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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC J. Lennox 3 Internet-Draft Vidyo 4 Intended status: Standards Track March 8, 2010 5 Expires: September 9, 2010 7 Encryption of Header Extensions in the Secure Real-Time Transport 8 Protocol (SRTP) 9 draft-lennox-avt-srtp-encrypted-extension-headers-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 to IETF 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), its areas, and its working groups. Note that 28 other groups may also distribute working documents as Internet- 29 Drafts. 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 The list of current Internet-Drafts can be accessed at 37 http://www.ietf.org/ietf/1id-abstracts.txt. 39 The list of Internet-Draft Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 This Internet-Draft will expire on September 9, 2010. 44 Copyright Notice 46 Copyright (c) 2010 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Encryption Mechanism . . . . . . . . . . . . . . . . . . . . . 3 64 3.1. Example Encryption Mask . . . . . . . . . . . . . . . . . . 4 65 4. Signaling (Setup) Information . . . . . . . . . . . . . . . . . 6 66 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 68 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 69 7.1. Normative References . . . . . . . . . . . . . . . . . . . 7 70 7.2. Informative References . . . . . . . . . . . . . . . . . . 7 71 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . . 8 72 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 74 1. Introduction 76 The Secure Real-Time Transport Protocol [RFC3711] specification 77 provides confidentiality, message authentication, and replay 78 protection for multimedia payloads sent using of the Real-Time 79 Protocol (RTP) [RFC3550]. However, in order to preserve RTP header 80 compression efficiency, SRTP provides only authentication and replay 81 protection for the headers of RTP packets, not confidentiality. 83 For the standard portions of an RTP header, this does not normally 84 present a problem, as the information carried in an RTP header does 85 not provide much information beyond that which an attacker could 86 infer by observing the size and timing of RTP packets. Thus, there 87 is little need for confidentiality of the header information. 89 However, this is not necessarily true for information carried in RTP 90 header extensions. A number of recent proposals for header 91 extensions using the General Mechanism for RTP Header Extensions 92 [RFC5285] carry information for which confidentiality could be 93 desired or essential. Notably, two recent drafts 94 ([I-D.lennox-avt-rtp-audio-level-exthdr] and [I-D.ivov-avt-slic]) 95 carry information about per-packet sound levels of the media data 96 carried in the RTP payload, and exposing this to an eavesdropper may 97 be unacceptable in many circumstances. 99 This document, therefore, defines a mechanism by which encryption can 100 be applied to RTP header extensions when they are transported using 101 SRTP. As an RTP sender may wish some extension information to be 102 sent in the clear (for example, it may be useful for a network 103 monitoring device to be aware of RTP transmission time offsets 104 [RFC5450]), this mechanism can be selectively applied to a subset of 105 the header extension elements carried in an SRTP packet. 107 2. Terminology 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in RFC 2119 [RFC2119] and 112 indicate requirement levels for compliant implementations. 114 3. Encryption Mechanism 116 Encrypted header extension elements are carried in the same manner as 117 non-encrypted header extension elements, as defined by [RFC5285]. 118 The (one- or two-byte) header of the extension elements is not 119 encrypted, nor is any of the header extension padding. If multiple 120 different header extension elements are being encrypted, they have 121 separate element identifier values, just as they would if they were 122 not encrypted; similarly, encrypted and non-encrypted header 123 extension elements have separate identifier values. 125 To encrypt (or decrypt) an encrypted extension header, an SRTP 126 participant first generates a keystream for the SRTP extension 127 header. This keystream is generated in the same manner as the 128 encryption keystream for the corersponding SRTP payload, except the 129 the SRTP encryption and salting keys k_e and k_s are replaced by the 130 keys k_he and k_hs, respectively. The keys k_he and k_hs are 131 computed in the same manner as k_e and k_s, except that the