AVTCORE                                                        J. Lennox
Internet-Draft                                                     Vidyo
Intended status: Standards Track                            June 1,                        October 28, 2011
Expires: December 3, 2011 April 30, 2012

   Encryption of Header Extensions in the Secure  Real-Time Transport
                            Protocol (SRTP)
            draft-ietf-avtcore-srtp-encrypted-header-ext-00
            draft-ietf-avtcore-srtp-encrypted-header-ext-01

Abstract

   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.

Status of this Memo

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   This Internet-Draft will expire on December 3, 2011. April 30, 2012.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Encryption Mechanism . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Example Encryption Mask  . . . . . . . . . . . . . . . . . .  5
   4.  Signaling (Setup) Information  . . . . . . . . . . . . . . . . .  6
     4.1.  Backward compatibility . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . . 6  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 7  8
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 7
     7.2.  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 7  9
   Appendix A.  Test Vectors  . . . . . . . . . . . . . . . . . . . . 10
     A.1.  Key derivation test vectors  . 8
   Appendix B.  Open issues . . . . . . . . . . . . . . 10
     A.2.  Header Encryption Test Vectors using AES-CM  . . . . . . . 8 11
   Appendix C. B.  Changes From Earlier Versions . . . . . . . . . . . . 8
     C.1. 12
     B.1.  Changes from draft-lennox-avtcore draft-ietf-avtcore -00  . . . . . . . . . . . 9
     C.2. 12
     B.2.  Changes from draft-lennox-avtcore -00  . . . . . . . . . . 13
     B.3.  Changes from draft-lennox-avt -02  . . . . . . . . . . . . . 9
     C.3. 13
     B.4.  Changes From Individual Submission Draft -01 . . . . . . . 9
     C.4. 13
     B.5.  Changes From Individual Submission Draft -00 . . . . . . . 9 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 13

1.  Introduction

   The Secure Real-Time Transport Protocol [RFC3711] specification
   provides confidentiality, message authentication, and replay
   protection for multimedia payloads sent using of 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, this 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, this is not necessarily true for information carried in RTP
   header extensions.  A number of recent proposals for header
   extensions using the General Mechanism for RTP Header Extensions
   [RFC5285] carry information for which confidentiality could be
   desired or essential.  Notably, two recent drafts
   ([I-D.ietf-avtext-client-to-mixer-audio-level] and
   [I-D.ietf-avtext-mixer-to-client-audio-level]) carry information
   about per-packet sound levels of the media data carried in the RTP
   payload, and exposing this to an eavesdropper may be unacceptable in
   many circumstances.

   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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in 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 extension headers are only carried in packets encrypted
   using the Secure Real-Time Transport Protocol [RFC3711].  To encrypt
   (or decrypt) an encrypted extension header, headers, an SRTP participant first generates a keystream for
   uses the SRTP extension
   header.  This keystream is generated Key Derivation Algorithm, specified in Section 4.3.1 of
   [RFC3711], to generate header encryption and header salting keys,
   using the same manner pseudo-random function family as the
   encryption keystream are used for the corresponding SRTP payload, except the key
   derivation for the SRTP encryption and salting session.  These keys k_e and k_s are replaced by the
   keys k_he and k_hs, respectively.  The keys derived as follows:
   o  k_he and (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
   computed in from the cryptographic context: the same manner as k_e size
   encryption key and k_s, except that the <label>
   values salting key are used as are 0x06 for k_he and and 0x07 used for k_hs. the SRTP
   payload.  (Note that since RTP headers, including extension headers,
   are authenticated in SRTP, no new authentication key is needed for
   extension headers.)

   The

   For SRTP participant then computes encryption transforms that operate by generating a
   keystream, a header keystream is generated for each packet containing
   an encrypted header, using the same encryption mask transform and
   Initialization Vector (IV) as is used for the header
   extension, identifying SRTP payload, except
   that the portions of SRTP encryption and salting keys k_e and k_s are replaced by
   the SRTP header extension that are,
   or are to be, encryption and header salting keys k_he and k_hs,
   respectively.

