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Internet Engineering Task Force (IETF)                         J. Lennox
Request for Comments: 6904                                         Vidyo
Updates: 3711                                                 April 2013
Category: Standards Track
ISSN: 2070-1721

                    Encryption of Header Extensions
           in the Secure Real-time Transport Protocol (SRTP)


   The Secure Real-time Transport Protocol (SRTP) provides
   authentication, but not encryption, of the headers of Real-time
   Transport Protocol (RTP) packets.  However, RTP header extensions may
   carry sensitive information for which participants in multimedia
   sessions want confidentiality.  This document provides a mechanism,
   extending the mechanisms of SRTP, to selectively encrypt RTP header
   extensions in SRTP.

   This document updates RFC 3711, the Secure Real-time Transport
   Protocol specification, to require that all future SRTP encryption
   transforms specify how RTP header extensions are to be encrypted.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 6904            Encrypted SRTP Header Extensions          April 2013

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Encryption Mechanism  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Example Encryption Mask . . . . . . . . . . . . . . . . .   6
     3.2.  Header Extension Keystream Generation for Existing
           Encryption Transforms . . . . . . . . . . . . . . . . . .   7
     3.3.  Header Extension Keystream Generation for Future
           Encryption Transforms . . . . . . . . . . . . . . . . . .   8
   4.  Signaling (Setup) Information . . . . . . . . . . . . . . . .   8
     4.1.  Backward Compatibility  . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Key Derivation Test Vectors . . . . . . . . . . . . . . .  13
     A.2.  Header Encryption Test Vectors Using AES-CM . . . . . . .  14

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1.  Introduction

   The Secure Real-time Transport Protocol [RFC3711] specification
   provides confidentiality, message authentication, and replay
   protection for multimedia payloads sent using the Real-time Protocol
   (RTP) [RFC3550].  However, in order to preserve RTP header
   compression efficiency, SRTP provides only authentication and replay
   protection for the headers of RTP packets, not confidentiality.

   For the standard portions of an RTP header, providing only
   authentication and replay protection does not normally present a
   problem, as the information carried in an RTP header does not provide
   much information beyond that which an attacker could infer by
   observing the size and timing of RTP packets.  Thus, there is little
   need for confidentiality of the header information.

   However, the security requirements can be different for information
   carried in RTP header extensions.  A number of recent proposals for
   header extensions using the mechanism described in "A General
   Mechanism for RTP Header Extensions" [RFC5285] carry information for
   which confidentiality could be desired or essential.  Notably, two
   recent specifications ([RFC6464] and [RFC6465]) contain information
   about per-packet sound levels of the media data carried in the RTP
   payload and specify that exposing this information to an eavesdropper
   is unacceptable in many circumstances (as described in the Security
   Considerations sections of those RFCs).

   This document, therefore, defines a mechanism by which encryption can
   be applied to RTP header extensions when they are transported using
   SRTP.  As an RTP sender may wish some extension information to be
   sent in the clear (for example, it may be useful for a network
   monitoring device to be aware of RTP transmission time offsets
   [RFC5450]), this mechanism can be selectively applied to a subset of
   the header extension elements carried in an SRTP packet.

   The mechanism defined by this document encrypts packets' header
   extensions using the same cryptographic algorithms and parameters as
   are used to encrypt the packets' RTP payloads.  This document defines
   how this is done for the encryption transforms defined in [RFC3711],
   [RFC5669], and [RFC6188], which are the SRTP encryption transforms
   defined by Standards Track RFCs at the time of this writing.  It also
   updates [RFC3711] to indicate that specifications of future SRTP
   encryption transforms must define how header extension encryption is
   to be performed.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119] and
   indicate requirement levels for compliant implementations.

