| Index: libsrtp/doc/rfc3711.txt
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| +
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| +
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| +
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| +
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| +
|
| +
|
| +Network Working Group M. Baugher
|
| +Request for Comments: 3711 D. McGrew
|
| +Category: Standards Track Cisco Systems, Inc.
|
| + M. Naslund
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| + E. Carrara
|
| + K. Norrman
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| + Ericsson Research
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| + March 2004
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| +
|
| +
|
| + The Secure Real-time Transport Protocol (SRTP)
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| +
|
| +Status of this Memo
|
| +
|
| + This document specifies an Internet standards track protocol for the
|
| + Internet community, and requests discussion and suggestions for
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| + improvements. Please refer to the current edition of the "Internet
|
| + Official Protocol Standards" (STD 1) for the standardization state
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| + and status of this protocol. Distribution of this memo is unlimited.
|
| +
|
| +Copyright Notice
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| +
|
| + Copyright (C) The Internet Society (2004). All Rights Reserved.
|
| +
|
| +Abstract
|
| +
|
| + This document describes the Secure Real-time Transport Protocol
|
| + (SRTP), a profile of the Real-time Transport Protocol (RTP), which
|
| + can provide confidentiality, message authentication, and replay
|
| + protection to the RTP traffic and to the control traffic for RTP, the
|
| + Real-time Transport Control Protocol (RTCP).
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| +
|
| +Table of Contents
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| +
|
| + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
|
| + 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
|
| + 2. Goals and Features . . . . . . . . . . . . . . . . . . . . . . 4
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| + 2.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 5
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| + 3. SRTP Framework . . . . . . . . . . . . . . . . . . . . . . . . 5
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| + 3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 6
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| + 3.2. SRTP Cryptographic Contexts. . . . . . . . . . . . . . . 7
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| + 3.2.1. Transform-independent parameters . . . . . . . . 8
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| + 3.2.2. Transform-dependent parameters . . . . . . . . . 10
|
| + 3.2.3. Mapping SRTP Packets to Cryptographic Contexts . 10
|
| + 3.3. SRTP Packet Processing . . . . . . . . . . . . . . . . . 11
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| + 3.3.1. Packet Index Determination, and ROC, s_l Update. 13
|
| + 3.3.2. Replay Protection. . . . . . . . . . . . . . . . 15
|
| + 3.4. Secure RTCP . . . . . . . . . . . . . . . . . . . . . . . 15
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| +
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| +
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| +
|
| +Baugher, et al. Standards Track [Page 1]
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| +
|
| +RFC 3711 SRTP March 2004
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| +
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| +
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| + 4. Pre-Defined Cryptographic Transforms . . . . . . . . . . . . . 19
|
| + 4.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . 19
|
| + 4.1.1. AES in Counter Mode. . . . . . . . . . . . . . . 21
|
| + 4.1.2. AES in f8-mode . . . . . . . . . . . . . . . . . 22
|
| + 4.1.3. NULL Cipher. . . . . . . . . . . . . . . . . . . 25
|
| + 4.2. Message Authentication and Integrity . . . . . . . . . . 25
|
| + 4.2.1. HMAC-SHA1. . . . . . . . . . . . . . . . . . . . 25
|
| + 4.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 26
|
| + 4.3.1. Key Derivation Algorithm . . . . . . . . . . . . 26
|
| + 4.3.2. SRTCP Key Derivation . . . . . . . . . . . . . . 28
|
| + 4.3.3. AES-CM PRF . . . . . . . . . . . . . . . . . . . 28
|
| + 5. Default and mandatory-to-implement Transforms. . . . . . . . . 28
|
| + 5.1. Encryption: AES-CM and NULL. . . . . . . . . . . . . . . 29
|
| + 5.2. Message Authentication/Integrity: HMAC-SHA1. . . . . . . 29
|
| + 5.3. Key Derivation: AES-CM PRF . . . . . . . . . . . . . . . 29
|
| + 6. Adding SRTP Transforms . . . . . . . . . . . . . . . . . . . . 29
|
| + 7. Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . 30
|
| + 7.1. Key derivation . . . . . . . . . . . . . . . . . . . . . 30
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| + 7.2. Salting key. . . . . . . . . . . . . . . . . . . . . . . 30
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| + 7.3. Message Integrity from Universal Hashing . . . . . . . . 31
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| + 7.4. Data Origin Authentication Considerations. . . . . . . . 31
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| + 7.5. Short and Zero-length Message Authentication . . . . . . 32
|
| + 8. Key Management Considerations. . . . . . . . . . . . . . . . . 33
|
| + 8.1. Re-keying . . . . . . . . . . . . . . . . . . . . . . . 34
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| + 8.1.1. Use of the <From, To> for re-keying. . . . . . . 34
|
| + 8.2. Key Management parameters. . . . . . . . . . . . . . . . 35
|
| + 9. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
|
| + 9.1. SSRC collision and two-time pad. . . . . . . . . . . . . 37
|
| + 9.2. Key Usage. . . . . . . . . . . . . . . . . . . . . . . . 38
|
| + 9.3. Confidentiality of the RTP Payload . . . . . . . . . . . 39
|
| + 9.4. Confidentiality of the RTP Header. . . . . . . . . . . . 40
|
| + 9.5. Integrity of the RTP payload and header. . . . . . . . . 40
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| + 9.5.1. Risks of Weak or Null Message Authentication. . . 42
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| + 9.5.2. Implicit Header Authentication . . . . . . . . . 43
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| + 10. Interaction with Forward Error Correction mechanisms. . . . . 43
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| + 11. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 43
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| + 11.1. Unicast. . . . . . . . . . . . . . . . . . . . . . . . . 43
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| + 11.2. Multicast (one sender) . . . . . . . . . . . . . . . . . 44
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| + 11.3. Re-keying and access control . . . . . . . . . . . . . . 45
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| + 11.4. Summary of basic scenarios . . . . . . . . . . . . . . . 46
|
| + 12. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 46
|
| + 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
|
| + 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
|
| + 14.1. Normative References . . . . . . . . . . . . . . . . . . 47
|
| + 14.2. Informative References . . . . . . . . . . . . . . . . . 48
|
| + Appendix A: Pseudocode for Index Determination . . . . . . . . . . 51
|
| + Appendix B: Test Vectors . . . . . . . . . . . . . . . . . . . . . 51
|
| + B.1. AES-f8 Test Vectors. . . . . . . . . . . . . . . . . . . 51
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| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 2]
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| +
|
| +RFC 3711 SRTP March 2004
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| +
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| +
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| + B.2. AES-CM Test Vectors. . . . . . . . . . . . . . . . . . . 52
|
| + B.3. Key Derivation Test Vectors. . . . . . . . . . . . . . . 53
|
| + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55
|
| + Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 56
|
| +
|
| +1. Introduction
|
| +
|
| + This document describes the Secure Real-time Transport Protocol
|
| + (SRTP), a profile of the Real-time Transport Protocol (RTP), which
|
| + can provide confidentiality, message authentication, and replay
|
| + protection to the RTP traffic and to the control traffic for RTP,
|
| + RTCP (the Real-time Transport Control Protocol) [RFC3350].
|
| +
|
| + SRTP provides a framework for encryption and message authentication
|
| + of RTP and RTCP streams (Section 3). SRTP defines a set of default
|
| + cryptographic transforms (Sections 4 and 5), and it allows new
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| + transforms to be introduced in the future (Section 6). With
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| + appropriate key management (Sections 7 and 8), SRTP is secure
|
| + (Sections 9) for unicast and multicast RTP applications (Section 11).
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| +
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| + SRTP can achieve high throughput and low packet expansion. SRTP
|
| + proves to be a suitable protection for heterogeneous environments
|
| + (mix of wired and wireless networks). To get such features, default
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| + transforms are described, based on an additive stream cipher for
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| + encryption, a keyed-hash based function for message authentication,
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| + and an "implicit" index for sequencing/synchronization based on the
|
| + RTP sequence number for SRTP and an index number for Secure RTCP
|
| + (SRTCP).
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| +
|
| +1.1. Notational Conventions
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| +
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| + The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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| + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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| + document are to be interpreted as described in [RFC2119]. The
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| + terminology conforms to [RFC2828] with the following exception. For
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| + simplicity we use the term "random" throughout the document to denote
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| + randomly or pseudo-randomly generated values. Large amounts of
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| + random bits may be difficult to obtain, and for the security of SRTP,
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| + pseudo-randomness is sufficient [RFC1750].
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| +
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| + By convention, the adopted representation is the network byte order,
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| + i.e., the left most bit (octet) is the most significant one. By XOR
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| + we mean bitwise addition modulo 2 of binary strings, and || denotes
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| + concatenation. In other words, if C = A || B, then the most
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| + significant bits of C are the bits of A, and the least significant
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| + bits of C equal the bits of B. Hexadecimal numbers are prefixed by
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| + 0x.
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| +
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| +
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| +
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| +
|
| +Baugher, et al. Standards Track [Page 3]
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| +
|
| +RFC 3711 SRTP March 2004
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| +
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| +
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| + The word "encryption" includes also use of the NULL algorithm (which
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| + in practice does leave the data in the clear).
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| +
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| + With slight abuse of notation, we use the terms "message
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| + authentication" and "authentication tag" as is common practice, even
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| + though in some circumstances, e.g., group communication, the service
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| + provided is actually only integrity protection and not data origin
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| + authentication.
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| +
|
| +2. Goals and Features
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| +
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| + The security goals for SRTP are to ensure:
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| +
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| + * the confidentiality of the RTP and RTCP payloads, and
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| +
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| + * the integrity of the entire RTP and RTCP packets, together with
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| + protection against replayed packets.
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| +
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| + These security services are optional and independent from each other,
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| + except that SRTCP integrity protection is mandatory (malicious or
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| + erroneous alteration of RTCP messages could otherwise disrupt the
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| + processing of the RTP stream).
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| +
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| + Other, functional, goals for the protocol are:
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| +
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| + * a framework that permits upgrading with new cryptographic
|
| + transforms,
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| +
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| + * low bandwidth cost, i.e., a framework preserving RTP header
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| + compression efficiency,
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| +
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| + and, asserted by the pre-defined transforms:
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| +
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| + * a low computational cost,
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| +
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| + * a small footprint (i.e., small code size and data memory for
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| + keying information and replay lists),
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| +
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| + * limited packet expansion to support the bandwidth economy goal,
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| +
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| + * independence from the underlying transport, network, and physical
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| + layers used by RTP, in particular high tolerance to packet loss
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| + and re-ordering.
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| +
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| + These properties ensure that SRTP is a suitable protection scheme for
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| + RTP/RTCP in both wired and wireless scenarios.
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| +
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| +
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| +
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| +
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| +
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| +Baugher, et al. Standards Track [Page 4]
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| +
|
| +RFC 3711 SRTP March 2004
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| +
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| +
|
| +2.1. Features
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| +
|
| + Besides the above mentioned direct goals, SRTP provides for some
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| + additional features. They have been introduced to lighten the burden
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| + on key management and to further increase security. They include:
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| +
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| + * A single "master key" can provide keying material for
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| + confidentiality and integrity protection, both for the SRTP stream
|
| + and the corresponding SRTCP stream. This is achieved with a key
|
| + derivation function (see Section 4.3), providing "session keys"
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| + for the respective security primitive, securely derived from the
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| + master key.
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| +
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| + * In addition, the key derivation can be configured to periodically
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| + refresh the session keys, which limits the amount of ciphertext
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| + produced by a fixed key, available for an adversary to
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| + cryptanalyze.
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| +
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| + * "Salting keys" are used to protect against pre-computation and
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| + time-memory tradeoff attacks [MF00] [BS00].
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| +
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| + Detailed rationale for these features can be found in Section 7.
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| +
|
| +3. SRTP Framework
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| +
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| + RTP is the Real-time Transport Protocol [RFC3550]. We define SRTP as
|
| + a profile of RTP. This profile is an extension to the RTP
|
| + Audio/Video Profile [RFC3551]. Except where explicitly noted, all
|
| + aspects of that profile apply, with the addition of the SRTP security
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| + features. Conceptually, we consider SRTP to be a "bump in the stack"
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| + implementation which resides between the RTP application and the
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| + transport layer. SRTP intercepts RTP packets and then forwards an
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| + equivalent SRTP packet on the sending side, and intercepts SRTP
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| + packets and passes an equivalent RTP packet up the stack on the
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| + receiving side.
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| +
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| + Secure RTCP (SRTCP) provides the same security services to RTCP as
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| + SRTP does to RTP. SRTCP message authentication is MANDATORY and
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| + thereby protects the RTCP fields to keep track of membership, provide
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| + feedback to RTP senders, or maintain packet sequence counters. SRTCP
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| + is described in Section 3.4.
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +Baugher, et al. Standards Track [Page 5]
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| +
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| +RFC 3711 SRTP March 2004
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| +
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| +
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| +3.1. Secure RTP
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| +
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| + The format of an SRTP packet is illustrated in Figure 1.
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| +
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| + 0 1 2 3
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| + 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
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| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
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| + |V=2|P|X| CC |M| PT | sequence number | |
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| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | timestamp | |
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| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | synchronization source (SSRC) identifier | |
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| + +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
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| + | contributing source (CSRC) identifiers | |
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| + | .... | |
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| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | RTP extension (OPTIONAL) | |
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| + +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | | payload ... | |
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| + | | +-------------------------------+ |
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| + | | | RTP padding | RTP pad count | |
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| + +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
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| + | ~ SRTP MKI (OPTIONAL) ~ |
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| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | : authentication tag (RECOMMENDED) : |
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| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| + | |
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| + +- Encrypted Portion* Authenticated Portion ---+
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| +
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| + Figure 1. The format of an SRTP packet. *Encrypted Portion is the
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| + same size as the plaintext for the Section 4 pre-defined transforms.
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| +
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| + The "Encrypted Portion" of an SRTP packet consists of the encryption
|
| + of the RTP payload (including RTP padding when present) of the
|
| + equivalent RTP packet. The Encrypted Portion MAY be the exact size
|
| + of the plaintext or MAY be larger. Figure 1 shows the RTP payload
|
| + including any possible padding for RTP [RFC3550].
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| +
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| + None of the pre-defined encryption transforms uses any padding; for
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| + these, the RTP and SRTP payload sizes match exactly. New transforms
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| + added to SRTP (following Section 6) may require padding, and may
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| + hence produce larger payloads. RTP provides its own padding format
|
| + (as seen in Fig. 1), which due to the padding indicator in the RTP
|
| + header has merits in terms of compactness relative to paddings using
|
| + prefix-free codes. This RTP padding SHALL be the default method for
|
| + transforms requiring padding. Transforms MAY specify other padding
|
| + methods, and MUST then specify the amount, format, and processing of
|
| + their padding. It is important to note that encryption transforms
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| +
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| +
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| +
|
| +Baugher, et al. Standards Track [Page 6]
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| +
|
| +RFC 3711 SRTP March 2004
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| +
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| +
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| + that use padding are vulnerable to subtle attacks, especially when
|
| + message authentication is not used [V02]. Each specification for a
|
| + new encryption transform needs to carefully consider and describe the
|
| + security implications of the padding that it uses. Message
|
| + authentication codes define their own padding, so this default does
|
| + not apply to authentication transforms.
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| +
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| + The OPTIONAL MKI and the RECOMMENDED authentication tag are the only
|
| + fields defined by SRTP that are not in RTP. Only 8-bit alignment is
|
| + assumed.
