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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4568

   Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                   Mark Baugher
   INTERNET-DRAFT                                             Dan Wing
   EXPIRES: April 2004                                   Cisco Systems
                                                      October 24, 2003

           Session Description Protocol Security Descriptions
                           for Media Streams
                <draft-ietf-mmusic-sdescriptions-02.txt>


Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   Drafts.

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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines a Session Description Protocol (SDP)
   cryptographic attribute for media streams.  The attribute describes
   a cryptographic key and other parameters, which serve to configure
   security for a media stream in either a single message or a
   roundtrip.  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) media streams.  The SDP crypto
   attribute requires the services of a data security protocol to
   secure the SDP message.

Table of Contents

1. Notational Conventions............................................3
2. Introduction......................................................3
3. SDP "Crypto" Attribute and Parameters.............................4
 3.1 Crypto-suite....................................................5



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 3.2 Key Parameters..................................................5
 3.3 Session Parameters..............................................6
 3.4 Example.........................................................6
4. General Use of the crypto Attribute...............................6
 4.1 Use With Offer/Answer...........................................7
   4.1.1  Generating the Initial Offer..............................7
   4.1.2  Generating the Initial Answer.............................8
   4.1.3  Offerer Processing of the Initial Answer..................9
   4.1.4  Modifying the Session....................................10
 4.2 Use Outside Offer/Answer: Advertising..........................10
 4.3 General Backwards Compatibility Considerations.................10
5. SRTP Security Descriptions.......................................11
 5.2 Crypto-suites..................................................14
   5.2.1  AES_CM_128_HMAC_SHA1_80..................................14
   5.2.2  AES_CM_128_HMAC_SHA1_32..................................14
   5.2.3  F8_128_HMAC_SHA1_80......................................15
   5.2.4  Adding new Crypto-suite Definitions......................15
 5.3 Session Parameters.............................................15
   5.3.1  SRC=SSRC/ROC/SEQ.........................................15
   5.3.2  KDR=n....................................................18
   5.3.3  UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................18
   5.3.4  UNAUTHENTICATED_SRTP.....................................19
   5.3.5  FEC_ORDER=order..........................................19
   5.3.6  Window Size Hint (WSH)...................................19
   5.3.7  SRTP Extension Session Parameters........................19
6. SRTP-Specific Use of the crypto Attribute........................20
 6.1 Use with Offer/Answer..........................................20
   6.1.1  Generating the Initial Offer.............................20
   6.1.2  Generating the Initial Answer............................21
   6.1.3  Offerer Processing of the Initial Answer.................22
   6.1.4  Modifying the Session....................................23
   6.1.5  Offer/Answer Example.....................................24
 6.2 SRTP-Specific Use Outside Offer/Answer: Advertising............25
 6.3 SRTP-Specific Backwards Compatibility Considerations...........25
 6.4 Operation with KEYMGT= and k= lines............................26
 6.5 Removal of Crypto Contexts.....................................26
7. Security Considerations..........................................26
 7.1 Authentication of packets......................................27
 7.2 Keystream Reuse................................................27
 7.3 Signaling Authentication and Signaling Encryption..............27
8. Grammar..........................................................29
 8.1 Generic "Crypto" Attribute Grammar.............................29
 8.2 SRTP "Crypto" Attribute Grammar................................29
9. Open Issues......................................................30
10. IANA Considerations.............................................31
 10.1 Registration of the "crypto" attribute........................31
 10.2 New IANA Registries and Registration Procedures...............31
   10.2.1 Security Descriptions Key Method Registry and Registration31
   10.2.2 SRTP Crypto Suite Registry and Registration..............31
   10.2.3 SRTP Session Parameter Registration......................32
 10.3 Initial Registrations.........................................32



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11. Acknowledgements................................................32
12. Authors' Addresses..............................................33
13. Normative References............................................33
14. Informative References..........................................34
Intellectual Property Statement.....................................35
Acknowledgement.....................................................36

1. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
   NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
   be interpreted as described in [RFC2119].  The terminology in this
   document conforms to [RFC2828], "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 abbreviation "iff" means "if and only if."

2. Introduction

   The Session Description Protocol (SDP) describes multimedia
   sessions, which can be audio, video, whiteboard, fax, modem, and
   other media sessions.  Security services such as data origin
   authentication, integrity and confidentiality are often needed for
   media streams.  The Secure Real-time Transport Protocol (SRTP)
   [srtp] provides such security services and is signaled by use of the
   "RTP/SAVP" transport 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 SRTP in particular.

   The crypto attribute is defined in a generic way to enable its use
   with secure transports besides SRTP that need to signal and
   negotiate cryptographic parameters, e.g. IPsec [ipsec], S/MIME
   [s/mime], or TLS [tls], if and only if such parameters can either be
   advertised in a single message, or negotiated in a single round-trip
   by use of the offer/answer model [RFC3264].  Such extensions,
   however, are beyond the scope of this document.  Each type of secure
   SDP 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 an Internet
   RFC and registered with IANA according to the procedures defined in
   Section 10.  This document defines the security parameters and
   keying material for SRTP only.

   It would be self-defeating not to secure cryptographic keys and
   other parameters at least as well as SRTP secures RTP packets or
   IPsec secures IP packets.  Data security protocols such as SRTP rely
   upon a separate key management system to securely establish
   encryption and/or authentication keys.  Key management protocols



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   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 [skeme].  Along with the key, an AKE
   protocol such as MIKEY [mikey], GDOI [GDOI], KINK [kink], IKE [ike]
   or TLS [tls] securely disseminates information describing both the
   key and the data-security session (for example, whether SRTCP
   payloads are encrypted or unencrypted in an SRTP 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 that follow 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
   [keymgt].  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 secure multiparts)
   protects the SDP message. This document adds security descriptions
   to those encrypted and/or authenticated SDP messages through the
   "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 frequently unique to a
   particular transport.  In Section 3, we introduce the general SDP
   crypto attribute, and in Section 4 we define how it is used with and
   without the offer/answer model. In Section 5, we define the crypto
   attribute details needed for SRTP, and in Section 6 we define SRTP-
   specific use of the attribute with and without the offer/answer
   model.  Section 7 recites security considerations, and Section 8
   gives an Augmented-BNF grammar for the general crypto attribute as
   well as the SRTP-specific use of the crypto attribute.  A list of
   open issues is provided in Section 9 and IANA considerations are
   provided in Section 10.

