Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                   Mark Baugher
   INTERNET-DRAFT                                             Dan Wing
   EXPIRES: December 2003                                Cisco Systems
                                                         June 27, 2003

              SDP Security Descriptions for Media Streams

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
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   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.  This document defines the Secure Real-
   time Transport Protocol (SRTP) parameters for the attribute.  The
   SDP crypto attribute requires the services of a data security
   protocol to secure the SDP message.

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1. Notational Conventions............................................2
2. Introduction......................................................3
3. SDP "Crypto" Attribute and Parameters.............................4
 3.1 Crypto-suite....................................................4
 3.2 Key Parameter...................................................4
 3.4 Session Parameters..............................................5
 3.5 Examples........................................................5
4. RTP/SAVP (SRTP) Security Descriptions.............................6
 4.1 Crypto-suites...................................................7
   4.1.1 AES_CM_128_HMAC_SHA1_80.....................................7
   4.1.2 AES_CM_128_HMAC_SHA1_32.....................................7
   4.1.3 F8_128_HMAC_SHA1_80.........................................7
   4.1.4 Adding new CRYPTO-SUITE definitions.........................8
 4.2 Key-param Parameter.............................................8
   4.2.1 Key Usage...................................................8
   4.2.2 INLINE Definition...........................................8
 4.3 Session Parameters.............................................10
   4.3.1 SRC=/SSRC/ROC/SEQ..........................................10
   4.3.2 KEY_DERIVATION_RATE=n......................................11
   4.3.3 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP.....................11
   4.3.4 FEC_ORDER=order............................................11
   4.3.5 UNAUTHENTICATED_SRTP.......................................12
5. Use with Offer/Answer............................................12
 5.1 Generating the Offer...........................................12
 5.2 Answerer Processing............................................12
 5.4 Non-RTP/SAVP Answerers.........................................14
 5.4 Offer/Answer Example: Receiver Supports SRTP...................14
 5.7 Use of a=crypto With Active Media Streams......................15
 5.8 Operation with KEYMGT and Key lines............................15
6. Security Considerations..........................................15
 6.1 Authentication of packets......................................16
 6.1 Key-stream Reuse...............................................16
 6.2 Signaling Authentication and Signaling Encryption..............17
7. SRTP "Crypto" Attribute Grammar..................................18
8. Open Issues......................................................19
9. Acknowledgements.................................................19
10. Authors' Addresses..............................................19
11. Normative References............................................20
Intellectual Property Statement.....................................21

1. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
   document are to be interpreted as described in [RFC2119]. The
   terminology conforms to [RFC2828].

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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
   these media streams.  The Secure Real-time Transport Protocol (SRTP)
   can be used to provide such security services, and use of it can be
   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 defaults values.  This document specifies a new
   SDP attribute called "crypto", which is used to signal and negotiate
   cryptographic parameters for SRTP.

   The crypto attribute might be extended to non-SRTP transports such
   as whiteboard, modem, fax, and other transports that could use
   various security protocols such as IPsec or TLS.  These extensions,
   however, are beyond the scope of this document.  Each type of SDP
   media stream needs its own definitions that assign values to its
   crypto-attribute parameters.  These definitions are unique to the
   particular SDP transport and SHOULD be specified in an Internet RFC.
   This document defines the parameter values for SRTP.

   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
   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, GDOI, KINK, IKE or 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 are suitable for restricted
   cases where IPsec, TLS, or some other encapsulating data-security
   protocol (e.g. SIP secure multiparts) protects the SDP message.
   This draft adds security descriptions to those encrypted and/or
   authenticated SDP messages through the "crypto" attribute, which

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   provides the cryptographic parameters of a media stream. The "
   crypto" attribute could be adapted to any media transport, but its
   definition is unique to a particular transport.  In Section 3, we
   introduce the SDP crypto attribute, and in Section 4, we define the
   crypto attribute details needed for SRTP.  In Section 5, we specify
   how to use the crypto attribute for SRTP streams with the
   Offer/Answer model [RFC3264].  Section 6 recites security
   considerations, and Section 7 gives an Augmented-BNF grammar for the
   SRTP security descriptions provided for the crypto attribute.  A
   list of open issues is provided in Section 8.