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| +
|
| + MKI (Master Key Identifier): configurable length, OPTIONAL. The
|
| + MKI is defined, signaled, and used by key management. The
|
| + MKI identifies the master key from which the session
|
| + key(s) were derived that authenticate and/or encrypt the
|
| + particular packet. Note that the MKI SHALL NOT identify
|
| + the SRTP cryptographic context, which is identified
|
| + according to Section 3.2.3. The MKI MAY be used by key
|
| + management for the purposes of re-keying, identifying a
|
| + particular master key within the cryptographic context
|
| + (Section 3.2.1).
|
| +
|
| + Authentication tag: configurable length, RECOMMENDED. The
|
| + authentication tag is used to carry message authentication
|
| + data. The Authenticated Portion of an SRTP packet
|
| + consists of the RTP header followed by the Encrypted
|
| + Portion of the SRTP packet. Thus, if both encryption and
|
| + authentication are applied, encryption SHALL be applied
|
| + before authentication on the sender side and conversely on
|
| + the receiver side. The authentication tag provides
|
| + authentication of the RTP header and payload, and it
|
| + indirectly provides replay protection by authenticating
|
| + the sequence number. Note that the MKI is not integrity
|
| + protected as this does not provide any extra protection.
|
| +
|
| +3.2. SRTP Cryptographic Contexts
|
| +
|
| + Each SRTP stream requires the sender and receiver to maintain
|
| + cryptographic state information. This information is called the
|
| + "cryptographic context".
|
| +
|
| + SRTP uses two types of keys: session keys and master keys. By a
|
| + "session key", we mean a key which is used directly in a
|
| + cryptographic transform (e.g., encryption or message authentication),
|
| + and by a "master key", we mean a random bit string (given by the key
|
| + management protocol) from which session keys are derived in a
|
| +
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| +
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| +
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| +
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| +
|
| +Baugher, et al. Standards Track [Page 7]
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| +
|
| +RFC 3711 SRTP March 2004
|
| +
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| +
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| + cryptographically secure way. The master key(s) and other parameters
|
| + in the cryptographic context are provided by key management
|
| + mechanisms external to SRTP, see Section 8.
|
| +
|
| +3.2.1. Transform-independent parameters
|
| +
|
| + Transform-independent parameters are present in the cryptographic
|
| + context independently of the particular encryption or authentication
|
| + transforms that are used. The transform-independent parameters of
|
| + the cryptographic context for SRTP consist of:
|
| +
|
| + * a 32-bit unsigned rollover counter (ROC), which records how many
|
| + times the 16-bit RTP sequence number has been reset to zero after
|
| + passing through 65,535. Unlike the sequence number (SEQ), which
|
| + SRTP extracts from the RTP packet header, the ROC is maintained by
|
| + SRTP as described in Section 3.3.1.
|
| +
|
| + We define the index of the SRTP packet corresponding to a given
|
| + ROC and RTP sequence number to be the 48-bit quantity
|
| +
|
| + i = 2^16 * ROC + SEQ.
|
| +
|
| + * for the receiver only, a 16-bit sequence number s_l, which can be
|
| + thought of as the highest received RTP sequence number (see
|
| + Section 3.3.1 for its handling), which SHOULD be authenticated
|
| + since message authentication is RECOMMENDED,
|
| +
|
| + * an identifier for the encryption algorithm, i.e., the cipher and
|
| + its mode of operation,
|
| +
|
| + * an identifier for the message authentication algorithm,
|
| +
|
| + * a replay list, maintained by the receiver only (when
|
| + authentication and replay protection are provided), containing
|
| + indices of recently received and authenticated SRTP packets,
|
| +
|
| + * an MKI indicator (0/1) as to whether an MKI is present in SRTP and
|
| + SRTCP packets,
|
| +
|
| + * if the MKI indicator is set to one, the length (in octets) of the
|
| + MKI field, and (for the sender) the actual value of the currently
|
| + active MKI (the value of the MKI indicator and length MUST be kept
|
| + fixed for the lifetime of the context),
|
| +
|
| + * the master key(s), which MUST be random and kept secret,
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 8]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + * for each master key, there is a counter of the number of SRTP
|
| + packets that have been processed (sent) with that master key
|
| + (essential for security, see Sections 3.3.1 and 9),
|
| +
|
| + * non-negative integers n_e, and n_a, determining the length of the
|
| + session keys for encryption, and message authentication.
|
| +
|
| + In addition, for each master key, an SRTP stream MAY use the
|
| + following associated values:
|
| +
|
| + * a master salt, to be used in the key derivation of session keys.
|
| + This value, when used, MUST be random, but MAY be public. Use of
|
| + master salt is strongly RECOMMENDED, see Section 9.2. A "NULL"
|
| + salt is treated as 00...0.
|
| +
|
| + * an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate",
|
| + where an unspecified value is treated as zero. The constraint to
|
| + be a power of 2 simplifies the session-key derivation
|
| + implementation, see Section 4.3.
|
| +
|
| + * an MKI value,
|
| +
|
| + * <From, To> values, specifying the lifetime for a master key,
|
| + expressed in terms of the two 48-bit index values inside whose
|
| + range (including the range end-points) the master key is valid.
|
| + For the use of <From, To>, see Section 8.1.1. <From, To> is an
|
| + alternative to the MKI and assumes that a master key is in one-
|
| + to-one correspondence with the SRTP session key on which the
|
| + <From, To> range is defined.
|
| +
|
| + SRTCP SHALL by default share the crypto context with SRTP, except:
|
| +
|
| + * no rollover counter and s_l-value need to be maintained as the
|
| + RTCP index is explicitly carried in each SRTCP packet,
|
| +
|
| + * a separate replay list is maintained (when replay protection is
|
| + provided),
|
| +
|
| + * SRTCP maintains a separate counter for its master key (even if the
|
| + master key is the same as that for SRTP, see below), as a means to
|
| + maintain a count of the number of SRTCP packets that have been
|
| + processed with that key.
|
| +
|
| + Note in particular that the master key(s) MAY be shared between SRTP
|
| + and the corresponding SRTCP, if the pre-defined transforms (including
|
| + the key derivation) are used but the session key(s) MUST NOT be so
|
| + shared.
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 9]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + In addition, there can be cases (see Sections 8 and 9.1) where
|
| + several SRTP streams within a given RTP session, identified by their
|
| + synchronization source (SSRCs, which is part of the RTP header),
|
| + share most of the crypto context parameters (including possibly
|
| + master and session keys). In such cases, just as in the normal
|
| + SRTP/SRTCP parameter sharing above, separate replay lists and packet
|
| + counters for each stream (SSRC) MUST still be maintained. Also,
|
| + separate SRTP indices MUST then be maintained.
|
| +
|
| + A summary of parameters, pre-defined transforms, and default values
|
| + for the above parameters (and other SRTP parameters) can be found in
|
| + Sections 5 and 8.2.
|
| +
|
| +3.2.2. Transform-dependent parameters
|
| +
|
| + All encryption, authentication/integrity, and key derivation
|
| + parameters are defined in the transforms section (Section 4).
|
| + Typical examples of such parameters are block size of ciphers,
|
| + session keys, data for the Initialization Vector (IV) formation, etc.
|
| + Future SRTP transform specifications MUST include a section to list
|
| + the additional cryptographic context's parameters for that transform,
|
| + if any.
|
| +
|
| +3.2.3. Mapping SRTP Packets to Cryptographic Contexts
|
| +
|
| + Recall that an RTP session for each participant is defined [RFC3550]
|
| + by a pair of destination transport addresses (one network address
|
| + plus a port pair for RTP and RTCP), and that a multimedia session is
|
| + defined as a collection of RTP sessions. For example, a particular
|
| + multimedia session could include an audio RTP session, a video RTP
|
| + session, and a text RTP session.
|
| +
|
| + A cryptographic context SHALL be uniquely identified by the triplet
|
| + context identifier:
|
| +
|
| + context id = <SSRC, destination network address, destination
|
| + transport port number>
|
| +
|
| + where the destination network address and the destination transport
|
| + port are the ones in the SRTP packet. It is assumed that, when
|
| + presented with this information, the key management returns a context
|
| + with the information as described in Section 3.2.
|
| +
|
| + As noted above, SRTP and SRTCP by default share the bulk of the
|
| + parameters in the cryptographic context. Thus, retrieving the crypto
|
| + context parameters for an SRTCP stream in practice may imply a
|
| + binding to the correspondent SRTP crypto context. It is up to the
|
| + implementation to assure such binding, since the RTCP port may not be
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 10]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + directly deducible from the RTP port only. Alternatively, the key
|
| + management may choose to provide separate SRTP- and SRTCP- contexts,
|
| + duplicating the common parameters (such as master key(s)). The
|
| + latter approach then also enables SRTP and SRTCP to use, e.g.,
|
| + distinct transforms, if so desired. Similar considerations arise
|
| + when multiple SRTP streams, forming part of one single RTP session,
|
| + share keys and other parameters.
|
| +
|
| + If no valid context can be found for a packet corresponding to a
|
| + certain context identifier, that packet MUST be discarded.
|
| +
|
| +3.3. SRTP Packet Processing
|
| +
|
| + The following applies to SRTP. SRTCP is described in Section 3.4.
|
| +
|
| + Assuming initialization of the cryptographic context(s) has taken
|
| + place via key management, the sender SHALL do the following to
|
| + construct an SRTP packet:
|
| +
|
| + 1. Determine which cryptographic context to use as described in
|
| + Section 3.2.3.
|
| +
|
| + 2. Determine the index of the SRTP packet using the rollover counter,
|
| + the highest sequence number in the cryptographic context, and the
|
| + sequence number in the RTP packet, as described in Section 3.3.1.
|
| +
|
| + 3. Determine the master key and master salt. This is done using the
|
| + index determined in the previous step or the current MKI in the
|
| + cryptographic context, according to Section 8.1.
|
| +
|
| + 4. Determine the session keys and session salt (if they are used by
|
| + the transform) as described in Section 4.3, using master key,
|
| + master salt, key_derivation_rate, and session key-lengths in the
|
| + cryptographic context with the index, determined in Steps 2 and 3.
|
| +
|
| + 5. Encrypt the RTP payload to produce the Encrypted Portion of the
|
| + packet (see Section 4.1, for the defined ciphers). This step uses
|
| + the encryption algorithm indicated in the cryptographic context,
|
| + the session encryption key and the session salt (if used) found in
|
| + Step 4 together with the index found in Step 2.
|
| +
|
| + 6. If the MKI indicator is set to one, append the MKI to the packet.
|
| +
|
| + 7. For message authentication, compute the authentication tag for the
|
| + Authenticated Portion of the packet, as described in Section 4.2.
|
| + This step uses the current rollover counter, the authentication
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 11]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + algorithm indicated in the cryptographic context, and the session
|
| + authentication key found in Step 4. Append the authentication tag
|
| + to the packet.
|
| +
|
| + 8. If necessary, update the ROC as in Section 3.3.1, using the packet
|
| + index determined in Step 2.
|
| +
|
| + To authenticate and decrypt an SRTP packet, the receiver SHALL do the
|
| + following:
|
| +
|
| + 1. Determine which cryptographic context to use as described in
|
| + Section 3.2.3.
|
| +
|
| + 2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
|
| + packet. The algorithm uses the rollover counter and highest
|
| + sequence number in the cryptographic context with the sequence
|
| + number in the SRTP packet, as described in Section 3.3.1.
|
| +
|
| + 3. Determine the master key and master salt. If the MKI indicator in
|
| + the context is set to one, use the MKI in the SRTP packet,
|
| + otherwise use the index from the previous step, according to
|
| + Section 8.1.
|
| +
|
| + 4. Determine the session keys, and session salt (if used by the
|
| + transform) as described in Section 4.3, using master key, master
|
| + salt, key_derivation_rate and session key-lengths in the
|
| + cryptographic context with the index, determined in Steps 2 and 3.
|
| +
|
| + 5. For message authentication and replay protection, first check if
|
| + the packet has been replayed (Section 3.3.2), using the Replay
|
| + List and the index as determined in Step 2. If the packet is
|
| + judged to be replayed, then the packet MUST be discarded, and the
|
| + event SHOULD be logged.
|
| +
|
| + Next, perform verification of the authentication tag, using the
|
| + rollover counter from Step 2, the authentication algorithm
|
| + indicated in the cryptographic context, and the session
|
| + authentication key from Step 4. If the result is "AUTHENTICATION
|
| + FAILURE" (see Section 4.2), the packet MUST be discarded from
|
| + further processing and the event SHOULD be logged.
|
| +
|
| + 6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
|
| + the defined ciphers), using the decryption algorithm indicated in
|
| + the cryptographic context, the session encryption key and salt (if
|
| + used) found in Step 4 with the index from Step 2.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 12]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + 7. Update the rollover counter and highest sequence number, s_l, in
|
| + the cryptographic context as in Section 3.3.1, using the packet
|
| + index estimated in Step 2. If replay protection is provided, also
|
| + update the Replay List as described in Section 3.3.2.
|
| +
|
| + 8. When present, remove the MKI and authentication tag fields from
|
| + the packet.
|
| +
|
| +3.3.1. Packet Index Determination, and ROC, s_l Update
|
| +
|
| + SRTP implementations use an "implicit" packet index for sequencing,
|
| + i.e., not all of the index is explicitly carried in the SRTP packet.
|
| + For the pre-defined transforms, the index i is used in replay
|
| + protection (Section 3.3.2), encryption (Section 4.1), message
|
| + authentication (Section 4.2), and for the key derivation (Section
|
| + 4.3).
|
| +
|
| + When the session starts, the sender side MUST set the rollover
|
| + counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps
|
| + modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
|
| + (see security aspects below). The sender's packet index is then
|
| + defined as
|
| +
|
| + i = 2^16 * ROC + SEQ.
|
| +
|
| + Receiver-side implementations use the RTP sequence number to
|
| + determine the correct index of a packet, which is the location of the
|
| + packet in the sequence of all SRTP packets. A robust approach for
|
| + the proper use of a rollover counter requires its handling and use to
|
| + be well defined. In particular, out-of-order RTP packets with
|
| + sequence numbers close to 2^16 or zero must be properly handled.
|
| +
|
| + The index estimate is based on the receiver's locally maintained ROC
|
| + and s_l values. At the setup of the session, the ROC MUST be set to
|
| + zero. Receivers joining an on-going session MUST be given the
|
| + current ROC value using out-of-band signaling such as key-management
|
| + signaling. Furthermore, the receiver SHALL initialize s_l to the RTP
|
| + sequence number (SEQ) of the first observed SRTP packet (unless the
|
| + initial value is provided by out of band signaling such as key
|
| + management).
|
| +
|
| + On consecutive SRTP packets, the receiver SHOULD estimate the index
|
| + as
|
| + i = 2^16 * v + SEQ,
|
| +
|
| + where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
|
| + such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
|
| + + s_l (see Appendix A for pseudocode).