   The AES-CM and AES-f8 transforms defined in [RFC3711] both operate in
   this keystream mode, as do the AES_192_CM and AES_256_CM transforms
   defined in [RFC6188].  For other transforms (for example,
   Authenticated Encryption with Associated Data (AEAD) cryptographic
   transforms, such as AES_GCM and AES_CCM [I-D.ietf-avt-srtp-aes-gcm])
   their usage of header extensions MUST be specified explicitly.  (As
   of this writing, it is believed that it will be sufficient for SRTP
   packets protected with AEAD transforms to use a CM transform with
   equivalent algorithms and key lengths for their encrypted headers;
   however, this guidance is not normative.)

   Once the header 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.
   This encryption mask corresponds to the entire payload of each header
   extension element that is encrypted.  It does not include any non-encrypted 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 which are to be
   encrypted, and all-bits-0 octets for header extension octets which
   are not to be.

   For those octets indicated in the encryption mask, the SRTP
   participant bitwise exclusive-ors the header extension with the
   keystream to produce the ciphertext version of the header extension.
   Those octets not indicated in the encryption mask are left
   unmodified.  Thus, conceptually, the encryption mask is logically
   ANDed with the keystream to produce a masked keystream.  The sender
   and receiver MUST use the same encryption mask.  The set of extension
   elements to be encrypted is communicated between the sender and the
   receiver using the signaling mechanisms described in Section 4.

   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 that
   could affect the security or proper functioning of the system, prior
   to validating the authentication tag.

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, and an encrypted audio level indication
   [I-D.ietf-avtext-client-to-mixer-audio-level] 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=15| len=7 |     SMTPE timecode (long form)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       SMTPE timecode (continued)                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SMTPE timecode (continued)                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       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.)        | padding = 0   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

   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|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|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 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 1 1|0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

   In the mask, the octets corresponding to the payloads of the
   encrypted header extension elements are set to all-1 values, and
   octets corresponding to non-encrypted elements, element headers, and
   header extension padding are set to all-0 values.

4.  Signaling (Setup) Information

   Encrypted header extension elements are signaled in the 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.
   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
   (line breaks added for formatting):

   m=audio 49170 RTP/SAVP 0
   a=crypto:1 AES_CM_128_HMAC_SHA1_32 \
     inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
   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 3

   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 only be
   specified for media which 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 which 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.

4.1.  Backward compatibility

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

   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.  Receivers which understand header
   extension encryption SHOULD choose the encrypted form and mark the
   unencrypted form "inactive", unless they have an explicit reason to
   prefer the unencrypted form.  (Note that, as always, users of best-
   effort encryption MUST be cautious of bid-down attacks, and 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 (0x100x),
   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 two-byte-headers' app bits (field 256, the lowest four bits of
   the "defined by profile" field).  Neither of these features are used
   in for one-byte-header form of header extension elements (0xBEDE), so
   these limitations do not apply in that case.

   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.

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 extension header element
   Contact:  jonathan@vidyo.com
   Reference:  RFC XXXX

   (Note to the RFC-Editor: please replace "XXXX" with the number of
   this document prior to publication as an RFC.)

7.  Acknowledgments

   Thanks to Roni Even, Kevin Igoe, David McGrew, David Singer, Qin Wu,
   and Felix Wyss for their comments and suggestions in the development
   of this specification.

8.  References

7.1.

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.

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

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

7.2.

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

8.2.  Informative References

   [I-D.ietf-avt-srtp-aes-gcm]
              McGrew, D., "AES-GCM and AES-CCM Authenticated Encryption
              in Secure RTP (SRTP)", draft-ietf-avt-srtp-aes-gcm-01
              (work in progress), January 2011.

   [I-D.ietf-avtext-client-to-mixer-audio-level]
              Lennox, J., Ivov, E., and E. Marocco, "A Real-Time
              Transport Protocol (RTP) Header Extension for Client-to-
              Mixer Audio Level Indication",
              draft-ietf-avtext-client-to-mixer-audio-level-01
              draft-ietf-avtext-client-to-mixer-audio-level-05 (work in
              progress), March September 2011.