3.  Encryption Mechanism

   Encrypted header extension elements are carried in the same manner as
   non-encrypted header extension elements, as defined by [RFC5285].
   The one- or two-byte header of the extension elements is not
   encrypted, nor is any of the header extension padding.  If multiple
   different header extension elements are being encrypted, they have
   separate element identifier values, just as they would if they were
   not encrypted.  Similarly, encrypted and non-encrypted header
   extension elements have separate identifier values.

   Encrypted header extension elements are carried only in packets
   encrypted using the Secure Real-time Transport Protocol [RFC3711].
   To encrypt (or decrypt) encrypted header extension elements, an SRTP
   participant first uses the SRTP key derivation algorithm, specified
   in Section 4.3.1 of [RFC3711], to generate header encryption and
   header salting keys, using the same pseudorandom function family as
   is used for the key derivation for the SRTP session.  These keys are
   derived as follows:

   o  k_he (SRTP header encryption): <label> = 0x06, n=n_e.

   o  k_hs (SRTP header salting key): <label> = 0x07, n=n_s.

   where n_e and n_s are from the cryptographic context: the same size
   encryption key and salting key are used as are used for the SRTP
   payload.  Additionally, the same master key, master salt, index, and
   key_derivation_rate are used as for the SRTP payload.  (Note that
   since RTP headers, including header extensions, are authenticated in
   SRTP, no new authentication key is needed for header extensions.)

   A header extension keystream is generated for each packet containing
   encrypted header extension elements.  The details of how this header
   extension keystream is generated depend on the encryption transform
   that is used for the SRTP packet.  For encryption transforms that
   have been standardized as of the date of publication of this
   document, see Section 3.2; for requirements for new transforms, see
   Section 3.3.

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   After the header extension keystream is generated, the SRTP
   participant then computes an encryption mask for the header
   extension, identifying the portions of the header extension that are,
   or are to be, encrypted.  (For an example of this procedure, see
   Section 3.1.)  This encryption mask corresponds to the entire
   payload of each header extension element that is encrypted.  It does
   not include any non-encrypted header extension elements, any
   extension element headers, or any padding octets.  The encryption
   mask has all-bits-1 octets (i.e., hexadecimal 0xff) for header
   extension octets that are to be encrypted and all-bits-0 octets for
   header extension octets that are not to be encrypted.  The set of
   extension elements to be encrypted is communicated between the sender
   and the receiver using the signaling mechanisms described in
   Section 4.

   This encryption mask is computed separately for every packet that
   carries a header extension.  Based on the non-encrypted portions of
   the headers and the signaled list of encrypted extension elements, a
   receiver can always determine the correct encryption mask for any
   encrypted header extension.

   The SRTP participant bitwise-ANDs the encryption mask with the
   keystream to produce a masked keystream.  It then bitwise
   exclusive-ORs the header extension with this masked keystream to
   produce the ciphertext version of the header extension.  (Thus,
   octets indicated as all-bits-1 in the encrypted mask are encrypted,
   whereas those indicated as all-bits-0 are not.)

   The header extension encryption process does not include the "defined
   by profile" or "length" fields of the header extension, only the
   field that Section 5.3.1 of [RFC3550] calls "header extension"
   proper, starting with the first [RFC5285] ID and length.  Thus, both
   the encryption mask and the keystream begin at this point.

   This header extension encryption process could, equivalently, be
   computed by considering the encryption mask as a mixture of the
   encrypted and unencrypted headers, i.e., as

       EncryptedHeader = (Encrypt(Key, Plaintext) AND MASK) OR
                         (Plaintext AND (NOT MASK))

   where Encrypt is the encryption function, MASK is the encryption
   mask, and AND, OR, and NOT are bitwise operations.  This formulation
   of the encryption process might be preferred by implementations for
   which encryption is performed by a separate module and cannot be
   modified easily.

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   The SRTP authentication tag is computed across the encrypted header
   extension, i.e., the data that is actually transmitted on the wire.
   Thus, header extension encryption MUST be done before the
   authentication tag is computed, and authentication tag validation
   MUST be done on the encrypted header extensions.  For receivers,
   header extension decryption SHOULD be done only after the receiver
   has validated the packet's message authentication tag, and the
   receiver MUST NOT take any actions based on decrypted headers, prior
   to validating the authentication tag, that could affect the security
   or proper functioning of the system.