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 13]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + After the packet has been processed and authenticated (when enabled
|
| + for SRTP packets for the session), the receiver MUST use v to
|
| + conditionally update its s_l and ROC variables as follows. If
|
| + v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC,
|
| + then s_l is set to SEQ if and only if SEQ is larger than the current
|
| + s_l; there is no change to ROC. If v=(ROC+1) mod 2^32, then s_l is
|
| + set to SEQ and ROC is set to v.
|
| +
|
| + After a re-keying occurs (changing to a new master key), the rollover
|
| + counter always maintains its sequence of values, i.e., it MUST NOT be
|
| + reset to zero.
|
| +
|
| + As the rollover counter is 32 bits long and the sequence number is 16
|
| + bits long, the maximum number of packets belonging to a given SRTP
|
| + stream that can be secured with the same key is 2^48 using the pre-
|
| + defined transforms. After that number of SRTP packets have been sent
|
| + with a given (master or session) key, the sender MUST NOT send any
|
| + more packets with that key. (There exists a similar limit for SRTCP,
|
| + which in practice may be more restrictive, see Section 9.2.) This
|
| + limitation enforces a security benefit by providing an upper bound on
|
| + the amount of traffic that can pass before cryptographic keys are
|
| + changed. Re-keying (see Section 8.1) MUST be triggered, before this
|
| + amount of traffic, and MAY be triggered earlier, e.g., for increased
|
| + security and access control to media. Recurring key derivation by
|
| + means of a non-zero key_derivation_rate (see Section 4.3), also gives
|
| + stronger security but does not change the above absolute maximum
|
| + value.
|
| +
|
| + On the receiver side, there is a caveat to updating s_l and ROC: if
|
| + message authentication is not present, neither the initialization of
|
| + s_l, nor the ROC update can be made completely robust. The
|
| + receiver's "implicit index" approach works for the pre-defined
|
| + transforms as long as the reorder and loss of the packets are not too
|
| + great and bit-errors do not occur in unfortunate ways. In
|
| + particular, 2^15 packets would need to be lost, or a packet would
|
| + need to be 2^15 packets out of sequence before synchronization is
|
| + lost. Such drastic loss or reorder is likely to disrupt the RTP
|
| + application itself.
|
| +
|
| + The algorithm for the index estimate and ROC update is a matter of
|
| + implementation, and should take into consideration the environment
|
| + (e.g., packet loss rate) and the cases when synchronization is likely
|
| + to be lost, e.g., when the initial sequence number (randomly chosen
|
| + by RTP) is not known in advance (not sent in the key management
|
| + protocol) but may be near to wrap modulo 2^16.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 14]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + A more elaborate and more robust scheme than the one given above is
|
| + the handling of RTP's own "rollover counter", see Appendix A.1 of
|
| + [RFC3550].
|
| +
|
| +3.3.2. Replay Protection
|
| +
|
| + Secure replay protection is only possible when integrity protection
|
| + is present. It is RECOMMENDED to use replay protection, both for RTP
|
| + and RTCP, as integrity protection alone cannot assure security
|
| + against replay attacks.
|
| +
|
| + A packet is "replayed" when it is stored by an adversary, and then
|
| + re-injected into the network. When message authentication is
|
| + provided, SRTP protects against such attacks through a Replay List.
|
| + Each SRTP receiver maintains a Replay List, which conceptually
|
| + contains the indices of all of the packets which have been received
|
| + and authenticated. In practice, the list can use a "sliding window"
|
| + approach, so that a fixed amount of storage suffices for replay
|
| + protection. Packet indices which lag behind the packet index in the
|
| + context by more than SRTP-WINDOW-SIZE can be assumed to have been
|
| + received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
|
| + dependent parameter and MUST be at least 64, but which MAY be set to
|
| + a higher value.
|
| +
|
| + The receiver checks the index of an incoming packet against the
|
| + replay list and the window. Only packets with index ahead of the
|
| + window, or, inside the window but not already received, SHALL be
|
| + accepted.
|
| +
|
| + After the packet has been authenticated (if necessary the window is
|
| + first moved ahead), the replay list SHALL be updated with the new
|
| + index.
|
| +
|
| + The Replay List can be efficiently implemented by using a bitmap to
|
| + represent which packets have been received, as described in the
|
| + Security Architecture for IP [RFC2401].
|
| +
|
| +3.4. Secure RTCP
|
| +
|
| + Secure RTCP follows the definition of Secure RTP. SRTCP adds three
|
| + mandatory new fields (the SRTCP index, an "encrypt-flag", and the
|
| + authentication tag) and one optional field (the MKI) to the RTCP
|
| + packet definition. The three mandatory fields MUST be appended to an
|
| + RTCP packet in order to form an equivalent SRTCP packet. The added
|
| + fields follow any other profile-specific extensions.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 15]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + According to Section 6.1 of [RFC3550], there is a REQUIRED packet
|
| + format for compound packets. SRTCP MUST be given packets according
|
| + to that requirement in the sense that the first part MUST be a sender
|
| + report or a receiver report. However, the RTCP encryption prefix (a
|
| + random 32-bit quantity) specified in that Section MUST NOT be used
|
| + since, as is stated there, it is only applicable to the encryption
|
| + method specified in [RFC3550] and is not needed by the cryptographic
|
| + mechanisms used in SRTP.
|
| +
|
| + 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
|
| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|
| + |V=2|P| RC | PT=SR or RR | length | |
|
| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | SSRC of sender | |
|
| + +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
|
| + | ~ sender info ~ |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | ~ report block 1 ~ |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | ~ report block 2 ~ |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | ~ ... ~ |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | |V=2|P| SC | PT=SDES=202 | length | |
|
| + | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
|
| + | | SSRC/CSRC_1 | |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | ~ SDES items ~ |
|
| + | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
|
| + | ~ ... ~ |
|
| + +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
|
| + | |E| SRTCP index | |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|
| + | ~ SRTCP MKI (OPTIONAL) ~ |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | : authentication tag : |
|
| + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|
| + | |
|
| + +-- Encrypted Portion Authenticated Portion -----+
|
| +
|
| +
|
| + Figure 2. An example of the format of a Secure RTCP packet,
|
| + consisting of an underlying RTCP compound packet with a Sender Report
|
| + and SDES packet.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 16]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + The Encrypted Portion of an SRTCP packet consists of the encryption
|
| + (Section 4.1) of the RTCP payload of the equivalent compound RTCP
|
| + packet, from the first RTCP packet, i.e., from the ninth (9) octet to
|
| + the end of the compound packet. The Authenticated Portion of an
|
| + SRTCP packet consists of the entire equivalent (eventually compound)
|
| + RTCP packet, the E flag, and the SRTCP index (after any encryption
|
| + has been applied to the payload).
|
| +
|
| + The added fields are:
|
| +
|
| + E-flag: 1 bit, REQUIRED
|
| + The E-flag indicates if the current SRTCP packet is
|
| + encrypted or unencrypted. Section 9.1 of [RFC3550] allows
|
| + the split of a compound RTCP packet into two lower-layer
|
| + packets, one to be encrypted and one to be sent in the
|
| + clear. The E bit set to "1" indicates encrypted packet, and
|
| + "0" indicates non-encrypted packet.
|
| +
|
| + SRTCP index: 31 bits, REQUIRED
|
| + The SRTCP index is a 31-bit counter for the SRTCP packet.
|
| + The index is explicitly included in each packet, in contrast
|
| + to the "implicit" index approach used for SRTP. The SRTCP
|
| + index MUST be set to zero before the first SRTCP packet is
|
| + sent, and MUST be incremented by one, modulo 2^31, after
|
| + each SRTCP packet is sent. In particular, after a re-key,
|
| + the SRTCP index MUST NOT be reset to zero again.
|
| +
|
| + Authentication Tag: configurable length, REQUIRED
|
| + The authentication tag is used to carry message
|
| + authentication data.
|
| +
|
| + MKI: configurable length, OPTIONAL
|
| + The MKI is the Master Key Indicator, and functions according
|
| + to the MKI definition in Section 3.
|
| +
|
| + SRTCP uses the cryptographic context parameters and packet processing
|
| + of SRTP by default, with the following changes:
|
| +
|
| + * The receiver does not need to "estimate" the index, as it is
|
| + explicitly signaled in the packet.
|
| +
|
| + * Pre-defined SRTCP encryption is as specified in Section 4.1, but
|
| + using the definition of the SRTCP Encrypted Portion given in this
|
| + section, and using the SRTCP index as the index i. The encryption
|
| + transform and related parameters SHALL by default be the same
|
| + selected for the protection of the associated SRTP stream(s),
|
| + while the NULL algorithm SHALL be applied to the RTCP packets not
|
| + to be encrypted. SRTCP may have a different encryption transform
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 17]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + than the one used by the corresponding SRTP. The expected use for
|
| + this feature is when the former has NULL-encryption and the latter
|
| + has a non NULL-encryption.
|
| +
|
| + The E-flag is assigned a value by the sender depending on whether the
|
| + packet was encrypted or not.
|
| +
|
| + * SRTCP decryption is performed as in Section 4, but only if the E
|
| + flag is equal to 1. If so, the Encrypted Portion is decrypted,
|
| + using the SRTCP index as the index i. In case the E-flag is 0,
|
| + the payload is simply left unmodified.
|
| +
|
| + * SRTCP replay protection is as defined in Section 3.3.2, but using
|
| + the SRTCP index as the index i and a separate Replay List that is
|
| + specific to SRTCP.
|
| +
|
| + * The pre-defined SRTCP authentication tag is specified as in
|
| + Section 4.2, but with the Authenticated Portion of the SRTCP
|
| + packet given in this section (which includes the index). The
|
| + authentication transform and related parameters (e.g., key size)
|
| + SHALL by default be the same as selected for the protection of the
|
| + associated SRTP stream(s).
|
| +
|
| + * In the last step of the processing, only the sender needs to
|
| + update the value of the SRTCP index by incrementing it modulo 2^31
|
| + and for security reasons the sender MUST also check the number of
|
| + SRTCP packets processed, see Section 9.2.
|
| +
|
| + Message authentication for RTCP is REQUIRED, as it is the control
|
| + protocol (e.g., it has a BYE packet) for RTP.
|
| +
|
| + Precautions must be taken so that the packet expansion in SRTCP (due
|
| + to the added fields) does not cause SRTCP messages to use more than
|
| + their share of RTCP bandwidth. To avoid this, the following two
|
| + measures MUST be taken:
|
| +
|
| + 1. When initializing the RTCP variable "avg_rtcp_size" defined in
|
| + chapter 6.3 of [RFC3550], it MUST include the size of the fields
|
| + that will be added by SRTCP (index, E-bit, authentication tag, and
|
| + when present, the MKI).
|
| +
|
| + 2. When updating the "avg_rtcp_size" using the variable "packet_size"
|
| + (section 6.3.3 of [RFC3550]), the value of "packet_size" MUST
|
| + include the size of the additional fields added by SRTCP.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 18]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + With these measures in place the SRTCP messages will not use more
|
| + than the allotted bandwidth. The effect of the size of the added
|
| + fields on the SRTCP traffic will be that messages will be sent with
|
| + longer packet intervals. The increase in the intervals will be
|
| + directly proportional to size of the added fields. For the pre-
|
| + defined transforms, the size of the added fields will be at least 14
|
| + octets, and upper bounded depending on MKI and the authentication tag
|
| + sizes.
|
| +
|
| +4. Pre-Defined Cryptographic Transforms
|
| +
|
| + While there are numerous encryption and message authentication
|
| + algorithms that can be used in SRTP, below we define default
|
| + algorithms in order to avoid the complexity of specifying the
|
| + encodings for the signaling of algorithm and parameter identifiers.
|
| + The defined algorithms have been chosen as they fulfill the goals
|
| + listed in Section 2. Recommendations on how to extend SRTP with new
|
| + transforms are given in Section 6.
|
| +
|
| +4.1. Encryption
|
| +
|
| + The following parameters are common to both pre-defined, non-NULL,
|
| + encryption transforms specified in this section.
|
| +
|
| + * BLOCK_CIPHER-MODE indicates the block cipher used and its mode of
|
| + operation
|
| + * n_b is the bit-size of the block for the block cipher
|
| + * k_e is the session encryption key
|
| + * n_e is the bit-length of k_e
|
| + * k_s is the session salting key
|
| + * n_s is the bit-length of k_s
|
| + * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, a
|
| + non-negative integer, specified by the message authentication code
|
| + in use.
|
| +
|
| + The distinct session keys and salts for SRTP/SRTCP are by default
|
| + derived as specified in Section 4.3.
|
| +
|
| + The encryption transforms defined in SRTP map the SRTP packet index
|
| + and secret key into a pseudo-random keystream segment. Each
|
| + keystream segment encrypts a single RTP packet. The process of
|
| + encrypting a packet consists of generating the keystream segment
|
| + corresponding to the packet, and then bitwise exclusive-oring that
|
| + keystream segment onto the payload of the RTP packet to produce the
|
| + Encrypted Portion of the SRTP packet. In case the payload size is
|
| + not an integer multiple of n_b bits, the excess (least significant)
|
| + bits of the keystream are simply discarded. Decryption is done the
|
| + same way, but swapping the roles of the plaintext and ciphertext.
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 19]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + +----+ +------------------+---------------------------------+
|
| + | KG |-->| Keystream Prefix | Keystream Suffix |---+
|
| + +----+ +------------------+---------------------------------+ |
|
| + |
|
| + +---------------------------------+ v
|
| + | Payload of RTP Packet |->(*)
|
| + +---------------------------------+ |
|
| + |
|
| + +---------------------------------+ |
|
| + | Encrypted Portion of SRTP Packet|<--+
|
| + +---------------------------------+
|
| +
|
| + Figure 3: Default SRTP Encryption Processing. Here KG denotes the
|
| + keystream generator, and (*) denotes bitwise exclusive-or.
|
| +
|
| + The definition of how the keystream is generated, given the index,
|
| + depends on the cipher and its mode of operation. Below, two such
|
| + keystream generators are defined. The NULL cipher is also defined,
|
| + to be used when encryption of RTP is not required.
|
| +
|
| + The SRTP definition of the keystream is illustrated in Figure 3. The
|
| + initial octets of each keystream segment MAY be reserved for use in a
|
| + message authentication code, in which case the keystream used for
|
| + encryption starts immediately after the last reserved octet. The
|
| + initial reserved octets are called the "keystream prefix" (not to be
|
| + confused with the "encryption prefix" of [RFC3550, Section 6.1]), and
|
| + the remaining octets are called the "keystream suffix". The
|
| + keystream prefix MUST NOT be used for encryption. The process is
|
| + illustrated in Figure 3.
|
| +
|
| + The number of octets in the keystream prefix is denoted as
|
| + SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a positive,
|
| + non-zero value of SRTP_PREFIX_LENGTH. This means that, even if
|
| + confidentiality is not to be provided, the keystream generator output
|
| + may still need to be computed for packet authentication, in which
|
| + case the default keystream generator (mode) SHALL be used.
|
| +
|
| + The default cipher is the Advanced Encryption Standard (AES) [AES],
|
| + and we define two modes of running AES, (1) Segmented Integer Counter
|
| + Mode AES and (2) AES in f8-mode. In the remainder of this section,
|
| + let E(k,x) be AES applied to key k and input block x.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 20]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +4.1.1. AES in Counter Mode
|
| +
|
| + Conceptually, counter mode [AES-CTR] consists of encrypting
|
| + successive integers. The actual definition is somewhat more
|
| + complicated, in order to randomize the starting point of the integer
|
| + sequence. Each packet is encrypted with a distinct keystream
|
| + segment, which SHALL be computed as follows.
|
| +
|
| + A keystream segment SHALL be the concatenation of the 128-bit output
|
| + blocks of the AES cipher in the encrypt direction, using key k = k_e,
|
| + in which the block indices are in increasing order. Symbolically,
|
| + each keystream segment looks like
|
| +
|
| + E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ...
|
| +
|
| + where the 128-bit integer value IV SHALL be defined by the SSRC, the
|
| + SRTP packet index i, and the SRTP session salting key k_s, as below.
