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)


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.

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Table of Contents

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.

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.

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   |

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|

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 \
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

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.

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:
Encrypted extension header element

(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.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, July 2008.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.

7.2. Informative References

[RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in RTP Streams", RFC 5450, March 2009.
[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", Internet-Draft draft-ietf-avtext-client-to-mixer-audio-level-06, November 2011.
[RFC4568] Andreasen, F., Baugher, M. and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, July 2006.
[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", Internet-Draft draft-ietf-avtext-mixer-to-client-audio-level-06, November 2011.
[RFC5484] Singer, D., "Associating Time-Codes with RTP Streams", RFC 5484, March 2009.

Appendix A. Test Vectors


Appendix B. Changes From Earlier Versions

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

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

Appendix B.2. Changes From Individual Submission Draft -01

Appendix B.3. Changes From Individual Submission Draft -00

Author's Address

Jonathan Lennox Vidyo, Inc. 433 Hackensack Avenue Sixth Floor Hackensack, NJ 07601 US EMail: jonathan@vidyo.com