3.1.  Example Encryption Mask

   If a sender wished to send a header extension containing an encrypted
   SMPTE timecode [RFC5484] with ID 1, a plaintext transmission time
   offset [RFC5450] with ID 2, an encrypted audio level indication
   [RFC6464] with ID 3, and an encrypted NTP timestamp [RFC6051] with ID
   4, the plaintext RTP header extension might look like this:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  ID=1 | len=7 |     SMTPE timecode (long form)                |
   |       SMTPE timecode (continued)                              |
   | SMTPE (cont'd)|  ID=2 | len=2 | toffset                       |
   | toffset (ct'd)|  ID=3 | len=0 | audio level   |  ID=4 | len=6 |
   |       NTP timestamp (Variant B)                               |
   |       NTP timestamp (Variant B, cont'd)       | padding = 0   |

         Figure 1: Structure of Plaintext Example Header Extension

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   The corresponding encryption mask would then be:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |0 0 0 0 0 0 0 0|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|
   |1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|
   |1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0|
   |1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|
   |1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0|

          Figure 2: Encryption Mask for Example Header Extension

   In the mask, the octets corresponding to the payloads of the
   encrypted header extension elements are set to all-1 values, and the
   octets corresponding to non-encrypted header extension elements,
   element headers, and header extension padding are set to all-zero

3.2.  Header Extension Keystream Generation for Existing Encryption

   For the AES-CM and AES-f8 transforms [RFC3711], the SEED-CTR
   transform [RFC5669], and the AES_192_CM and AES_256_CM transforms
   [RFC6188], the header extension keystream SHALL be generated for each
   packet containing encrypted header extension elements using the same
   encryption transform and Initialization Vector (IV) as are used for
   that packet's SRTP payload, except that the SRTP encryption and
   salting keys k_e and k_s are replaced by the SRTP header encryption
   and header salting keys k_he and k_hs, respectively, as defined

   For the SEED-CCM and SEED-GCM transforms [RFC5669], the header
   extension keystream SHALL be generated using the algorithm specified
   above for the SEED-CTR algorithm.  (Because the Authenticated
   Encryption with Associated Data (AEAD) transform used on the payload
   in these algorithms includes the RTP header, including the RTP header
   extension, in its Associated Authenticated Data (AAD), counter-mode
   encryption for the header extension is believed to be of equivalent
   cryptographic strength to the CCM and GCM transforms.)

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   For the NULL encryption transform [RFC3711], the header extension
   keystream SHALL be all-zero.

3.3.  Header Extension Keystream Generation for Future Encryption

   When new SRTP encryption transforms are defined, this document
   updates [RFC3711] as follows: in addition to the rules specified in
   Section 6 of RFC 3711, the Standards Track RFC defining the new
   transform MUST specify how the encryption transform is to be used
   with header extension encryption.

   It is RECOMMENDED that new transformations follow the same mechanisms
   as are defined in Section 3.2 of this document if they are applicable
   and are believed to be cryptographically adequate for the transform
   in question.

4.  Signaling (Setup) Information

   Encrypted header extension elements are signaled in the Session
   Description Protocol (SDP) extmap attribute using the URI
   "urn:ietf:params:rtp-hdrext:encrypt" followed by the URI of the
   header extension element being encrypted, as well as any
   extensionattributes that extension normally takes.  Figure 3 gives a
   formal Augmented Backus-Naur Form (ABNF) [RFC5234] showing this
   grammar extension, extending the grammar defined in [RFC5285].

   enc-extensionname = %x75.72.6e.3a.
       ; "urn:ietf:params:rtp-hdrext:encrypt" in lower case

   extmap =/ mapentry SP enc-extensionname SP extensionname
       [SP extensionattributes]

