP) unicast media
   streams.  The SDP crypto attribute requires the services of a data
   security protocol to secure the SDP message.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Notational Conventions ..........................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. Applicability ...................................................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. SDP "Crypto" Attribute and Parameters ...........................<a href="#page-5">5</a>
      <a href="#section-4.1">4.1</a>. Tag ........................................................<a href="#page-6">6</a>
      <a href="#section-4.2">4.2</a>. Crypto-Suite ...............................................<a href="#page-6">6</a>
      <a href="#section-4.3">4.3</a>. Key Parameters .............................................<a href="#page-7">7</a>
      <a href="#section-4.4">4.4</a>. Session Parameters .........................................<a href="#page-8">8</a>
      <a href="#section-4.5">4.5</a>. Example ....................................................<a href="#page-8">8</a>
   <a href="#section-5">5</a>. General Use of the crypto Attribute .............................<a href="#page-9">9</a>
      <a href="#section-5.1">5.1</a>. Use with Offer/Answer ......................................<a href="#page-9">9</a>
           <a href="#section-5.1.1">5.1.1</a>. Generating the Initial Offer - Unicast Streams ......<a href="#page-9">9</a>



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           <a href="#section-5.1.2">5.1.2</a>. Generating the Initial Answer - Unicast Streams ....<a href="#page-10">10</a>
           5.1.3. Processing of the Initial Answer - Unicast
                  Streams ............................................<a href="#page-11">11</a>
           <a href="#section-5.1.4">5.1.4</a>. Modifying the Session ..............................<a href="#page-11">11</a>
      <a href="#section-5.2">5.2</a>. Use Outside Offer/Answer ..................................<a href="#page-11">11</a>
      <a href="#section-5.3">5.3</a>. General Backwards Compatibility Considerations ............<a href="#page-12">12</a>
   <a href="#section-6">6</a>. SRTP Security Descriptions .....................................<a href="#page-12">12</a>
      <a href="#section-6.1">6.1</a>. SRTP Key Parameter ........................................<a href="#page-13">13</a>
      <a href="#section-6.2">6.2</a>. Crypto-Suites .............................................<a href="#page-16">16</a>
           <a href="#section-6.2.1">6.2.1</a>. AES_CM_128_HMAC_SHA1_80 ............................<a href="#page-16">16</a>
           <a href="#section-6.2.2">6.2.2</a>. AES_CM_128_HMAC_SHA1_32 ............................<a href="#page-17">17</a>
           <a href="#section-6.2.3">6.2.3</a>. F8_128_HMAC_SHA1_80 ................................<a href="#page-17">17</a>
           <a href="#section-6.2.4">6.2.4</a>. Adding New Crypto-Suite Definitions ................<a href="#page-17">17</a>
      <a href="#section-6.3">6.3</a>. Session Parameters ........................................<a href="#page-17">17</a>
           <a href="#section-6.3.1">6.3.1</a>. KDR=n ..............................................<a href="#page-18">18</a>
           <a href="#section-6.3.2">6.3.2</a>. UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP .............<a href="#page-18">18</a>
           <a href="#section-6.3.3">6.3.3</a>. UNAUTHENTICATED_SRTP ...............................<a href="#page-18">18</a>
           <a href="#section-6.3.4">6.3.4</a>. FEC_ORDER=order ....................................<a href="#page-19">19</a>
           <a href="#section-6.3.5">6.3.5</a>. FEC_KEY=key-params .................................<a href="#page-19">19</a>
           <a href="#section-6.3.6">6.3.6</a>. Window Size Hint (WSH) .............................<a href="#page-19">19</a>
           <a href="#section-6.3.7">6.3.7</a>. Defining New SRTP Session Parameters ...............<a href="#page-20">20</a>
      <a href="#section-6.4">6.4</a>. SRTP Crypto Context Initialization ........................<a href="#page-20">20</a>
           6.4.1. Late Binding of One or More SSRCs to a
                  Crypto Context .....................................<a href="#page-21">21</a>
           6.4.2. Sharing Cryptographic Contexts among
                  Sessions or SSRCs ..................................<a href="#page-22">22</a>
      <a href="#section-6.5">6.5</a>. Removal of Crypto Contexts ................................<a href="#page-23">23</a>
   <a href="#section-7">7</a>. SRTP-Specific Use of the Crypto Attribute ......................<a href="#page-23">23</a>
      <a href="#section-7.1">7.1</a>. Use with Offer/Answer .....................................<a href="#page-23">23</a>
           <a href="#section-7.1.1">7.1.1</a>. Generating the Initial Offer - Unicast Streams .....<a href="#page-23">23</a>
           <a href="#section-7.1.2">7.1.2</a>. Generating the Initial Answer - Unicast Streams ....<a href="#page-24">24</a>
           7.1.3. Processing of the Initial Answer - Unicast
                  Streams ............................................<a href="#page-25">25</a>
           <a href="#section-7.1.4">7.1.4</a>. Modifying the Session ..............................<a href="#page-25">25</a>
           <a href="#section-7.1.5">7.1.5</a>. Offer/Answer Example ...............................<a href="#page-27">27</a>
      <a href="#section-7.2">7.2</a>. SRTP-Specific Use Outside Offer/Answer ....................<a href="#page-28">28</a>
      <a href="#section-7.3">7.3</a>. Support for SIP Forking ...................................<a href="#page-28">28</a>
      <a href="#section-7.4">7.4</a>. SRTP-Specific Backwards Compatibility Considerations ......<a href="#page-29">29</a>
      <a href="#section-7.5">7.5</a>. Operation with KEYMGT= and k= lines .......................<a href="#page-29">29</a>
   <a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-29">29</a>
      <a href="#section-8.1">8.1</a>. Authentication of Packets .................................<a href="#page-30">30</a>
      <a href="#section-8.2">8.2</a>. Keystream Reuse ...........................................<a href="#page-30">30</a>
      <a href="#section-8.3">8.3</a>. Signaling Authentication and Signaling Encryption .........<a href="#page-31">31</a>
   <a href="#section-9">9</a>. Grammar ........................................................<a href="#page-32">32</a>
      <a href="#section-9.1">9.1</a>. Generic "Crypto" Attribute Grammar ........................<a href="#page-32">32</a>
      <a href="#section-9.2">9.2</a>. SRTP "Crypto" Attribute Grammar ...........................<a href="#page-32">32</a>
   <a href="#section-10">10</a>. IANA Considerations ...........................................<a href="#page-34">34</a>
      <a href="#section-10.1">10.1</a>. Registration of the "crypto" Attribute ...................<a href="#page-34">34</a>



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      <a href="#section-10.2">10.2</a>. New IANA Registries and Registration Procedures ..........<a href="#page-34">34</a>
           <a href="#section-10.2.1">10.2.1</a>. Key Method Registry and Registration ..............<a href="#page-34">34</a>
           <a href="#section-10.2.2">10.2.2</a>. Media Stream Transport Registry and Registration ..35
      <a href="#section-10.3">10.3</a>. Initial Registrations ....................................<a href="#page-35">35</a>
           <a href="#section-10.3.1">10.3.1</a>. Key Method ........................................<a href="#page-35">35</a>
           <a href="#section-10.3.2">10.3.2</a>. SRTP Media Stream Transport .......................<a href="#page-35">35</a>
                  10.3.2.1. SRTP Crypto Suite Registry and
                            Registration .............................<a href="#page-35">35</a>
                  <a href="#section-10.3.2.2">10.3.2.2</a>. SRTP Session Parameter Registration ......<a href="#page-36">36</a>
   <a href="#section-11">11</a>. Acknowledgements ..............................................<a href="#page-36">36</a>
   <a href="#section-12">12</a>. Normative References ..........................................<a href="#page-36">36</a>
   <a href="#section-13">13</a>. Informative References ........................................<a href="#page-37">37</a>
   <a href="#appendix-A">Appendix A</a> - Rationale for Keying Material Directionality .........<a href="#page-40">40</a>

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

   The Session Description Protocol (SDP) [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] describes multimedia
   sessions, which can be audio, video, whiteboard, fax, modem, and
   other media streams.  Security services such as data origin
   authentication, integrity, and confidentiality are often needed for
   those streams.  The Secure Real-time Transport Protocol (SRTP)
   [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>] provides security services for RTP media and is signaled by
   use of secure RTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in an
   SDP media (m=) line.  However, there are no means within SDP itself
   to configure SRTP beyond using default values.  This document
   specifies a new SDP attribute called "crypto", which is used to
   signal and negotiate cryptographic parameters for media streams in
   general, and for SRTP in particular.  The definition of the crypto
   attribute in this document is limited to two-party unicast media
   streams where each source has a unique cryptographic key; support for
   multicast media streams or multipoint unicast streams is for further
   study.

   The crypto attribute is defined in a generic way to enable its use
   with SRTP and any other secure transports that can establish
   cryptographic parameters with only a single message or in a single
   round-trip exchange using the offer/answer model [<a href="./rfc3264" title="&quot;An Offer/Answer Model with Session Description Protocol (SDP)&quot;">RFC3264</a>].
   Extensions to transports other than SRTP, however, is beyond the
   scope of this document.  Each type of secure media transport needs
   its own specification for the crypto-attribute parameter.  These
   definitions are frequently unique to the particular type of transport
   and must be specified in a Standards-Track RFC and registered with
   IANA according to the procedures defined in <a href="#section-10">Section 10</a>.  This
   document defines the security parameters and keying material for SRTP
   only.






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   It would be self-defeating not to secure cryptographic keys and other
   parameters at least as well as the data are secured.  Data security
   protocols such as SRTP rely upon a separate key management system to
   securely establish encryption and/or authentication keys.  Key
   management protocols provide authenticated key establishment (AKE)
   procedures to authenticate the identity of each endpoint and protect
   against man-in-the-middle, reflection/replay, connection hijacking,
   and some denial-of-service attacks [<a href="#ref-skeme" title="&quot;SKEME: A Versatile Secure Key Exchange Mechanism for the Internet&quot;">skeme</a>].  Along with the key, an
   AKE protocol such as MIKEY [<a href="#ref-mikey" title="&quot;MIKEY: Multimedia Internet KEYing&quot;">mikey</a>], GDOI [<a href="#ref-GDOI" title="&quot;The Group Domain of Interpretation&quot;">GDOI</a>], KINK [<a href="#ref-kink" title="&quot;Kerberized Internet Negotiation of Keys (KINK)&quot;">kink</a>], IKE
   [<a href="#ref-ike" title="&quot;Internet Key Exchange (IKEv2) Protocol&quot;">ike</a>], Secure Multiparts [s/mime, pgp/mime], or TLS [<a href="#ref-TLS" title="&quot;The TLS Protocol Version 1.0&quot;">TLS</a>] securely
   disseminates information describing both the key and the data-
   security session.  AKE is needed because it is pointless to provide a
   key over a medium where an attacker can snoop the key, alter the
   definition of the key to render it useless, or change the parameters
   of the security session to gain unauthorized access to session-
   related information.

   SDP, however, was not designed to provide AKE services, and the media
   security descriptions defined in this document do not add AKE
   services to SDP.  This specification is no replacement for a key
   management protocol or for the conveyance of key management messages
   in SDP [<a href="#ref-keymgt" title="&quot;Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)&quot;">keymgt</a>].  The SDP security descriptions defined here are
   suitable for restricted cases only where IPsec, TLS, or some other
   encapsulating data-security protocol (e.g., SIP S/MIME) protects the
   SDP message.  This document adds security descriptions to those
   encrypted and/or authenticated SDP messages through the new SDP
   "crypto" attribute, which provides the cryptographic parameters of a
   media stream.

   The "crypto" attribute can be adapted to any media transport, but its
   precise definition is unique to a particular transport.