|
| +
|
| + IV = (k_s * 2^16) XOR (SSRC * 2^64) XOR (i * 2^16)
|
| +
|
| + Each of the three terms in the XOR-sum above is padded with as many
|
| + leading zeros as needed to make the operation well-defined,
|
| + considered as a 128-bit value.
|
| +
|
| + The inclusion of the SSRC allows the use of the same key to protect
|
| + distinct SRTP streams within the same RTP session, see the security
|
| + caveats in Section 9.1.
|
| +
|
| + In the case of SRTCP, the SSRC of the first header of the compound
|
| + packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s
|
| + SHALL be replaced by the SRTCP encryption session key and salt.
|
| +
|
| + Note that the initial value, IV, is fixed for each packet and is
|
| + formed by "reserving" 16 zeros in the least significant bits for the
|
| + purpose of the counter. The number of blocks of keystream generated
|
| + for any fixed value of IV MUST NOT exceed 2^16 to avoid keystream
|
| + re-use, see below. The AES has a block size of 128 bits, so 2^16
|
| + output blocks are sufficient to generate the 2^23 bits of keystream
|
| + needed to encrypt the largest possible RTP packet (except for IPv6
|
| + "jumbograms" [RFC2675], which are not likely to be used for RTP-based
|
| + multimedia traffic). This restriction on the maximum bit-size of the
|
| + packet that can be encrypted ensures the security of the encryption
|
| + method by limiting the effectiveness of probabilistic attacks [BDJR].
|
| +
|
| + For a particular Counter Mode key, each IV value used as an input
|
| + MUST be distinct, in order to avoid the security exposure of a two-
|
| + time pad situation (Section 9.1). To satisfy this constraint, an
|
| + implementation MUST ensure that the combination of the SRTP packet
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 21]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + index of ROC || SEQ, and the SSRC used in the construction of the IV
|
| + are distinct for any particular key. The failure to ensure this
|
| + uniqueness could be catastrophic for Secure RTP. This is in contrast
|
| + to the situation for RTP itself, which may be able to tolerate such
|
| + failures. It is RECOMMENDED that, if a dedicated security module is
|
| + present, the RTP sequence numbers and SSRC either be generated or
|
| + checked by that module (i.e., sequence-number and SSRC processing in
|
| + an SRTP system needs to be protected as well as the key).
|
| +
|
| +4.1.2. AES in f8-mode
|
| +
|
| + To encrypt UMTS (Universal Mobile Telecommunications System, as 3G
|
| + networks) data, a solution (see [f8-a] [f8-b]) known as the f8-
|
| + algorithm has been developed. On a high level, the proposed scheme
|
| + is a variant of Output Feedback Mode (OFB) [HAC], with a more
|
| + elaborate initialization and feedback function. As in normal OFB,
|
| + the core consists of a block cipher. We also define here the use of
|
| + AES as a block cipher to be used in what we shall call "f8-mode of
|
| + operation" RTP encryption. The AES f8-mode SHALL use the same
|
| + default sizes for session key and salt as AES counter mode.
|
| +
|
| + Figure 4 shows the structure of block cipher, E, running in f8-mode.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 22]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + IV
|
| + |
|
| + v
|
| + +------+
|
| + | |
|
| + +--->| E |
|
| + | +------+
|
| + | |
|
| + m -> (*) +-----------+-------------+-- ... ------+
|
| + | IV' | | | |
|
| + | | j=1 -> (*) j=2 -> (*) ... j=L-1 ->(*)
|
| + | | | | |
|
| + | | +-> (*) +-> (*) ... +-> (*)
|
| + | | | | | | | |
|
| + | v | v | v | v
|
| + | +------+ | +------+ | +------+ | +------+
|
| + k_e ---+--->| E | | | E | | | E | | | E |
|
| + | | | | | | | | | | |
|
| + +------+ | +------+ | +------+ | +------+
|
| + | | | | | | |
|
| + +------+ +--------+ +-- ... ----+ |
|
| + | | | |
|
| + v v v v
|
| + S(0) S(1) S(2) . . . S(L-1)
|
| +
|
| + Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise XOR).
|
| + The figure represents the KG in Figure 3, when AES-f8 is used.
|
| +
|
| +4.1.2.1. f8 Keystream Generation
|
| +
|
| + The Initialization Vector (IV) SHALL be determined as described in
|
| + Section 4.1.2.2 (and in Section 4.1.2.3 for SRTCP).
|
| +
|
| + Let IV', S(j), and m denote n_b-bit blocks. The keystream,
|
| + S(0) ||... || S(L-1), for an N-bit message SHALL be defined by
|
| + setting IV' = E(k_e XOR m, IV), and S(-1) = 00..0. For
|
| + j = 0,1,..,L-1 where L = N/n_b (rounded up to nearest integer if it
|
| + is not already an integer) compute
|
| +
|
| + S(j) = E(k_e, IV' XOR j XOR S(j-1))
|
| +
|
| + Notice that the IV is not used directly. Instead it is fed through E
|
| + under another key to produce an internal, "masked" value (denoted
|
| + IV') to prevent an attacker from gaining known input/output pairs.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 23]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + The role of the internal counter, j, is to prevent short keystream
|
| + cycles. The value of the key mask m SHALL be
|
| +
|
| + m = k_s || 0x555..5,
|
| +
|
| + i.e., the session salting key, appended by the binary pattern 0101..
|
| + to fill out the entire desired key size, n_e.
|
| +
|
| + The sender SHOULD NOT generate more than 2^32 blocks, which is
|
| + sufficient to generate 2^39 bits of keystream. Unlike counter mode,
|
| + there is no absolute threshold above (below) which f8 is guaranteed
|
| + to be insecure (secure). The above bound has been chosen to limit,
|
| + with sufficient security margin, the probability of degenerative
|
| + behavior in the f8 keystream generation.
|
| +
|
| +4.1.2.2. f8 SRTP IV Formation
|
| +
|
| + The purpose of the following IV formation is to provide a feature
|
| + which we call implicit header authentication (IHA), see Section 9.5.
|
| +
|
| + The SRTP IV for 128-bit block AES-f8 SHALL be formed in the following
|
| + way:
|
| +
|
| + IV = 0x00 || M || PT || SEQ || TS || SSRC || ROC
|
| +
|
| + M, PT, SEQ, TS, SSRC SHALL be taken from the RTP header; ROC is from
|
| + the cryptographic context.
|
| +
|
| + The presence of the SSRC as part of the IV allows AES-f8 to be used
|
| + when a master key is shared between multiple streams within the same
|
| + RTP session, see Section 9.1.
|
| +
|
| +4.1.2.3. f8 SRTCP IV Formation
|
| +
|
| + The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the
|
| + following way:
|
| +
|
| + IV= 0..0 || E || SRTCP index || V || P || RC || PT || length || SSRC
|
| +
|
| + where V, P, RC, PT, length, SSRC SHALL be taken from the first header
|
| + in the RTCP compound packet. E and SRTCP index are the 1-bit and
|
| + 31-bit fields added to the packet.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 24]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +4.1.3. NULL Cipher
|
| +
|
| + The NULL cipher is used when no confidentiality for RTP/RTCP is
|
| + requested. The keystream can be thought of as "000..0", i.e., the
|
| + encryption SHALL simply copy the plaintext input into the ciphertext
|
| + output.
|
| +
|
| +4.2. Message Authentication and Integrity
|
| +
|
| + Throughout this section, M will denote data to be integrity
|
| + protected. In the case of SRTP, M SHALL consist of the Authenticated
|
| + Portion of the packet (as specified in Figure 1) concatenated with
|
| + the ROC, M = Authenticated Portion || ROC; in the case of SRTCP, M
|
| + SHALL consist of the Authenticated Portion (as specified in Figure 2)
|
| + only.
|
| +
|
| + Common parameters:
|
| +
|
| + * AUTH_ALG is the authentication algorithm
|
| + * k_a is the session message authentication key
|
| + * n_a is the bit-length of the authentication key
|
| + * n_tag is the bit-length of the output authentication tag
|
| + * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as
|
| + defined above, a parameter of AUTH_ALG
|
| +
|
| + The distinct session authentication keys for SRTP/SRTCP are by
|
| + default derived as specified in Section 4.3.
|
| +
|
| + The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for
|
| + any particular fixed value of the key.
|
| +
|
| + We describe the process of computing authentication tags as follows.
|
| + The sender computes the tag of M and appends it to the packet. The
|
| + SRTP receiver verifies a message/authentication tag pair by computing
|
| + a new authentication tag over M using the selected algorithm and key,
|
| + and then compares it to the tag associated with the received message.
|
| + If the two tags are equal, then the message/tag pair is valid;
|
| + otherwise, it is invalid and the error audit message "AUTHENTICATION
|
| + FAILURE" MUST be returned.
|
| +
|
| +4.2.1. HMAC-SHA1
|
| +
|
| + The pre-defined authentication transform for SRTP is HMAC-SHA1
|
| + [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
|
| + be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
|
| + the session authentication key and M as specified above, i.e.,
|
| + HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
|
| + left-most bits.
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 25]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +4.3. Key Derivation
|
| +
|
| +4.3.1. Key Derivation Algorithm
|
| +
|
| + Regardless of the encryption or message authentication transform that
|
| + is employed (it may be an SRTP pre-defined transform or newly
|
| + introduced according to Section 6), interoperable SRTP
|
| + implementations MUST use the SRTP key derivation to generate session
|
| + keys. Once the key derivation rate is properly signaled at the start
|
| + of the session, there is no need for extra communication between the
|
| + parties that use SRTP key derivation.
|
| +
|
| + packet index ---+
|
| + |
|
| + v
|
| + +-----------+ master +--------+ session encr_key
|
| + | ext | key | |---------->
|
| + | key mgmt |-------->| key | session auth_key
|
| + | (optional | | deriv |---------->
|
| + | rekey) |-------->| | session salt_key
|
| + | | master | |---------->
|
| + +-----------+ salt +--------+
|
| +
|
| + Figure 5: SRTP key derivation.
|
| +
|
| + At least one initial key derivation SHALL be performed by SRTP, i.e.,
|
| + the first key derivation is REQUIRED. Further applications of the
|
| + key derivation MAY be performed, according to the
|
| + "key_derivation_rate" value in the cryptographic context. The key
|
| + derivation function SHALL initially be invoked before the first
|
| + packet and then, when r > 0, a key derivation is performed whenever
|
| + index mod r equals zero. This can be thought of as "refreshing" the
|
| + session keys. The value of "key_derivation_rate" MUST be kept fixed
|
| + for the lifetime of the associated master key.
|
| +
|
| + Interoperable SRTP implementations MAY also derive session salting
|
| + keys for encryption transforms, as is done in both of the pre-
|
| + defined transforms.
|
| +
|
| + Let m and n be positive integers. A pseudo-random function family is
|
| + a set of keyed functions {PRF_n(k,x)} such that for the (secret)
|
| + random key k, given m-bit x, PRF_n(k,x) is an n-bit string,
|
| + computationally indistinguishable from random n-bit strings, see
|
| + [HAC]. For the purpose of key derivation in SRTP, a secure PRF with
|
| + m = 128 (or more) MUST be used, and a default PRF transform is
|
| + defined in Section 4.3.3.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 26]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Let "a DIV t" denote integer division of a by t, rounded down, and
|
| + with the convention that "a DIV 0 = 0" for all a. We also make the
|
| + convention of treating "a DIV t" as a bit string of the same length
|
| + as a, and thus "a DIV t" will in general have leading zeros.
|
| +
|
| + Key derivation SHALL be defined as follows in terms of <label>, an
|
| + 8-bit constant (see below), master_salt and key_derivation_rate, as
|
| + determined in the cryptographic context, and index, the packet index
|
| + (i.e., the 48-bit ROC || SEQ for SRTP):
|
| +
|
| + * Let r = index DIV key_derivation_rate (with DIV as defined above).
|
| +
|
| + * Let key_id = <label> || r.
|
| +
|
| + * Let x = key_id XOR master_salt, where key_id and master_salt are
|
| + aligned so that their least significant bits agree (right-
|
| + alignment).
|
| +
|
| + <label> MUST be unique for each type of key to be derived. We
|
| + currently define <label> 0x00 to 0x05 (see below), and future
|
| + extensions MAY specify new values in the range 0x06 to 0xff for other
|
| + purposes. The n-bit SRTP key (or salt) for this packet SHALL then be
|
| + derived from the master key, k_master as follows:
|
| +
|
| + PRF_n(k_master, x).
|
| +
|
| + (The PRF may internally specify additional formatting and padding of
|
| + x, see e.g., Section 4.3.3 for the default PRF.)
|
| +
|
| + The session keys and salt SHALL now be derived using:
|
| +
|
| + - k_e (SRTP encryption): <label> = 0x00, n = n_e.
|
| +
|
| + - k_a (SRTP message authentication): <label> = 0x01, n = n_a.
|
| +
|
| + - k_s (SRTP salting key): <label> = 0x02, n = n_s.
|
| +
|
| + where n_e, n_s, and n_a are from the cryptographic context.
|
| +
|
| + The master key and master salt MUST be random, but the master salt
|
| + MAY be public.
|
| +
|
| + Note that for a key_derivation_rate of 0, the application of the key
|
| + derivation SHALL take place exactly once.
|
| +
|
| + The definition of DIV above is purely for notational convenience.
|
| + For a non-zero t among the set of allowed key derivation rates, "a
|
| + DIV t" can be implemented as a right-shift by the base-2 logarithm of
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 27]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + t. The derivation operation is further facilitated if the rates are
|
| + chosen to be powers of 256, but that granularity was considered too
|
| + coarse to be a requirement of this specification.
|
| +
|
| + The upper limit on the number of packets that can be secured using
|
| + the same master key (see Section 9.2) is independent of the key
|
| + derivation.
|
| +
|
| +4.3.2. SRTCP Key Derivation
|
| +
|
| + SRTCP SHALL by default use the same master key (and master salt) as
|
| + SRTP. To do this securely, the following changes SHALL be done to
|
| + the definitions in Section 4.3.1 when applying session key derivation
|
| + for SRTCP.
|
| +
|
| + Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index
|
| + (i.e., excluding the E-bit, replacing it with a fixed 0-bit), and use
|
| + <label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the
|
| + SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting
|
| + key.
|
| +
|
| +4.3.3. AES-CM PRF
|
| +
|
| + The currently defined PRF, keyed by 128, 192, or 256 bit master key,
|
| + has input block size m = 128 and can produce n-bit outputs for n up
|
| + to 2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as described
|
| + in Section 4.1.1, applied to key k_master, and IV equal to (x*2^16),
|
| + and with the output keystream truncated to the n first (left-most)
|
| + bits. (Requiring n/128, rounded up, applications of AES.)
|
| +
|
| +5. Default and mandatory-to-implement Transforms
|
| +
|
| + The default transforms also are mandatory-to-implement transforms in
|
| + SRTP. Of course, "mandatory-to-implement" does not imply
|
| + "mandatory-to-use". Table 1 summarizes the pre-defined transforms.
|
| + The default values below are valid for the pre-defined transforms.
|
| +
|
| + mandatory-to-impl. optional default
|
| +
|
| + encryption AES-CM, NULL AES-f8 AES-CM
|
| + message integrity HMAC-SHA1 - HMAC-SHA1
|
| + key derivation (PRF) AES-CM - AES-CM
|
| +
|
| + Table 1: Mandatory-to-implement, optional and default transforms in
|
| + SRTP and SRTCP.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 28]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +5.1. Encryption: AES-CM and NULL
|
| +
|
| + AES running in Segmented Integer Counter Mode, as defined in Section
|
| + 4.1.1, SHALL be the default encryption algorithm. The default key
|
| + lengths SHALL be 128-bit for the session encryption key (n_e). The
|
| + default session salt key-length (n_s) SHALL be 112 bits.
