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+
+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). This document is subject
+ to the rights, licenses and restrictions contained in BCP 78 and
+ except as set forth therein, the authors retain all their rights.
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Intellectual Property
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; nor does it represent that it has
+ made any independent effort to identify any such rights. Information
+ on the procedures with respect to rights in RFC documents can be
+ found in BCP 78 and BCP 79.
+
+ Copies of IPR disclosures made to the IETF Secretariat and any
+ assurances of licenses to be made available, or the result of an
+ attempt made to obtain a general license or permission for the use of
+ such proprietary rights by implementers or users of this
+ specification can be obtained from the IETF on-line IPR repository at
+ http://www.ietf.org/ipr.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights that may cover technology that may be required to implement
+ this standard. Please address the information to the IETF at ietf-
+ ipr@ietf.org.
+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+
+
+
+
+
+Baugher, et al. Standards Track [Page 56]
+