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
   proceeding media line.  The "crypto" attribute MUST only appear at
   the SDP media level (not the session level).  The "crypto" attribute
   is defined by the following ABNF [RFC2234]:

     "a=crypto:" crypto-suite SP key-param *(SP session-param)

   where "SP" is the space character (see [RFC2234]); the fields
   crypto-suite, key-param, and session-param are described in Section
   3.1, 3.2, and 3.3.

   The ordering of multiple "a=crypto" lines is significant:  The most-
   preferred crypto line is listed first; see section 5 for details. We
   now describe the crypto fields in more detail.

3.1 Crypto-suite

   The crypto-suite field describes all needed information about the
   encryption and authentication algorithms for the RTP/SAVP transport.
   The ABNF grammar for crypto-suite is:

     crypto-suite = VCHAR

   where VCHAR is defined in [RFC2234]. The possible values for the
   crypto-suite parameter is unique to the transport.

3.2 Key Parameter

   The key-param field  MUST either contain an inline key descriptor,
   or it MUST be a pointer to a uri which contains the actual key. The
   ABNF grammar for key-param is:

     key-param           = inline-key / uri-key
     inline-key          = "inline:" key-descriptor
     key-descriptor      = VCHAR
     uri-key             = "uri:" absolute-uri
     absolute-uri        = VCHAR

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   where VCHAR is defined in [RFC2234].

   If the key parameter starts with the string "uri:", the URI method
   is used and the value that follows MUST be a Uniform Resource
   Identifier. The URI is a resource that SHOULD be queried to obtain
   the cryptographic key for the session.  The format or protocols used
   for the uri are beyond the scope of this document, however it is
   RECOMMENDED that such protocols provide both integrity and

   The INLINE method is invoked when the key parameter starts with the
   string "inline:"; the cryptographic key is encoded according to a
   transport-specific syntax subject to the general format provided
   above.  Thus, the URI method is transport generic and the INLINE
   method is transport specific.  Section 4 describes the INLINE key-
   parameter syntax for RTP/SAVP (the SRTP media transport type).

3.4 Session Parameters

   The session parameters are specific to the SDP media stream
   transport and are OPTIONAL. The ABNF grammar for session-param is:

     session-param  = VCHAR

   where VCHAR is defined in [RFC2234]. Section 4 describes the session
   parameters for RTP/SAVP.

3.5 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.

     o=jdoe 2890844526 2890842807 IN IP4
     s=SDP Seminar
     i=A Seminar on the session description protocol
     u= (Jane Doe)
     c=IN IP4
     t=2873397496 2873404696
     m=video 51372 RTP/SAVP 31
     m=audio 49170 RTP/SAVP 0
     m=application 32416 udp wb

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   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
   next section.

4. RTP/SAVP (SRTP) Security Descriptions

   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 crypto attribute defined in this
   document.  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 5.0).

   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
   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.

     SRTP Crypto Parameter    Description
     ---------------------    -----------
     CRYPTO-SUITE             Encryption and authentication transforms
     INLINE                   SRTP and associated parameters
     SRC                      An <SSRC, ROC, SEQ> triple
     KEY_DERIVATION_RATE      Rate that the 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

         Table 4-1: SRTP Crypto-suite, Key and Session Parameters

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   Please refer to the SRTP specification for a complete list of
   parameters and their descriptions [Section 8.2, srtp].  The CRYPTO-
   SUITE, the key parameter, and the session parameters shown in the
   table above are described in the following sections. If a receiver
   cannot recognize a parameter or value, then the receiver MUST NOT
   participate in the media stream and SHOULD log an "invalid name"
   condition unless the receiver is participating in an Offer/Answer
   exchange (Section 5).

4.1 Crypto-suites

   A crypto-suite value appears as the first parameter in a crypto
   attribute.  If a receiver does not support the particular crypto-
   suite, then the receiver MUST NOT participate in the media stream
   and SHOULD log an "unrecognized crypto-suite" condition unless the
   receiver is participating in an Offer/Answer exchange (Section 5).
   RTP/SAVP has three crypto-suites as described below.