   In <a href="#section-2">Section 2</a>, we provide notational conventions followed by an
   applicability statement for the crypto attribute in Section 3.  In
   <a href="#section-4">Section 4</a>, we introduce the general SDP crypto attribute, and in
   <a href="#section-5">Section 5</a>, we define how it is used with and without the offer/answer
   model.  In <a href="#section-6">Section 6</a>, we define the crypto attribute details needed
   for SRTP, and in <a href="#section-7">Section 7</a>, we define SRTP-specific use of the
   attribute with and without the offer/answer model.  <a href="#section-8">Section 8</a> recites
   security considerations, and <a href="#section-9">Section 9</a> gives an Augmented-BNF grammar
   for the general crypto attribute as well as the SRTP-specific use of
   the crypto attribute.  IANA considerations are provided in <a href="#section-10">Section</a>
   <a href="#section-10">10</a>.








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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Notational Conventions</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].  The terminology in this
   document conforms to [<a href="./rfc2828" title="&quot;Internet Security Glossary&quot;">RFC2828</a>], "Internet Security Glossary".

   n^r is exponentiation, where n is multiplied by itself r times; n and
   r are integers.  0..k is an integer range of all integers from 0
   through k, inclusive.

   The terms 'transport' and 'media transport' are used to mean
   'transport protocol' as defined in <a href="./rfc4566">RFC 4566</a>.

   Explanatory notes are provided in several places throughout the
   document; these notes are indented three spaces from the surrounding
   text.

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

   <a href="./rfc4567">RFC 4567</a> provides similar cryptographic key distribution capabilities
   and is intended for use when the signaling is to be confidential
   and/or integrity-protected separately from the keying material.

   In contrast, this specification carries the keying material within
   the SDP message, and it is intended for use when the keying material
   is protected along with the signaling.  Implementations MUST employ
   security mechanisms that provide confidentiality and integrity for
   the keying material.  When this specification is used in the context
   of SIP [<a href="./rfc3261" title="&quot;SIP: Session Initiation Protocol&quot;">RFC3261</a>], the application SHOULD employ either the SIPS URI
   or S/MIME to provide protection for the SDP message and the keying
   material that it contains.  The use of transport layer or IP layer
   security in lieu of the SIPS URI or S/MIME protection is NOT
   RECOMMENDED since the protection of the SDP message and the keying
   material that it contains cannot be ensured through all intermediate
   entities such as SIP proxies.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  SDP "Crypto" Attribute and Parameters</span>

   A new media-level SDP attribute called "crypto" describes the
   cryptographic suite, key parameters, and session parameters for the
   preceding unicast media line.  The "crypto" attribute MUST only
   appear at the SDP media level (not at the session level).  The
   "crypto" attribute follows the format (see <a href="#section-9.1">Section 9.1</a> for the formal
   ABNF grammar):

      a=crypto:&lt;tag&gt; &lt;crypto-suite&gt; &lt;key-params&gt; [&lt;session-params&gt;]




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   The fields tag, crypto-suite, key-params, and session-params are
   described in the following sub-sections.  The values of each of these
   fields is case-insensitive, unless otherwise noted.  However,
   implementers are encouraged to use the actual case shown in this
   document and any extensions to it.  Note that per normal SDP rules,
   the "crypto" attribute name itself is case-sensitive.  Below, we show
   an example of the crypto attribute for the "RTP/SAVP" transport,
   i.e., the secure RTP extension to the Audio/Video Profile [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].
   In the following, newlines are included for formatting reasons only:

      a=crypto:1 AES_CM_128_HMAC_SHA1_80
      inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32

   The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined by
   the text starting with "inline:", and session-params is omitted.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Tag</span>

   The tag is a decimal number used as an identifier for a particular
   crypto attribute (see <a href="#section-9.1">Section 9.1</a> for details); leading zeroes MUST
   NOT be used.  The tag MUST be unique among all crypto attributes for
   a given media line.  It is used with the offer/answer model to
   determine which of several offered crypto attributes were chosen by
   the answerer (see <a href="#section-5.1">Section 5.1</a>).

   In the offer/answer model, the tag is a negotiated parameter.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Crypto-Suite</span>

   The crypto-suite field is an identifier that describes the encryption
   and authentication algorithms (e.g., AES_CM_128_HMAC_SHA1_80) for the
   transport in question (see <a href="#section-9.1">Section 9.1</a> for details).  The possible
   values for the crypto-suite parameter are defined within the context
   of the transport, i.e., each transport defines a separate namespace
   for the set of crypto-suites.  For example, the crypto-suite
   "AES_CM_128_HMAC_SHA1_80" defined within the context "RTP/SAVP"
   transport applies to Secure RTP only; the string may be reused for
   another transport (e.g., "RTP/SAVPF" [<a href="#ref-srtpf" title="&quot;Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF)&quot;">srtpf</a>]), but a separate
   definition would be needed.

   In the offer/answer model, the crypto-suite is a negotiated
   parameter.









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<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Key Parameters</span>

   The key-params field provides one or more sets of keying material for
   the crypto-suite in question.  The field consists of a method
   indicator followed by a colon, and the actual keying information as
   shown below (the formal grammar is provided in <a href="#section-9.1">Section 9.1</a>):

      key-params = &lt;key-method&gt; ":" &lt;key-info&gt;

   Keying material might be provided by different means from that for
   key-params; however, this is out of scope.  Only one method is
   defined in this document, namely, "inline", which indicates that the
   actual keying material is provided in the key-info field itself.
   There is a single name space for the key-method, i.e., the key-method
   is transport independent.  New key-methods (e.g., use of a URL) may
   be defined in a Standards-Track RFC in the future.  Although the
   key-method itself may be generic, the accompanying key-info
   definition is specific not only to the key-method, but also to the
   transport in question.  Key-info encodes keying material for a crypto
   suite, which defines that keying material.  New key methods MUST be
   registered with the IANA according to the procedures defined in
   <a href="#section-10.2.1">Section 10.2.1</a>.

   Key-info is defined as a general octet string (see <a href="#section-9.1">Section 9.1</a> for
   details); further transport and key-method specific syntax and
   semantics MUST be provided in a Standards-Track RFC for each
   combination of transport and key-method that uses it; definitions for
   SRTP are provided in <a href="#section-6">Section 6</a>.  Note that such definitions are
   provided within the context of both a particular transport (e.g.,
   "RTP/SAVP") and a specific key-method (e.g., "inline").  IANA will
   register the list of supported key methods for each transport.

   When multiple keys are included in the key parameters, it MUST be
   possible to determine which of the keys is being used in a given
   media packet by a simple inspection of the media packet received; a
   trial-and-error approach between the possible keys MUST NOT be
   performed.

      For SRTP, this could be achieved by use of Master Key Identifiers
      (MKI) [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].  Use of &lt;"From, "To"&gt; values are not supported in
      SRTP security descriptions for reasons explained in <a href="#section-6.1">Section 6.1</a>,
      below.

   In the offer/answer model, the key parameter is a declarative
   parameter.






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<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Session Parameters</span>

   Session parameters are specific to a given transport and use of them
   is OPTIONAL in the security descriptions framework, where they are
   just defined as general character strings.  If session parameters are
   to be used for a given transport, then transport-specific syntax and
   semantics MUST be provided in a Standards-Track RFC; definitions for
   SRTP are provided in <a href="#section-6">Section 6</a>.

   In the offer/answer model, session parameters may be either
   negotiated or declarative; the definition of specific session
   parameters MUST indicate whether they are negotiated or declarative.
   Negotiated parameters apply to data sent in both directions, whereas
   declarative parameters apply only to media sent by the entity that
   generated the SDP.  Thus, a declarative parameter in an offer applies
   to media sent by the offerer, whereas a declarative parameter in an
   answer applies to media sent by the answerer.

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

   This example shows use of the crypto attribute for the "RTP/SAVP"
   media transport type (as defined in <a href="#section-5">Section 5</a>).  The "a=crypto" line
   is actually one long line; it is shown as two lines due to page
   formatting.

      v=0
      o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
      s=SDP Seminar
      i=A Seminar on the session description protocol
      u=http://www.example.com/seminars/sdp.pdf
      e=j.doe@example.com (Jane Doe)
      c=IN IP4 161.44.17.12/127
      t=2873397496 2873404696
      m=video 51372 RTP/SAVP 31
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      m=audio 49170 RTP/SAVP 0
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      m=application 32416 udp wb
      a=orient:portrait

   This SDP message describes three media streams, two of which use the
   "RTP/SAVP" transport.  Each has a crypto attribute for the "RTP/SAVP"
   transport.  These secure-RTP specific descriptions are defined in
   <a href="#section-6">Section 6</a>.





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<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  General Use of the crypto Attribute</span>

   In this section, we describe the general use of the crypto attribute
   outside of any transport or key-method specific rules.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Use with Offer/Answer</span>

   The general offer/answer rules for the crypto attribute are in
   addition to the rules specified in <a href="./rfc3264">RFC 3264</a>, which MUST be followed,
   unless otherwise noted.  <a href="./rfc3264">RFC 3264</a> defines operation for both unicast
   and multicast streams; the sections below describe operation for
   two-party unicast streams only, since support for multicast streams
   (and multipoint unicast streams) is for further study.

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Generating the Initial Offer - Unicast Streams</span>

   When generating an initial offer for a unicast stream, there MUST be
   one or more crypto attributes present for each media stream for which
   security is desired.  Each crypto attribute for a given media stream
   MUST contain a unique tag.

   The ordering of multiple "a=crypto" lines is significant: the most
   preferred crypto line is listed first.  Each crypto attribute
   describes the crypto-suite, key(s), and possibly session parameters
   offered for the media stream.  In general, a "more preferred"
   crypto-suite SHOULD be cryptographically stronger than a "less
   preferred" crypto-suite.

   The crypto-suite always applies to media in the directions supported
   by the media stream (e.g., send and receive).  The key(s), however,
   apply to data packets (e.g., SRTP and SRTCP packets) that will be
   sent by the same party that generated the SDP.  That is, each
   endpoint determines its own transmission keys and sends those keys,
   in SDP, to the other endpoint.

      This is done for consistency.  Also, in the case of SRTP, for
      example, secure RTCP will still be flowing in both the send and
      receive direction for a unidirectional stream.

   The inline parameter conveys the keying material used by an endpoint
   to encrypt the media streams transmitted by that endpoint.  The same
   keying material is used by the recipient to decrypt those streams.

   The offer may include session parameters.  There are no general offer
   rules for the session parameters; instead, specific rules may be
   provided as part of the transport-specific definitions of any session
   parameters.




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   When issuing an offer, the offerer MUST be prepared to support media
   security in accordance with any of the crypto attributes included in
   the offer.  There are, however, two problems associated with this.
   First of all, the offerer does not know which key the answerer will
   be using for media sent to the offerer.  Second, the offerer may not
   be able to deduce which of the offered crypto attributes were
   accepted.  Since media may arrive prior to the answer, delay or
   clipping can occur.  If this is unacceptable to the offerer, the
   offerer SHOULD use a mechanism outside the scope of this document to
   prevent the above problem.

      For example, in SIP [<a href="./rfc3261" title="&quot;SIP: Session Initiation Protocol&quot;">RFC3261</a>], a "security" precondition as
      defined in [<a href="#ref-sprecon" title="&quot;Security Preconditions for Session Description Protocol Media Streams&quot;">sprecon</a>] could solve the above problem.

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  Generating the Initial Answer - Unicast Streams</span>

   When the answerer receives the initial offer with one or more crypto
   attributes for a given unicast media stream, the answerer MUST either
   accept exactly one of the offered crypto attributes, or the offered
   stream MUST be rejected.

      If the answerer wishes to indicate support for other crypto
      attributes, those can be listed by use of the SDP Simple
      Capability Declaration [<a href="./rfc3407" title="&quot;Session Description Protocol (SDP) Simple Capability Declaration&quot;">RFC3407</a>] extensions.