|
| +
|
| + The NULL cipher SHALL also be mandatory-to-implement.
|
| +
|
| +5.2. Message Authentication/Integrity: HMAC-SHA1
|
| +
|
| + HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message
|
| + authentication code. The default session authentication key-length
|
| + (n_a) SHALL be 160 bits, the default authentication tag length
|
| + (n_tag) SHALL be 80 bits, and the SRTP_PREFIX_LENGTH SHALL be zero
|
| + for HMAC-SHA1. In addition, for SRTCP, the pre-defined HMAC-SHA1
|
| + MUST NOT be applied with a value of n_tag, nor n_a, that are smaller
|
| + than these defaults. For SRTP, smaller values are NOT RECOMMENDED,
|
| + but MAY be used after careful consideration of the issues in Section
|
| + 7.5 and 9.5.
|
| +
|
| +5.3. Key Derivation: AES-CM PRF
|
| +
|
| + The AES Counter Mode based key derivation and PRF defined in Sections
|
| + 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the default
|
| + method for generating session keys. The default master salt length
|
| + SHALL be 112 bits and the default key-derivation rate SHALL be zero.
|
| +
|
| +6. Adding SRTP Transforms
|
| +
|
| + Section 4 provides examples of the level of detail needed for
|
| + defining transforms. Whenever a new transform is to be added to
|
| + SRTP, a companion standard track RFC MUST be written to exactly
|
| + define how the new transform can be used with SRTP (and SRTCP). Such
|
| + a companion RFC SHOULD avoid overlap with the SRTP protocol document.
|
| + Note however, that it MAY be necessary to extend the SRTP or SRTCP
|
| + cryptographic context definition with new parameters (including fixed
|
| + or default values), add steps to the packet processing, or even add
|
| + fields to the SRTP/SRTCP packets. The companion RFC SHALL explain
|
| + any known issues regarding interactions between the transform and
|
| + other aspects of SRTP.
|
| +
|
| + Each new transform document SHOULD specify its key attributes, e.g.,
|
| + size of keys (minimum, maximum, recommended), format of keys,
|
| + recommended/required processing of input keying material,
|
| + requirements/recommendations on key lifetime, re-keying and key
|
| + derivation, whether sharing of keys between SRTP and SRTCP is allowed
|
| + or not, etc.
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 29]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + An added message integrity transform SHOULD define a minimum
|
| + acceptable key/tag size for SRTCP, equivalent in strength to the
|
| + minimum values as defined in Section 5.2.
|
| +
|
| +7. Rationale
|
| +
|
| + This section explains the rationale behind several important features
|
| + of SRTP.
|
| +
|
| +7.1. Key derivation
|
| +
|
| + Key derivation reduces the burden on the key establishment. As many
|
| + as six different keys are needed per crypto context (SRTP and SRTCP
|
| + encryption keys and salts, SRTP and SRTCP authentication keys), but
|
| + these are derived from a single master key in a cryptographically
|
| + secure way. Thus, the key management protocol needs to exchange only
|
| + one master key (plus master salt when required), and then SRTP itself
|
| + derives all the necessary session keys (via the first, mandatory
|
| + application of the key derivation function).
|
| +
|
| + Multiple applications of the key derivation function are optional,
|
| + but will give security benefits when enabled. They prevent an
|
| + attacker from obtaining large amounts of ciphertext produced by a
|
| + single fixed session key. If the attacker was able to collect a
|
| + large amount of ciphertext for a certain session key, he might be
|
| + helped in mounting certain attacks.
|
| +
|
| + Multiple applications of the key derivation function provide
|
| + backwards and forward security in the sense that a compromised
|
| + session key does not compromise other session keys derived from the
|
| + same master key. This means that the attacker who is able to recover
|
| + a certain session key, is anyway not able to have access to messages
|
| + secured under previous and later session keys (derived from the same
|
| + master key). (Note that, of course, a leaked master key reveals all
|
| + the session keys derived from it.)
|
| +
|
| + Considerations arise with high-rate key refresh, especially in large
|
| + multicast settings, see Section 11.
|
| +
|
| +7.2. Salting key
|
| +
|
| + The master salt guarantees security against off-line key-collision
|
| + attacks on the key derivation that might otherwise reduce the
|
| + effective key size [MF00].
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 30]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + The derived session salting key used in the encryption, has been
|
| + introduced to protect against some attacks on additive stream
|
| + ciphers, see Section 9.2. The explicit inclusion method of the salt
|
| + in the IV has been selected for ease of hardware implementation.
|
| +
|
| +7.3. Message Integrity from Universal Hashing
|
| +
|
| + The particular definition of the keystream given in Section 4.1 (the
|
| + keystream prefix) is to give provision for particular universal hash
|
| + functions, suitable for message authentication in the Wegman-Carter
|
| + paradigm [WC81]. Such functions are provably secure, simple, quick,
|
| + and especially appropriate for Digital Signal Processors and other
|
| + processors with a fast multiply operation.
|
| +
|
| + No authentication transforms are currently provided in SRTP other
|
| + than HMAC-SHA1. Future transforms, like the above mentioned
|
| + universal hash functions, MAY be added following the guidelines in
|
| + Section 6.
|
| +
|
| +7.4. Data Origin Authentication Considerations
|
| +
|
| + Note that in pair-wise communications, integrity and data origin
|
| + authentication are provided together. However, in group scenarios
|
| + where the keys are shared between members, the MAC tag only proves
|
| + that a member of the group sent the packet, but does not prevent
|
| + against a member impersonating another. Data origin authentication
|
| + (DOA) for multicast and group RTP sessions is a hard problem that
|
| + needs a solution; while some promising proposals are being
|
| + investigated [PCST1] [PCST2], more work is needed to rigorously
|
| + specify these technologies. Thus SRTP data origin authentication in
|
| + groups is for further study.
|
| +
|
| + DOA can be done otherwise using signatures. However, this has high
|
| + impact in terms of bandwidth and processing time, therefore we do not
|
| + offer this form of authentication in the pre-defined packet-integrity
|
| + transform.
|
| +
|
| + The presence of mixers and translators does not allow data origin
|
| + authentication in case the RTP payload and/or the RTP header are
|
| + manipulated. Note that these types of middle entities also disrupt
|
| + end-to-end confidentiality (as the IV formation depends e.g., on the
|
| + RTP header preservation). A certain trust model may choose to trust
|
| + the mixers/translators to decrypt/re-encrypt the media (this would
|
| + imply breaking the end-to-end security, with related security
|
| + implications).
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 31]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +7.5. Short and Zero-length Message Authentication
|
| +
|
| + As shown in Figure 1, the authentication tag is RECOMMENDED in SRTP.
|
| + A full 80-bit authentication-tag SHOULD be used, but a shorter tag or
|
| + even a zero-length tag (i.e., no message authentication) MAY be used
|
| + under certain conditions to support either of the following two
|
| + application environments.
|
| +
|
| + 1. Strong authentication can be impractical in environments where
|
| + bandwidth preservation is imperative. An important special
|
| + case is wireless communication systems, in which bandwidth is a
|
| + scarce and expensive resource. Studies have shown that for
|
| + certain applications and link technologies, additional bytes
|
| + may result in a significant decrease in spectrum efficiency
|
| + [SWO]. Considerable effort has been made to design IP header
|
| + compression techniques to improve spectrum efficiency
|
| + [RFC3095]. A typical voice application produces 20 byte
|
| + samples, and the RTP, UDP and IP headers need to be jointly
|
| + compressed to one or two bytes on average in order to obtain
|
| + acceptable wireless bandwidth economy [RFC3095]. In this case,
|
| + strong authentication would impose nearly fifty percent
|
| + overhead.
|
| +
|
| + 2. Authentication is impractical for applications that use data
|
| + links with fixed-width fields that cannot accommodate the
|
| + expansion due to the authentication tag. This is the case for
|
| + some important existing wireless channels. For example, zero-
|
| + byte header compression is used to adapt EVRC/SMV voice with
|
| + the legacy IS-95 bearer channel in CDMA2000 VoIP services. It
|
| + was found that not a single additional octet could be added to
|
| + the data, which motivated the creation of a zero-byte profile
|
| + for ROHC [RFC3242].
|
| +
|
| + A short tag is secure for a restricted set of applications. Consider
|
| + a voice telephony application, for example, such as a G.729 audio
|
| + codec with a 20-millisecond packetization interval, protected by a
|
| + 32-bit message authentication tag. The likelihood of any given
|
| + packet being successfully forged is only one in 2^32. Thus an
|
| + adversary can control no more than 20 milliseconds of audio output
|
| + during a 994-day period, on average. In contrast, the effect of a
|
| + single forged packet can be much larger if the application is
|
| + stateful. A codec that uses relative or predictive compression
|
| + across packets will propagate the maliciously generated state,
|
| + affecting a longer duration of output.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 32]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Certainly not all SRTP or telephony applications meet the criteria
|
| + for short or zero-length authentication tags. Section 9.5.1
|
| + discusses the risks of weak or no message authentication, and section
|
| + 9.5 describes the circumstances when it is acceptable and when it is
|
| + unacceptable.
|
| +
|
| +8. Key Management Considerations
|
| +
|
| + There are emerging key management standards [MIKEY] [KEYMGT] [SDMS]
|
| + for establishing an SRTP cryptographic context (e.g., an SRTP master
|
| + key). Both proprietary and open-standard key management methods are
|
| + likely to be used for telephony applications [MIKEY] [KINK] and
|
| + multicast applications [GDOI]. This section provides guidance for
|
| + key management systems that service SRTP session.
|
| +
|
| + For initialization, an interoperable SRTP implementation SHOULD be
|
| + given the SSRC and MAY be given the initial RTP sequence number for
|
| + the RTP stream by key management (thus, key management has a
|
| + dependency on RTP operational parameters). Sending the RTP sequence
|
| + number in the key management may be useful e.g., when the initial
|
| + sequence number is close to wrapping (to avoid synchronization
|
| + problems), and to communicate the current sequence number to a
|
| + joining endpoint (to properly initialize its replay list).
|
| +
|
| + If the pre-defined transforms are used, SRTP allows sharing of the
|
| + same master key between SRTP/SRTCP streams belonging to the same RTP
|
| + session.
|
| +
|
| + First, sharing between SRTP streams belonging to the same RTP session
|
| + is secure if the design of the synchronization mechanism, i.e., the
|
| + IV, avoids keystream re-use (the two-time pad, Section 9.1). This is
|
| + taken care of by the fact that RTP provides for unique SSRCs for
|
| + streams belonging to the same RTP session. See Section 9.1 for
|
| + further discussion.
|
| +
|
| + Second, sharing between SRTP and the corresponding SRTCP is secure.
|
| + The fact that an SRTP stream and its associated SRTCP stream both
|
| + carry the same SSRC does not constitute a problem for the two-time
|
| + pad due to the key derivation. Thus, SRTP and SRTCP corresponding to
|
| + one RTP session MAY share master keys (as they do by default).
|
| +
|
| + Note that message authentication also has a dependency on SSRC
|
| + uniqueness that is unrelated to the problem of keystream reuse: SRTP
|
| + streams authenticated under the same key MUST have a distinct SSRC in
|
| + order to identify the sender of the message. This requirement is
|
| + needed because the SSRC is the cryptographically authenticated field
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 33]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + used to distinguish between different SRTP streams. Were two streams
|
| + to use identical SSRC values, then an adversary could substitute
|
| + messages from one stream into the other without detection.
|
| +
|
| + SRTP/SRTCP MUST NOT share master keys under any other circumstances
|
| + than the ones given above, i.e., between SRTP and its corresponding
|
| + SRTCP, and, between streams belonging to the same RTP session.
|
| +
|
| +8.1. Re-keying
|
| +
|
| + The recommended way for a particular key management system to provide
|
| + re-key within SRTP is by associating a master key in a crypto context
|
| + with an MKI.
|
| +
|
| + This provides for easy master key retrieval (see Scenarios in Section
|
| + 11), but has the disadvantage of adding extra bits to each packet.
|
| + As noted in Section 7.5, some wireless links do not cater for added
|
| + bits, therefore SRTP also defines a more economic way of triggering
|
| + re-keying, via use of <From, To>, which works in some specific,
|
| + simple scenarios (see Section 8.1.1).
|
| +
|
| + SRTP senders SHALL count the amount of SRTP and SRTCP traffic being
|
| + used for a master key and invoke key management to re-key if needed
|
| + (Section 9.2). These interactions are defined by the key management
|
| + interface to SRTP and are not defined by this protocol specification.
|
| +
|
| +8.1.1. Use of the <From, To> for re-keying
|
| +
|
| + In addition to the use of the MKI, SRTP defines another optional
|
| + mechanism for master key retrieval, the <From, To>. The <From, To>
|
| + specifies the range of SRTP indices (a pair of sequence number and
|
| + ROC) within which a certain master key is valid, and is (when used)
|
| + part of the crypto context. By looking at the 48-bit SRTP index of
|
| + the current SRTP packet, the corresponding master key can be found by
|
| + determining which From-To interval it belongs to. For SRTCP, the
|
| + most recently observed/used SRTP index (which can be obtained from
|
| + the cryptographic context) is used for this purpose, even though
|
| + SRTCP has its own (31-bit) index (see caveat below).
|
| +
|
| + This method, compared to the MKI, has the advantage of identifying
|
| + the master key and defining its lifetime without adding extra bits to
|
| + each packet. This could be useful, as already noted, for some
|
| + wireless links that do not cater for added bits. However, its use
|
| + SHOULD be limited to specific, very simple scenarios. We recommend
|
| + to limit its use when the RTP session is a simple unidirectional or
|
| + bi-directional stream. This is because in case of multiple streams,
|
| + it is difficult to trigger the re-key based on the <From, To> of a
|
| + single RTP stream. For example, if several streams share a master
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 34]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + key, there is no simple one-to-one correspondence between the index
|
| + sequence space of a certain stream, and the index sequence space on
|
| + which the <From, To> values are based. Consequently, when a master
|
| + key is shared between streams, one of these streams MUST be
|
| + designated by key management as the one whose index space defines the
|
| + re-keying points. Also, the re-key triggering on SRTCP is based on
|
| + the correspondent SRTP stream, i.e., when the SRTP stream changes the
|
| + master key, so does the correspondent SRTCP. This becomes obviously
|
| + more and more complex with multiple streams.
|
| +
|
| + The default values for the <From, To> are "from the first observed
|
| + packet" and "until further notice". However, the maximum limit of
|
| + SRTP/SRTCP packets that are sent under each given master/session key
|
| + (Section 9.2) MUST NOT be exceeded.
|
| +
|
| + In case the <From, To> is used as key retrieval, then the MKI is not
|
| + inserted in the packet (and its indicator in the crypto context is
|
| + zero). However, using the MKI does not exclude using <From, To> key
|
| + lifetime simultaneously. This can for instance be useful to signal
|
| + at the sender side at which point in time an MKI is to be made
|
| + active.