3. SDP "Crypto" Attribute and Parameters

   A new media-level SDP attribute called "crypto" describes the
   cryptographic suite, key parameters, and session parameters for the
   preceding media line.  The "crypto" attribute MUST only appear at
   the SDP media level (not the session level).  The "crypto" attribute
   follows the format (see Section 8.1 for a formal ABNF grammar):

     a=crypto:<crypto-suite> <key-params> *<session-params>

   The fields crypto-suite, key-params, and session-param are described
   in the following sub-sections.  Below we show an example of the
   crypto attribute for the "RTP/SAVP" transport, i.e. SRTP (newlines
   included for formatting reasons only):




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     a=crypto:AES_CM_128_HMAC_SHA1_80
      inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32
      SRC=/721/13

   The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined
   by the line starting with "inline:", and there is a single session-
   param named "SRC".

3.1 Crypto-suite

   The crypto-suite field is an identifier (see Section 8.1 for
   details) that describes the encryption and authentication algorithms
   (e.g. AES_CM_128_HMAC_SHA1_80) for the transport in question.  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 of the
   "RTP/SAVP" transport applies to Secure RTP only; the string may be
   reused for another transport, however a separate definition would be
   needed.

3.2 Key Parameters

   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 (a formal grammar is provided in Section 8.1):

     key-params = <key-method> ":" <key-info>

   Keying material may be provided by different means. One method is
   defined in this document, namely "inline", which indicates that the
   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 an IETF RFC in the future, in which case they may be used
   with any transport, provided the definitions for that transport
   support use of the new key-method.  New key methods MUST be
   registered with the IANA according to the procedures defined in
   Section 10.2.1.

   Key-info is here just defined as a general character string (see
   Section 8.1 for details); further transport and key-method specific
   syntax and semantics MUST be provided in an IETF RFC for each
   combination of transport and key-method that wants to use it;
   definitions for SRTP are provided in Section 5.  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").





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

    For SRTP, this could for example be achieved by use of Master Key
    Identifiers (MKI), or <"From", "To"> values.

3.3 Session Parameters

   Session parameters are specific to a given transport and use of them
   is OPTIONAL in the general framework, where they are just defined as
   a general character string.  If session parameters are to be used
   for a given transport, then key-method and transport-specific syntax
   and semantics MUST be provided in an IETF RFC for each transport
   that wants to use it; definitions for SRTP are provided in Section
   5.  Note that such definitions are provided within the context of
   both a specific key-method (e.g. "inline") and a particular
   transport (e.g. "RTP/SAVP").

3.4 Example

   The first example shows use of the crypto attribute for the RTP/SAVP
   media transport type (as defined in Section 4).  The a=crypto line
   is actually one long line, although it is shown as two lines in this
   document 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:AES_CM_128_HMAC_SHA1_80
      inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
     m=audio 49170 RTP/SAVP 0
     a=crypto: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 RTP/SAVP-specific descriptions are defined in the
   Section 5.

4. General Use of the crypto Attribute




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

4.1 Use With Offer/Answer

   In this section, we define the general rules for use of the crypto
   attribute with the offer/answer [RFC3264] model.  These rules are in
   addition to the rules specified in RFC 3264, which MUST be followed,
   unless otherwise noted.

4.1.1 Generating the Initial Offer

4.1.1.1   Unicast Streams

   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.  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 stronger
   cryptographically than a "less preferred" crypto suite.

   The crypto-suite always applies to media in all directions supported
   by the media stream (e.g. send and receive).

   The key(s) apply to media in the direction from the offerer to the
   answerer; if the media stream is marked as "recvonly", a key MUST
   still be provided.

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

   There are no general offer/answer rules for the session parameters;
   instead, specific rules are provided as part of the transport and
   key-method specific definitions of any session parameters.

   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; the answerer may or may not
   choose the same key as the offerer chose in his sending direction
   (in fact, the answerer SHOULD NOT use the same key as explained in
   Section 4.1.2.1).  Since media may arrive prior to the answer, delay
   or clipping may 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.





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     For example, a "security" precondition [RFC3312] could be defined
     to solve the above problem.

   Another problem can occur when the offerer includes multiple crypto
   attributes, since the offerer may not be able to deduce which of the
   offered crypto attributes was accepted by the answerer until the
   answer is received, yet media may arrive before the answer.

   If this is unacceptable to the offerer, the offerer either SHOULD
   NOT include multiple crypto attributes in the offer, or a mechanism
   outside the scope of this document SHOULD be used to prevent the
   above problem (e.g. a "security" precondition).

4.1.1.2   Multicast Streams

   The rules for multicast streams are similar to those for unicast
   streams, except as noted below:

   * In order to ensure that all participants use the same crypto
     parameters, there MUST be exactly one crypto attribute per media
     stream.

   * The key(s) provided apply to media in all directions supported by
     the media stream, as opposed to just the sending direction.

4.1.2 Generating the Initial Answer

4.1.2.1   Unicast Streams

   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 [RFC3407] extensions.

   Only crypto attributes that are valid, i.e. do not violate any of
   general rules defined for security descriptions as well as any
   specific rules defined for the transport and key method in question
   can be accepted.  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, considering the answerer's capabilities and security policies.

   If there is 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.

   The crypto attribute in the answer MUST contain the following:



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   * The 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.

   There are no general offer/answer rules for the session parameters;
   instead, specific rules are provided as part of the transport and
   key-method specific definitions of any 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.

4.1.2.2   Multicast Streams

   The rules for multicast streams are similar to those for unicast
   streams, except as noted below:

   * The crypto-suite in the answer MUST be the same as the one in the
     offer (unless the offered media stream is rejected).  Since no
     more than one crypto attribute can be offered for a multicast
     stream, this is satisfied trivially.

   * The key(s) provided apply to media in all directions supported by
     the media stream, as opposed to just the sending direction.
     Consequently, the key(s) in the answer MUST be the same as the
     key(s) in the offer.

4.1.3 Offerer Processing of the Initial Answer

4.1.3.1   Unicast Streams

   When the offerer receives the answer, the offerer MUST verify, that
   exactly one of the offered crypto attributes was accepted.
   Otherwise, the offerer MUST consider the offer/answer negotiation to
   have failed for that stream.