4.1.1 AES_CM_128_HMAC_SHA1_80

   This 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 lifetime of a maximum of
   2^48 SRTP packets or 2^31 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).  The master salt value is 112 bits and the
   session salt value is 112 bits.  The PRF is the default SRTP pseudo-
   random function that uses AES Counter Mode with a 128-bit key

4.1.2 AES_CM_128_HMAC_SHA1_32

   The SRTP AES Counter Mode cipher is used with HMAC-SHA1 message
   authentication having a 32-bit authentication tag for SRTP packets;
   SRTCP uses an 80-bit tag.  The master-key length is 128 bits and has
   a lifetime of a maximum of 2^48 SRTP packets or 2^31 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).  The master salt value
   is 112 bits and the session salt value is 112 bits.  The PRF is the
   default SRTP pseudo-random function that uses AES Counter Mode with
   a 128-bit key length.

4.1.3 F8_128_HMAC_SHA1_80

   The SRTP f8 cipher is used with HMAC-SHA1 message authentication
   having a 80-bit authentication tag.  The master-key length is 128
   bits and has a lifetime of a maximum of 2^48 SRTP packets or 2^31
   SRTCP packets, whichever comes first [srtp].  The SRTP and SRTCP
   encryption key lengths are 128 bits.  The SRTP and SRTCP

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   authentication key lengths are 160 bits (see Security
   Considerations).  The master salt value is 112 bits and the session
   salt value is 112 bits.  The PRF is the default SRTP pseudo-random
   function that uses AES Counter Mode with a 128-bit key length.

4.1.4 Adding new CRYPTO-SUITE definitions

   If new transforms are added to SRTP, new definitions for those
   transforms SHOULD be given for the SDP crypto attribute and
   published in an Internet RFC. Sections 4.1.1 through 4.1.3
   illustrate how to define CRYPTO-SUITE values for particular
   cryptographic transforms.  New definitions MAY be added to existing
   transforms, moreover, to augment or modify definitions 4.1.1 through
   4.1.3.  For example, if AES_CM_128_HMAC_SHA1_80 were desired that
   used a 160-bit master key, then a new crypto-suite MUST be defined
   having a new name that is identical to AES_CM_128_HMAC_SHA1_80 but
   with the size of the master key defined to be 160 bits instead of
   128 bits.

4.2 Key-param Parameter

   If the key-param parameter has a "uri:" descriptor, the value is a
   Uniform Resource Identifier value as described in Section 3.  When
   the key-param parameter has an "inline:" descriptor, the value
   contains a cryptographic master key that MUST be a unique
   cryptographically random [RFC1750] value with respect to other
   "inline:" values in the SDP message.

4.2.1 Key Usage

   The "inline" type of key contains the keying material and all policy
   relating to that key, including how it can be used (for encryption,
   decryption, or both encryption and decryption), how long it can be
   used (lifetime) and whether or not it uses a master key index
   (master key index or MKI) to associate an incoming SRTP packet with
   a 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,
   for example).

4.2.2 INLINE Definition

   If the identifier is "inline", the key-param descriptor has the
   format described in Section 7 (Grammar) and contains the following

     use            key use indicator
     key_length     key length
     salt_length    salt length
     key||salt      concatenated key and salt, BASE64-encoded
     lifetime       key lifetime (number of packets)

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     MKI:length     MKI and length of the MKI field in SRTP packets.

   The "use" indicator defines usage as one of three values which are
   all provided from the perspective of the recipient of the SDP: "d"
   means the key is used for decryption only, "e" means the key is used
   for encryption only, and "b" means the key is used for both
   encryption and decryption.  If the crypto suite uses the same key
   for both encryption and decryption, "b" MUST be specified.

   The "key_length" is the integer length of the SRTP master key in
   bytes, and "salt_length" is the integer length of the master salt in
   bytes.  Their sum MUST be exactly equal to the length of the
   concatenated master key and salt provided in the fourth field.  The
   key_length and salt_length MUST appear in the "inline" encoding. For


   is a decryption key with a key length of 16 and a salt length of 14.