|
| +
|
| +8.2. Key Management parameters
|
| +
|
| + The table below lists all SRTP parameters that key management can
|
| + supply. For reference, it also provides a summary of the default and
|
| + mandatory-to-support values for an SRTP implementation as described
|
| + in Section 5.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 35]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Parameter Mandatory-to-support Default
|
| + --------- -------------------- -------
|
| +
|
| + SRTP and SRTCP encr transf. AES_CM, NULL AES_CM
|
| + (Other possible values: AES_f8)
|
| +
|
| + SRTP and SRTCP auth transf. HMAC-SHA1 HMAC-SHA1
|
| +
|
| + SRTP and SRTCP auth params:
|
| + n_tag (tag length) 80 80
|
| + SRTP prefix_length 0 0
|
| +
|
| + Key derivation PRF AES_CM AES_CM
|
| +
|
| + Key material params
|
| + (for each master key):
|
| + master key length 128 128
|
| + n_e (encr session key length) 128 128
|
| + n_a (auth session key length) 160 160
|
| + master salt key
|
| + length of the master salt 112 112
|
| + n_s (session salt key length) 112 112
|
| + key derivation rate 0 0
|
| +
|
| + key lifetime
|
| + SRTP-packets-max-lifetime 2^48 2^48
|
| + SRTCP-packets-max-lifetime 2^31 2^31
|
| + from-to-lifetime <From, To>
|
| + MKI indicator 0 0
|
| + length of the MKI 0 0
|
| + value of the MKI
|
| +
|
| + Crypto context index params:
|
| + SSRC value
|
| + ROC
|
| + SEQ
|
| + SRTCP Index
|
| + Transport address
|
| + Port number
|
| +
|
| + Relation to other RTP profiles:
|
| + sender's order between FEC and SRTP FEC-SRTP FEC-SRTP
|
| + (see Section 10)
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 36]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +9. Security Considerations
|
| +
|
| +9.1. SSRC collision and two-time pad
|
| +
|
| + Any fixed keystream output, generated from the same key and index
|
| + MUST only be used to encrypt once. Re-using such keystream (jokingly
|
| + called a "two-time pad" system by cryptographers), can seriously
|
| + compromise security. The NSA's VENONA project [C99] provides a
|
| + historical example of such a compromise. It is REQUIRED that
|
| + automatic key management be used for establishing and maintaining
|
| + SRTP and SRTCP keying material; this requirement is to avoid
|
| + keystream reuse, which is more likely to occur with manual key
|
| + management. Furthermore, in SRTP, a "two-time pad" is avoided by
|
| + requiring the key, or some other parameter of cryptographic
|
| + significance, to be unique per RTP/RTCP stream and packet. The pre-
|
| + defined SRTP transforms accomplish packet-uniqueness by including the
|
| + packet index and stream-uniqueness by inclusion of the SSRC.
|
| +
|
| + The pre-defined transforms (AES-CM and AES-f8) allow master keys to
|
| + be shared across streams belonging to the same RTP session by the
|
| + inclusion of the SSRC in the IV. A master key MUST NOT be shared
|
| + among different RTP sessions.
|
| +
|
| + Thus, the SSRC MUST be unique between all the RTP streams within the
|
| + same RTP session that share the same master key. RTP itself provides
|
| + an algorithm for detecting SSRC collisions within the same RTP
|
| + session. Thus, temporary collisions could lead to temporary two-time
|
| + pad, in the unfortunate event that SSRCs collide at a point in time
|
| + when the streams also have identical sequence numbers (occurring with
|
| + probability roughly 2^(-48)). Therefore, the key management SHOULD
|
| + take care of avoiding such SSRC collisions by including the SSRCs to
|
| + be used in the session as negotiation parameters, proactively
|
| + assuring their uniqueness. This is a strong requirements in
|
| + scenarios where for example, there are multiple senders that can
|
| + start to transmit simultaneously, before SSRC collision are detected
|
| + at the RTP level.
|
| +
|
| + Note also that even with distinct SSRCs, extensive use of the same
|
| + key might improve chances of probabilistic collision and time-
|
| + memory-tradeoff attacks succeeding.
|
| +
|
| + As described, master keys MAY be shared between streams belonging to
|
| + the same RTP session, but it is RECOMMENDED that each SSRC have its
|
| + own master key. When master keys are shared among SSRC participants
|
| + and SSRCs are managed by a key management module as recommended
|
| + above, the RECOMMENDED policy for an SSRC collision error is for the
|
| + participant to leave the SRTP session as it is a sign of malfunction.
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 37]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +9.2. Key Usage
|
| +
|
| + The effective key size is determined (upper bounded) by the size of
|
| + the master key and, for encryption, the size of the salting key. Any
|
| + additive stream cipher is vulnerable to attacks that use statistical
|
| + knowledge about the plaintext source to enable key collision and
|
| + time-memory tradeoff attacks [MF00] [H80] [BS00]. These attacks take
|
| + advantage of commonalities among plaintexts, and provide a way for a
|
| + cryptanalyst to amortize the computational effort of decryption over
|
| + many keys, or over many bytes of output, thus reducing the effective
|
| + key size of the cipher. A detailed analysis of these attacks and
|
| + their applicability to the encryption of Internet traffic is provided
|
| + in [MF00]. In summary, the effective key size of SRTP when used in a
|
| + security system in which m distinct keys are used, is equal to the
|
| + key size of the cipher less the logarithm (base two) of m.
|
| + Protection against such attacks can be provided simply by increasing
|
| + the size of the keys used, which here can be accomplished by the use
|
| + of the salting key. Note that the salting key MUST be random but MAY
|
| + be public. A salt size of (the suggested) size 112 bits protects
|
| + against attacks in scenarios where at most 2^112 keys are in use.
|
| + This is sufficient for all practical purposes.
|
| +
|
| + Implementations SHOULD use keys that are as large as possible.
|
| + Please note that in many cases increasing the key size of a cipher
|
| + does not affect the throughput of that cipher.
|
| +
|
| + The use of the SRTP and SRTCP indices in the pre-defined transforms
|
| + fixes the maximum number of packets that can be secured with the same
|
| + key. This limit is fixed to 2^48 SRTP packets for an SRTP stream,
|
| + and 2^31 SRTCP packets, when SRTP and SRTCP are considered
|
| + independently. Due to for example re-keying, reaching this limit may
|
| + or may not coincide with wrapping of the indices, and thus the sender
|
| + MUST keep packet counts. However, when the session keys for related
|
| + SRTP and SRTCP streams are derived from the same master key (the
|
| + default behavior, Section 4.3), the upper bound that has to be
|
| + considered is in practice the minimum of the two quantities. That
|
| + is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured
|
| + with the same key (whichever occurs before), the key management MUST
|
| + be called to provide new master key(s) (previously stored and used
|
| + keys MUST NOT be used again), or the session MUST be terminated. If
|
| + a sender of RTCP discovers that the sender of SRTP (or SRTCP) has not
|
| + updated the master or session key prior to sending 2^48 SRTP (or 2^31
|
| + SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is up to
|
| + the security policy of the RTCP sender how to behave, e.g., whether
|
| + an RTCP BYE-packet should be sent and/or if the event should be
|
| + logged.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 38]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Note: in most typical applications (assuming at least one RTCP packet
|
| + for every 128,000 RTP packets), it will be the SRTCP index that first
|
| + reaches the upper limit, although the time until this occurs is very
|
| + long: even at 200 SRTCP packets/sec, the 2^31 index space of SRTCP is
|
| + enough to secure approximately 4 months of communication.
|
| +
|
| + Note that if the master key is to be shared between SRTP streams
|
| + within the same RTP session (Section 9.1), although the above bounds
|
| + are on a per stream (i.e., per SSRC) basis, the sender MUST base re-
|
| + key decision on the stream whose sequence number space is the first
|
| + to be exhausted.
|
| +
|
| + Key derivation limits the amount of plaintext that is encrypted with
|
| + a fixed session key, and made available to an attacker for analysis,
|
| + but key derivation does not extend the master key's lifetime. To see
|
| + this, simply consider our requirements to avoid two-time pad: two
|
| + distinct packets MUST either be processed with distinct IVs, or with
|
| + distinct session keys, and both the distinctness of IV and of the
|
| + session keys are (for the pre-defined transforms) dependent on the
|
| + distinctness of the packet indices.
|
| +
|
| + Note that with the key derivation, the effective key size is at most
|
| + that of the master key, even if the derived session key is
|
| + considerably longer. With the pre-defined authentication transform,
|
| + the session authentication key is 160 bits, but the master key by
|
| + default is only 128 bits. This design choice was made to comply with
|
| + certain recommendations in [RFC2104] so that an existing HMAC
|
| + implementation can be plugged into SRTP without problems. Since the
|
| + default tag size is 80 bits, it is, for the applications in mind,
|
| + also considered acceptable from security point of view. Users having
|
| + concerns about this are RECOMMENDED to instead use a 192 bit master
|
| + key in the key derivation. It was, however, chosen not to mandate
|
| + 192-bit keys since existing AES implementations to be used in the
|
| + key-derivation may not always support key-lengths other than 128
|
| + bits. Since AES is not defined (or properly analyzed) for use with
|
| + 160 bit keys it is NOT RECOMMENDED that ad-hoc key-padding schemes
|
| + are used to pad shorter keys to 192 or 256 bits.
|
| +
|
| +9.3. Confidentiality of the RTP Payload
|
| +
|
| + SRTP's pre-defined ciphers are "seekable" stream ciphers, i.e.,
|
| + ciphers able to efficiently seek to arbitrary locations in their
|
| + keystream (so that the encryption or decryption of one packet does
|
| + not depend on preceding packets). By using seekable stream ciphers,
|
| + SRTP avoids the denial of service attacks that are possible on stream
|
| + ciphers that lack this property. It is important to be aware that,
|
| + as with any stream cipher, the exact length of the payload is
|
| + revealed by the encryption. This means that it may be possible to
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 39]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + deduce certain "formatting bits" of the payload, as the length of the
|
| + codec output might vary due to certain parameter settings etc. This,
|
| + in turn, implies that the corresponding bit of the keystream can be
|
| + deduced. However, if the stream cipher is secure (counter mode and
|
| + f8 are provably secure under certain assumptions [BDJR] [KSYH] [IK]),
|
| + knowledge of a few bits of the keystream will not aid an attacker in
|
| + predicting subsequent keystream bits. Thus, the payload length (and
|
| + information deducible from this) will leak, but nothing else.
|
| +
|
| + As some RTP packet could contain highly predictable data, e.g., SID,
|
| + it is important to use a cipher designed to resist known plaintext
|
| + attacks (which is the current practice).
|
| +
|
| +9.4. Confidentiality of the RTP Header
|
| +
|
| + In SRTP, RTP headers are sent in the clear to allow for header
|
| + compression. This means that data such as payload type,
|
| + synchronization source identifier, and timestamp are available to an
|
| + eavesdropper. Moreover, since RTP allows for future extensions of
|
| + headers, we cannot foresee what kind of possibly sensitive
|
| + information might also be "leaked".
|
| +
|
| + SRTP is a low-cost method, which allows header compression to reduce
|
| + bandwidth. It is up to the endpoints' policies to decide about the
|
| + security protocol to employ. If one really needs to protect headers,
|
| + and is allowed to do so by the surrounding environment, then one
|
| + should also look at alternatives, e.g., IPsec [RFC2401].
|
| +
|
| +9.5. Integrity of the RTP payload and header
|
| +
|
| + SRTP messages are subject to attacks on their integrity and source
|
| + identification, and these risks are discussed in Section 9.5.1. To
|
| + protect against these attacks, each SRTP stream SHOULD be protected
|
| + by HMAC-SHA1 [RFC2104] with an 80-bit output tag and a 160-bit key,
|
| + or a message authentication code with equivalent strength. Secure
|
| + RTP SHOULD NOT be used without message authentication, except under
|
| + the circumstances described in this section. It is important to note
|
| + that encryption algorithms, including AES Counter Mode and f8, do not
|
| + provide message authentication. SRTCP MUST NOT be used with weak (or
|
| + NULL) authentication.
|
| +
|
| + SRTP MAY be used with weak authentication (e.g., a 32-bit
|
| + authentication tag), or with no authentication (the NULL
|
| + authentication algorithm). These options allow SRTP to be used to
|
| + provide confidentiality in situations where
|
| +
|
| + * weak or null authentication is an acceptable security risk, and
|
| + * it is impractical to provide strong message authentication.
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 40]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + These conditions are described below and in Section 7.5. Note that
|
| + both conditions MUST hold in order for weak or null authentication to
|
| + be used. The risks associated with exercising the weak or null
|
| + authentication options need to be considered by a security audit
|
| + prior to their use for a particular application or environment given
|
| + the risks, which are discussed in Section 9.5.1.
|
| +
|
| + Weak authentication is acceptable when the RTP application is such
|
| + that the effect of a small fraction of successful forgeries is
|
| + negligible. If the application is stateless, then the effect of a
|
| + single forged RTP packet is limited to the decoding of that
|
| + particular packet. Under this condition, the size of the
|
| + authentication tag MUST ensure that only a negligible fraction of the
|
| + packets passed to the RTP application by the SRTP receiver can be
|
| + forgeries. This fraction is negligible when an adversary, if given
|
| + control of the forged packets, is not able to make a significant
|
| + impact on the output of the RTP application (see the example of
|
| + Section 7.5).
|
| +
|
| + Weak or null authentication MAY be acceptable when it is unlikely
|
| + that an adversary can modify ciphertext so that it decrypts to an
|
| + intelligible value. One important case is when it is difficult for
|
| + an adversary to acquire the RTP plaintext data, since for many
|
| + codecs, an adversary that does not know the input signal cannot
|
| + manipulate the output signal in a controlled way. In many cases it
|
| + may be difficult for the adversary to determine the actual value of
|
| + the plaintext. For example, a hidden snooping device might be
|
| + required in order to know a live audio or video signal. The
|
| + adversary's signal must have a quality equivalent to or greater than
|
| + that of the signal under attack, since otherwise the adversary would
|
| + not have enough information to encode that signal with the codec used
|
| + by the victim. Plaintext prediction may also be especially difficult
|
| + for an interactive application such as a telephone call.
|
| +
|
| + Weak or null authentication MUST NOT be used when the RTP application
|
| + makes data forwarding or access control decisions based on the RTP
|
| + data. In such a case, an attacker may be able to subvert
|
| + confidentiality by causing the receiver to forward data to an
|
| + attacker. See Section 3 of [B96] for a real-life example of such
|
| + attacks.
|
| +
|
| + Null authentication MUST NOT be used when a replay attack, in which
|
| + an adversary stores packets then replays them later in the session,
|
| + could have a non-negligible impact on the receiver. An example of a
|
| + successful replay attack is the storing of the output of a
|
| + surveillance camera for a period of time, later followed by the
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 41]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + injection of that output to the monitoring station to avoid
|
| + surveillance. Encryption does not protect against this attack, and
|
| + non-null authentication is REQUIRED in order to defeat it.
|
| +
|
| + If existential message forgery is an issue, i.e., when the accuracy
|
| + of the received data is of non-negligible importance, null
|
| + authentication MUST NOT be used.
|
| +
|
| +9.5.1. Risks of Weak or Null Message Authentication
|
| +
|
| + During a security audit considering the use of weak or null
|
| + authentication, it is important to keep in mind the following attacks
|
| + which are possible when no message authentication algorithm is used.
|
| +
|
| + An attacker who cannot predict the plaintext is still always able to
|
| + modify the message sent between the sender and the receiver so that
|
| + it decrypts to a random plaintext value, or to send a stream of bogus
|
| + packets to the receiver that will decrypt to random plaintext values.
