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Lennox 3 Internet-Draft Vidyo 4 Intended status: Standards Track October 21, 2010 5 Expires: April 24, 2011 7 Encryption of Header Extensions in the Secure Real-Time Transport 8 Protocol (SRTP) 9 draft-lennox-avt-srtp-encrypted-extension-headers-02 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 24, 2011. 38 Copyright Notice 40 Copyright (c) 2010 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 . . . . . . . . . . . . . . . . . . 4 59 4. Signaling (Setup) Information . . . . . . . . . . . . . . . . . 6 60 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 61 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 62 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 7.1. Normative References . . . . . . . . . . . . . . . . . . . 7 64 7.2. Informative References . . . . . . . . . . . . . . . . . . 7 65 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . . 8 66 Appendix B. Changes From Earlier Versions . . . . . . . . . . . . 8 67 B.1. Changes From Individual Submission Draft -01 . . . . . . . 8 68 B.2. Changes From Individual Submission Draft -00 . . . . . . . 8 69 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 71 1. Introduction 73 The Secure Real-Time Transport Protocol [RFC3711] specification 74 provides confidentiality, message authentication, and replay 75 protection for multimedia payloads sent using of the Real-Time 76 Protocol (RTP) [RFC3550]. However, in order to preserve RTP header 77 compression efficiency, SRTP provides only authentication and replay 78 protection for the headers of RTP packets, not confidentiality. 80 For the standard portions of an RTP header, this does not normally 81 present a problem, as the information carried in an RTP header does 82 not provide much information beyond that which an attacker could 83 infer by observing the size and timing of RTP packets. Thus, there 84 is little need for confidentiality of the header information. 86 However, this is not necessarily true for information carried in RTP 87 header extensions. A number of recent proposals for header 88 extensions using the General Mechanism for RTP Header Extensions 89 [RFC5285] carry information for which confidentiality could be 90 desired or essential. Notably, two recent drafts 91 ([I-D.lennox-avt-rtp-audio-level-exthdr] and [I-D.ivov-avt-slic]) 92 carry information about per-packet sound levels of the media data 93 carried in the RTP payload, and exposing this to an eavesdropper may 94 be unacceptable in many circumstances. 96 This document, therefore, defines a mechanism by which encryption can 97 be applied to RTP header extensions when they are transported using 98 SRTP. As an RTP sender may wish some extension information to be 99 sent in the clear (for example, it may be useful for a network 100 monitoring device to be aware of RTP transmission time offsets 101 [RFC5450]), this mechanism can be selectively applied to a subset of 102 the header extension elements carried in an SRTP packet. 104 2. Terminology 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 108 document are to be interpreted as described in RFC 2119 [RFC2119] and 109 indicate requirement levels for compliant implementations. 111 3. Encryption Mechanism 113 Encrypted header extension elements are carried in the same manner as 114 non-encrypted header extension elements, as defined by [RFC5285]. 115 The (one- or two-byte) header of the extension elements is not 116 encrypted, nor is any of the header extension padding. If multiple 117 different header extension elements are being encrypted, they have 118 separate element identifier values, just as they would if they were 119 not encrypted; similarly, encrypted and non-encrypted header 120 extension elements have separate identifier values. 122 To encrypt (or decrypt) an encrypted extension header, an SRTP 123 participant first generates a keystream for the SRTP extension 124 header. This keystream is generated in the same manner as the 125 encryption keystream for the corresponding SRTP payload, except the 126 the SRTP encryption and salting keys k_e and k_s are replaced by the 127 keys k_he and k_hs, respectively. The keys k_he and k_hs are 128 computed in the same manner as k_e and k_s, except that the