   The key(s) included in the answer are the key(s) that will be used
   for media sent from the answerer to the offerer and hence the
   offerer MUST use those key(s) to process media received; the key(s)
   might not be the same as the key(s) used by the offerer for sending
   media to the answerer.

   There are no general offer/answer rules for the session parameters;
   instead, specific rules are provided as part of the transport and
   key-method specific definitions of any session parameters.




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4.1.3.2   Multicast Streams

   When the offerer receives the answer, the offerer MUST verify, that
   the offered crypto attribute and key(s) were accepted and echoed in
   the answer.  Otherwise, the offerer MUST consider the offer/answer
   negotiation to have failed for that stream for *that answerer* and
   hence the answerer is not considered a participant in that media
   stream.  If there are other participants in the multimedia session,
   the session may continue unaffected by this particular answerer's
   failure.

   There are no general offer/answer rules for the session parameters;
   instead, specific rules are provided as part of the transport and
   key-method specific definitions of any session parameters.

4.1.4 Modifying the Session

   Once a media stream has been established, it MAY be modified at any
   time, as described in RFC 3264, Section 8.  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 is still desired, the
   crypto attribute MUST be included in these new offer/answer
   exchanges.  The procedures are similar to those defined in Section
   4.1.1, 4.1.2, 4.1.3 subject to the considerations provided in RFC
   3264 Section 8.

4.2  Use Outside Offer/Answer: Advertising

   The crypto attribute can also be used outside the context of
   offer/answer.  For example, when using the Session Announcement
   Protocol (SAP) [RFC2974], there is no negotiation of the media
   streams described by the SDP; instead media streams are simply
   advertised.

   The crypto attribute defined here can be used in such environments
   where the crypto parameters are advertised in a single message
   rather than being negotiated in a roundtrip (an offer and an
   answer), albeit with certain restrictions:

   * There MUST be exactly one crypto attribute.


   There are no general rules for the session parameters; instead,
   specific rules for advertising session parameters are provided as
   part of the transport and key-method specific definitions of any
   session parameters.

4.3  General Backwards Compatibility Considerations





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   It is possible that the answerer supports a given secure transport
   and accepts the offered media stream, yet the answerer does not
   support the crypto attribute defined here.  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 be deemed to have failed.

5. SRTP Security Descriptions

   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 for a media stream MUST only be used for
   media streams that use the "RTP/SAVP" transport in the media (m=)
   line and SHALL apply to that media stream only.

   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 [srtp].  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 (per [SDPnew]) and hence, if it supports SRTP, it will
   simply assume use of default SRTP parameters.  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 Section 6.1).

   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, moreover, there is usually no need to provide unique
   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:













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     * crypto-suite:     Identifies the encryption and authentication
                         transforms
     * key parameter:    SRTP keying material and parameters
     * session parameters:    The following parameters are defined:
          - SRC:    An <SSRC, ROC, SEQ> triple
          - KDR:    The SRTP Key Derivation Rate is the rate that 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
          - 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 [Section 8.2, srtp].  The key
   parameter, the crypto-suite, and the session parameters shown above
   are described in detail in the following sections.

5.1.1.1   SRTP Key Parameter

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

   The "inline" type of key contains the keying material and all policy
   relating to that key, including how long it can be used (lifetime)
   and whether or not 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 key lifetime has expired).

   The key parameter contains a semi-colon separated list of
   cryptographic master keys, each of which MUST be a unique
   cryptographically random [RFC1750] value with respect to other
   master keys in the entire SDP message (i.e. including master keys
   for other streams).  Each key in the list follows the format (a
   formal definition is provided in Section 8.2):

     "inline:" <key salt> "|" [<lifetime] "|" [MKI:length / FromTo]

     key||salt      concatenated key and salt, base64 encoded
     lifetime       key lifetime (number of packets)
     MKI:length     MKI and length of the MKI field in SRTP packets.
     FromTo         <"From", "To"> values, specifying the lifetime for
                    a master key.

   The following definition provides an example for
   AES_CM_128_HMAC_SHA1_80:



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     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
   entire crypto attribute MUST be considered invalid and an "invalid
   key/salt" condition SHOULD be logged.  Each master key and salt MUST
   be a cryptographically random number and MUST be unique to the SDP
   message.

   The second field, is the OPTIONAL lifetime of the master key as
   measured in maximum total number of packets using that key.  The
   lifetime value MAY be written as a non-zero, positive integer or as
   a power of 2 (see the grammar in Section 8.2 for details).  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 and an
   "invalid lifetime" condition SHOULD be logged.  The default MAY be
   implicitly signaled by omitting the lifetime value (i.e. "||").
   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 the SRTP's and SRTCP's default
   values will be used (2^31):

     inline: YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6MTIzNDU2||1066:4

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

   The third field, which is also OPTIONAL, is either the Master Key
   Identifier (MKI) and its byte length, or a <"From", "To"> value.

   "MKI" is the master key identifier associated with the SRTP master
   key.  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.
   If the MKI length is not given or if it exceeds 128 (bytes), then
   the entire crypto attribute MUST be considered invalid and an
   "invalid MKI length" condition SHOULD be logged.  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 key identifier of 1066 to the key.



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   <"From", "To"> specifies the lifetime for a master key, expressed in
   terms of the ROC and SEQ values inside whose range (including the
   range end-points) the master key is valid.  <"From", "To"> is an
   alternative to the MKI and assumes that a master key is in one-to-
   one correspondence with the SRTP session key on which the <"From",
   "To"> range is defined.  The following example illustrates the use
   of the <"From", "To"> parameter:

    inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|FT=0:0,1:0

   As mentioned above, the key parameter can contain one or more master
   keys.  When the key parameter contains more than one master key, all
   of the master keys MUST either include an MKI or a <"From", "To">
   value.  Note that it is not permissible to mix and match use of the
   two within a single key parameter (i.e., one crypto attribute); all
   master keys in a given key parameter must use one or the other.

5.2 Crypto-suites

   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 below are
   described in the context of the SRTP security descriptions.

5.2.1     AES_CM_128_HMAC_SHA1_80

   AES_CM_128_HMAC_SHA1_80 is the SRTP default AES Counter Mode cipher
   and HMAC-SHA1 message authentication having an 80-bit authentication
   tag.  The master-key length is 128 bits and has a default lifetime
   of a maximum of 2^31 SRTP packets or SRTCP packets, whichever comes
   first [srtp].  The SRTP and SRTCP encryption key lengths are 128
   bits.  The SRTP and SRTCP authentication key lengths are 160 bits
   (see Security Considerations in Section 7).  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.