   ; extmap, mapentry, extensionname, and extensionattributes
   ; are defined in [RFC5285]

                 Figure 3: Syntax of the "encrypt" extmap

   Thus, for example, to signal an SRTP session using encrypted SMPTE
   timecodes [RFC5484], while simultaneously signaling plaintext
   transmission time offsets [RFC5450], an SDP document could contain
   the text shown in Figure 4 (line breaks have been added for

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   m=audio 49170 RTP/SAVP 0
   a=crypto:1 AES_CM_128_HMAC_SHA1_32 \
   a=extmap:1 urn:ietf:params:rtp-hdrext:encrypt \
       urn:ietf:params:rtp-hdrext:smpte-tc 25@600/24
   a=extmap:2 urn:ietf:params:rtp-hdrext:toffset

         Figure 4: Sample SDP Document Offering Encrypted Headers

   This example uses SDP security descriptions [RFC4568] for SRTP
   keying, but this is merely for illustration.  Any SRTP keying
   mechanism to establish session keys will work.

   The extmap SDP attribute is defined in [RFC5285] as being either a
   session or media attribute.  If the extmap for an encrypted header
   extension is specified as a media attribute, it MUST be specified
   only for media that use SRTP-based RTP profiles.  If such an extmap
   is specified as a session attribute, there MUST be at least one media
   in the SDP session that uses an SRTP-based RTP profile.  The session-
   level extmap applies to all the SRTP-based media in the session and
   MUST be ignored for all other (non-SRTP or non-RTP) media.

   The "urn:ietf:params:rtp-hdrext:encrypt" extension MUST NOT be
   recursively applied to itself.

4.1.  Backward Compatibility

   Following the procedures in [RFC5285], an SDP endpoint that does not
   understand the "urn:ietf:params:rtp-hdrext:encrypt" extension URI
   will ignore the extension and, for SDP offer/answer, will negotiate
   not to use it.

   For backward compatibility with endpoints that do not implement this
   specification, in a negotiated session (whether using offer/answer or
   some other means), best-effort encryption of a header extension
   element is possible: an endpoint MAY offer the same header extension
   element both encrypted and unencrypted.  An offerer MUST offer only
   best-effort negotiation when lack of confidentiality would be
   acceptable in the backward-compatible case.  Answerers (or equivalent
   peers in a negotiation) that understand header extension encryption
   SHOULD choose the encrypted form of the offered header extension
   element and mark the unencrypted form "inactive", unless they have an
   explicit reason to prefer the unencrypted form.  In all cases,
   answerers MUST NOT negotiate the use of, and senders MUST NOT send,
   both encrypted and unencrypted forms of the same header extension.

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   Note that, as always, users of best-effort encryption MUST be
   cautious of bid-down attacks, where a man-in-the-middle attacker
   removes a higher-security option, forcing endpoints to negotiate a
   lower-security one.  Appropriate countermeasures depend on the
   signaling protocol in use, but users can ensure, for example, that
   signaling is integrity-protected.

5.  Security Considerations

   The security properties of header extension elements protected by the
   mechanism in this document are equivalent to those for SRTP payloads.

   The mechanism defined in this document does not provide
   confidentiality about which header extension elements are used for a
   given SRTP packet, only for the content of those header extension
   elements.  This appears to be in the spirit of SRTP itself, which
   does not encrypt RTP headers.  If this is a concern, an alternate
   mechanism would be needed to provide confidentiality.

   For the two-byte-header form of header extension elements (0x100N,
   where "N" is the appbits field), this mechanism does not provide any
   protection to zero-length header extension elements (for which their
   presence or absence is the only information they carry).  It also
   does not provide any protection for the appbits (field 256, the
   lowest four bits of the "defined by profile" field) of the two-byte
   headers.  Neither of these features is present in the one-byte-header
   form of header extension elements (0xBEDE), so these limitations do
   not apply in that case.