   Only crypto attributes that are valid can be accepted; valid
   attributes do not violate any of the general rules defined for
   security descriptions, nor any specific rules defined for the
   transport and key-method in question.  When selecting one of the
   valid crypto attributes, the answerer SHOULD select the most
   preferred crypto attribute it can support, i.e., the first valid
   supported crypto attribute in the list, according to the answerer's
   capabilities and security policies.

   If there are one or more crypto attributes in the offer, but none of
   them are valid or none of the valid ones are supported, the offered
   media stream MUST be rejected.

   When an offered crypto attribute is accepted, the crypto attribute in
   the answer MUST contain the following:

   *  The tag and crypto-suite from the accepted crypto attribute in the
      offer (the same crypto-suite MUST be used in the send and receive
      direction).

   *  The key(s) the answerer will be using for media sent to the
      offerer.  Note that a key MUST be provided, irrespective of any
      direction attributes in the offer or answer.



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   Furthermore, any session parameters that are negotiated MUST be
   included in the answer.  Declarative session parameters provided by
   the offerer are not included in the answer; however, the answerer may
   provide its own set of declarative session parameters.

   Once the answerer has accepted one of the offered crypto attributes,
   the answerer MAY begin sending media to the offerer in accordance
   with the selected crypto attribute.  Note, however, that the offerer
   may not be able to process such media packets correctly until the
   answer has been received.

<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  Processing of the Initial Answer - Unicast Streams</span>

   When the offerer receives the answer, the offerer MUST verify that
   one of the initially offered crypto suites and its accompanying tag
   were accepted and echoed in the answer.  Also, the answer MUST
   include one or more keys, which will be used for media sent from the
   answerer to the offerer.

   If the offer contained any mandatory negotiated session parameters
   (see <a href="#section-6.3.7">Section 6.3.7</a>), the offerer MUST verify that said parameters are
   included in the answer and support them.  If the answer contains any
   mandatory declarative session parameters, the offerer MUST be able to
   support those.

   If any of the above fails, the negotiation MUST fail.

<span class="h4"><a class="selflink" id="section-5.1.4" href="#section-5.1.4">5.1.4</a>.  Modifying the Session</span>

   Once a media stream has been established, it MAY be modified at any
   time, as described in <a href="./rfc3264#section-8">RFC 3264, Section&nbsp;8</a>.  Such a modification MAY
   be triggered by the security service, e.g., in order to perform a
   re-keying or change the crypto-suite.  If media stream security using
   the general security descriptions defined here is still desired, the
   crypto attribute MUST be included in these new offer/answer
   exchanges.  The procedures are similar to those defined in <a href="#section-5.1.1">Section</a>
   <a href="#section-5.1.1">5.1.1</a>, 5.1.2, and 5.1.3 of this document, subject to the
   considerations provided in <a href="./rfc3264#section-8">RFC 3264, Section&nbsp;8</a>.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Use Outside Offer/Answer</span>

   The crypto attribute can also be used outside the context of
   offer/answer where there is no negotiation of the crypto suite,
   cryptographic key, or session parameters.  In this case, the sender
   determines security parameters for the stream.  Since there is no
   negotiation mechanism, the sender MUST include exactly one crypto
   attribute, and the receiver MUST either accept it or SHOULD NOT




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   receive the associated stream.  The sender SHOULD select the security
   description that it deems most secure for its purposes.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  General Backwards Compatibility Considerations</span>

   In the offer/answer model, it is possible that the answerer supports
   a given secure transport (e.g., "RTP/SAVP") and accepts the offered
   media stream, but that the answerer does not support the crypto
   attribute defined in this document and hence ignores it.  The offerer
   can recognize this situation by seeing an accepted media stream in
   the answer that does not include a crypto line.  In that case, the
   security negotiation defined here MUST fail.

   Similar issues exist when security descriptions are used outside the
   offer/answer model.  But the source of a non-negotiated security
   description has no indication that the receiver has ignored the
   crypto attribute.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  SRTP Security Descriptions</span>

   In this section, we provide definitions for security descriptions for
   SRTP media streams.  In the next section, we define how to use SRTP
   security descriptions with and without the offer/answer model.

   SRTP security descriptions MUST only be used with the SRTP transport
   (e.g., "RTP/SAVP" or "RTP/SAVPF").  The following specifies security
   descriptions for the "RTP/SAVP" profile, defined in [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].
   However, it is expected that other secure RTP profiles (e.g.,
   "RTP/SAVPF") can use the same descriptions, which are in accordance
   with the SRTP protocol specification [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   There is no assurance that an endpoint is capable of configuring its
   SRTP service with a particular crypto attribute parameter, but SRTP
   guarantees minimal interoperability among SRTP endpoints through the
   default SRTP parameters [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].  More capable SRTP endpoints
   support a variety of parameter values beyond the SRTP defaults, and
   these values can be configured by the SRTP security descriptions
   defined here.  An endpoint that does not support the crypto attribute
   will ignore it according to the SDP.  Such an endpoint will not
   correctly process the particular media stream.  By using the
   Offer/Answer model, the offerer and answerer can negotiate the crypto
   parameters to be used before commencement of the multimedia session
   (see <a href="#section-7.1">Section 7.1</a>).

   There are over twenty cryptographic parameters listed in the SRTP
   specification.  Many of these parameters have fixed values for
   particular cryptographic transforms.  At the time of session
   establishment, however, there is usually no need to provide unique



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   settings for many of the SRTP parameters, such as salt length and
   pseudo-random function (PRF).  Thus, it is possible to simplify the
   list of parameters by defining "cryptographic suites" that fix a set
   of SRTP parameter values for the security session.  This approach is
   followed by the SRTP security descriptions, which uses the general
   security description parameters as follows:

      * crypto-suite:     Identifies the encryption and authentication
                          transforms.
      * key parameter:    SRTP keying material and parameters
      * session parameters:    The following parameters are defined:
           - KDR:    The SRTP Key Derivation Rate is the rate at which a
                     pseudo-random function is applied to a master key.
           - UNENCRYPTED_SRTP:      SRTP messages are not encrypted.
           - UNENCRYPTED_SRTCP:     SRTCP messages are not encrypted.
           - UNAUTHENTICATED_SRTP:  SRTP messages are not authenticated.
           - FEC_ORDER:   Order of forward error correction (FEC)
                          relative to SRTP services.
           - FEC_KEY:     Master Key for FEC when the FEC stream is sent
                          to a separate address and/or port.
           - WSH:         Window Size Hint.
           - Extensions:  Extension parameters can be defined.

   Please refer to the SRTP specification for a complete list of
   parameters and their descriptions [<a href="#section-8.2">Section 8.2</a>, srtp].  Regarding the
   UNENCRYPTED_SRTCP parameter, offerers and answerers of SDP security
   descriptions MUST NOT use the SRTCP E-bit to override
   UNENCRYPTED_SRTCP or the default, which is to encrypt all SRTCP
   messages (see <a href="#section-6.3.2">Section 6.3.2</a>).  The key parameter, the crypto-suite,
   and the session parameters shown above are described in detail in the
   following subsections.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  SRTP Key Parameter</span>

   SRTP security descriptions define the use of the "inline" key method
   as described in the following.  Use of any other keying method (e.g.,
   URL) for SRTP security descriptions is for further study.

   The "inline" type of key contains the keying material (master key and
   salt) and all policy related to that master key, including how long
   it can be used (lifetime) and whether it uses a master key identifier
   (MKI) to associate an incoming SRTP packet with a particular master
   key.  Compliant implementations obey the policies associated with a
   master key and MUST NOT accept incoming packets that violate the
   policy (e.g., after the master key lifetime has expired).

   The key parameter contains one or more cryptographic master keys,
   each of which MUST be a unique cryptographically random [<a href="./rfc1750" title="&quot;Randomness Recommendations for Security&quot;">RFC1750</a>]



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   value with respect to other master keys in the entire SDP message
   (i.e., including master keys for other streams).  Each key follows
   the format (the formal definition is provided in <a href="#section-9.2">Section 9.2</a>):

      "inline:" &lt;key||salt&gt; ["|" lifetime] ["|" MKI ":" length]

      key||salt      concatenated master key and salt, base64 encoded
                     (see <a href="./rfc3548#section-3">[RFC3548], Section&nbsp;3</a>)
      lifetime       master key lifetime (max number of SRTP or SRTCP
                     packets using this master key)
      MKI:length     MKI and length of the MKI field in SRTP packets

   The following definition provides an example for
   AES_CM_128_HMAC_SHA1_80:

      inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4

   The first field ("d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj") of the
   parameter is the cryptographic master key appended with the master
   salt; the two are first concatenated and then base64 encoded.  The
   length of the concatenated key and salt is determined by the crypto-
   suite for which the key applies.  If the length (after being decoded
   from base64) does not match that specified for the crypto-suite, the
   crypto attribute in question MUST be considered invalid.  Each master
   key and salt MUST be a cryptographically random number and MUST be
   unique to the entire SDP message.  When base64 decoding the key and
   salt, padding characters (i.e., one or two "=" at the end of the
   base64-encoded data) are discarded (see [<a href="./rfc3548" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC3548</a>] for details).
   Base64 encoding assumes that the base64 encoding input is an integral
   number of octets.  If a given crypto-suite requires the use of a
   concatenated key and salt with a length that is not an integral
   number of octets, said crypto-suite MUST define a padding scheme that
   results in the base64 input being an integral number of octets.  For
   example, if the length defined were 250 bits, then 6 padding bits
   would be needed, which could be defined to be the last 6 bits in a
   256 bit input.

   The second field is the OPTIONAL lifetime of the master key as
   measured in maximum number of SRTP or SRTCP packets using that master
   key (i.e., the number of SRTP packets and the number of SRTCP packets
   each have to be less than the lifetime).  The lifetime value MAY be
   written as a non-zero, positive decimal integer or as a power of 2
   (see the grammar in <a href="#section-9.2">Section 9.2</a> for details); leading zeroes MUST NOT
   be used.  The "lifetime" value MUST NOT exceed the maximum packet
   lifetime for the crypto-suite.  If the lifetime is too large or
   otherwise invalid, then the entire crypto attribute MUST be
   considered invalid.  The default MAY be implicitly signaled by
   omitting the lifetime (note that the lifetime field never includes a



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   colon, whereas the third field always does).  This is convenient when
   the SRTP cryptographic key lifetime is the default value.  As a
   shortcut to avoid long decimal values, the syntax of the lifetime
   allows using the literal "2^", which indicates "two to the power of".
   The example above shows a case where the lifetime is specified as
   2^20.  The following example, which is for the
   AES_CM_128_HMAC_SHA1_80 crypto-suite, has a default for the lifetime
   field, which means that SRTP's and SRTCP's default values will be
   used (see [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>]):

      inline:YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6MTIzNDU2|1066:4

   The example shows a 30-octet key and concatenated salt that is base64
   encoded:  The 30-octet key/salt concatenation is expanded to 40
   characters (octets) by the three-in-four encoding of base64.

   The third field, which is also OPTIONAL, is the Master Key Identifier
   (MKI) and its byte length.

   "MKI" is the master key identifier associated with the SRTP master
   key.  The MKI is here defined as a positive decimal integer that is
   encoded as a big-endian integer in the actual SRTP packets; leading
   zeroes MUST NOT be used in the integer representation.  If the MKI is
   given, then the length of the MKI MUST also be given and separated
   from the MKI by a colon (":").  The MKI length is the size of the MKI
   field in the SRTP packet, specified in bytes as a decimal integer;
   leading zeroes MUST NOT be used.  If the MKI length is not given or
   its value exceeds 128 (bytes), then the entire crypto attribute MUST
   be considered invalid.  The substring "1:4" in the first example
   assigns to the key a master key identifier of 1 that is 4 bytes long,
   and the second example assigns a 4-byte master key identifier of 1066
   to the key.  One or more master keys with their associated MKI can be
   initially defined, and then later updated, or deleted and new ones
   defined.