5.2.2 AES_CM_128_HMAC_SHA1_32

   This crypto suite is identical to AES_CM_128_HMAC_SHA1_80 except
   that the SRTP authentication key is 32 bits and the SRTCP
   authentication key is 80 bits.






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   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.

5.2.3 F8_128_HMAC_SHA1_80

   This crypto suite is identical to AES_CM_128_HMAC_SHA1_80 except the
   cipher is F8 [srtp].

   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.

5.2.4 Adding new Crypto-suite Definitions

   If new transforms are added to SRTP, new definitions for those
   transforms SHOULD be given for the SRTP security descriptions and
   published in an IETF RFC.  Sections 5.2.1 through 5.2.3 illustrate
   how to define crypto-suite values for particular cryptographic
   transforms.  Any new crypto suites MUST be registered with IANA
   following the guidelines in section 10.

5.3 Session Parameters

   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 and are described in the following.

5.3.1     SRC=SSRC/ROC/SEQ

   The SRTP cryptographic context for a given SRTP session is
   identified by the synchronization source (SSRC).  Furthermore,
   associated with a cryptographic context is the SRTP packet index
   which is derived from the RTP sequence number (SEQ) and a rollover
   counter (ROC).  The SSRC and SEQ are included in the SRTP packets,
   however they are not included in standard SDP (for various reasons).
   The ROC is neither included in the SRTP packets nor standard SDP but
   is instead derived algorithmically based on the total number of
   packets sent.  This presents a couple of challenges:

   * If the master key is shared between two or more session
     participants, SSRC collisions MUST be avoided; SSRC collision
     detection and resolution is not an acceptable alternative as this
     can lead to the two-time pad problem [srtp].

   * If a participant joins an ongoing session (where the ROC is non-
     zero), the participant needs to learn the ROC somehow.






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   * If the initial sequence number is close to the maximum sequence
     number and the initial SRTP packets are lost, the receiver may not
     update his ROC correctly.

   * When joining a multicast RTP session with multiple participants, a
     separate crypto context needs to be established for each
     participant (SSRC).  Even if the same master key is used by all
     participants, the ROC for each still needs to be learned somehow.


   The SRC session parameter provides information to establish the SRTP
   cryptographic context.  It contains information about one or more of
   the following:

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

   The ROC and sequence number are typically only needed for sessions
   already in progress (as when rekeying or when joining a multicast
   session).

   Zero or more SRC parameters MAY appear in a crypto attribute.  When
   more than one SRC parameter is present, each of them MUST contain a
   distinct SSRC value.  Each SRC parameter defines a separate SRTP
   crypto context (see section 3.2 of [srtp]) that SHALL share the
   master key and salt defined by one or more inline key parameters.
   The total number of all packets that are encrypted by keys derived
   from this master key MUST NOT exceed the lifetime of the inline key.
   The SRTP crypto contexts so defined SHALL also have a common
   definition for the crypto-suite and all other crypto parameters.

   SSRC is the RTP SSRC that is associated with the crypto context, and
   is an integer in the range of 0..2^32-1.  If an SSRC value is
   invalid, the entire crypto attribute line MUST be considered invalid
   and an "invalid SSRC" condition SHOULD be logged.  If an SSRC value
   collides with an SSRC for an existing participant in the session,
   the entire crypto attribute line MUST be considered invalid and an
   "SSRC collision" condition SHOULD be logged.

     OPEN ISSUE: It would be nice to have a way of indicating this
     condition in an answer SDP, but we quickly end up duplicating the
     RTP collision detection and resolution, which we don't want to.

   ROC is the SRTP rollover counter (ROC) in the range of 0..2^32-1 and
   is zero by default.  Typically the ROC value is specified as a non-
   zero value for an ongoing SRTP stream in which the ROC has cycled
   one or more times [srtp].  The receiver of the SDP message SHOULD
   refresh the ROC value before joining an ongoing session.  Depending
   on the nature of the session control, the late-joining receiver
   might need to refresh its ROC value through a unicast exchange or
   through receipt of a multicast or unicast message containing a ROC



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   SRTP description.  If the ROC is greater than 2^32-1, then the
   entire crypto attribute line MUST be considered invalid and an
   "invalid ROC" condition SHOULD be logged.

   SEQ is the SRTP sequence number (SEQ), which MUST be in the range of
   0..2^16-1.  SRTP uses the RTP sequence number and the ROC to compute
   the packet index [srtp].  For this reason, the initial SEQ SHOULD be
   in the range of 0..2^15-1 to avoid an ambiguity when packets are
   lost at the start of the session.  At the start of a session, an
   SSRC source that randomly selects a high sequence-number value can
   put the receiver in an ambiguous situation:  If initial packets are
   lost in transit up to the point that the sequence number wraps
   (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 of the first 2^15 packets are lost
   (which seems, if not impossible, then rather unlikely).  See Section
   3.3.1 of the SRTP specification regarding packet-index determination
   [srtp].

   It is not necessary to signal SEQ and ROC at the start of the SRTP
   session if the receivers do not join the session late, which is
   typical in IP telephony, multimedia client/server, and similar
   applications.  Large-scale multicast applications, however, will
   sometimes have late joiners to the session and MAY choose to use the
   SRC session parameter to set the SEQ and the ROC.  The SSRC MAY also
   be initialized in the SRC parameter; this can for example be useful
   to establish the crypto contexts (in particular the ROC) for all the
   session participants.

   Like SEQ and ROC, SSRC is OPTIONAL (unless there are multiple SRC
   parameters in which case it is mandatory) and often need not be
   signaled.  If the master key is not shared among senders for their
   encryption services, then SSRC uniqueness is NOT REQUIRED (see
   Section 7.2) and the SSRC need not be signaled.  In this way, each
   master key is used for encryption by exactly one sender and used for
   decryption by one or more receivers: In this case, there is no risk
   of keystream reuse for the crypto-suite ciphers of Section 5.2.1,
   5.2.2, and 5.2.3.