   The fourth part of the "inline" encoding is the cryptographic master
   key appended with the master salt.  Each master key and salt MUST be
   a cryptographically random number and MUST be unique to the SDP
   message.  Both are concatenated and then base-64 encoded.  If the
   length of the concatenated key and salt (after being decoded from
   base 64) does not equal the sum of the key_length and salt_length
   indicated, the receiver MUST NOT use this crypto attribute line for
   the media stream and SHOULD log a "inline encoding too short"
   condition.  For example,


   describes a decryption key with a key_length of 16, a salt_length of
   8, and a 32-character key and concatenated salt that is base-64
   encoded: The 24-character key/salt concatenation is expanded to 32
   characters by the three-in-four encoding of base 64.

   The fifth part of the of the "inline" encoding is the OPTIONAL
   lifetime of the master key as measured in number of packets using
   that key.  The lifetime value MAY be written as an non-zero,
   positive integer or as a power of 2, see the ABNF in Section 7 for
   details. The "lifetime" value MUST NOT exceed the maximum packet
   lifetime for the crypto-suite.  If lifetime is too large or
   otherwise invalid, then the receiver MUST NOT use this crypto
   attribute line for the media stream and SHOULD log an "invalid
   lifetime" condition.  The default MAY be implicitly signaled by
   having no described value for lifetime (i.e. "//").  This is
   convenient when the srtp crypto_key lifetime is allowed to default.
   The first example, above, shows a case where the lifetime is
   specified as 2^20 while the second example shows an empty lifetime,

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   which means the SRTP default value of 2^48 will be used with
   UNENCRYPTED_SRTCP and 2^31 will be used otherwise.

   The MKI and its byte length are OPTIONAL (see Section 7).  "MKI" is
   the master key index 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 the value exceeds 128, then the
   receiver MUST NOT use this crypto attribute line for the media
   stream and SHOULD log an "invalid MKI_length" condition.  If the
   value of the MKI is larger than allowed by MKI_length, then the
   receiver MUST NOT use this crypto attribute line for the media
   stream and SHOULD log an "invalid MKI" condition.  The substring
   "1:4" in the first example assigns to the key a master key index of
   1 that is 4 bytes long, and the second example assigns a 4-byte key
   index of 1066 to the key.

4.3 Session Parameters

   The "session" parameters are OPTIONAL and serve to override SRTP
   session defaults for the SRTP and SRTCP streams.  These parameters
   configure an RTP session for SRTP services.


   The SRC session parameter provides information to establish the SRTP
   cryptographic context.  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.  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
   an INLINE parameter.  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 OPTIONAL provided that either a ROC or a SEQ appear in the
   SRC parameter.  SSRC is an integer in the range of 0..2^32-1 for the
   RTP SSRC parameter, which is undefined by default.  This is the RTP
   SSRC that is associated with the inline key. If the SSRC value is
   invalid, the receiver MUST NOT use this crypto attribute line for
   the media stream but SHOULD log an "invalid SSRC" condition.  If
   SSRC is specified and an SRTP packet is received with a different
   SSRC value, the receiver SHOULD discard the packet and log an error.

   ROC is OPTIONAL provided that either an SSRC or a SEQ appear in the
   SRC parameter.  ROC is an integer in the range of 0..2^32-1 for the

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   SRTP rollover counter (ROC), which is zero by default.  The ROC MAY
   be set to a non-zero value for an ongoing RTP/SAVP stream in which
   the SRTP 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 SDP message containing a ROC SRTP description. If ROC is
   greater than 2^32-1, then the receiver MUST NOT use this crypto
   attribute line for the media stream but SHOULD log an "invalid ROC"

   SEQ is OPTIONAL provided that either an SSRC or a ROC appear in the
   SRC parameter.  SEQ is an integer in the range of 0..2^16-1 for the
   SRTP sequence number (SEQ).  SRTP uses the RTP sequence number (and
   the ROC) to compute the packet index [srtp].  At the start of a
   session, an SSRC 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.  See also section 3.3.1 of [srtp].  If SEQ
   is greater than 2^16-1, then the receiver MUST NOT use this crypto
   attribute line for the media stream but SHOULD log an "invalid SEQ"

4.3.2 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).


