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Versions: 00 draft-ietf-avtcore-srtp-encrypted-header-ext

AVTCORE                                                        J. Lennox
Internet-Draft                                                     Vidyo
Intended status: Standards Track                          March 28, 2011
Expires: September 29, 2011


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

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 to cite them other than as "work in progress."

   This Internet-Draft will expire on September 29, 2011.

Copyright Notice

   Copyright (c) 2011 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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Encryption Mechanism  . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Example Encryption Mask . . . . . . . . . . . . . . . . . . 5
   4.  Signaling (Setup) Information . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . . . 8
   Appendix B.  Changes From Earlier Versions  . . . . . . . . . . . . 8
     B.1.  Changes from draft-lennox-avt -02 . . . . . . . . . . . . . 8
     B.2.  Changes From Individual Submission Draft -01  . . . . . . . 8
     B.3.  Changes From Individual Submission Draft -00  . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8




























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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 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



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   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.

   To encrypt (or decrypt) an encrypted extension header, an SRTP
   participant first generates a keystream for the SRTP extension
   header.  This keystream is generated in the same manner as the
   encryption keystream for the corresponding SRTP payload, except the
   the SRTP encryption and salting keys k_e and k_s are replaced by the
   keys k_he and k_hs, respectively.  The keys k_he and k_hs are
   computed in the same manner as k_e and k_s, except that the <label>
   values used are 0x06 for k_he and and 0x07 for k_hs.  (Note that
   since RTP headers, including extension headers, are authenticated in
   SRTP, no new authentication key is needed for extension headers.)

   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 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.







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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, 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|     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   | 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 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|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

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



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   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.


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.

   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.





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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.  References

7.1.  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.  Informative References

   [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 (work in
              progress), March 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-01 (work in
              progress), March 2011.



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   [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.


Appendix A.  Test Vectors

   TODO


Appendix B.  Changes From Earlier Versions

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

B.1.  Changes from draft-lennox-avt -02

   o  Retargeted at AVTCORE working group.
   o  Updated references.

B.2.  Changes From Individual Submission Draft -01

   o  Minor editorial changes.

B.3.  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
   Sixth Floor
   Hackensack, NJ  07601
   US

   Email: jonathan@vidyo.com




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