   The SRTP crypto context can be established for the SRTP session
   address in the connection (c=) line and the port in the media (m=)
   line (or rtpmap) without having specified an SSRC value in the SRTP
   security descriptions. This is called "late binding" by this
   specification.  If late binding is used, then when a packet arrives,
   the SSRC that is contained in it can be bound to the crypto context
   at the time of session commencement rather than at the time of
   session signaling.  With the arrival of the packet containing the
   SSRC, all the data items (except the ROC if it is non-zero) needed
   for the SRTP crypto context are held by the receiver.  In other




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   words, the crypto context for an RTP/SAVP session using late binding
   is initially identified by the SDP as:

          <*, address, port>

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

          <ssrcX, address, port>

   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.

   It should be noted, that use of late binding when there is no
   authentication of the SRTP media packets is subject to numerous
   security attacks and consequently it is NOT RECOMMENDED (of course,
   this can be said for unauthenticated SRTP in general).  Endpoints
   that do not wish to subject themselves to such security risks can
   either signal the SSRC by out-of-band mechanisms (as defined here),
   or ensure that only authenticated SRTP is being used.

5.3.2     KDR=n

   KDR specifies the Key Derivation Rate, as described in section 4.3.1
   of [srtp].

   The value n MUST be an integer in the set {0,1,2,...,24}, which
   denotes a power of 2 from 2^0 to 2^24, inclusive.  The SRTP key
   derivation rate controls how frequently a new session key is derived
   from an SRTP master key [srtp].  The default value is 0, which
   causes the key derivation function to be invoked exactly once (since
   2^0 is 1).

5.3.3     UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP

   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.




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5.3.4     UNAUTHENTICATED_SRTP

   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 [srtp].

5.3.5     FEC_ORDER=order

   FEC_ORDER signals the use of forward error correction for the RTP
   packets [rfc2733].  The forward error correction values for "order"
   are FEC_SRTP, SRTP_FEC, or SPLIT [mikey].  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.  SPLIT
   signals that the sender performs SRTP encryption, followed by FEC
   processing, followed by SRTP authentication; processing is reversed
   on the receiver.

5.3.6     Window Size Hint (WSH)

   SRTP defines the SRTP-WINDOW-SIZE [SRTP, section 3.3.2] parameter to
   protect against replay attacks.  The minimum value, per [srtp], is
   64, 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 number of packets per second).  However, there
   might be enough information given in SDP attributes like
   "a=maxprate" 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 which MAY choose
   to ignore the value provided.

5.3.7     SRTP Extension Session Parameters

   New SRTP session parameters for the SRTP security descriptions can
   be defined in an IETF RFC and registered with IANA according to the
   registration procedures defined in Section 10.

   SRTP extension session parameters are by default mandatory.  An SRTP
   extension session parameter that is prefixed with the dash character
   ("-") however is considered optional and MAY be ignored.  If a SDP
   is received with an unknown mandatory session parameter in a crypto
   attribute, that crypto attribute MUST be considered invalid and a
   "unknown session parameter" condition SHOULD be logged.




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6. SRTP-Specific Use of the crypto Attribute

   In this section, we describe the SRTP-specific use of the crypto
   attribute.

6.1 Use with Offer/Answer

   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 Section
   4.1, which MUST be followed, unless otherwise specified.

6.1.1     Generating the Initial Offer

6.1.1.1   Unicast Streams

   When the initial offer is generated, the offerer MUST follow the
   steps in Section 4.1.1.1 as well as the following steps.

   For each unicast media line (m=) using the "RTP/SAVP" 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 Section 5.

   The offerer MAY include one or more SRTP session parameters as
   defined in Section 5.3.  Note however, that if any extension SRTP
   session parameters are included, the negotiation will fail if the
   answerer does not support them.

6.1.1.2   Multicast Streams

   When the initial offer is generated, the offerer MUST follow the
   steps in Section 4.1.1.2 as well as the following steps.

   For each multicast media line (m=) using the "RTP/SAVP" 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 Section 5.  Furthermore, the
   <"From", "To"> parameter in the key parameter MUST NOT be used,
   unless the media stream is marked as "recvonly".

     The <"From", "To"> value is SSRC specific, and hence will only
     work when there is a single sender in the multicast case, i.e. all
     invited participants only receive media.

   The offerer MAY include one or more SRTP session parameters as
   defined in Section 5.3.  Note however, that if any extension SRTP
   session parameters are included, the negotiation will fail if the
   answerer does not support them.




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6.1.2     Generating the Initial Answer

6.1.2.1   Unicast Streams

   When the initial answer is generated, the answerer MUST follow the
   steps in Section 4.1.2.1 as well as the following steps.

   For each unicast media line using the "RTP/SAVP" transport that
   contains one or more "a=crypto" lines in the offer, the answerer
   MUST either accept 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 Section 5
   can be accepted.  Furthermore, all parameters (crypto-suite, key
   parameter, and session parameters) MUST be acceptable to the
   answerer in order for the offered media stream to be accepted.

   When the answerer accepts an "RTP/SAVP" unicast media stream with a
   crypto line, the answerer indicates acceptance by including its own
   "a=crypto" line in the answer.  The answer crypto line MUST include
   at least the selected SRTP crypto-suite and one or more master keys
   appropriate for the selected crypto algorithm; the master key(s)
   included in the answer SHOULD be different from those in the offer.

     If the master key(s) are not shared between the offerer and
     answerer, SSRC collisions are acceptable, which simplifies the
     overall operation.

   Session parameters MAY be included in the answer as well; any
   session parameters included in the answer are independent of session
   parameters included in the offer.  Use of extension SRTP session
   parameters SHOULD be avoided unless it is known that the offerer
   supports these.

   If the answerer cannot find any valid crypto line that it supports,
   or 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 "RTP/SAVP" by other means outside the
   scope of this document (e.g., by use of MIKEY [mikey]).

6.1.2.2   Multicast Streams

   When the initial answer is generated, the answerer MUST follow the
   steps in Section 4.1.2.2 as well as the following steps.

   For each multicast media stream using the "RTP/SAVP" transport that
   contains an "a=crypto" line in the offer, the answerer MUST either
   accept the first crypto line for that media stream (note that there
   should only be one crypto line), or it MUST reject the media stream.
   The crypto line MUST only be accepted if it is considered a valid
   SRTP security description as defined in Section 5.  Furthermore, all



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   parameters (crypto-suite, key parameter, and session parameters)
   MUST be acceptable to the answerer in order for the offered media
   stream to be accepted.