   This mechanism cannot protect RTP header extensions that do not use
   the mechanism defined in [RFC5285].

   This document does not specify the circumstances in which extension
   header encryption should be used.  Documents defining specific header
   extension elements should provide guidance on when encryption is
   appropriate for these elements.

   If a middlebox does not have access to the SRTP authentication keys,
   it has no way to verify the authenticity of unencrypted RTP header
   extension elements (or the unencrypted RTP header), even though it
   can monitor them.  Therefore, such middleboxes MUST treat such
   headers as untrusted and potentially generated by an attacker, in the
   same way as they treat unauthenticated traffic.  (This does not mean
   that middleboxes cannot view and interpret such traffic, of course,
   only that appropriate skepticism needs to be maintained about the
   results of such interpretation.)

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   There is no mechanism defined to protect header extensions with
   different algorithms or encryption keys than are used to protect the
   RTP payloads.  In particular, it is not possible to provide
   confidentiality for a header extension while leaving the payload in

   The dangers of using weak or NULL authentication with SRTP, described
   in Section 9.5 of [RFC3711], apply to encrypted header extensions as
   well.  In particular, since some header extension elements will have
   some easily guessed plaintext bits, strong authentication is REQUIRED
   if an attacker setting such bits could have a meaningful effect on
   the behavior of the system.

   The technique defined in this document can be applied only to
   encryption transforms that work by generating a pseudorandom
   keystream and bitwise exclusive-ORing it with the plaintext, such as
   CTR or f8.  It will not work with ECB, CBC, or any other encryption
   method that does not use a keystream.

6.  IANA Considerations

   This document defines a new extension URI to the RTP Compact Header
   Extensions subregistry of the Real-Time Transport Protocol (RTP)
   Parameters registry, according to the following data:

      Extension URI:  urn:ietf:params:rtp-hdrext:encrypt
      Description:    Encrypted header extension element
      Contact:        jonathan@vidyo.com
      Reference:      RFC 6904

7.  Acknowledgments

   Thanks to Benoit Claise, Roni Even, Stephen Farrell, Kevin Igoe, Joel
   Jaeggli, David McGrew, David Singer, Robert Sparks, Magnus
   Westerlund, Qin Wu, and Felix Wyss for their comments and suggestions
   in the development of this specification.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, July 2008.

   [RFC5669]  Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The
              SEED Cipher Algorithm and Its Use with the Secure Real-
              Time Transport Protocol (SRTP)", RFC 5669, August 2010.

   [RFC6188]  McGrew, D., "The Use of AES-192 and AES-256 in Secure
              RTP", RFC 6188, March 2011.

8.2.  Informative References

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC5450]  Singer, D. and H. Desineni, "Transmission Time Offsets in
              RTP Streams", RFC 5450, March 2009.

   [RFC5484]  Singer, D., "Associating Time-Codes with RTP Streams", RFC
              5484, March 2009.

   [RFC6051]  Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", RFC 6051, November 2010.

   [RFC6464]  Lennox, J., Ivov, E., and E. Marocco, "A Real-time
              Transport Protocol (RTP) Header Extension for Client-to-
              Mixer Audio Level Indication", RFC 6464, December 2011.

   [RFC6465]  Ivov, E., Marocco, E., and J. Lennox, "A Real-time
              Transport Protocol (RTP) Header Extension for Mixer-to-
              Client Audio Level Indication", RFC 6465, December 2011.

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Appendix A.  Test Vectors

A.1.  Key Derivation Test Vectors

   This section provides test data for the header extension key
   derivation function, using AES-128 in Counter Mode.  (The algorithms
   and keys used are the same as those for the test vectors in Appendix
   B.3 of [RFC3711].)