   UNENCRYPTED_SRTCP indicates that the SRTCP packet payloads are not
   encrypted.  UNENCRYPTED_SRTP indicates that the SRTP packet payloads
   are not encrypted.  SRTP and SRTCP packet payloads are encrypted by

4.3.4 FEC_ORDER=order

   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 on the sender and after SRTP processing on
   the receiver; FEC_SRTP is the default. SRTP_FEC is the reverse
   processing.  SPLIT signals that SRTP encryption occurs on the

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   sender, followed by FEC processing, followed by SRTP authentication;
   processing is reversed on the receiver.  If the receiver cannot
   recognize the order value, then the receiver MUST NOT use this
   crypto attribute line for the media stream but SHOULD log an
   "invalid FEC_ORDER" condition.


   This parameter signals that SRTP messages are not authenticated.
   SRTP authenticates SRTP messages by default.  Use of

5. Use with Offer/Answer

   The Offer/Answer model [RFC 3264] enables parties that wish to
   engage in a multimedia session to negotiate the media stream
   parameters that will be used for the multimedia session.  In this
   section, we detail how the security descriptions defined for SRTP
   are used with the offer/answer model to negotiate cryptographic
   capabilities and communicate SRTP master keys.

5.1 Generating the Offer

   For each SDP media line (m=) using the "RTP/SAVP" transport where
   the offerer wants to specify cryptographic parameters, the offerer
   MUST provide at least one "a=crypto" line.  When multiple crypto
   lines are provided, the a=crypto lines are specified in preference
   order, with the most preferred listed first.  The offerer determines
   the crypto parameters based on its capabilities and its security

   The offerer obtains keying material for "inline", or obtains the uri
   pointing to a key server.  The mechanism to generate or obtain a key
   is outside the scope of this specification.

5.2 Answerer Processing

   For each SDP media line using the "RTP/SAVP" transport that contains
   an "a=crypto" line in the offer, the answerer MUST either accept
   exactly one of the crypto lines for that media stream, or it MUST
   reject the media stream as described in RFC 3264.  Note that if
   there are no "a=crypto" lines for the media stream in the offer,
   then the answerer MUST skip the following steps and instead use the
   default SRTP/SRTCP parameters for that media stream (note that the
   endpoint will then need to somehow obtain the correct master key as
   well).  When the answerer accepts an "RTP/SAVP" media stream with a
   crypto line, the answerer MUST include exactly one "a=crypto" line
   in the answer.  The answer crypto line MUST include at least the
   selected crypto-suite and a key-param parameter.

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   To determine if the answerer can accept any of the provided
   "a=crypto" lines, the answerer examines the crypto lines in order.
   If an "a=crypto" line does not satisfy the constraints provided in
   Section 4, that crypto line is deemed invalid and MUST be discarded.
   The answerer selects the first valid crypto line that it supports,
   considering the answerer's capabilities and security policies.  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. SSRC,
   ROC, 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]).

   After selecting a single crypto line, the answerer generates a
   master key appropriate for the selected crypto algorithm(s), unless
   the offered master key was specified to apply to both encryption and
   decryption, in which case the offered master key MUST be used
   instead.  If the offered master key was for decryption, then the
   answerer MUST use it to decrypt any incoming packets; the key
   provided in the answer MUST also be marked as being for decryption,
   since the answerer will be using it when encrypting it's packets.
   Similarly, if the offered key was for encryption, then the answerer
   MUST use it to encrypt any packets it sends and the key it provides
   in its answer MUST be used to decrypt any incoming packets. The
   master key in the answer MUST have the same key length and salt
   length as the offer.

   To set up the bi-directional media with transport set to RTP/SAVP,
   the answerer includes one "a=crypto" line after its media line with
   transport set to RTP/SAVP.

5.3 Offerer Processing of the Answer

   When the offerer receives the answer, it MUST perform the following
   steps for each "RTP/SAVP" media stream it offerered 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 media line is valid according to the
   constraints specified in Section 4.  Furthermore, the offerer MUST
   validate, that the crypto-suite selected was one of the offered
   crypto-suites for the media stream.  If any of these checks fails,
   the security negotiation defined here MUST be deemed to have failed.