   When the answerer accepts an "RTP/SAVP" multicast media stream with
   a crypto line, the answerer indicates acceptance by repeating the
   crypto line from the offer in the answer, except for the session
   parameters which SHOULD be excluded.

     There is only a single view of a multicast stream (unlike
     unicast), and hence there is no reason to repeat optional
     parameters that cannot change anyway.

     OPEN ISSUE:  It is not clear that all session parameters should be
     excluded from the answer.  In particular, we may want to allow for
     inclusion of the SRC parameter, as this would enable a new-comer
     to instantiate crypto-contexts for other participants in a
     multicast conference, provided the conference is using a shared
     key.  If each sender uses a unique key, something else would be
     needed (e.g. an offer/answer exchange with each participant or an
     entirely different mechanism).

   If the answerer cannot find any valid crypto line that it supports,
   or 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.

   It should be noted, that multicast streams with more than one sender
   that are negotiated by use of this mechanism will be using the same
   master key for sending and receiving and hence SSRC collisions must
   be avoided.  The mechanism defined here does not provide a way to
   avoid such SSRC collisions for multicast streams, and hence means
   outside of the scope of this document are needed to ensure that SSRC
   collisions are avoided.  Examples of how this can be achieved
   include a centralized controller supplying unique SSRCs to the
   session participants or a separate protocol that can ensure SSRC
   uniqueness prior to sending any SRTP packets.

6.1.3     Offerer Processing of the Initial Answer

6.1.3.1   Unicast Streams

   When the offerer receives the answer, it MUST perform the steps in
   Section 4.1.3.1 as well as the following steps for each "RTP/SAVP"
   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 Section 5.





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   If the crypto line contains any SRTP session parameters, those
   parameters define SRTP behavior for media sent from the answerer to
   the offerer.  If the offerer either does not support or is not
   willing to honor one or more of the SRTP session 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.

6.1.3.2   Multicast Streams

   When the offerer receives the answer, it MUST perform the steps in
   Section 4.1.3.2 as well as the following steps for each "RTP/SAVP"
   media stream it offered with a crypto line 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 Section 5.  If the crypto line includes any
   session parameters, those are simply ignored.

     OPEN ISSUE:  As noted in Section 6.1.2.2, it may make sense to
     allow for some session parameters, e.g. SRC, to be included.

   If the crypto line is not valid, the SRTP security negotiation MUST
   be deemed to have failed for that particular answerer.

6.1.4     Modifying the Session

   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 Section 4.1.4 as well
   as the following steps.

   Unicast Streams:
   * The offerer SHOULD include the ROC and SEQ (unless both are made
     available to the answerer by other means); this enables the
     answerer to establish the complete crypto context in case he
     currently does not have the ROC.

   Multicast Streams:
   * When the media stream is "recvonly", the offerer SHOULD include
     the ROC and SEQ (unless both are made available to the answerer by
     other means); this enables the answerer to establish the complete
     crypto context in case he currently does not have the ROC.

   It should be noted, that the mechanism defined here does not provide
   a way to communicate the ROC for multiple senders, which may be
   needed in some multicast scenarios, e.g. conferencing.  If
   renegotiation is needed, a separate mechanism, such as [GDOI], will



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   be needed for this.  These methods are beyond the scope of this
   document.

     OPEN ISSUE:  As noted in Section 6.1.2.2, it is not clear that we
     couldn't do that with the SRC parameter.

   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 RFC 3264,
   when doing a new offer/answer exchange 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 the new offer/answer exchange.
   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 the answerer to determine if a given packet
     is encrypted according to the old or new offer/answer exchange.
     RFC 3264 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 firewall and NAT
     traversal.  The SRTP security descriptions offers two other ways
     of dealing with this; use the MKI (which adds a few bytes to each
     SRTP packet) or the <"From","To"> mechanism (which doesn't add
     bytes to each SRTP packet) as described in Section 5.1.1.1.  For
     further details on MKI and "<"From","To">, please refer to [srtp].

   * 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.

     As noted in Section 4.1.1.1, this could for example be addressed
     by defining a security "precondition" [RFC3312]

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

6.1.5 Offer/Answer Example

   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.











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   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:AES_CM_128_HMAC_SHA1_80
      inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
      FEC_ORDER=FEC_SRTP SRC=//49126
     a=crypto:F8_128_HMAC_SHA1_80
      inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4
      FEC_ORDER=FEC_SRTP SRC=//49126

   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:AES_CM_128_HMAC_SHA1_80
      inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
      SRC=/721/13

   In this case, the session would use the AES_CM_128_HMAC_SHA1_80
   crypto suite for the RTP and RTCP traffic.  The answerer is also
   specifying both its current rollover counter (721), and sequence
   number (13).

6.2 SRTP-Specific Use Outside Offer/Answer: Advertising

   The SRTP security descriptions can be used outside the context of
   offer/answer as described in Section 4.2.  In those cases, the
   general rules defined in Section 4.2 as well as the SRTP-specific
   rule defined below MUST be followed:

   * If any SRTP session parameters are included, they MUST be
     supported by the recipient of the SDP; otherwise, the recipient
     MUST NOT join the SRTP session.

6.3  SRTP-Specific Backwards Compatibility Considerations

   It is possible that the answerer supports the "RTP/SAVP" transport
   and accepts the offered media stream, yet it does not support the



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   crypto attribute defined here.  The offerer can recognize this
   situation by seeing an accepted "RTP/SAVP" 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 transport set to "RTP/SAVP" is sent to
   a device that does not support "RTP/SAVP", that media stream will be
   rejected.

6.4  Operation with KEYMGT= and k= lines

   An offer MAY include both "a=crypto" and "a=keymgt" lines [keymgt].
   Per SDP rules, the answerer will ignore attribute lines 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 [SDPnew].  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.

6.5  Removal of Crypto Contexts

   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 can not automatically be recovered
   (e.g. ROC and SEQ), 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 once the crypt context
     is removed.

   We resolve this problem as follows.  When SRTP security descriptions
   are being used, crypto contexts removal MUST follow the same rules
   as SSRC removal from the member table [RFC 3550]; 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.