   SRTP offers a second feature for specifying the lifetime of a master
   key in terms of two values, called "From" and "To," which are defined
   on the SRTP sequence number space [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].  This SRTP Security
   Descriptions specification, however, does not support the &lt;"From",
   "To"&gt; feature since the lifetime of an AES master key is 2^48 SRTP
   packets, which means that there is no cryptographic reason to replace
   a master key for practical point-to-point applications.  For this
   reason, there is no need to support two means for signaling key
   update.  The MKI is chosen over &lt;"From", "To"&gt; by this specification
   for the very few applications that need it since the MKI feature is
   simpler (though the MKI adds additional bytes to each packet, whereas
   &lt;"From", "To"&gt; does not).




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   As mentioned above, the key parameter can contain one or more master
   keys.  When the key parameter contains more than one master key, all
   the master keys in that key parameter MUST include an MKI value.

   When using the MKI, the MKI length MUST be the same for all keys in a
   given crypto attribute.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Crypto-Suites</span>

   The SRTP crypto-suites define the encryption and authentication
   transforms to be used for the SRTP media stream.  The SRTP
   specification has defined three crypto-suites, which are described
   further in the following subsections in the context of the SRTP
   security descriptions.  The table below provides an overview of the
   crypto-suites and their parameters:

   +---------------------+-------------+--------------+---------------+
   |                     |AES_CM_128_  | AES_CM_128_  | F8_128_       |
   |                     |HMAC_SHA1_80 | HMAC_SHA1_32 |  HMAC_SHA1_80 |
   +---------------------+-------------+--------------+---------------+
   | Master key length   |   128 bits  |   128 bits   |   128 bits    |
   | Master salt length  |   112 bits  |   112 bits   |   112 bits    |
   | SRTP lifetime       | 2^48 packets| 2^48 packets | 2^48 packets  |
   | SRTCP lifetime      | 2^31 packets| 2^31 packets | 2^31 packets  |
   | Cipher              | AES Counter | AES Counter  | AES F8 Mode   |
   |                     | Mode        | Mode         |               |
   | Encryption key      |   128 bits  |   128 bits   |   128 bits    |
   | MAC                 |  HMAC-SHA1  |  HMAC-SHA1   |  HMAC-SHA1    |
   | SRTP auth. tag      |    80 bits  |    32 bits   |    80 bits    |
   | SRTCP auth. tag     |    80 bits  |    80 bits   |    80 bits    |
   | SRTP auth. key len. |   160 bits  |   160 bits   |   160 bits    |
   | SRTCP auth. key len.|   160 bits  |   160 bits   |   160 bits    |
   +---------------------+-------------+--------------+---------------+

<span class="h4"><a class="selflink" id="section-6.2.1" href="#section-6.2.1">6.2.1</a>.  AES_CM_128_HMAC_SHA1_80</span>

   AES_CM_128_HMAC_SHA1_80 is the SRTP default AES Counter Mode cipher
   and HMAC-SHA1 message authentication with an 80-bit authentication
   tag.  The master-key length is 128 bits and has a default lifetime of
   a maximum of 2^48 SRTP packets or 2^31 SRTCP packets, whichever comes
   first [Page 39, srtp].

      SRTP allows 2^48 SRTP packets or 2^31 SRTCP packets, whichever
      comes first.  However, it is RECOMMENDED that automated key
      management allow easy and efficient rekeying at intervals far
      smaller than 2^31 packets given today's media rates or even HDTV
      media rates.




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   The SRTP and SRTCP encryption key lengths are 128 bits.  The SRTP and
   SRTCP authentication key lengths are 160 bits (see Security
   Considerations in <a href="#section-8">Section 8</a>).  The master salt value is 112 bits in
   length and the session salt value is 112 bits in length.  The
   pseudo-random function (PRF) is the default SRTP pseudo-random
   function that uses AES Counter Mode with a 128-bit key length.

   The length of the base64-decoded key and salt value for this crypto-
   suite MUST be 30 characters (i.e., 240 bits); otherwise, the crypto
   attribute is considered invalid.

<span class="h4"><a class="selflink" id="section-6.2.2" href="#section-6.2.2">6.2.2</a>.  AES_CM_128_HMAC_SHA1_32</span>

   This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except that
   the authentication tag is 32 bits.

   The length of the base64-decoded key and salt value for this crypto-
   suite MUST be 30 octets i.e., 240 bits; otherwise, the crypto
   attribute is considered invalid.

<span class="h4"><a class="selflink" id="section-6.2.3" href="#section-6.2.3">6.2.3</a>.  F8_128_HMAC_SHA1_80</span>

   This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except that
   the cipher is F8 [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   The length of the base64-decoded key and salt value for this crypto-
   suite MUST be 30 octets, i.e., 240 bits; otherwise the crypto
   attribute is considered invalid.

<span class="h4"><a class="selflink" id="section-6.2.4" href="#section-6.2.4">6.2.4</a>.  Adding New Crypto-Suite Definitions</span>

   If new transforms are added to SRTP, new definitions for those
   transforms SHOULD be given for the SRTP security descriptions and
   published in a Standards-Track RFC.  Sections <a href="#section-6.2.1">6.2.1</a> through <a href="#section-6.2.3">6.2.3</a>
   illustrate how to define crypto-suite values for particular
   cryptographic transforms.  Any new crypto-suites MUST be registered
   with IANA following the procedures in <a href="#section-10">Section 10</a>.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Session Parameters</span>

   SRTP security descriptions define a set of "session" parameters,
   which OPTIONALLY may be used to override SRTP session defaults for
   the SRTP and SRTCP streams.  These parameters configure an RTP
   session for SRTP services.  The session parameters provide session-
   specific information to establish the SRTP cryptographic context.






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<span class="h4"><a class="selflink" id="section-6.3.1" href="#section-6.3.1">6.3.1</a>.  KDR=n</span>

   KDR specifies the Key Derivation Rate, as described in <a href="./rfc3711#section-4.3.1">Section&nbsp;4.3.1
   of [RFC3711]</a>.

   The value n MUST be a decimal integer in the set {1,2,...,24}, which
   denotes a power of 2 from 2^1 to 2^24, inclusive; leading zeroes MUST
   NOT be used.  The SRTP key derivation rate controls how frequently a
   new session key is derived from an SRTP master key(s) [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>] given
   in the declaration.  When the key derivation rate is not specified
   (i.e., the KDR parameter is omitted), a single initial key derivation
   is performed [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   In the offer/answer model, KDR is a declarative parameter.

<span class="h4"><a class="selflink" id="section-6.3.2" href="#section-6.3.2">6.3.2</a>.  UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP</span>

   SRTP and SRTCP packet payloads are encrypted by default.  The
   UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP session parameters modify the
   default behavior of the crypto-suites with which they are used:

   *  UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not
      encrypted.

   *  UNENCRYPTED_SRTP signals that the SRTP packet payloads are not
      encrypted.

   In the offer/answer model, these parameters are negotiated.  If
   UNENCRYPTED_SRTCP is signaled for the session, then the SRTCP E bit
   MUST be clear (0) in all SRTCP messages.  If the default is used, all
   SRTCP messages are encrypted, and the E bit MUST be set (1) on all
   SRTCP messages.

<span class="h4"><a class="selflink" id="section-6.3.3" href="#section-6.3.3">6.3.3</a>.  UNAUTHENTICATED_SRTP</span>

   SRTP and SRTCP packet payloads are authenticated by default.  The
   UNAUTHENTICATED_SRTP session parameter signals that SRTP messages are
   not authenticated.  Use of UNAUTHENTICATED_SRTP is NOT RECOMMENDED
   (see Security Considerations).

      The SRTP specification requires use of message authentication for
      SRTCP, but not for SRTP [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   In the offer/answer model, this parameter is negotiated.







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<span class="h4"><a class="selflink" id="section-6.3.4" href="#section-6.3.4">6.3.4</a>.  FEC_ORDER=order</span>

   FEC_ORDER signals the use of forward error correction for the RTP
   packets [<a href="./rfc2733" title="&quot;An RTP Payload Format for Generic Forward Error Correction&quot;">RFC2733</a>].  The forward error correction values for "order"
   are FEC_SRTP or SRTP_FEC.  FEC_SRTP signals that FEC is applied
   before SRTP processing by the sender of the SRTP media and after SRTP
   processing by the receiver of the SRTP media; FEC_SRTP is the
   default.  SRTP_FEC is the reverse processing.

   In the offer/answer model, FEC_ORDER is a declarative parameter.

<span class="h4"><a class="selflink" id="section-6.3.5" href="#section-6.3.5">6.3.5</a>.  FEC_KEY=key-params</span>

   FEC_KEY signals the use of separate master key(s) for a Forward Error
   Correction (FEC) stream.  The master key(s) are specified with the
   exact same format as the SRTP Key Parameter defined in <a href="#section-6.1">Section 6.1</a>,
   and the semantic rules are the same - in particular, the master
   key(s) MUST be different from all other master key(s) in the SDP.  An
   FEC_KEY MUST be specified when the FEC stream is sent to a different
   IP-address and/or port than the media stream to which it applies
   (i.e., the "m=" line), e.g., as described in <a href="./rfc2733#section-11.1">RFC 2733, Section&nbsp;11.1</a>.
   When an FEC stream is sent to the same IP-address and port as the
   media stream to which it applies, an FEC_KEY MUST NOT be specified.
   If an FEC_KEY is specified in this latter case, the crypto attribute
   in question MUST be considered invalid.

   In the offer/answer model, FEC_KEY is a declarative parameter.

<span class="h4"><a class="selflink" id="section-6.3.6" href="#section-6.3.6">6.3.6</a>.  Window Size Hint (WSH)</span>

   SRTP defines the SRTP-WINDOW-SIZE [RFC3711, <a href="#section-3.3.2">Section 3.3.2</a>] parameter
   to protect against replay attacks.  The minimum value is 64
   [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>]; however, this value may be considered too low for some
   applications (e.g., video).

   The Window Size Hint (WSH) session parameter provides a hint for how
   big this window should be to work satisfactorily (e.g., based on
   sender knowledge of the number of packets per second).  However,
   there might be enough information given in SDP attributes like
   "a=maxprate" [<a href="#ref-maxprate" title="&quot;A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)&quot;">maxprate</a>] and the bandwidth modifiers to allow a
   receiver to derive the parameter satisfactorily.  Consequently, this
   value is only considered a hint to the receiver of the SDP that MAY
   choose to ignore the value provided.  The value is a decimal integer;
   leading zeroes MUST NOT be used.

   In the offer/answer model, WSH is a declarative parameter.





<span class="grey">Andreasen, et al.           Standards Track                    [Page 19]</span></pre>
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<span class="h4"><a class="selflink" id="section-6.3.7" href="#section-6.3.7">6.3.7</a>.  Defining New SRTP Session Parameters</span>

   New SRTP session parameters for the SRTP security descriptions can be
   defined in a Standards-Track RFC and registered with IANA according
   to the registration procedures defined in <a href="#section-10">Section 10</a>.

   New SRTP session parameters are by default mandatory.  A newly
   defined SRTP session parameter that is prefixed with the dash
   character ("-"), however, is considered optional and MAY be ignored.
   If an SDP crypto attribute is received with an unknown session
   parameter that is not prefixed with a "-" character, that crypto
   attribute MUST be considered invalid.

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  SRTP Crypto Context Initialization</span>

   In addition to the various SRTP parameters defined above, there are
   three pieces of information that are critical to the operation of the
   default SRTP ciphers:

   * SSRC:     Synchronization source
   * ROC:      Roll-over counter for a given SSRC
   * SEQ:      Sequence number for a given SSRC

   In a unicast session, as defined here, there are three constraints on
   these values.