|
| + This attack is essentially a denial of service attack, though in the
|
| + absence of message authentication, the RTP application will have
|
| + inputs that are bit-wise correlated with the true value. Some
|
| + multimedia codecs and common operating systems will crash when such
|
| + data are accepted as valid video data. This denial of service attack
|
| + may be a much larger threat than that due to an attacker dropping,
|
| + delaying, or re-ordering packets.
|
| +
|
| + An attacker who cannot predict the plaintext can still replay a
|
| + previous message with certainty that the receiver will accept it.
|
| + Applications with stateless codecs might be robust against this type
|
| + of attack, but for other, more complex applications these attacks may
|
| + be far more grave.
|
| +
|
| + An attacker who can predict the plaintext can modify the ciphertext
|
| + so that it will decrypt to any value of her choosing. With an
|
| + additive stream cipher, an attacker will always be able to change
|
| + individual bits.
|
| +
|
| + An attacker may be able to subvert confidentiality due to the lack of
|
| + authentication when a data forwarding or access control decision is
|
| + made on decrypted but unauthenticated plaintext. This is because the
|
| + receiver may be fooled into forwarding data to an attacker, leading
|
| + to an indirect breach of confidentiality (see Section 3 of [B96]).
|
| + This is because data-forwarding decisions are made on the decrypted
|
| + plaintext; information in the plaintext will determine to what subnet
|
| + (or process) the plaintext is forwarded in ESP [RFC2401] tunnel mode
|
| + (respectively, transport mode). When Secure RTP is used without
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 42]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + message authentication, it should be verified that the application
|
| + does not make data forwarding or access control decisions based on
|
| + the decrypted plaintext.
|
| +
|
| + Some cipher modes of operation that require padding, e.g., standard
|
| + cipher block chaining (CBC) are very sensitive to attacks on
|
| + confidentiality if certain padding types are used in the absence of
|
| + integrity. The attack [V02] shows that this is indeed the case for
|
| + the standard RTP padding as discussed in reference to Figure 1, when
|
| + used together with CBC mode. Later transform additions to SRTP MUST
|
| + therefore carefully consider the risk of using this padding without
|
| + proper integrity protection.
|
| +
|
| +9.5.2. Implicit Header Authentication
|
| +
|
| + The IV formation of the f8-mode gives implicit authentication (IHA)
|
| + of the RTP header, even when message authentication is not used.
|
| + When IHA is used, an attacker that modifies the value of the RTP
|
| + header will cause the decryption process at the receiver to produce
|
| + random plaintext values. While this protection is not equivalent to
|
| + message authentication, it may be useful for some applications.
|
| +
|
| +10. Interaction with Forward Error Correction mechanisms
|
| +
|
| + The default processing when using Forward Error Correction (e.g., RFC
|
| + 2733) processing with SRTP SHALL be to perform FEC processing prior
|
| + to SRTP processing on the sender side and to perform SRTP processing
|
| + prior to FEC processing on the receiver side. Any change to this
|
| + ordering (reversing it, or, placing FEC between SRTP encryption and
|
| + SRTP authentication) SHALL be signaled out of band.
|
| +
|
| +11. Scenarios
|
| +
|
| + SRTP can be used as security protocol for the RTP/RTCP traffic in
|
| + many different scenarios. SRTP has a number of configuration
|
| + options, in particular regarding key usage, and can have impact on
|
| + the total performance of the application according to the way it is
|
| + used. Hence, the use of SRTP is dependent on the kind of scenario
|
| + and application it is used with. In the following, we briefly
|
| + illustrate some use cases for SRTP, and give some guidelines for
|
| + recommended setting of its options.
|
| +
|
| +11.1. Unicast
|
| +
|
| + A typical example would be a voice call or video-on-demand
|
| + application.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 43]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Consider one bi-directional RTP stream, as one RTP session. It is
|
| + possible for the two parties to share the same master key in the two
|
| + directions according to the principles of Section 9.1. The first
|
| + round of the key derivation splits the master key into any or all of
|
| + the following session keys (according to the provided security
|
| + functions):
|
| +
|
| + SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.
|
| +
|
| + (For simplicity, we omit discussion of the salts, which are also
|
| + derived.) In this scenario, it will in most cases suffice to have a
|
| + single master key with the default lifetime. This guarantees
|
| + sufficiently long lifetime of the keys and a minimum set of keys in
|
| + place for most practical purposes. Also, in this case RTCP
|
| + protection can be applied smoothly. Under these assumptions, use of
|
| + the MKI can be omitted. As the key-derivation in combination with
|
| + large difference in the packet rate in the respective directions may
|
| + require simultaneous storage of several session keys, if storage is
|
| + an issue, we recommended to use low-rate key derivation.
|
| +
|
| + The same considerations can be extended to the unicast scenario with
|
| + multiple RTP sessions, where each session would have a distinct
|
| + master key.
|
| +
|
| +11.2. Multicast (one sender)
|
| +
|
| + Just as with (unprotected) RTP, a scalability issue arises in big
|
| + groups due to the possibly very large amount of SRTCP Receiver
|
| + Reports that the sender might need to process. In SRTP, the sender
|
| + may have to keep state (the cryptographic context) for each receiver,
|
| + or more precisely, for the SRTCP used to protect Receiver Reports.
|
| + The overhead increases proportionally to the size of the group. In
|
| + particular, re-keying requires special concern, see below.
|
| +
|
| + Consider first a small group of receivers. There are a few possible
|
| + setups with the distribution of master keys among the receivers.
|
| + Given a single RTP session, one possibility is that the receivers
|
| + share the same master key as per Section 9.1 to secure all their
|
| + respective RTCP traffic. This shared master key could then be the
|
| + same one used by the sender to protect its outbound SRTP traffic.
|
| + Alternatively, it could be a master key shared only among the
|
| + receivers and used solely for their SRTCP traffic. Both alternatives
|
| + require the receivers to trust each other.
|
| +
|
| + Considering SRTCP and key storage, it is recommended to use low-rate
|
| + (or zero) key_derivation (except the mandatory initial one), so that
|
| + the sender does not need to store too many session keys (each SRTCP
|
| + stream might otherwise have a different session key at a given point
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 44]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + in time, as the SRTCP sources send at different times). Thus, in
|
| + case key derivation is wanted for SRTP, the cryptographic context for
|
| + SRTP can be kept separate from the SRTCP crypto context, so that it
|
| + is possible to have a key_derivation_rate of 0 for SRTCP and a non-
|
| + zero value for SRTP.
|
| +
|
| + Use of the MKI for re-keying is RECOMMENDED for most applications
|
| + (see Section 8.1).
|
| +
|
| + If there are more than one SRTP/SRTCP stream (within the same RTP
|
| + session) that share the master key, the upper limit of 2^48 SRTP
|
| + packets / 2^31 SRTCP packets means that, before one of the streams
|
| + reaches its maximum number of packets, re-keying MUST be triggered on
|
| + ALL streams sharing the master key. (From strict security point of
|
| + view, only the stream reaching the maximum would need to be re-keyed,
|
| + but then the streams would no longer be sharing master key, which is
|
| + the intention.) A local policy at the sender side should force
|
| + rekeying in a way that the maximum packet limit is not reached on any
|
| + of the streams. Use of the MKI for re-keying is RECOMMENDED.
|
| +
|
| + In large multicast with one sender, the same considerations as for
|
| + the small group multicast hold. The biggest issue in this scenario
|
| + is the additional load placed at the sender side, due to the state
|
| + (cryptographic contexts) that has to be maintained for each receiver,
|
| + sending back RTCP Receiver Reports. At minimum, a replay window
|
| + might need to be maintained for each RTCP source.
|
| +
|
| +11.3. Re-keying and access control
|
| +
|
| + Re-keying may occur due to access control (e.g., when a member is
|
| + removed during a multicast RTP session), or for pure cryptographic
|
| + reasons (e.g., the key is at the end of its lifetime). When using
|
| + SRTP default transforms, the master key MUST be replaced before any
|
| + of the index spaces are exhausted for any of the streams protected by
|
| + one and the same master key.
|
| +
|
| + How key management re-keys SRTP implementations is out of scope, but
|
| + it is clear that there are straightforward ways to manage keys for a
|
| + multicast group. In one-sender multicast, for example, it is
|
| + typically the responsibility of the sender to determine when a new
|
| + key is needed. The sender is the one entity that can keep track of
|
| + when the maximum number of packets has been sent, as receivers may
|
| + join and leave the session at any time, there may be packet loss and
|
| + delay etc. In scenarios other than one-sender multicast, other
|
| + methods can be used. Here, one must take into consideration that key
|
| + exchange can be a costly operation, taking several seconds for a
|
| + single exchange. Hence, some time before the master key is
|
| + exhausted/expires, out-of-band key management is initiated, resulting
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 45]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + in a new master key that is shared with the receiver(s). In any
|
| + event, to maintain synchronization when switching to the new key,
|
| + group policy might choose between using the MKI and the <From, To>,
|
| + as described in Section 8.1.
|
| +
|
| + For access control purposes, the <From, To> periods are set at the
|
| + desired granularity, dependent on the packet rate. High rate re-
|
| + keying can be problematic for SRTCP in some large-group scenarios.
|
| + As mentioned, there are potential problems in using the SRTP index,
|
| + rather than the SRTCP index, for determining the master key. In
|
| + particular, for short periods during switching of master keys, it may
|
| + be the case that SRTCP packets are not under the current master key
|
| + of the correspondent SRTP. Therefore, using the MKI for re-keying in
|
| + such scenarios will produce better results.
|
| +
|
| +11.4. Summary of basic scenarios
|
| +
|
| + The description of these scenarios highlights some recommendations on
|
| + the use of SRTP, mainly related to re-keying and large scale
|
| + multicast:
|
| +
|
| + - Do not use fast re-keying with the <From, To> feature. It may, in
|
| + particular, give problems in retrieving the correct SRTCP key, if
|
| + an SRTCP packet arrives close to the re-keying time. The MKI
|
| + SHOULD be used in this case.
|
| +
|
| + - If multiple SRTP streams in the same RTP session share the same
|
| + master key, also moderate rate re-keying MAY have the same
|
| + problems, and the MKI SHOULD be used.
|
| +
|
| + - Though offering increased security, a non-zero key_derivation_rate
|
| + is NOT RECOMMENDED when trying to minimize the number of keys in
|
| + use with multiple streams.
|
| +
|
| +12. IANA Considerations
|
| +
|
| + The RTP specification establishes a registry of profile names for use
|
| + by higher-level control protocols, such as the Session Description
|
| + Protocol (SDP), to refer to transport methods. This profile
|
| + registers the name "RTP/SAVP".
|
| +
|
| + SRTP uses cryptographic transforms which a key management protocol
|
| + signals. It is the task of each particular key management protocol
|
| + to register the cryptographic transforms or suites of transforms with
|
| + IANA. The key management protocol conveys these protocol numbers,
|
| + not SRTP, and each key management protocol chooses the numbering
|
| + scheme and syntax that it requires.
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 46]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Specification of a key management protocol for SRTP is out of scope
|
| + here. Section 8.2, however, provides guidance on the parameters that
|
| + need to be defined for the default and mandatory transforms.
|
| +
|
| +13. Acknowledgements
|
| +
|
| + David Oran (Cisco) and Rolf Blom (Ericsson) are co-authors of this
|
| + document but their valuable contributions are acknowledged here to
|
| + keep the length of the author list down.
|
| +
|
| + The authors would in addition like to thank Magnus Westerlund, Brian
|
| + Weis, Ghyslain Pelletier, Morgan Lindqvist, Robert Fairlie-
|
| + Cuninghame, Adrian Perrig, the AVT WG and in particular the chairmen
|
| + Colin Perkins and Stephen Casner, the Transport and Security Area
|
| + Directors, and Eric Rescorla for their reviews and support.
|
| +
|
| +14. References
|
| +
|
| +14.1. Normative References
|
| +
|
| + [AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
|
| + http://www.nist.gov/aes/
|
| +
|
| + [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
|
| + Hashing for Message Authentication", RFC 2104, February
|
| + 1997.
|
| +
|
| + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
|
| + Requirement Levels", BCP 14, RFC 2119, March 1997.
|
| +
|
| + [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for
|
| + Internet Protocol", RFC 2401, November 1998.
|
| +
|
| + [RFC2828] Shirey, R., "Internet Security Glossary", FYI 36, RFC 2828,
|
| + May 2000.
|
| +
|
| + [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
|
| + "RTP: A Transport Protocol for Real-time Applications", RFC
|
| + 3550, July 2003.
|
| +
|
| + [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
|
| + Video Conferences with Minimal Control", RFC 3551, July
|
| + 2003.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 47]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +14.2. Informative References
|
| +
|
| + [AES-CTR] Lipmaa, H., Rogaway, P. and D. Wagner, "CTR-Mode
|
| + Encryption", NIST, http://csrc.nist.gov/encryption/modes/
|
| + workshop1/papers/lipmaa-ctr.pdf
|
| +
|
| + [B96] Bellovin, S., "Problem Areas for the IP Security
|
| + Protocols," in Proceedings of the Sixth Usenix Unix
|
| + Security Symposium, pp. 1-16, San Jose, CA, July 1996
|
| + (http://www.research.att.com/~smb/papers/index.html).
|
| +
|
| + [BDJR] Bellare, M., Desai, A., Jokipii, E. and P. Rogaway, "A
|
| + Concrete Treatment of Symmetric Encryption: Analysis of DES
|
| + Modes of Operation", Proceedings 38th IEEE FOCS, pp. 394-
|
| + 403, 1997.
|
| +
|
| + [BS00] Biryukov, A. and A. Shamir, "Cryptanalytic Time/Memory/Data
|
| + Tradeoffs for Stream Ciphers", Proceedings, ASIACRYPT 2000,
|
| + LNCS 1976, pp. 1-13, Springer Verlag.
|
| +
|
| + [C99] Crowell, W. P., "Introduction to the VENONA Project",
|
| + http://www.nsa.gov:8080/docs/venona/index.html.
|
| +
|
| + [CTR] Dworkin, M., NIST Special Publication 800-38A,
|
| + "Recommendation for Block Cipher Modes of Operation:
|
| + Methods and Techniques", 2001.
|
| + http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-
|
| + 38a.pdf.
|
| +
|
| + [f8-a] 3GPP TS 35.201 V4.1.0 (2001-12) Technical Specification 3rd
|
| + Generation Partnership Project; Technical Specification
|
| + Group Services and System Aspects; 3G Security;
|
| + Specification of the 3GPP Confidentiality and Integrity
|
| + Algorithms; Document 1: f8 and f9 Specification (Release
|
| + 4).
|
| +
|
| + [f8-b] 3GPP TR 33.908 V4.0.0 (2001-09) Technical Report 3rd
|
| + Generation Partnership Project; Technical Specification
|
| + Group Services and System Aspects; 3G Security; General
|
| + Report on the Design, Specification and Evaluation of 3GPP
|
| + Standard Confidentiality and Integrity Algorithms (Release
|
| + 4).
|
| +
|
| + [GDOI] Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The
|
| + Group Domain of Interpretation, RFC 3547, July 2003.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 48]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + [HAC] Menezes, A., Van Oorschot, P. and S. Vanstone, "Handbook
|
| + of Applied Cryptography", CRC Press, 1997, ISBN 0-8493-
|
| + 8523-7.
|
| +
|
| + [H80] Hellman, M. E., "A cryptanalytic time-memory trade-off",
|
| + IEEE Transactions on Information Theory, July 1980, pp.
|
| + 401-406.
|
| +
|
| + [IK] T. Iwata and T. Kohno: "New Security Proofs for the 3GPP
|
| + Confidentiality and Integrity Algorithms", Proceedings of
|
| + FSE 2004.
|
| +
|
| + [KINK] Thomas, M. and J. Vilhuber, "Kerberized Internet
|
| + Negotiation of Keys (KINK)", Work in Progress.
|
| +
|
| + [KEYMGT] Arrko, J., et al., "Key Management Extensions for Session
|
| + Description Protocol (SDP) and Real Time Streaming Protocol
|
| + (RTSP)", Work in Progress.
|
| +
|
| + [KSYH] Kang, J-S., Shin, S-U., Hong, D. and O. Yi, "Provable
|
| + Security of KASUMI and 3GPP Encryption Mode f8",
|
| + Proceedings Asiacrypt 2001, Springer Verlag LNCS 2248, pp.
|
| + 255-271, 2001.
|
| +
|
| + [MIKEY] Arrko, J., et. al., "MIKEY: Multimedia Internet KEYing",
|
| + Work in Progress.
|
| +
|
| + [MF00] McGrew, D. and S. Fluhrer, "Attacks on Encryption of
|
| + Redundant Plaintext and Implications on Internet Security",
|
| + the Proceedings of the Seventh Annual Workshop on Selected
|
| + Areas in Cryptography (SAC 2000), Springer-Verlag.
|
| +
|
| + [PCST1] Perrig, A., Canetti, R., Tygar, D. and D. Song, "Efficient
|
| + and Secure Source Authentication for Multicast", in Proc.
|
| + of Network and Distributed System Security Symposium NDSS
|
| + 2001, pp. 35-46, 2001.
|
| +
|
| + [PCST2] Perrig, A., Canetti, R., Tygar, D. and D. Song, "Efficient
|
| + Authentication and Signing of Multicast Streams over Lossy
|
| + Channels", in Proc. of IEEE Security and Privacy Symposium
|
| + S&P2000, pp. 56-73, 2000.
|
| +
|
| + [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
|
| + Recommendations for Security", RFC 1750, December 1994.
|
| +
|
| + [RFC2675] Borman, D., Deering, S. and R. Hinden, "IPv6 Jumbograms",
|
| + RFC 2675, August 1999.