   The inputs to the key derivation function are the 16-octet master key
   and the 14-octet master salt:

      master key: E1F97A0D3E018BE0D64FA32C06DE4139

      master salt: 0EC675AD498AFEEBB6960B3AABE6

   Following [RFC3711], the input block for AES-CM is generated by
   exclusive-ORing the master salt with the concatenation of the
   encryption key label 0x06 with (index DIV kdr), then padding on the
   right with two null octets, which implements the multiply-by-2^16
   operation (see Section 4.3.3 of [RFC3711]).  The resulting value is
   then AES-CM-encrypted using the master key to get the cipher key.

     index DIV kdr:                    000000000000
     label:                          06
     master salt:      0EC675AD498AFEEBB6960B3AABE6
     XOR:              0EC675AD498AFEEDB6960B3AABE6     (x, PRF input)

     x*2^16:           0EC675AD498AFEEDB6960B3AABE60000 (AES-CM input)

     hdr. cipher key:  549752054D6FB708622C4A2E596A1B93 (AES-CM output)

   Next, we show how the cipher salt is generated.  The input block for
   AES-CM is generated by exclusive-ORing the master salt with the
   concatenation of the encryption salt label.  That value is padded and
   encrypted as above.

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     index DIV kdr:                    000000000000
     label:                          07
     master salt:      0EC675AD498AFEEBB6960B3AABE6

     XOR:              0EC675AD498AFEECB6960B3AABE6     (x, PRF input)

     x*2^16:           0EC675AD498AFEECB6960B3AABE60000 (AES-CM input)

                       AB01818174C40D39A3781F7C2D270733 (AES-CM ouptut)

     hdr. cipher salt: AB01818174C40D39A3781F7C2D27

A.2.  Header Encryption Test Vectors Using AES-CM

   This section provides test vectors for the encryption of a header
   extension using the AES_CM cryptographic transform.

   The header extension is encrypted using the header cipher key and
   header cipher salt computed in Appendix A.1.  The header extension is
   carried in an SRTP-encrypted RTP packet with SSRC 0xCAFEBABE,
   sequence number 0x1234, and an all-zero rollover counter.

       Session Key:      549752054D6FB708622C4A2E596A1B93
       Session Salt:     AB01818174C40D39A3781F7C2D27

       SSRC:                     CAFEBABE
       Rollover Counter:                 00000000
       Sequence Number:                          1234
       Init. Counter:    AB018181BE3AB787A3781F7C3F130000

   The SRTP session was negotiated to indicate that header extension ID
   values 1, 3, and 4 are encrypted.

   In hexadecimal, the header extension being encrypted is as follows
   (spaces have been added to show the internal structure of the header

     17 414273A475262748 22 0000C8 30 8E 46 55996386B395FB 00

   This header extension is 24 bytes long.  (Its values are intended to
   represent plausible values of the header extension elements shown in
   Section 3.1, but their specific meaning is not important for the
   example.)  The header extension "defined by profile" and "length"
   fields, which in this case are BEDE 0006 in hexadecimal, are not
   included in the encryption process.

Lennox                       Standards Track                   [Page 14]

RFC 6904            Encrypted SRTP Header Extensions          April 2013

   In hexadecimal, the corresponding encryption mask selecting the
   bodies of header extensions 1, 2, and 4 (corresponding to the mask in
   Figure 2) is:


   Finally, we compute the keystream from the session key and the
   initial counter, apply the mask to the keystream, and then exclusive-
   OR the keystream with the plaintext:

       Initial keystream:  1E19C8E1D481C779549ED1617AAA1B7A
       Mask (hex):         00FFFFFFFFFFFFFFFF0000000000FF00
       Masked keystream:   0019C8E1D481C7795400000000001B00
       Plaintext:          17414273A475262748220000C8308E46
       Ciphertext:         17588A9270F4E15E1C220000C8309546

Author's Address

   Jonathan Lennox
   Vidyo, Inc.
   433 Hackensack Avenue
   Seventh Floor
   Hackensack, NJ  07601

   EMail: jonathan@vidyo.com

Lennox                       Standards Track                   [Page 15]