   The first constraint is on the SSRC, which makes an SRTP keystream
   unique from other participants.  As explained in SRTP, the keystream
   MUST NOT be reused on two or more different pieces of plaintext.
   Keystream reuse makes the ciphertext vulnerable to cryptanalysis.
   One vulnerability is that known-plaintext fields in one stream can
   expose portions of the reused keystream, and this could further
   expose more plaintext in other streams.  Since all current SRTP
   encryption transforms use keystreams, key sharing is a general
   problem [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].  SRTP mitigates this problem by including the SSRC
   of the sender in the keystream.  But SRTP does not solve this problem
   in its entirety because the Real-time Transport Protocol has SSRC
   collisions, which although very rare [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>] are quite possible.
   During a collision, two or more SSRCs that share a master key will
   have identical keystreams for overlapping portions of the RTP
   sequence number space.  SRTP Security Descriptions avoid keystream
   reuse by making unique master keys REQUIRED for the sender and
   receiver of the security description.  Thus, the first constraint is
   satisfied.

      Also note that there is a second problem with SSRC collisions: the
      SSRC is used to identify the crypto context and thereby the
      cipher, key, ROC, etc. to process incoming packets.  In case of



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      SSRC collisions, crypto context identification becomes ambiguous
      and correct packet processing may not occur.  Furthermore, if an
      RTCP BYE packet is to be sent for a colliding SSRC, that packet
      may also have to be secured.  In a (unicast) point-to-multipoint
      scenario, this can be problematic for the same reasons, i.e., it
      is not known which of the possible crypto contexts to use.  Note
      that these problems are not unique to the SDP security
      descriptions; any use of SRTP needs to consider them.

   The second constraint is that the ROC MUST be zero at the time that
   each SSRC commences sending packets.  Thus, there is no concept of a
   "late joiner" in SRTP security descriptions, which are constrained to
   be unicast and pairwise.  The ROC and SEQ form a "packet index" in
   the default SRTP transforms and the ROC is consistently set to zero
   at session commencement, according to this document.

   The third constraint is that the initial value of SEQ SHOULD be
   chosen to be within the range of 0..2^15-1; this avoids an ambiguity
   when packets are lost at the start of the session.  If it is at the
   start of a session, an SSRC source might randomly select a high
   sequence-number value and put the receiver in an ambiguous situation:
   if initial packets are lost in transit up to the point that the
   sequence number wraps (i.e., exceeds 2^16-1), then the receiver might
   not recognize that its ROC needs to be incremented.  By restricting
   the initial SEQ to the range of 0..2^15-1, SRTP packet-index
   determination will find the correct ROC value, unless all the first
   2^15 packets are lost (which seems, if not impossible, rather
   unlikely).  See <a href="#section-3.3.1">Section 3.3.1</a> of the SRTP specification regarding
   packet-index determination [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

<span class="h4"><a class="selflink" id="section-6.4.1" href="#section-6.4.1">6.4.1</a>.  Late Binding of One or More SSRCs to a Crypto Context</span>

   The packet index, therefore, depends on the SSRC, the SEQ of an
   incoming packet, and the ROC, which is an SRTP crypto context
   variable.  Thus, SRTP has a big security dependency on SSRC
   uniqueness.

   Given the above constraints, unicast SRTP crypto contexts can be
   established without the need to negotiate SSRC values in the SRTP
   security descriptions.  Instead, an approach called "late binding" is
   RECOMMENDED by this specification.  When a packet arrives, the SSRC
   that is contained in it can be bound to the crypto context at the
   time of session commencement (i.e., SRTP packet arrival) rather than
   at the time of session signaling (i.e., receipt of an SDP).  With the
   arrival of the packet containing the SSRC, all the data items needed
   for the SRTP crypto context are held by the receiver.  (Note that the
   ROC value by definition is zero; if non-zero values were to be
   supported, additional signaling would be required.)  In other words,



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   the crypto context for a secure RTP session using late binding is
   initially identified by the SDP as

      &lt;*, address, port&gt;

   where '*' is a wildcard SSRC, "address" is the local receive address
   from the "c=" line, and "port" is the local receive port from the
   "m=" line.  When the first packet arrives with ssrcX in its SSRC
   field, the crypto context

      &lt;ssrcX, address, port&gt;

   is instantiated subject to the following constraints:

   *  Media packets are authenticated: authentication MUST succeed;
      otherwise, the crypto context is not instantiated.

   *  Media packets are not authenticated: crypto context is
      automatically instantiated.

   Note that use of late binding when there is no authentication of the
   SRTP media packets is subject to numerous security attacks, and that
   consequently it is NOT RECOMMENDED (of course, this can be said for
   unauthenticated SRTP in general).

      Note that use of late binding without authentication will result
      in the creation of local state as a result of receiving a packet
      from any unknown SSRC.  UNAUTHENTICATED_SRTP, therefore, is NOT
      RECOMMENDED because it invites easy denial-of-service attack.  In
      contrast, late binding with authentication does not suffer from
      this weakness.

<span class="h4"><a class="selflink" id="section-6.4.2" href="#section-6.4.2">6.4.2</a>.  Sharing Cryptographic Contexts among Sessions or SSRCs</span>

   With the constraints and procedures described above, it is not
   necessary to explicitly signal the SSRC, ROC, and SEQ for a unicast
   RTP session.  So there are no a=crypto parameters for signaling SSRC,
   ROC, or SEQ.  Thus, multiple SSRCs from the same entity will share
   a=crypto parameters when late binding is used.  Multiple SSRCs from
   the same entity arise due to either multiple sources (microphones,
   cameras, etc.) or RTP payloads requiring SSRC multiplexing within
   that same session.  SDP also allows multiple RTP sessions to be
   defined in the same media description ("m="); these RTP sessions will
   also share the a=crypto parameters.  An application that uses
   a=crypto in this way serially shares a master key among RTP sessions
   or SSRCs and MUST replace the master key when the aggregate number of
   packets among all SSRCs approaches 2^31 packets.  SSRCs that share a
   master key MUST be unique from one another.



<span class="grey">Andreasen, et al.           Standards Track                    [Page 22]</span></pre>
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<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>.  Removal of Crypto Contexts</span>

   The mechanism defined above addresses the issue of creating crypto
   contexts.  However, in practice, session participants may want to
   remove crypto contexts prior to session termination.  Since a crypto
   context contains information that cannot automatically be recovered
   (e.g., ROC), it is important that the sender and receiver agree on
   when a crypto context can be removed, and perhaps more importantly
   when it cannot.

      Even when late binding is used for a unicast stream, the ROC is
      lost and cannot be recovered automatically (unless it is zero)
      once the crypto context is removed.

   We resolve this problem as follows.  When SRTP security descriptions
   are being used, crypto-context removal MUST follow the same rules as
   SSRC removal from the member table [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>]; note that this can
   happen as the result of an SRTCP BYE packet or a simple time-out due
   to inactivity.  Inactive session participants that wish to ensure
   their crypto contexts are not timed out MUST thus send SRTCP packets
   at regular intervals.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  SRTP-Specific Use of the Crypto Attribute</span>

   <a href="#section-5">Section 5</a> describes general use of the crypto attribute, and this
   section completes it by describing SRTP-specific use.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Use with Offer/Answer</span>

   In this section, we describe how the SRTP security descriptions are
   used with the offer/answer model to negotiate cryptographic
   capabilities and communicate SRTP master keys.  The rules defined
   below complement the general offer/answer rules defined in <a href="#section-5.1">Section</a>
   <a href="#section-5.1">5.1</a>, which MUST be followed, unless otherwise specified.  Note that
   the rules below define unicast operation only; support for multicast
   and multipoint unicast streams is for further study.

<span class="h4"><a class="selflink" id="section-7.1.1" href="#section-7.1.1">7.1.1</a>.  Generating the Initial Offer - Unicast Streams</span>

   When the initial offer is generated, the offerer MUST follow the
   steps in <a href="#section-5.1.1">Section 5.1.1</a>, as well as the following steps.

   For each unicast media line (m=) using the secure RTP transport where
   the offerer wants to specify cryptographic parameters, the offerer
   MUST provide at least one valid SRTP security description ("a=crypto"
   line), as defined in <a href="#section-6">Section 6</a>.  If the media stream includes Forward





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   Error Correction with a different IP-address and/or port from that of
   the media stream itself, an FEC_KEY parameter MUST be included, as
   described in <a href="#section-6.3.5">Section 6.3.5</a>.

   The inline parameter conveys the SRTP master key used by an endpoint
   to encrypt the SRTP and SRTCP streams transmitted by that endpoint.
   The same key is used by the recipient to decrypt those streams.
   However, the receiver MUST NOT use that same key for the SRTP or
   SRTCP packets that it sends to the session because the default SRTP
   cipher and mode is insecure when the master key is reused across
   distinct SRTP streams.

   The offerer MAY include one or more other SRTP session parameters, as
   defined in <a href="#section-6.3">Section 6.3</a>.  Note, however, that if any SRTP session
   parameters are included that are not known to the answerer, but that
   are nonetheless mandatory (see <a href="#section-6.3.6">Section 6.3.6</a>), the negotiation will
   fail if the answerer does not support them.

<span class="h4"><a class="selflink" id="section-7.1.2" href="#section-7.1.2">7.1.2</a>.  Generating the Initial Answer - Unicast Streams</span>

   When the initial answer is generated, the answerer MUST follow the
   steps in <a href="#section-5.1.2">Section 5.1.2</a>, as well as the following steps.

   For each unicast media line that uses the secure RTP transport and
   contains one or more "a=crypto" lines in the offer, the answerer MUST
   either accept one (and only one) of the crypto lines for that media
   stream, or it MUST reject the media stream.  Only "a=crypto" lines
   that are considered valid SRTP security descriptions, as defined in
   <a href="#section-6">Section 6</a>, can be accepted.  Furthermore, all parameters (crypto-
   suite, key parameter, and mandatory session parameters) MUST be
   acceptable to the answerer in order for the offered media stream to
   be accepted.  Note that if the media stream includes Forward Error
   Correction with a different IP-address and/or port from that of the
   media stream itself, an FEC_KEY parameter MUST be included, as
   described in <a href="#section-6.3.5">Section 6.3.5</a>.

   When the answerer accepts an SRTP unicast media stream with a crypto
   line, the answerer MUST include one or more master keys appropriate
   for the selected crypto algorithm; the master key(s) included in the
   answer MUST be different from those in the offer.

      When the master key(s) are not shared between the offerer and
      answerer, SSRC collisions between the offerer and answerer will
      not lead to keystream reuse, and hence SSRC collisions do not
      necessarily have to be prevented.






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   If Forward Error Correction to a separate IP-address and/or port is
   included, the answer MUST include an FEC_KEY parameter, as described
   in <a href="#section-6.3.5">Section 6.3.5</a>.

   Declarative session parameters may be added to the answer as usual;
   however, the answerer SHOULD NOT add any mandatory session parameter
   (see <a href="#section-6.3.6">Section 6.3.6</a>) that might be unknown to the offerer.

   If the answerer cannot find any valid crypto line that it supports,
   or if its configured policy prohibits any cryptographic key parameter
   (e.g., key length) or cryptographic session parameter (e.g., KDR,
   FEC_ORDER), it MUST reject the media stream, unless it is able to
   successfully negotiate use of SRTP by other means outside the scope
   of this document (e.g., by use of MIKEY [<a href="#ref-mikey" title="&quot;MIKEY: Multimedia Internet KEYing&quot;">mikey</a>]).

<span class="h4"><a class="selflink" id="section-7.1.3" href="#section-7.1.3">7.1.3</a>.  Processing of the Initial Answer - Unicast Streams</span>

   When the offerer receives the answer, it MUST perform the steps in
   <a href="#section-5.1.3">Section 5.1.3</a>, as well as the following steps for each SRTP media
   stream it offered with one or more crypto lines in it.

   If the media stream was accepted and it contains a crypto line, it
   MUST be checked that the crypto line is valid according to the
   constraints specified in <a href="#section-6">Section 6</a> (including any FEC constraints).