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 49]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukuhsima, H.,
|
| + Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
|
| + Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke,
|
| + T., Yoshimura, T. and H. Zheng, "RObust Header Compression:
|
| + Framework and Four Profiles: RTP, UDP, ESP, and
|
| + uncompressed (ROHC)", RFC 3095, July 2001.
|
| +
|
| + [RFC3242] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
|
| + (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP ", RFC
|
| + 3242, April 2002.
|
| +
|
| + [SDMS] Andreasen, F., Baugher, M. and D. Wing, "Session
|
| + Description Protocol Security Descriptions for Media
|
| + Streams", Work in Progress.
|
| +
|
| + [SWO] Svanbro, K., Wiorek, J. and B. Olin, "Voice-over-IP-over-
|
| + wireless", Proc. PIMRC 2000, London, Sept. 2000.
|
| +
|
| + [V02] Vaudenay, S., "Security Flaws Induced by CBC Padding -
|
| + Application to SSL, IPsec, WTLS...", Advances in
|
| + Cryptology, EUROCRYPT'02, LNCS 2332, pp. 534-545.
|
| +
|
| + [WC81] Wegman, M. N., and J.L. Carter, "New Hash Functions and
|
| + Their Use in Authentication and Set Equality", JCSS 22,
|
| + 265-279, 1981.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 50]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +Appendix A: Pseudocode for Index Determination
|
| +
|
| + The following is an example of pseudo-code for the algorithm to
|
| + determine the index i of an SRTP packet with sequence number SEQ. In
|
| + the following, signed arithmetic is assumed.
|
| +
|
| + if (s_l < 32,768)
|
| + if (SEQ - s_l > 32,768)
|
| + set v to (ROC-1) mod 2^32
|
| + else
|
| + set v to ROC
|
| + endif
|
| + else
|
| + if (s_l - 32,768 > SEQ)
|
| + set v to (ROC+1) mod 2^32
|
| + else
|
| + set v to ROC
|
| + endif
|
| + endif
|
| + return SEQ + v*65,536
|
| +
|
| +Appendix B: Test Vectors
|
| +
|
| + All values are in hexadecimal.
|
| +
|
| +B.1. AES-f8 Test Vectors
|
| +
|
| + SRTP PREFIX LENGTH : 0
|
| +
|
| + RTP packet header : 806e5cba50681de55c621599
|
| +
|
| + RTP packet payload : 70736575646f72616e646f6d6e657373
|
| + 20697320746865206e65787420626573
|
| + 74207468696e67
|
| +
|
| + ROC : d462564a
|
| + key : 234829008467be186c3de14aae72d62c
|
| + salt key : 32f2870d
|
| + key-mask (m) : 32f2870d555555555555555555555555
|
| + key XOR key-mask : 11baae0dd132eb4d3968b41ffb278379
|
| +
|
| + IV : 006e5cba50681de55c621599d462564a
|
| + IV' : 595b699bbd3bc0df26062093c1ad8f73
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 51]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + j = 0
|
| + IV' xor j : 595b699bbd3bc0df26062093c1ad8f73
|
| + S(-1) : 00000000000000000000000000000000
|
| + IV' xor S(-1) xor j : 595b699bbd3bc0df26062093c1ad8f73
|
| + S(0) : 71ef82d70a172660240709c7fbb19d8e
|
| + plaintext : 70736575646f72616e646f6d6e657373
|
| + ciphertext : 019ce7a26e7854014a6366aa95d4eefd
|
| +
|
| + j = 1
|
| + IV' xor j : 595b699bbd3bc0df26062093c1ad8f72
|
| + S(0) : 71ef82d70a172660240709c7fbb19d8e
|
| + IV' xor S(0) xor j : 28b4eb4cb72ce6bf020129543a1c12fc
|
| + S(1) : 3abd640a60919fd43bd289a09649b5fc
|
| + plaintext : 20697320746865206e65787420626573
|
| + ciphertext : 1ad4172a14f9faf455b7f1d4b62bd08f
|
| +
|
| + j = 2
|
| + IV' xor j : 595b699bbd3bc0df26062093c1ad8f71
|
| + S(1) : 3abd640a60919fd43bd289a09649b5fc
|
| + IV' xor S(1) xor j : 63e60d91ddaa5f0b1dd4a93357e43a8d
|
| + S(2) : 220c7a8715266565b09ecc8a2a62b11b
|
| + plaintext : 74207468696e67
|
| + ciphertext : 562c0eef7c4802
|
| +
|
| +B.2. AES-CM Test Vectors
|
| +
|
| + Keystream segment length: 1044512 octets (65282 AES blocks)
|
| + Session Key: 2B7E151628AED2A6ABF7158809CF4F3C
|
| + Rollover Counter: 00000000
|
| + Sequence Number: 0000
|
| + SSRC: 00000000
|
| + Session Salt: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 (already shifted)
|
| + Offset: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000
|
| +
|
| + Counter Keystream
|
| +
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 E03EAD0935C95E80E166B16DD92B4EB4
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001 D23513162B02D0F72A43A2FE4A5F97AB
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002 41E95B3BB0A2E8DD477901E4FCA894C0
|
| + ... ...
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF EC8CDF7398607CB0F2D21675EA9EA1E4
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00 362B7C3C6773516318A077D7FC5073AE
|
| + F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01 6A2CC3787889374FBEB4C81B17BA6C44
|
| +
|
| + Nota Bene: this test case is contrived so that the latter part of the
|
| + keystream segment coincides with the test case in Section F.5.1 of
|
| + [CTR].
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 52]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +B.3. Key Derivation Test Vectors
|
| +
|
| + This section provides test data for the default key derivation
|
| + function, which uses AES-128 in Counter Mode. In the following, we
|
| + walk through the initial key derivation for the AES-128 Counter Mode
|
| + cipher, which requires a 16 octet session encryption key and a 14
|
| + octet session salt, and an authentication function which requires a
|
| + 94-octet session authentication key. These values are called the
|
| + cipher key, the cipher salt, and the auth key in the following.
|
| + Since this is the initial key derivation and the key derivation rate
|
| + is equal to zero, the value of (index DIV key_derivation_rate) is
|
| + zero (actually, a six-octet string of zeros). In the following, we
|
| + shorten key_derivation_rate to kdr.
|
| +
|
| + The inputs to the key derivation function are the 16 octet master key
|
| + and the 14 octet master salt:
|
| +
|
| + master key: E1F97A0D3E018BE0D64FA32C06DE4139
|
| + master salt: 0EC675AD498AFEEBB6960B3AABE6
|
| +
|
| + We first show how the cipher key is generated. The input block for
|
| + AES-CM is generated by exclusive-oring the master salt with the
|
| + concatenation of the encryption key label 0x00 with (index DIV kdr),
|
| + then padding on the right with two null octets (which implements the
|
| + multiply-by-2^16 operation, see Section 4.3.3). The resulting value
|
| + is then AES-CM- encrypted using the master key to get the cipher key.
|
| +
|
| + index DIV kdr: 000000000000
|
| + label: 00
|
| + master salt: 0EC675AD498AFEEBB6960B3AABE6
|
| + -----------------------------------------------
|
| + xor: 0EC675AD498AFEEBB6960B3AABE6 (x, PRF input)
|
| +
|
| + x*2^16: 0EC675AD498AFEEBB6960B3AABE60000 (AES-CM input)
|
| +
|
| + cipher key: C61E7A93744F39EE10734AFE3FF7A087 (AES-CM output)
|
| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
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| +
|
| +Baugher, et al. Standards Track [Page 53]
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| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| + Next, we show how the cipher salt is generated. The input block for
|
| + AES-CM is generated by exclusive-oring the master salt with the
|
| + concatenation of the encryption salt label. That value is padded and
|
| + encrypted as above.
|
| +
|
| + index DIV kdr: 000000000000
|
| + label: 02
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| + master salt: 0EC675AD498AFEEBB6960B3AABE6
|
| +
|
| + ----------------------------------------------
|
| + xor: 0EC675AD498AFEE9B6960B3AABE6 (x, PRF input)
|
| +
|
| + x*2^16: 0EC675AD498AFEE9B6960B3AABE60000 (AES-CM input)
|
| +
|
| + 30CBBC08863D8C85D49DB34A9AE17AC6 (AES-CM ouptut)
|
| +
|
| + cipher salt: 30CBBC08863D8C85D49DB34A9AE1
|
| +
|
| + We now show how the auth key is generated. The input block for AES-
|
| + CM is generated as above, but using the authentication key label.
|
| +
|
| + index DIV kdr: 000000000000
|
| + label: 01
|
| + master salt: 0EC675AD498AFEEBB6960B3AABE6
|
| + -----------------------------------------------
|
| + xor: 0EC675AD498AFEEAB6960B3AABE6 (x, PRF input)
|
| +
|
| + x*2^16: 0EC675AD498AFEEAB6960B3AABE60000 (AES-CM input)
|
| +
|
| + Below, the auth key is shown on the left, while the corresponding AES
|
| + input blocks are shown on the right.
|
| +
|
| + auth key AES input blocks
|
| + CEBE321F6FF7716B6FD4AB49AF256A15 0EC675AD498AFEEAB6960B3AABE60000
|
| + 6D38BAA48F0A0ACF3C34E2359E6CDBCE 0EC675AD498AFEEAB6960B3AABE60001
|
| + E049646C43D9327AD175578EF7227098 0EC675AD498AFEEAB6960B3AABE60002
|
| + 6371C10C9A369AC2F94A8C5FBCDDDC25 0EC675AD498AFEEAB6960B3AABE60003
|
| + 6D6E919A48B610EF17C2041E47403576 0EC675AD498AFEEAB6960B3AABE60004
|
| + 6B68642C59BBFC2F34DB60DBDFB2 0EC675AD498AFEEAB6960B3AABE60005
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 54]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +Authors' Addresses
|
| +
|
| + Questions and comments should be directed to the authors and
|
| + avt@ietf.org:
|
| +
|
| + Mark Baugher
|
| + Cisco Systems, Inc.
|
| + 5510 SW Orchid Street
|
| + Portland, OR 97219 USA
|
| +
|
| + Phone: +1 408-853-4418
|
| + EMail: mbaugher@cisco.com
|
| +
|
| +
|
| + Elisabetta Carrara
|
| + Ericsson Research
|
| + SE-16480 Stockholm
|
| + Sweden
|
| +
|
| + Phone: +46 8 50877040
|
| + EMail: elisabetta.carrara@ericsson.com
|
| +
|
| +
|
| + David A. McGrew
|
| + Cisco Systems, Inc.
|
| + San Jose, CA 95134-1706
|
| + USA
|
| +
|
| + Phone: +1 301-349-5815
|
| + EMail: mcgrew@cisco.com
|
| +
|
| +
|
| + Mats Naslund
|
| + Ericsson Research
|
| + SE-16480 Stockholm
|
| + Sweden
|
| +
|
| + Phone: +46 8 58533739
|
| + EMail: mats.naslund@ericsson.com
|
| +
|
| +
|
| + Karl Norrman
|
| + Ericsson Research
|
| + SE-16480 Stockholm
|
| + Sweden
|
| +
|
| + Phone: +46 8 4044502
|
| + EMail: karl.norrman@ericsson.com
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 55]
|
| +
|
| +RFC 3711 SRTP March 2004
|
| +
|
| +
|
| +Full Copyright Statement
|
| +
|
| + Copyright (C) The Internet Society (2004). This document is subject
|
| + to the rights, licenses and restrictions contained in BCP 78 and
|
| + except as set forth therein, the authors retain all their rights.
|
| +
|
| + This document and the information contained herein are provided on an
|
| + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
|
| + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
|
| + ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
|
| + INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
|
| + INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
|
| + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
|
| +
|
| +Intellectual Property
|
| +
|
| + The IETF takes no position regarding the validity or scope of any
|
| + Intellectual Property Rights or other rights that might be claimed to
|
| + pertain to the implementation or use of the technology described in
|
| + this document or the extent to which any license under such rights
|
| + might or might not be available; nor does it represent that it has
|
| + made any independent effort to identify any such rights. Information
|
| + on the procedures with respect to rights in RFC documents can be
|
| + found in BCP 78 and BCP 79.
|
| +
|
| + Copies of IPR disclosures made to the IETF Secretariat and any
|
| + assurances of licenses to be made available, or the result of an
|
| + attempt made to obtain a general license or permission for the use of
|
| + such proprietary rights by implementers or users of this
|
| + specification can be obtained from the IETF on-line IPR repository at
|
| + http://www.ietf.org/ipr.
|
| +
|
| + The IETF invites any interested party to bring to its attention any
|
| + copyrights, patents or patent applications, or other proprietary
|
| + rights that may cover technology that may be required to implement
|
| + this standard. Please address the information to the IETF at ietf-
|
| + ipr@ietf.org.
|
| +
|
| +Acknowledgement
|
| +
|
| + Funding for the RFC Editor function is currently provided by the
|
| + Internet Society.
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +Baugher, et al. Standards Track [Page 56]
|
| +
|
|
|
| Property changes on: libsrtp/doc/rfc3711.txt
|
| ___________________________________________________________________
|
| Added: svn:eol-style
|
| + LF
|
|
|
|
|
Chromium Code Reviews
Issue 3423016:
Add current version of libSRTP from CVS. (Closed)
Base URL: svn://svn.chromium.org/chrome/trunk/deps/third_party/