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diff --git a/doc/rfc3711-srtp.txt b/doc/rfc3711-srtp.txt deleted file mode 100644 index ecc0648..0000000 --- a/doc/rfc3711-srtp.txt +++ /dev/null @@ -1,3139 +0,0 @@ - - - - - - -Network Working Group M. Baugher -Request for Comments: 3711 D. McGrew -Category: Standards Track Cisco Systems, Inc. - M. Naslund - E. Carrara - K. Norrman - Ericsson Research - March 2004 - - - The Secure Real-time Transport Protocol (SRTP) - -Status of this Memo - - This document specifies an Internet standards track protocol for the - Internet community, and requests discussion and suggestions for - improvements. Please refer to the current edition of the "Internet - Official Protocol Standards" (STD 1) for the standardization state - and status of this protocol. Distribution of this memo is unlimited. - -Copyright Notice - - 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). - -Table of Contents - - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 - 2. Goals and Features . . . . . . . . . . . . . . . . . . . . . . 4 - 2.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 5 - 3. SRTP Framework . . . . . . . . . . . . . . . . . . . . . . . . 5 - 3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 6 - 3.2. SRTP Cryptographic Contexts. . . . . . . . . . . . . . . 7 - 3.2.1. Transform-independent parameters . . . . . . . . 8 - 3.2.2. Transform-dependent parameters . . . . . . . . . 10 - 3.2.3. Mapping SRTP Packets to Cryptographic Contexts . 10 - 3.3. SRTP Packet Processing . . . . . . . . . . . . . . . . . 11 - 3.3.1. Packet Index Determination, and ROC, s_l Update. 13 - 3.3.2. Replay Protection. . . . . . . . . . . . . . . . 15 - 3.4. Secure RTCP . . . . . . . . . . . . . . . . . . . . . . . 15 - - - -Baugher, et al. Standards Track [Page 1] - -RFC 3711 SRTP March 2004 - - - 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 - 7.2. Salting key. . . . . . . . . . . . . . . . . . . . . . . 30 - 7.3. Message Integrity from Universal Hashing . . . . . . . . 31 - 7.4. Data Origin Authentication Considerations. . . . . . . . 31 - 7.5. Short and Zero-length Message Authentication . . . . . . 32 - 8. Key Management Considerations. . . . . . . . . . . . . . . . . 33 - 8.1. Re-keying . . . . . . . . . . . . . . . . . . . . . . . 34 - 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 - 9.5.1. Risks of Weak or Null Message Authentication. . . 42 - 9.5.2. Implicit Header Authentication . . . . . . . . . 43 - 10. Interaction with Forward Error Correction mechanisms. . . . . 43 - 11. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 43 - 11.1. Unicast. . . . . . . . . . . . . . . . . . . . . . . . . 43 - 11.2. Multicast (one sender) . . . . . . . . . . . . . . . . . 44 - 11.3. Re-keying and access control . . . . . . . . . . . . . . 45 - 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 - - - -Baugher, et al. Standards Track [Page 2] - -RFC 3711 SRTP March 2004 - - - 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 - transforms to be introduced in the future (Section 6). With - appropriate key management (Sections 7 and 8), SRTP is secure - (Sections 9) for unicast and multicast RTP applications (Section 11). - - 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 - transforms are described, based on an additive stream cipher for - encryption, a keyed-hash based function for message authentication, - and an "implicit" index for sequencing/synchronization based on the - RTP sequence number for SRTP and an index number for Secure RTCP - (SRTCP). - -1.1. Notational Conventions - - The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", - "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this - document are to be interpreted as described in [RFC2119]. The - terminology conforms to [RFC2828] with the following exception. For - simplicity we use the term "random" throughout the document to denote - randomly or pseudo-randomly generated values. Large amounts of - random bits may be difficult to obtain, and for the security of SRTP, - pseudo-randomness is sufficient [RFC1750]. - - By convention, the adopted representation is the network byte order, - i.e., the left most bit (octet) is the most significant one. By XOR - we mean bitwise addition modulo 2 of binary strings, and || denotes - concatenation. In other words, if C = A || B, then the most - significant bits of C are the bits of A, and the least significant - bits of C equal the bits of B. Hexadecimal numbers are prefixed by - 0x. - - - - -Baugher, et al. Standards Track [Page 3] - -RFC 3711 SRTP March 2004 - - - The word "encryption" includes also use of the NULL algorithm (which - in practice does leave the data in the clear). - - With slight abuse of notation, we use the terms "message - authentication" and "authentication tag" as is common practice, even - though in some circumstances, e.g., group communication, the service - provided is actually only integrity protection and not data origin - authentication. - -2. Goals and Features - - The security goals for SRTP are to