7. Security Considerations

   Like all SDP messages, SDP messages containing security
   descriptions, are conveyed in an encapsulating application protocol
   (e.g. SIP, MGCP, RTSP, SAP, etc.).  It is the responsibility of the
   encapsulating protocol to ensure the protection of the SDP security



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   descriptions.  Therefore, the application protocol SHOULD either
   invoke its own security mechanisms to do so, or alternatively
   utilize a lower-layer security service (e.g. TLS, IPSEC).  This
   security service SHOULD provide strong message authentication and
   packet-payload encryption as well as effective replay protection.

7.1  Authentication of packets

   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).  Source authentication (i.e. data-origin
   authentication) of unicast SRTP messages SHOULD be performed [srtp].

7.2  Keystream Reuse

   Security descriptions as defined herein signal configuration
   parameters for SRTP sessions.  Misconfigured SRTP sessions  are
   vulnerable to attacks on their encryption services when running the
   crypto suites defined in Sections 5.2.1, 5.2.2, and 5.2.3.  An SRTP
   encryption service is "misconfigured" when two or more media streams
   are encrypted using the same AES keystream.  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 were used [srtp].  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, as this document
   RECOMMENDS (see Section 6.1.2.1), i.e. keys are not shared between
   multiple media streams, and the keys used in the send and receive
   direction for a given media stream are unique.

   SRTP multicast operation requires that each host-sender have a
   unique SRTP keystream.  This can be accomplished by ensuring that
   each sender be allocated a unique key or by ensuring that the SSRC
   of each sender will not collide.  Since SSRC collision might occur,
   the latter condition is avoided when all SSRCs are assigned by a
   central authority such as a 3rd-party key server [srtp].  The
   RECOMMENDED approach of this document is to allocate a different
   master key for each host-participant of an SRTP session.

7.3  Signaling Authentication and Signaling Encryption

   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



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   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 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, IPsec, TLS, or some other data security service SHOULD be used
   to provide message authentication for the encapsulating protocol
   that carries the SDP messages having a crypto attribute (a=crypto).
   Furthermore, encryption of the encapsulating payload SHOULD be used
   because 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 SDP parameters cannot be carried in an encrypted and
   authenticated SDP message, it is RECOMMENDED that a key management
   protocol be used instead of the security descriptions defined here
   (a=crypto).  The proposed SDP key-mgmt extension [keymgt] allows
   authentication and encryption of the key management protocol data
   independently of the SDP message that carries it.  The security of
   the SDP SRTP attribute, however, is as good as the data security
   protocol that protects the SDP message.  For example, if an IPsec
   security association exists between the source and destination
   endpoints, then this solution is more secure than use of the key-
   mgmt statement in an unauthenticated SDP message, which is
   vulnerable to tampering.

   There are practical cases, however, where SDP security is not end-
   to-end: If there is a third-party provider between the sender and
   receiver, then the data-security session might not be end-to-end.
   That is, one possible configuration might have an IPsec or TLS
   connection between the sender of the SDP message and the provider,
   such as a VoIP service provider, with a second secure connection
   between the provider and the receiver:

     signaling controller---(network-b)---signaling controller
          |                                                |
     (network a)                                   (network c)
          |                                                |
     sender----------------(SRTP bearer)--------------receiver

   where all of link a, b, and c are encrypted with TLS or IPsec.

   In this case, the third-party provider has access to the contents of
   the SRTP descriptions in the SDP message. SDP key-mgmt statement,
   however, allows true end-to-end security that is independent of the
   service provider, who often needs access to some parts of the SDP



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   message to render its services.  The SRTP attribute SHOULD NOT be
   used when end-to-end authentication or confidentiality is needed but
   the SDP message is not secured end-to-end (such as the above example
   where a third-party provider maintains the security associations
   with the endpoints for the SDP message).

8. Grammar

8.1 Generic "Crypto" Attribute Grammar

   The ABNF grammar for the crypto attribute is defined below:

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

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

   key-params       = key-param *(";" key-params)
   key-param        = key-method ":" key-info
   key-method       = "inline" | key-method-ext
   key-method-ext   = 1*(ALPHA / DIGIT / "_")
   key-info         = %x21-3A / %x3B-7E ; visible (printing) characters
                                        ; except semi-colon
   session-param    = VCHAR   ; visible (printing) characters

   where WSP, ALPHA, DIGIT, and VCHAR are defined in [RFC2234].

8.2 SRTP "Crypto" Attribute Grammar

   This section provides an Augmented BNF [RFC2234] 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" /
                           "AES_CM_128_HMAC_SHA1_80" /
                           srtp-crypto-suite-ext

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

     key-salt       = 1*(base64)   ; binary key and salt values
                                   ; concatenated together, and then
                                   ; base64 encoded [section 6.8 of
                                   ; RFC2046]

     lifetime      = ["2^"] 1*(DIGIT)   ; see section 5.1.1.1 for "2^"
     mki            = mki-value ":" mki-length



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     mki-value      = 1*DIGIT
     mki-length     = 1*3DIGIT   ; range 1..128.
     FromTo         = "FT=" ftval "," ftval
     ftval          = roc ":" seq  ; packet index expressed in terms
                                   ; of ROC and SEQ.

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

     src  = "SRC=" [ssrc] "/" [roc] "/" [seq]

     ssrc = 1*DIGIT                 ; range 0..2^32-1
     roc  = 1*DIGIT                 ; range 0..2^32-1
     seq  = 1*DIGIT                 ; range 0..2^16-1

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

     fec-order = "FEC_ORDER=" fec-type
     fec-type  = "FEC_SRTP" / "SRTP_FEC" / "SPLIT"

     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 [RFC2234]
                              ; first character must not be dash ("-")

9. Open Issues

   The following is a list of open issues in this document:

   * The use of security descriptions, and in particular SRTP security
     descriptions, with multicast streams where offer/answer is being
     used is not well understood and requires further consideration.

   * The security descriptions do not deal with hierarchically encoded
     streams (or at least they have not been considered).

   * The current mechanism does not allow for a key to be specified as
     being an encryption or decryption key or both; instead this is
     inferred from the context (e.g. unicast offer).  Should there be a
     mechanism to allow a key to be tagged as an encryption, decryption
     or both key ?