   [I-D.ietf-avtext-mixer-to-client-audio-level]
              Ivov, E., Marocco, E., and J. Lennox, "A Real-Time
              Transport Protocol (RTP) Header Extension for Mixer-to-
              Client Audio Level Indication",
              draft-ietf-avtext-mixer-to-client-audio-level-02
              draft-ietf-avtext-mixer-to-client-audio-level-05 (work in
              progress), May September 2011.

   [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.

Appendix A.  Test Vectors

   TODO

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 the test vectors in
   Appendix B.  Open issues

   o  It 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 not clear how best 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 create get the keystream 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 extension
      headers carried in SRTP packets protected
   AES-CM is generated by exclusive-oring the master salt with Authenticated the
   concatenation of the encryption salt label.  That value is padded and
   encrypted as above.

     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 with Associated Data (AEAD) Test Vectors using AES-CM

   This section provides test vectors for the encryption of a header
   extension, using the AES_CM cryptographic transforms,
      such as AES_GCM transform.

   The header extension element is encrypted using the header cipher key
   and AES_CCM [I-D.ietf-avt-srtp-aes-gcm].  Header
      extensions are already protected as ancillary data by AEAD
      mechanisms, header cipher salt computed in Appendix A.1.

       Session Key:      549752054D6FB708622C4A2E596A1B93
       Session Salt:     AB01818174C40D39A3781F7C2D27

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

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

   In hexidecimal, the mechanism defined header extension being encrypted is (spaces added
   to show the internal structure of the header extension):

     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 this document does
   Section 3.1, but their specific meaning is not
      have any location important for the
   example.)

   In hexidecimal, the corresponding encryption mask selecting the
   bodies of header extensions 1, 2, and 4 (corresponding to insert an additional authentication tag. the mask in
   Figure 2 is:

      00 FFFFFFFFFFFFFFFF 00 000000 00 FF 00 FFFFFFFFFFFFFF 00

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

       Initial keystream:  1E19C8E1D481C779549ED1617AAA1B7A
                           FC0D933AE7ED6CC8
       Mask (Hex):         00FFFFFFFFFFFFFFFF0000000000FF00
                           FFFFFFFFFFFFFF00
       Masked keystream:   0019C8E1D481C7795400000000001B00
                           FC0D933AE7ED6C00
       Plaintext:          17414273A475262748220000C8308E46
                           55996386B395FB00
       Ciphertext:         17588A9270F4E15E1C220000C8309546
                           A994F0BC54789700

Appendix C. B.  Changes From Earlier Versions

   Note to the RFC-Editor: please remove this section prior to
   publication as an RFC.

C.1.

B.1.  Changes from draft-ietf-avtcore -00

   Clarified usage of Key Derivation Algorithm

   Provided non-normative guidance for how to use this mechanism with
   Authenticated Encryption with Associated Data (AEAD) transforms.

   Corrected SMPTE Timecode header extension element in example header
   extension (it's eight bytes, not sixteen).  Added an NTP timestamp to
   the example to fill it back out to original size.

   Specified applicability of the extmap attribute if it's specified as
   a session-level attribute.

   Added description of backward compatibility, including a description
   of how you can negotiate best-effort encryption.

   Added a note to the security considerations about the dangers for
   middleboxes observing unencrypted headers (both header extension
   elements and RTP headers) without being able to verify the
   authentication keys.

   Added test vectors.

   Added acknowledgments section.

B.2.  Changes from draft-lennox-avtcore -00

   o  Published as working group item.
   o  Added discussion of limitations when used with the two-byte-header
      form of header extension elements.
   o  Added open issue about how to use this mechanism with
      Authenticated Encryption with Associated Data (AEAD) transforms.
   o  Updated references.

C.2.

B.3.  Changes from draft-lennox-avt -02

   o  Retargeted at AVTCORE working group.
   o  Updated references.

C.3.

B.4.  Changes From Individual Submission Draft -01

   o  Minor editorial changes.

C.4.

B.5.  Changes From Individual Submission Draft -00

   o  Clarified description of encryption mask creation.
   o  Added example encryption mask.
   o  Editorial changes.

Author's Address

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

   Email: jonathan@vidyo.com