ensure: - - * the confidentiality of the RTP and RTCP payloads, and - - * the integrity of the entire RTP and RTCP packets, together with - protection against replayed packets. - - These security services are optional and independent from each other, - except that SRTCP integrity protection is mandatory (malicious or - erroneous alteration of RTCP messages could otherwise disrupt the - processing of the RTP stream). - - Other, functional, goals for the protocol are: - - * a framework that permits upgrading with new cryptographic - transforms, - - * low bandwidth cost, i.e., a framework preserving RTP header - compression efficiency, - - and, asserted by the pre-defined transforms: - - * a low computational cost, - - * a small footprint (i.e., small code size and data memory for - keying information and replay lists), - - * limited packet expansion to support the bandwidth economy goal, - - * independence from the underlying transport, network, and physical - layers used by RTP, in particular high tolerance to packet loss - and re-ordering. - - These properties ensure that SRTP is a suitable protection scheme for - RTP/RTCP in both wired and wireless scenarios. - - - - - -Baugher, et al. Standards Track [Page 4] - -RFC 3711 SRTP March 2004 - - -2.1. Features - - Besides the above mentioned direct goals, SRTP provides for some - additional features. They have been introduced to lighten the burden - on key management and to further increase security. They include: - - * A single "master key" can provide keying material for - 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" - for the respective security primitive, securely derived from the - master key. - - * In addition, the key derivation can be configured to periodically - refresh the session keys, which limits the amount of ciphertext - produced by a fixed key, available for an adversary to - cryptanalyze. - - * "Salting keys" are used to protect against pre-computation and - time-memory tradeoff attacks [MF00] [BS00]. - - Detailed rationale for these features can be found in Section 7. - -3. SRTP Framework - - 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 - features. Conceptually, we consider SRTP to be a "bump in the stack" - implementation which resides between the RTP application and the - transport layer. SRTP intercepts RTP packets and then forwards an - equivalent SRTP packet on the sending side, and intercepts SRTP - packets and passes an equivalent RTP packet up the stack on the - receiving side. - - Secure RTCP (SRTCP) provides the same security services to RTCP as - SRTP does to RTP. SRTCP message authentication is MANDATORY and - thereby protects the RTCP fields to keep track of membership, provide - feedback to RTP senders, or maintain packet sequence counters. SRTCP - is described in Section 3.4. - - - - - - - - - - -Baugher, et al. Standards Track [Page 5] - -RFC 3711 SRTP March 2004 - - -3.1. Secure RTP - - The format of an SRTP packet is illustrated in Figure 1. - - 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|X| CC |M| PT | sequence number | | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | timestamp | | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | synchronization source (SSRC) identifier | | - +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | - | contributing source (CSRC) identifiers | | - | .... | | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | RTP extension (OPTIONAL) | | - +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | | payload ... | | - | | +-------------------------------+ | - | | | RTP padding | RTP pad count | | - +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ - | ~ SRTP MKI (OPTIONAL) ~ | - | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | : authentication tag (RECOMMENDED) : | - | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | - | | - +- Encrypted Portion* Authenticated Portion ---+ - - Figure 1. The format of an SRTP packet. *Encrypted Portion is the - same size as the plaintext for the Section 4 pre-defined transforms. - - 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]. - - None of the pre-defined encryption transforms uses any padding; for - these, the RTP and SRTP payload sizes match exactly. New transforms - added to SRTP (following Section 6) may require padding, and may - 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 - - - -Baugher, et al. Standards Track [Page 6] - -RFC 3711 SRTP March 2004 - - - 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. - - 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. - - 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 - - - - - -Baugher, et al. Standards Track [Page 7] - -RFC 3711 SRTP March 2004 - - - 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) - - - - - - - - - - - - - - - -Baugher, et al. Standards Track [Page 53] - -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 - 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). 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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] - |