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10. IANA Considerations

10.1 Registration of the "crypto" attribute

   The IANA is hereby requested to register 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 Section 3

10.2 New IANA Registries and Registration Procedures

   The following sub-sections define several new IANA registries to be
   used for the security descriptions.  It is suggested that the
   following registry structure be used for these:

   Security Descriptions
     |
     +- Key Methods (described in 10.2.1)
     |
     +- Media Stream Transports
          |
          +- SRTP
               |
               +- SRTP crypto suites (described in Section 10.2.2)
               |
               +- SRTP session parameters (described in Section 10.2.3)


10.2.1    Security Descriptions Key Method Registry and Registration

   The IANA is hereby requested to create a new registry for SDP
   security description key methods.  An IANA key method registration
   MUST be documented in an IETF RFC and it MUST provide the name of
   the key method in accordance with the grammar for key-method-ext
   defined in Section 8.1.

10.2.2    SRTP Crypto Suite Registry and Registration

   The IANA is hereby requested to create a new registry for SRTP
   crypto suites. An IANA crypto suite registration MUST indicate the
   crypto suite name in accordance with the grammar for srtp-crypto-
   suite-ext defined in Section 8.2.






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   The semantics of the crypto suite MUST be described in an IETF RFC,
   including the semantics of the "inline" key-method and any special
   semantics of parameters.

10.2.3    SRTP Session Parameter Registration

   The IANA is hereby requested to create a new registry for SRTP
   session parameters.  An IANA SRTP session parameter registration
   MUST indicate the session parameter name (srtp-session-extension as
   defined in Section 8.2); the name MUST NOT begin with the dash
   character ("-").

   The semantics of the parameter MUST be described in an IETF RFC.  If
   values can be assigned to the parameter, then the format and
   possible values that can be assigned MUST be described in the IETF
   RFC as well.

10.3 Initial Registrations

   The following security descriptions key methods are hereby
   registered:

     inline

   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

   The following SRTP session parameters are hereby registered:

     SRC
     KDR
     UNENCRYPTED_SRTP
     UNENCRYPTED_SRTCP
     UNAUTHENTICATED_SRTP
     FEC_ORDER
     WSH

   The ABNF for all of the above is already included in the ABNF
   section of this document.

11. Acknowledgements

   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 David McGrew, Mats Naslund, Mike
   Thomas, Elisabetta Cararra, Brian Weis, Dave Oran, Bill Foster, Earl
   Carter, Matt Hammer and Dave Singer.  These people shared




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   observations, identified errors and made suggestions for improving
   the specification.  Mats made several valuable suggestions on
   parameters and syntax that are in the current draft.  Dave Oran and
   Mike Thomas encouraged us to bring this work to the IETF for
   standardization.  David McGrew suggested the conservative approach
   of requiring unique master keys for each unicast SDP media stream as
   followed in this document.  Jonathan Rosenberg suggested reducing
   the complexity by specifying only one security parameter for each
   media stream.

12. Authors' Addresses

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

   Mark Baugher
   5510 SW Orchid Street
   Portland, Oregon  97219 USA
   mbaugher@cisco.com
   +1-408-853-4418

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134  USA
   dwing@cisco.com
   +1-408-902-3348

13. Normative References

   [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
   "RTP: A Transport Protocol for Real-Time Applications", RFC 3550,
   July 2003, http://www.ietf.org/rfc/rfc3550.txt.

   [RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
   Specifications: ABNF," RFC 2234, November 1997,
   http://www.ietf.org/rfc/rfc2234.txt.

   [SDPnew] M. Handley, V. Jacobson, C. Perkins, "SDP: Session
   Description Protocol", Work in Progress.

   [RFC2733] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for
   Generic Forward Error Correction", RFC 2733, December 1999,
   http://www.ietf.org/rfc/rfc2733.txt.

   [RFC2828] R. Shirey, "Internet Security Glossary", RFC 2828, May
   2000, http://www.ietf.org/rfc/rfc2828.txt.




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   [RFC3264] J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
   the Session Description Protocol (SDP)", RFC 3264, June 2202,
   http://www.ietf.org/rfc/rfc3264.txt.

   [srtp] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K.
   Norrman, D. Oran, "The Secure Real-time Transport Protocol", Work in
   Progress.

   [RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness
   Recommendations for Security", RFC 1750, December 1994,
   http://www.ietf.org/rfc/rfc1750.txt.

14. Informative References

   [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
   Capability Declaration", RFC 3407, October 2002,
   http://www.ietf.org/rfc/rfc3407.txt.

   [Bellovin] Steven M. Bellovin, "Problem Areas for the IP Security
   Protocols," in Proceedings of the Sixth Usenix Unix Security
   Symposium, pp. 1-16, San Jose, CA, July 1996.

   [GDOI] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group
   Domain of Interpretation", RFC 3547, July 2003,
   http://www.ietf.org/rfc/rfc3547.txt.

   [kink] M. Thomas, J. Vilhuber, "Kerberized Internet Negotiation of
   Keys (KINK)", Work in Progress.

   [ike] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)", RFC
   2409, November 1998, http://www.ietf.org/rfc/rfc2409.txt.

   [ipsec] Kent, S. and R. Atkinson, "Security Architecture for the
   Internet Protocol", RFC 2401, November 1998,
   http://www.ietf.org/rfc/rfc2401.txt.

   [s/mime] Ramsdell B., "S/MIME Version 3 Message Specification", RFC
   2633, June 1999, http://www.ietf.org/rfc/rfc2633.txt.

   [tls] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
   2246, January 1999, http://www.ietf.org/rfc/rfc2246.txt.

   [keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "Key Management Extensions for SDP and RTSP", Work in Progress.

   [mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "MIKEY: Multimedia Internet KEYing", Work in Progress.

   [RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
   2045, November 1996, http://www.ietf.org/rfc/rfc2045.txt.



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   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
   for Message Authentication", RFC 2014, November 1997,
   http://www.ietf.org/rfc/rfc2104.txt.

   [skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
   Mechanism for the Internet", ISOC Secure Networks and Distributed
   Systems Symposium, San Diego, 1996.

   [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
   Resource Management and Session Initiation Protocol (SIP)", RFC
   3312, October 2002, http://www.ietf.org/rfc/rfc3312.txt.

   [RFC2974] M. Handley, C. Perkins, E. Whelan, "Session Announcement
   Protocol", RFC 2974, October 2000,
   http://www.ietf.org/rfc/rfc2974.txt .

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
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INTERNET-DRAFT         SDP Security Descriptions     October 24, 2003


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