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

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5.4 Non-RTP/SAVP Answerers

   If a media stream with transport set to "RTP/SAVP" is sent to a
   device that doesn't support "RTP/SAVP", that media stream will be

   In some cases, it is desirable to send SRTP when possible, but allow
   use of RTP if SRTP isn't supported by a remote device.  However, it
   is ambiguous to send an extra media line with transport set to
   "RTP/AVP" and with the same port as the "RTP/SAVP" line.  Thus, an
   offerer MUST NOT specify multiple media lines with the same port.

   Instead, such interoperability is obtained by first sending an offer
   with transport set to "RTP/SAVP".  If that media line is rejected by
   the answerer, the offerer can, if its policy permits, send a new
   offer with transport set to "RTP/AVP".  Also, the SDP extensions
   defined in RFC 3407 [RFC3407] can be used by both the offerer and
   answerer to indicate capabilities above and beyond what is being
   negotiated for the media stream.  Another offer/answer exchange will
   then be needed to negotiate use of any of these latent capabilities.

5.4 Offer/Answer Example: Receiver Supports SRTP

   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.

   Offerer sends:
     o=sam 2890844526 2890842807 IN IP4
     s=SRTP Discussion
     i=A discussion of Secure RTP
     u= (Marge Simpson)
     c=IN IP4
     t=2873397496 2873404696
     m=audio 49170 RTP/SAVP 0
      FEC_ORDER=FEC_SRTP SRC=17174//49126
      FEC_ORDER=FEC_SRTP SRC=17174//49126

   Answerer replies:
     o=jill 25690844 8070842634 IN IP4
     s=SRTP Discussion
     i=A discussion of Secure RTP
     u= (Homer Simpson)

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     c=IN IP4
     t=2873397526 2873405696
     m=audio 32640 RTP/SAVP 0

   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 initial SSRC (88131), rollover counter (721),
   and rollover counter (13).

5.7 Use of a=crypto With Active Media Streams

   When an active SRTP session is rekeyed, this is indicated by sending
   a new SDP.  Rekeying is done following the rules described for a
   normal Offer/Answer exchange.  The Answerer can take this
   opportunity to rekey the traffic it is sending, if the Answerer
   desires.  During rekeying, the session parameters cannot be changed
   and MUST NOT be specified in the Offer or the Answer.

   When the Offerer needs to rekey, the offerer MUST send an "a=crypto"
   line with same crypto-suite, key length, and salt length that was
   previously accepted by the Answerer.

   If the answerer selected "a=crypto" lines using the "inline" method,
   the exact same "a=crypto" line(s) as agreed to in the answer MUST be
   sent and a new new inline key MUST be sent.

   If the answerer selected "a=crypto" lines using the "uri" method,
   the sender MAY transmit the same uri, and the recipient MUST fetch
   the new key using the uri.

5.8 Operation with KEYMGT and Key lines

   An Offer MAY include both a=crypto and a=keymgt lines [keymgt].  Per
   SDP rules, the Answerer will ignore attribute lines it doesn't
   understand.  If the Answerer supports both a=crypto and [keymgt],
   the Answer MUST include either a=crypto or [keymgt], as including
   both is undefined.

6. 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
   descriptions.  Therefore, that protocol should either invoke its own
   security mechanisms to do so, or alternatively utilize a lower-layer
   security service (e.g. TLS, IPSEC) where that service is deemed
   adequate for protecting the encapsulating protocol itself.  Where

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   the encapsulating protocol is used in both a hop-by-hop and end-to-
   end context (e.g. SIP), an end-to-end security service SHOULD be
   provided by that protocol for all sensitive information including
   SDP's security parameters.  This service SHOULD provide strong
   message authentication and packet-payload encryption as well as
   effective replay protection.  As an example, SIP with S/MIME bodies
   satisfies these requirements.