   If the offerer either does not support or is not willing to honor one
   or more of the SRTP parameters in the answer, the offerer MUST
   consider the crypto line invalid.

   If the crypto line is not valid, or the offerer's configured policy
   prohibits any cryptographic key parameter (e.g., key length) or
   cryptographic session parameter, the SRTP security negotiation MUST
   be deemed to have failed.

<span class="h4"><a class="selflink" id="section-7.1.4" href="#section-7.1.4">7.1.4</a>.  Modifying the Session</span>

   When a media stream using the SRTP security descriptions has been
   established and a new offer/answer exchange is performed, the offerer
   and answerer MUST follow the steps in <a href="#section-5.1.4">Section 5.1.4</a>, as well as the
   following steps.

   When modifying the session, all negotiated aspects of the SRTP media
   stream can be modified.  For example, a new crypto suite can be used
   or a new master key can be established.  As described in <a href="./rfc3264">RFC 3264</a>,
   when a new offer/answer exchange is made, there will be a window of
   time where the offerer and the answerer must be prepared to receive
   media according to both the old and new offer/answer exchange.




<span class="grey">Andreasen, et al.           Standards Track                    [Page 25]</span></pre>
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   This requirement applies here as well; however, the following should
   be noted:

   *  When authentication is not being used, it may not be possible for
      either the offerer or answerer to determine if a given packet is
      encrypted according to the old or new offer/answer exchange.  <a href="./rfc3264">RFC</a>
      <a href="./rfc3264">3264</a> defines a couple of techniques to address this problem, e.g.,
      changing the payload types used and/or the transport addresses.
      Note, however, that a change in transport addresses may have an
      impact on quality of service as well as on firewall and NAT
      traversal.  The SRTP security descriptions use the MKI to deal
      with this (which adds a few bytes to each SRTP packet), as
      described in <a href="#section-6.1">Section 6.1</a>.  For further details on the MKI, please
      refer to [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   *  If the answerer changes its master key, the offerer will not be
      able to process packets secured via this master key until the
      answer is received.  This could be addressed by using a security
      "precondition" [<a href="#ref-sprecon" title="&quot;Security Preconditions for Session Description Protocol Media Streams&quot;">sprecon</a>].

   If the offerer includes an IP address and/or port that differs from
   that used previously for a media stream (or FEC stream), the offerer
   MUST include a new master key with the offer (and in so doing, it
   will be creating a new crypto context where the ROC is set to zero).
   Similarly, if the answerer includes an IP address and/or port that
   differs from that used previously for a media stream (or FEC stream),
   the answerer MUST include a new master key with the answer (and hence
   create a new crypto context with the ROC set to zero).  The reason
   for this is that when the answerer receives an offer or the offerer
   receives an answer with an updated IP address and/or port, it is not
   possible to determine if the other side has access to the old crypto
   context parameters (and in particular the ROC).  For example, if one
   side is a decomposed media gateway, or if a SIP back-to-back user
   agent is involved, it is possible that the media endpoint changed and
   no longer has access to the old crypto context.  By always requiring
   a new master key in this case, the answerer/offerer will know that
   the ROC is zero for this offer/answer, and any key lifetime
   constraints will trivially be satisfied too.  Another consideration
   here applies to media relays; if the relay changes the media endpoint
   on one side transparently to the other side, the relay cannot operate
   as a simple packet reflector but will have to actively engage in SRTP
   packet processing and transformation (i.e., decryption and re-
   encryption, etc.).

   Finally, note that if the new offer is rejected, the old crypto
   parameters remain in place.





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<span class="h4"><a class="selflink" id="section-7.1.5" href="#section-7.1.5">7.1.5</a>.  Offer/Answer Example</span>

   In this example, the offerer supports two crypto suites (f8 and AES).
   The a=crypto line is actually one long line, although it is shown as
   two lines in this document due to page formatting.  The f8 example
   shows two inline parameters; as explained in <a href="#section-6.1">Section 6.1</a>, there may
   be one or more key (i.e., inline) parameters in a crypto attribute.
   In this way, multiple keys are offered to support key rotation using
   a Master Key Identifier (MKI).

   Offerer sends:

      v=0
      o=sam 2890844526 2890842807 IN IP4 10.47.16.5
      s=SRTP Discussion
      i=A discussion of Secure RTP
      u=http://www.example.com/seminars/srtp.pdf
      e=marge@example.com (Marge Simpson)
      c=IN IP4 168.2.17.12
      t=2873397496 2873404696
      m=audio 49170 RTP/SAVP 0
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
       FEC_ORDER=FEC_SRTP
      a=crypto:2 F8_128_HMAC_SHA1_80
       inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
       inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
       FEC_ORDER=FEC_SRTP

   Answerer replies:

      v=0
      o=jill 25690844 8070842634 IN IP4 10.47.16.5
      s=SRTP Discussion
      i=A discussion of Secure RTP
      u=http://www.example.com/seminars/srtp.pdf
      e=homer@example.com (Homer Simpson)
      c=IN IP4 168.2.17.11
      t=2873397526 2873405696
      m=audio 32640 RTP/SAVP 0
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4

   In this case, the session would use the AES_CM_128_HMAC_SHA1_80
   crypto suite for the RTP and RTCP traffic.  If F8_128_HMAC_SHA1_80
   were selected by the answerer, there would be two inline keys
   associated with the SRTP cryptographic context.  One key has an MKI
   value of 1 and the second has an MKI of 2.



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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  SRTP-Specific Use Outside Offer/Answer</span>

   Use of SRTP security descriptions outside the offer/answer model is
   not defined.

      Use of SRTP security descriptions outside the offer/answer model
      could have been defined for sendonly media streams; however, there
      would not be a way to indicate the key to use for SRTCP by the
      receiver of said media stream.

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Support for SIP Forking</span>

   As mentioned earlier, the security descriptions defined here do not
   support multicast media streams or multipoint unicast streams.
   However, in the SIP protocol, it is possible to receive several
   answers to a single offer due to the use of forking (see [SIP]).
   Receiving multiple answers leads to a couple of problems for the SRTP
   security descriptions:

   *  Different answerers may choose different ciphers, keys, etc.;
      however, there is no way for the offerer to associate a particular
      incoming media packet with a particular answer.

   *  Two or more answerers may pick the same SSRC, and hence the SSRC
      collision problems mentioned earlier may arise.

   As stated earlier, the above point-to-multipoint cases are outside
   the scope of the SDP security descriptions.  However, there are still
   ways of supporting SIP forking, e.g., by changing the multipoint
   scenario resulting from SIP forking into multiple two-party unicast
   cases.  This can be done as follows:

   For each answer received beyond the initial answer, issue a new offer
   to that particular answerer using a new receive transport address (IP
   address and port); note that this requires support for the SIP UPDATE
   method [<a href="./rfc3311" title="&quot;The Session Initiation Protocol (SIP) UPDATE Method&quot;">RFC3311</a>].  Also, to ensure that two media sessions are not
   inadvertently established prior to the UPDATE being processed by one
   of them, use security preconditions [<a href="#ref-sprecon" title="&quot;Security Preconditions for Session Description Protocol Media Streams&quot;">sprecon</a>].

   Finally, note that all SIP User Agents that received the offer will
   know the key(s) being proposed by the initial offer.  If the offerer
   wants to ensure security with respect to all other User Agents that
   may have received the offer, a new offer/answer exchange with a new
   key needs to be performed with the answerer as well.  Note that the
   offerer cannot determine whether a single or multiple SIP User Agents
   received the offer, since intermediate forking proxies may only
   forward a single answer to the offerer.




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   The above description is intended to suggest one possible way of
   supporting SIP forking.  There are many details missing and it should
   not be considered a normative specification.  Alternative approaches
   may also be possible

<span class="h3"><a class="selflink" id="section-7.4" href="#section-7.4">7.4</a>.  SRTP-Specific Backwards Compatibility Considerations</span>

   It is possible that the answerer supports the SRTP transport and
   accepts the offered media stream, but that it does not support the
   crypto attribute defined here.  The offerer can recognize this
   situation by seeing an accepted SRTP media stream in the answer that
   does not include a crypto line.  In that case, the security
   negotiation defined here MUST be deemed to have failed.

   Also, if a media stream with a given SRTP transport (e.g.,
   "RTP/SAVP") is sent to a device that does not support SRTP, that
   media stream will be rejected.

<span class="h3"><a class="selflink" id="section-7.5" href="#section-7.5">7.5</a>.  Operation with KEYMGT= and k= lines</span>

   An offer MAY include both "a=crypto" and "a=keymgt" lines [<a href="#ref-keymgt" title="&quot;Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)&quot;">keymgt</a>].
   Per SDP rules, the answerer will ignore attribute lines that it does
   not understand.  If the answerer supports both "a=crypto" and
   "a=keymgt", the answer MUST include either "a=crypto" or "a=keymgt",
   but not both, as including both is undefined.

   An offer MAY include both "a=crypto" and "k=" lines [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  Per
   SDP rules, the answerer will ignore attribute lines it does not
   understand.  If the answerer supports both "a=crypto" and "k=", the
   answer MUST include either "a=crypto" or "k=" but not both, as
   including both is undefined.

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

   Like all SDP messages, SDP messages containing security descriptions
   are conveyed in an encapsulating application protocol (e.g., SIP,
   MGCP).  It is the responsibility of the encapsulating protocol to
   ensure the protection of the SDP security descriptions.  Therefore,
   IT IS REQUIRED that the application invoke its own security
   mechanisms (e.g., secure multiparts such as S/MIME [smime]) or,
   alternatively, utilize a lower-layer security service (e.g., TLS or
   IPsec).  IT IS REQUIRED that this security service provide strong
   message authentication and packet-payload encryption, as well as
   effective replay protection.

   "Replay protection" is needed against an attacker that has enough
   access to the communications channel to intercept messages and to
   deliver copies to the destination.  A successful replay attack will



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   cause the recipient to perform duplicate processing on a message; the
   attack is worse when the duped recipient sends a duplicate reply to
   the initiator.  Replay protections are not found in S/MIME or in the
   other secure-multiparts standard, PGP/MIME.  S/MIME and PGP/MIME,
   therefore, need to be augmented with some replay-protection mechanism
   that is appropriate to the encapsulating application protocol (e.g.,
   SIP, MGCP).  Three common ways to provide replay protection are to
   place a sequence number in the message, to use a timestamp, or for
   the receiver to keep a hash of the message to be compared with
   incoming messages.  There typically needs to be a replay "window" and
   some policy for keeping state information from previous messages in a
   "replay table" or list.

   The discussion that follows uses "message authentication" and
   "message confidentiality" in a manner consistent with SRTP [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].
   "Message confidentiality" means that only the holder of the secret
   decryption key can access the plain-text content of the message.  The
   decryption key is the same key as the encryption key, using SRTP
   counter mode and f8 encryption transforms, which are vulnerable to
   message tampering and need SRTP message authentication to detect such
   tampering. "Message authentication" and "message integrity
   validation" generally mean the same thing in IETF security standards:
   an SRTP message is authenticated following a successful HMAC
   integrity check [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>], which proves that the message originated
   from the holder of an SRTP master key and was not altered en route.
   Such an "authentic" message, however, can be captured by an attacker
   and "replayed" when the attacker re-inserts the packet into the
   channel.  A replayed packet can have a variety of bad effects on the
   session, and SRTP uses the extended sequence number to detect
   replayed SRTP packets [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].

   The SRTP specification identifies which services and features are
   default values that are normative-to-implement (such as
   AES_CM_128_80) versus normative-to-use (such as AES_CM_128_32).

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

   Security descriptions as defined herein signal security services for
   RTP packets.  RTP messages are vulnerable to a variety of attacks,
   such as replay and forging.  To limit these attacks, SRTP message
   integrity mechanisms SHOULD be used (SRTP replay protection is always
   enabled).

