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rfc:rfc3711

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:
  1. k_e (SRTP encryption): <label> = 0x00, n = n_e.
  1. k_a (SRTP message authentication): <label> = 0x01, n = n_a.
  1. 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:
  1. 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.
  1. 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.
  1. 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
  1. ———————————————

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

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 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
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Baugher, et al. Standards Track [Page 56]

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