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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5285 (Obsoleted by RFC 8285) == Outdated reference: A later version (-02) exists of draft-ietf-avt-srtp-aes-gcm-01 == Outdated reference: A later version (-06) exists of draft-ietf-avtext-client-to-mixer-audio-level-05 == Outdated reference: A later version (-06) exists of draft-ietf-avtext-mixer-to-client-audio-level-05 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVTCORE J. Lennox 3 Internet-Draft Vidyo 4 Intended status: Standards Track October 28, 2011 5 Expires: April 30, 2012 7 Encryption of Header Extensions in the Secure Real-Time Transport 8 Protocol (SRTP) 9 draft-ietf-avtcore-srtp-encrypted-header-ext-01 11 Abstract 13 The Secure Real-Time Transport Protocol (SRTP) provides 14 authentication, but not encryption, of the headers of Real-Time 15 Transport Protocol (RTP) packets. However, RTP header extensions may 16 carry sensitive information for which participants in multimedia 17 sessions want confidentiality. This document provides a mechanism, 18 extending the mechanisms of SRTP, to selectively encrypt RTP header 19 extensions in SRTP. 21 Status of this Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on April 30, 2012. 38 Copyright Notice 40 Copyright (c) 2011 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 3. Encryption Mechanism . . . . . . . . . . . . . . . . . . . . . 3 58 3.1. Example Encryption Mask . . . . . . . . . . . . . . . . . 5 59 4. Signaling (Setup) Information . . . . . . . . . . . . . . . . 6 60 4.1. Backward compatibility . . . . . . . . . . . . . . . . . . 7 61 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 62 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 63 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 65 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 66 8.2. Informative References . . . . . . . . . . . . . . . . . . 9 67 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10 68 A.1. Key derivation test vectors . . . . . . . . . . . . . . . 10 69 A.2. Header Encryption Test Vectors using AES-CM . . . . . . . 11 70 Appendix B. Changes From Earlier Versions . . . . . . . . . . . . 12 71 B.1. Changes from draft-ietf-avtcore -00 . . . . . . . . . . . 12 72 B.2. Changes from draft-lennox-avtcore -00 . . . . . . . . . . 13 73 B.3. Changes from draft-lennox-avt -02 . . . . . . . . . . . . 13 74 B.4. Changes From Individual Submission Draft -01 . . . . . . . 13 75 B.5. Changes From Individual Submission Draft -00 . . . . . . . 13 76 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 78 1. Introduction 80 The Secure Real-Time Transport Protocol [RFC3711] specification 81 provides confidentiality, message authentication, and replay 82 protection for multimedia payloads sent using of the Real-Time 83 Protocol (RTP) [RFC3550]. However, in order to preserve RTP header 84 compression efficiency, SRTP provides only authentication and replay 85 protection for the headers of RTP packets, not confidentiality. 87 For the standard portions of an RTP header, this does not normally 88 present a problem, as the information carried in an RTP header does 89 not provide much information beyond that which an attacker could 90 infer by observing the size and timing of RTP packets. Thus, there 91 is little need for confidentiality of the header information. 93 However, this is not necessarily true for information carried in RTP 94 header extensions. A number of recent proposals for header 95 extensions using the General Mechanism for RTP Header Extensions 96 [RFC5285] carry information for which confidentiality could be 97 desired or essential. Notably, two recent drafts 98 ([I-D.ietf-avtext-client-to-mixer-audio-level] and 99 [I-D.ietf-avtext-mixer-to-client-audio-level]) carry information 100 about per-packet sound levels of the media data carried in the RTP 101 payload, and exposing this to an eavesdropper may be unacceptable in 102 many circumstances. 104 This document, therefore, defines a mechanism by which encryption can 105 be applied to RTP header extensions when they are transported using 106 SRTP. As an RTP sender may wish some extension information to be 107 sent in the clear (for example, it may be useful for a network 108 monitoring device to be aware of RTP transmission time offsets 109 [RFC5450]), this mechanism can be selectively applied to a subset of 110 the header extension elements carried in an SRTP packet. 112 2. Terminology 114 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 115 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 116 document are to be interpreted as described in RFC 2119 [RFC2119] and 117 indicate requirement levels for compliant implementations. 119 3. Encryption Mechanism 121 Encrypted header extension elements are carried in the same manner as 122 non-encrypted header extension elements, as defined by [RFC5285]. 123 The (one- or two-byte) header of the extension elements is not 124 encrypted, nor is any of the header extension padding. If multiple 125 different header extension elements are being encrypted, they have 126 separate element identifier values, just as they would if they were 127 not encrypted; similarly, encrypted and non-encrypted header 128 extension elements have separate identifier values. 130 Encrypted extension headers are only carried in packets encrypted 131 using the Secure Real-Time Transport Protocol [RFC3711]. To encrypt 132 (or decrypt) encrypted extension headers, an SRTP participant first 133 uses the SRTP Key Derivation Algorithm, specified in Section 4.3.1 of 134 [RFC3711], to generate header encryption and header salting keys, 135 using the same pseudo-random function family as are used for the key 136 derivation for the SRTP session. These keys are derived as follows: 137 o k_he (SRTP header encryption):