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Keystream Reuse</span>

   SRTP security descriptions signal configuration parameters for SRTP
   sessions.  Misconfigured SRTP sessions are vulnerable to attacks on
   their encryption services when running the crypto suites defined in



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   Sections <a href="#section-6.2.1">6.2.1</a>, <a href="#section-6.2.2">6.2.2</a>, and <a href="#section-6.2.3">6.2.3</a>.  An SRTP encryption service is
   "misconfigured" when two or more media streams are encrypted using
   the same keystream of AES blocks.  When senders and receivers share
   derived session keys, SRTP requires that the SSRCs of session
   participants serve to make their corresponding keystreams unique,
   which is violated in the case of SSRC collision: SRTP SSRC collision
   drastically weakens SRTP or SRTCP payload encryption during the time
   that identical keystreams are used [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>].  An attacker, for
   example, might collect SRTP and SRTCP messages and await a collision.
   This attack on the AES-CM and AES-f8 encryption is avoided entirely
   when each media stream has its own unique master key in both the send
   and receive direction.  This specification restricts use of SDP
   security description to unicast point-to-point streams so that keys
   are not shared between SRTP hosts, and the master keys used in the
   send and receive direction for a given media stream are unique.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Signaling Authentication and Signaling Encryption</span>

   There is no reason to incur the complexity and computational expense
   of SRTP, however, when its key establishment is exposed to
   unauthorized parties.  In most cases, the SRTP crypto attribute and
   its parameters are vulnerable to denial-of-service attacks when they
   are carried in an unauthenticated SDP message.  In some cases, the
   integrity or confidentiality of the RTP stream can be compromised.
   For example, if an attacker sets UNENCRYPTED for the SRTP stream in
   an offer, this could result in the answerer's not decrypting the
   encrypted SRTP messages.  In the worst case, the answerer might
   itself send unencrypted SRTP and leave its data exposed to snooping.

   Thus, IT IS REQUIRED that MIME secure multiparts, IPsec, TLS, or some
   other data security service be used to provide message authentication
   for the encapsulating protocol that carries the SDP messages having a
   crypto attribute (a=crypto).  Furthermore, IT IS REQUIRED that
   encryption of the encapsulating payload be used whenever a master key
   parameter (inline) appears in the message.  Failure to encrypt the
   SDP message containing an inline SRTP master key renders the SRTP
   authentication or encryption service useless in practically all
   circumstances.  Failure to authenticate an SDP message that carries
   SRTP parameters renders the SRTP authentication or encryption service
   useless in most practical applications.

   When the communication path of the SDP message is routed through
   intermediate systems that inspect parts of the SDP message, security
   protocols such as [IPsec] or TLS SHOULD NOT be used for encrypting
   and/or authenticating the security description.  In the case of
   intermediate-system processing of a message containing SDP security
   descriptions, the "a=crypto" attributes SHOULD be protected end-to-
   end so that the intermediate system can neither modify the security



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   description nor access the keying material.  Network or transport
   security protocols that terminate at each intermediate system,
   therefore, SHOULD NOT be used for protecting SDP security
   descriptions.  A security protocol SHOULD allow the security
   descriptions to be encrypted and authenticated end-to-end
   independently of the portions of the SDP message that any
   intermediate system modifies or inspects: MIME secure multiparts are
   RECOMMENDED for the protection of SDP messages that are processed by
   intermediate systems.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Grammar</span>

   In this section, we first provide the ABNF grammar for the generic
   crypto attribute, and then we provide the ABNF grammar for the SRTP-
   specific use of the crypto attribute.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Generic "Crypto" Attribute Grammar</span>

   The ABNF grammar for the crypto attribute is defined below:

   "a=crypto:" tag 1*WSP crypto-suite 1*WSP key-params
                                           *(1*WSP session-param)

   tag              = 1*9DIGIT
   crypto-suite     = 1*(ALPHA / DIGIT / "_")

   key-params       = key-param *(";" key-param)
   key-param        = key-method ":" key-info
   key-method       = "inline" / key-method-ext
   key-method-ext   = 1*(ALPHA / DIGIT / "_")
   key-info         = 1*(%x21-3A / %x3C-7E) ; visible (printing) chars
                                        ; except semi-colon
   session-param    = 1*(VCHAR)         ; visible (printing) characters

   where WSP, ALPHA, DIGIT, and VCHAR are defined in [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>].

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  SRTP "Crypto" Attribute Grammar</span>

   This section provides an Augmented BNF [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>] grammar for the
   SRTP-specific use of the SDP crypto attribute:

      crypto-suite        = srtp-crypto-suite
      key-method          = srtp-key-method
      key-info            = srtp-key-info
      session-param       = srtp-session-param

      srtp-crypto-suite   = "AES_CM_128_HMAC_SHA1_32" /
                            "F8_128_HMAC_SHA1_32" /



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                            "AES_CM_128_HMAC_SHA1_80" /
                            srtp-crypto-suite-ext

      srtp-key-method     = "inline"
      srtp-key-info       = key-salt ["|" lifetime] ["|" mki]

      key-salt            = 1*(base64)   ; binary key and salt values
                                    ; concatenated together, and then
                                    ; base64 encoded [<a href="#section-3">section 3</a> of
                                    ; <a href="./rfc3548">RFC3548</a>

      lifetime           = ["2^"] 1*(DIGIT)   ; see <a href="#section-6.1">section 6.1</a> for "2^"
      mki                 = mki-value ":" mki-length
      mki-value           = 1*DIGIT
      mki-length          = 1*3DIGIT   ; range 1..128.

      srtp-session-param  = kdr /
                            "UNENCRYPTED_SRTP" /
                            "UNENCRYPTED_SRTCP" /
                            "UNAUTHENTICATED_SRTP" /
                            fec-order /
                            fec-key /
                            wsh /
                            srtp-session-extension

      kdr                 = "KDR=" 1*2(DIGIT)  ; range 0..24,
                                               ; power of two

      fec-order           = "FEC_ORDER=" fec-type
      fec-type            = "FEC_SRTP" / "SRTP_FEC"
      fec-key             = "FEC_KEY=" key-params

      wsh                 = "WSH=" 2*DIGIT    ; minimum value is 64
      base64              =  ALPHA / DIGIT / "+" / "/" / "="

      srtp-crypto-suite-ext  = 1*(ALPHA / DIGIT / "_")
      srtp-session-extension = ["-"] 1*(VCHAR)  ;visible chars [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>]
                               ; first character must not be dash ("-")













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<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>.  Registration of the "crypto" Attribute</span>

   The IANA has registered a new SDP attribute as
   follows:

   Attribute name:      crypto
   Long form name:      Security description cryptographic attribute
                        for media streams
   Type of attribute:   Media-level
   Subject to charset:  No
   Purpose:             Security descriptions
   Appropriate values:  See <a href="#section-4">Section 4</a>

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>.  New IANA Registries and Registration Procedures</span>

   The following sub-sections define a new IANA registry with associated
   sub-registries to be used for the SDP security descriptions.  The
   IANA has created an SDP Security Description registry as shown below
   and further described in the following sections:

   SDP Security Descriptions
     |
     +- Key Methods (described in 10.2.1)
     |
     +- Media Stream Transports (described in 10.2.2)
          |
          +- Transport1 (e.g., SRTP)
          |    |
          |    +- Supported Key Methods (e.g., inline)
          |    |
          |    +- crypto suites
          |    |
          |    +- session parameters
          |
          +- Transport2
          :    :

<span class="h4"><a class="selflink" id="section-10.2.1" href="#section-10.2.1">10.2.1</a>.  Key Method Registry and Registration</span>

   The IANA has created a new subregistry for SDP security description
   key methods.  An IANA key method registration MUST be documented in
   an RFC in accordance with the [<a href="./rfc2434" title="">RFC2434</a>] Standards Action, and it MUST
   provide the name of the key method in accordance with the grammar for
   key-method-ext defined in <a href="#section-9.1">Section 9.1</a>.





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<span class="h4"><a class="selflink" id="section-10.2.2" href="#section-10.2.2">10.2.2</a>.  Media Stream Transport Registry and Registration</span>

   The IANA has created a new subregistry for SDP security description
   Media Stream Transports.  An IANA media stream transport registration
   MUST be documented in an RFC in accordance with the <a href="./rfc2434">RFC 2434</a>
   Standards Action and the procedures defined in Sections <a href="#section-4">4</a> and <a href="#section-5">5</a> of
   this document.  The registration MUST provide the name of the
   transport and a list of supported key methods.

   In addition, each new media stream transport registry must contain a
   crypto-suite registry and a session parameter registry, as well as
   IANA instructions for how to populate these registries.

<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a>.  Initial Registrations</span>

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

   The following security descriptions key methods are hereby
   registered:

      inline

<span class="h4"><a class="selflink" id="section-10.3.2" href="#section-10.3.2">10.3.2</a>.  SRTP Media Stream Transport</span>

   The IANA has created an SDP Security Description Media Stream
   Transport subregistry for "SRTP".  The key methods supported is
   "inline".  The reference for the SDP security description for SRTP is
   this document.

<span class="h5"><a class="selflink" id="section-10.3.2.1" href="#section-10.3.2.1">10.3.2.1</a>.  SRTP Crypto Suite Registry and Registration</span>

   The IANA has created a new subregistry for SRTP crypto suites under
   the SRTP transport of the SDP Security Descriptions.  An IANA SRTP
   crypto suite registration MUST indicate the crypto suite name in
   accordance with the grammar for srtp-crypto-suite-ext defined in
   <a href="#section-9.2">Section 9.2</a>.

   The semantics of the SRTP crypto suite MUST be described in an RFC in
   accordance with the <a href="./rfc2434">RFC 2434</a> Standards Action, including the
   semantics of the "inline" key-method and any special semantics of
   parameters.

   The following SRTP crypto suites are hereby registered:

      AES_CM_128_HMAC_SHA1_80
      AES_CM_128_HMAC_SHA1_32
      F8_128_HMAC_SHA1_80




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   The reference for these crypto suites is provided in this document.

<span class="h5"><a class="selflink" id="section-10.3.2.2" href="#section-10.3.2.2">10.3.2.2</a>.  SRTP Session Parameter Registration</span>

   The IANA has created a new subregistry for SRTP session parameters
   under the SRTP transport of the SDP Security Descriptions.  An IANA
   SRTP session parameter registration MUST indicate the session
   parameter name (srtp-session-extension as defined in <a href="#section-9.2">Section 9.2</a>);
   the name MUST NOT begin with the dash character ("-").

   The semantics of the parameter MUST be described in an RFC in
   accordance with the <a href="./rfc2434">RFC 2434</a> Standards Action.  If values can be
   assigned to the parameter, then the format and possible values that
   can be assigned MUST be described in the RFC in accordance with the
   <a href="./rfc2434">RFC 2434</a> Standards Action as well.  Also, it MUST be specified
   whether the parameter is declarative or negotiated in the
   offer/answer model.

   The following SRTP session parameters are hereby registered:

      KDR
      UNENCRYPTED_SRTP
      UNENCRYPTED_SRTCP
      UNAUTHENTICATED_SRTP
      FEC_ORDER
      FEC_KEY
      WSH

   The reference for these parameters is this document.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Acknowledgements</span>

   This document is a product of the IETF MMUSIC working group and has
   benefited from comments from its participants.  This document also
   benefited from discussions with Elisabetta Cararra, Earl Carter, Per
   Cederqvist, Bill Foster, Matt Hammer, Cullen Jennings, Paul Kyzivat,
   David McGrew, Mats Naslund, Dave Oran, Jonathan Rosenberg, Dave
   Singer, Mike Thomas, Brian Weis, and Magnus Westerlund.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Normative References</span>

   [<a id="ref-RFC3550">RFC3550</a>]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, <a href="./rfc3550">RFC 3550</a>, July 2003.

   [<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.