6.1 Authentication of 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 of unicast SRTP messages SHOULD be
   performed [srtp].  Note that SRTP source-message authentication does
   not authenticate the IP-address of the SRTP source, but ensures that
   the SRTP message that the SRTP receiver had received is exactly what
   the SRTP sender had sent.  Source authentication of multicast SRTP
   messages is today non-standard and hence for further study.  But
   even in multicast sessions, SRTP packet authentication ensures that
   the packet originated from a member of the secure group.  Use of the

6.1 Key-stream Reuse

   Misconfigured SRTP sessions, moreover, are vulnerable to attacks on
   their encryption services when running the crypto suites defined in
   Sections 4.1.1, 4.1.2 and 4.1.3.  An SRTP encryption service is
   "mis-configured" 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
   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, as this document
   RECOMMENDS (Section 4.2).

   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].  This is
   for further study.  The RECOMMENDED approach of this document is to
   allocate a different master key for each host-participant of an SRTP

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6.2 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
   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.

   IPsec, TLS, encapsulating-protocol security or some other data
   security service SHOULD be used to provide message authentication
   for SDP messages that carry the SRTP attribute.  Message encryption
   SHOULD be used because a master key parameter 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

   When the SDP parameters cannot be carried in an encrypted and
   authenticated SDP message, it is RECOMMENDED that a key management
   protocol be used.  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 endpoint, its
   signaling controller, and the 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.

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   In this case, the third-party provider gets 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 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).

7. SRTP "Crypto" Attribute Grammar

   This section provides an Augmented BNF grammar for the SRTP profile
   of the SDP crypto attribute.  ABNF is defined in [RFC2234].

     key-param      = method-inline / method-uri

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

     method-inline   = "inline:" key-info *(SP session-param)
     method-uri      = "uri:<" absoluteURI ">" ; absoluteURI defined in
                                               ; [RFC2396]

     key-info = key-use "/" key-length "/" salt-length "/" key-salt
                "/" [lifetime] "/" [mki]

     key-use =   "d" / "e" / "b"   ; decrypt, encrypt, or both
     key-length = 1*DIGIT
     salt-length = 1*DIGIT

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

     lifetime = ["2^"] 1*(DIGIT)
     mki = mki-length ":" mki-value
     mki-length = 1*DIGIT         ; real value is 2^mki-length, max 128
     mki-value = 1*DIGIT

     session-param  = src /
                      kdr /
                      "UNENCRYPTED_SRTP" /
                      "UNENCRYPTED_SRTCP" /
                      "UNAUTHENTICATED_SRTP" /

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     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*(DIGIT)

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

     base64 =  ALPHA / DIGIT / "+" / "/" / "="

8. Open Issues

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

   * The crypto attribute can be used with or without offer/answer,
     however, details on usage outside of offer/answer are missing.

   * The offer/answer procedures need to be expanded to better describe
     unidirectional and inactive streams, unicast versus multicast, as
     well as additional detail for some of the session parameters.

9. Acknowledgements

   This document 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 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 using unique master keys for each SDP media stream as followed in
   this document.  Jonathan Rosenberg suggested reducing the complexity
   by specifying only one security parameter for each media stream.

10. Authors' Addresses

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

   Mark Baugher
   5510 SW Orchid Street
   Portland, Oregon  97219 USA

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

11. Normative References

   [RFC1889] H. Schulzrinne, S. Casner, R. Fredrick, V. Jacobson, "RTP:
   A Transport Protocol for Real-Time Applications", January 1996,

   [RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
   Specifications: ABNF," November 1997,

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

   [RFC2828] R. Shirey, "Internet Security Glossary", May 2000,

   [RFC3264] "J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
   the Session Description Protocol (SDP)", June 2202,

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

   [RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness
   Recommendations for Security", RFC 1750, December 1994.

12. Informative References

   [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
   Capability Declaration", RFC 3407, October 2002.

   [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.

   [keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "Key Management Extensions for SDP and RTSP", February 2003,
   07.txt, Work in Progress.

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   [mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "MIKEY: Multimedia Internet KEYing", July 2002,,
   Work in Progress.

   [RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies",
   November 1996,

   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
   for Message Authentication", November 1997,

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

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