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   [<a id="ref-RFC4566">RFC4566</a>]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", <a href="./rfc4566">RFC 4566</a>, July 2006.

   [<a id="ref-RFC4234">RFC4234</a>]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", <a href="./rfc4234">RFC 4234</a>, October 2005.

   [<a id="ref-RFC2828">RFC2828</a>]  Shirey, R., "Internet Security Glossary", FYI 36, <a href="./rfc2828">RFC</a>
              <a href="./rfc2828">2828</a>, May 2000.

   [<a id="ref-RFC3264">RFC3264</a>]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", <a href="./rfc3264">RFC 3264</a>, June
              2002.

   [<a id="ref-RFC3711">RFC3711</a>]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              <a href="./rfc3711">RFC 3711</a>, March 2004.

   [<a id="ref-RFC1750">RFC1750</a>]  Eastlake 3rd, D., Crocker, S., and J. Schiller,
              "Randomness Recommendations for Security", <a href="./rfc1750">RFC 1750</a>,
              December 1994.

   [<a id="ref-RFC3548">RFC3548</a>]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", <a href="./rfc3548">RFC 3548</a>, July 2003.

   [<a id="ref-RFC2434">RFC2434</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc2434">RFC 2434</a>,
              October 1998.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Informative References</span>

   [<a id="ref-sprecon">sprecon</a>]  Andreasen, F. and D. Wing, "Security Preconditions for
              Session Description Protocol Media Streams", Work in
              Progress, October 2005.

   [<a id="ref-RFC3407">RFC3407</a>]  Andreasen, F., "Session Description Protocol (SDP) Simple
              Capability Declaration", <a href="./rfc3407">RFC 3407</a>, October 2002.

   [<a id="ref-Bellovin">Bellovin</a>] Bellovin, S., "Problem Areas for the IP Security
              Protocols," in Proceedings of the Sixth Usenix Unix
              Security Symposium, pp. 1-16, San Jose, CA, July 1996.

   [<a id="ref-GDOI">GDOI</a>]     Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", <a href="./rfc3547">RFC 3547</a>, July 2003.

   [<a id="ref-kink">kink</a>]     Sakane, S., Kamada, K., Thomas, M. and J. Vilhuber,
              "Kerberized Internet Negotiation of Keys (KINK)", <a href="./rfc4430">RFC</a>
              <a href="./rfc4430">4430</a>, March 2006.




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   [<a id="ref-ike">ike</a>]      Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", <a href="./rfc4306">RFC</a>
              <a href="./rfc4306">4306</a>, December 2005.

   [<a id="ref-ipsec">ipsec</a>]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", <a href="./rfc4301">RFC 4301</a>, December 2005.

   [<a id="ref-maxprate">maxprate</a>] Westerlund, M., "A Transport Independent Bandwidth
              Modifier for the Session Description Protocol (SDP)", <a href="./rfc3890">RFC</a>
              <a href="./rfc3890">3890</a>, September 2004.

   [<a id="ref-RFC2733">RFC2733</a>]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
              for Generic Forward Error Correction", <a href="./rfc2733">RFC 2733</a>, December
              1999.

   [s/mime]   Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              <a href="./rfc3851">RFC 3851</a>, July 2004.

   [pgp/mime] Elkins, M., "MIME Security with Pretty Good Privacy
              (PGP)", <a href="./rfc2015">RFC 2015</a>, October 1996.

   [<a id="ref-TLS">TLS</a>]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              <a href="./rfc2246">RFC 2246</a>, January 1999.

   [<a id="ref-keymgt">keymgt</a>]   Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", <a href="./rfc4567">RFC 4567</a>, July 2006.

   [<a id="ref-mikey">mikey</a>]    Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", <a href="./rfc3830">RFC 3830</a>,
              August 2004.

   [<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>, February
              1997.

   [<a id="ref-skeme">skeme</a>]    Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
              Mechanism for the Internet", ISOC Secure Networks and
              Distributed Systems Symposium, San Diego, 1996.

   [<a id="ref-RFC3312">RFC3312</a>]  Camarillo, G., Marshall, W., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", <a href="./rfc3312">RFC 3312</a>, October 2002.

   [<a id="ref-RFC2974">RFC2974</a>]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", <a href="./rfc2974">RFC 2974</a>, October 2000.




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   [<a id="ref-srtpf">srtpf</a>]    Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              RTCP-based Feedback (RTP/SAVPF)", work in progress,
              October 2003.

   [<a id="ref-RFC3261">RFC3261</a>]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP:  Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>,
              June 2002.

   [<a id="ref-RFC3311">RFC3311</a>]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", <a href="./rfc3311">RFC 3311</a>, September 2002.








































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Appendix A - Rationale for Keying Material Directionality

   SDP security descriptions define the keying material for the sending
   direction, which is included in the SDP.  Thus, the key that is
   carried in an SDP message is a decryption key for the receiver of
   that SDP message.  This is in contrast to the majority of information
   included in SDP, which describes information for the receiving (or
   receiving and sending) direction.  This reversed information
   directionality generates some challenges with using the mechanism in
   the offer/answer model and in particular with SIP, where early media
   and forking require special consideration (as described in <a href="#section-7.3">Section</a>
   <a href="#section-7.3">7.3</a>).  There are however good reasons for why this was done, which
   can be summarized as follows:

   First of all, there is the general security philosophy of letting the
   entity that sends traffic decide what key to use for protecting it.
   SRTP uses counter mode, which is secure when counters do not overlap
   among senders who share a master key; the surest way to avoid counter
   overlap is for each endpoint to generate its own master key.
   Secondly, if SDP security descriptions had been designed to keep the
   normal SDP information directionality, it would have resulted in
   problems with supporting early media and SIP forking: If an offer
   generates multiple answers and the keying material was for the
   receive direction, some of the parameter values (e.g. lifetime) would
   have to be shared between all the answerers (senders of media), which
   would lead to considerable complexity, possibly requiring changes or
   extensions to SRTP.  Other problems were discovered as well, which we
   describe further below.

   In the following scenarios, we analyze what would occur if SDP
   security descriptions had been designed so that the keying material
   was the receive keying material (rather than its actual design, where
   the keying material is the sending keying material):


















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   Scenario A: Non-Forking Case

      In this scenario, the offer includes the receiving keying
      material, the answerer receives it and starts sending data packets
      towards the offerer.  If there was a single crypto attribute in
      the offer, there would be no ambiguity about which crypto suite
      was being used and, hence, the incoming packet could be processed.
      However, in the case where the offer included multiple alternative
      crypto-attributes, the offerer would not know which one was
      chosen, and hence, if the offerer received packets before the
      answer came back, the offerer would be unable to process those
      packets (problem 1).  (Use of the MKI has been suggested as one
      possible solution to that, however it incurs a per-packet
      overhead.)

   Scenario B: Serial Forking Case

      In this scenario, Alice generates an offer to Bob, who starts
      sending (early) media towards Alice (no answer returned yet).  In
      this scenario, we assume we aren't also encountering Scenario A
      (e.g., the offer includes only a single crypto-attribute) and that
      Bob is using a Synchronization Source (SSRC) value of 1 for his
      SRTP and SRTCP packets.  Alice thus has a crypto-context for SSRC
      1, including the associated ROC (Roll Over Counter) and SEQ (RTP
      Sequence Number).  Bob now forwards the call to Carol (Bob still
      has not generated an answer).  At this point, Bob has Alice's key,
      which sometimes might be a security weakness.  As the exchange
      proceeds, Carol gets the original offer, including the offered
      crypto-attribute and starts sending media packets towards Alice.
      It just so happens that Carol chooses an SSRC value of 1, as did
      Bob.  When Carol starts generating packets, there is a potential
      for what <a href="./rfc3711">RFC 3711</a> calls a "two-time pad" issue (problem 2), as
      well as the potential for the ROC to be out of sync between Alice
      and Carol (problem 3).  Note that since Bob and Carol are
      (presumably) using different source transport addresses, the SSRC
      reuse does not constitute an SSRC collision (although it may still
      be interpreted as such by Alice).  Per <a href="./rfc3711">RFC 3711</a>, since the master
      key would be shared between Bob and Carol in this case, it is
      RECOMMENDED that Alice leave the session at that point in order to
      avoid the two-time pad issue.  It should also be noted that <a href="./rfc3711">RFC</a>
      <a href="./rfc3711">3711</a> recommends against sharing SRTP master keys, which forking
      may accidentally introduce when the keying material is for the
      receiving direction.

      If we consider the above scenario again, but this time with keying
      material in the offer (and answer) being the sending keying
      material (as specified by SDP security descriptions), the scenario
      instead looks as follows: Bob again chooses SSRC 1, and Bob will



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      need to send back an answer to Alice, since Alice needs to learn
      Bob's sending key.  Bob also starts sending media towards Alice
      (clipping may occur until Alice receives Bob's answer).  Bob again
      forwards the call to Carol who also starts sending early media
      using SSRC 1.  However, Carol needs to generate a new answer (for
      the dialog between Alice and Carol) in order for Alice to process
      Carol's packets . Upon receiving this answer, Alice can initiate a
      new offer/answer exchange (to move the session to another
      transport address as described in <a href="#section-7.3">Section 7.3</a>).  In this case,
      there is one master key per session and a unique keystream
      regardless of whether or not SSRCs collide.

   Scenario C: Parallel Forking Case

      In this scenario, Alice generates an offer (with receive keying
      material) that gets forked to Bob and Carol in parallel.  Bob and
      Carol both start sending packets (early media) to Alice.  If Bob
      and Carol choose different SSRCs, everything is fine initially.
      However, one of the crypto context parameters is the master key
      lifetime, and since Bob and Carol are sharing the same master key
      (unbeknownst to either), they do not know when they need to rekey
      (problem 4).  If they choose the same SSRC, we have the two-time
      pad problem again (problem 2).

   In summary, if keying material were for the receive direction, we
   would have the following problems:

      - Problem 1: Offerer does not know which of multiple crypto offers
                   was chosen by answerer.

      - Problem 2: SSRC reuse (or SSRC collisions) between multiple
                   answerers (serial or parallel forking) may lead to
                   the two-time pad issue.

      - Problem 3: Part of the crypto context parameters (specifically
                   the ROC) is not communicated but derived, and if we
                   allow multiple entities to use the same SSRC
                   (sequentially), the ROC can be wrong.

      - Problem 4: All crypto contexts that share a master key need to
                   maintain a shared set of counters (master key
                   lifetime), and if we allow for multiple entities on
                   different platforms to share a master key, we would
                   need a mechanism to synchronize these counters.

      Problem 1 could be addressed by using the MKI as proposed
      separately; however, it would result in using extra bandwidth for
      each SRTP media packet.  Solving problem 2 implies a need for



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      being able to synchronize SSRC values with the answerer (or
      abandon the session when SSRC reuse or SSRC collisions occur).
      Problem 3 implies a need for being able to synchronize ROC values
      on a per SSRC basis (or abandon the session when SSRC reuse
      occurs).  Problem 4 could be solved by having the offerer (Alice,
      i.e., the entity receiving media) determine how many packets have
      actually been generated by the total set of senders to Alice and,
      hence, be the one to initiate the rekeying.  In the case of packet
      losses, etc. this is not foolproof, but in practice it could
      probably be addressed by use of a reasonable safety margin.

      In conclusion, it would be expected from an offer/answer and SIP
      point of view to have the offer (and answer) keying material be
      the receive keying material; however, doing so would trade
      security for SIP friendliness, e.g., two-time pad and master key
      lifetime issues, and violate the <a href="./rfc3711">RFC 3711</a> rule for sharing an SRTP
      master key across SRTP sessions.

Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, New Jersey  08837 USA

   EMail: fandreas@cisco.com


   Mark Baugher
   5510 SW Orchid Street
   Portland, Oregon  97219 USA

   EMail: mbaugher@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134  USA

   EMail: dwing@cisco.com










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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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