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RFC 3711

 Errata 
Proposed STD
Pages: 56
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The Secure Real-time Transport Protocol (SRTP)

Part 1 of 3, p. 1 to 19
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Updated by:    5506    6904


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Network Working Group                                         M. Baugher
Request for Comments: 3711                                     D. McGrew
Category: Standards Track                            Cisco Systems, Inc.
                                                              M. Naslund
                                                              E. Carrara
                                                              K. Norrman
                                                       Ericsson Research
                                                              March 2004


             The Secure Real-time Transport Protocol (SRTP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document describes the Secure Real-time Transport Protocol
   (SRTP), a profile of the Real-time Transport Protocol (RTP), which
   can provide confidentiality, message authentication, and replay
   protection to the RTP traffic and to the control traffic for RTP, the
   Real-time Transport Control Protocol (RTCP).

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Notational Conventions . . . . . . . . . . . . . . . . .  3
   2.  Goals and Features . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.  Features . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  SRTP Framework . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.  Secure RTP . . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.  SRTP Cryptographic Contexts. . . . . . . . . . . . . . .  7
             3.2.1.  Transform-independent parameters . . . . . . . .  8
             3.2.2.  Transform-dependent parameters . . . . . . . . . 10
             3.2.3.  Mapping SRTP Packets to Cryptographic Contexts . 10
       3.3.  SRTP Packet Processing . . . . . . . . . . . . . . . . . 11
             3.3.1.  Packet Index Determination, and ROC, s_l Update. 13
             3.3.2.  Replay Protection. . . . . . . . . . . . . . . . 15
      3.4.  Secure RTCP . . . . . . . . . . . . . . . . . . . . . . . 15

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   4.  Pre-Defined Cryptographic Transforms . . . . . . . . . . . . . 19
       4.1.  Encryption . . . . . . . . . . . . . . . . . . . . . . . 19
             4.1.1.  AES in Counter Mode. . . . . . . . . . . . . . . 21
             4.1.2.  AES in f8-mode . . . . . . . . . . . . . . . . . 22
             4.1.3.  NULL Cipher. . . . . . . . . . . . . . . . . . . 25
       4.2.  Message Authentication and Integrity . . . . . . . . . . 25
             4.2.1.  HMAC-SHA1. . . . . . . . . . . . . . . . . . . . 25
       4.3.  Key Derivation . . . . . . . . . . . . . . . . . . . . . 26
             4.3.1.  Key Derivation Algorithm . . . . . . . . . . . . 26
             4.3.2.  SRTCP Key Derivation . . . . . . . . . . . . . . 28
             4.3.3.  AES-CM PRF . . . . . . . . . . . . . . . . . . . 28
   5.  Default and mandatory-to-implement Transforms. . . . . . . . . 28
       5.1.  Encryption: AES-CM and NULL. . . . . . . . . . . . . . . 29
       5.2.  Message Authentication/Integrity: HMAC-SHA1. . . . . . . 29
       5.3.  Key Derivation: AES-CM PRF . . . . . . . . . . . . . . . 29
   6.  Adding SRTP Transforms . . . . . . . . . . . . . . . . . . . . 29
   7.  Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . 30
       7.1.  Key derivation . . . . . . . . . . . . . . . . . . . . . 30
       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

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       B.2.  AES-CM Test Vectors. . . . . . . . . . . . . . . . . . . 52
       B.3.  Key Derivation Test Vectors. . . . . . . . . . . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 56

1.  Introduction

   This document describes the Secure Real-time Transport Protocol
   (SRTP), a profile of the Real-time Transport Protocol (RTP), which
   can provide confidentiality, message authentication, and replay
   protection to the RTP traffic and to the control traffic for RTP,
   RTCP (the Real-time Transport Control Protocol) [RFC3350].

   SRTP provides a framework for encryption and message authentication
   of RTP and RTCP streams (Section 3).  SRTP defines a set of default
   cryptographic transforms (Sections 4 and 5), and it allows new
   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.

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   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.

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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.

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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

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   that use padding are vulnerable to subtle attacks, especially when
   message authentication is not used [V02].  Each specification for a
   new encryption transform needs to carefully consider and describe the
   security implications of the padding that it uses.  Message
   authentication codes define their own padding, so this default does
   not apply to authentication transforms.

   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

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   cryptographically secure way.  The master key(s) and other parameters
   in the cryptographic context are provided by key management
   mechanisms external to SRTP, see Section 8.

3.2.1.  Transform-independent parameters

   Transform-independent parameters are present in the cryptographic
   context independently of the particular encryption or authentication
   transforms that are used.  The transform-independent parameters of
   the cryptographic context for SRTP consist of:

   *  a 32-bit unsigned rollover counter (ROC), which records how many
      times the 16-bit RTP sequence number has been reset to zero after
      passing through 65,535.  Unlike the sequence number (SEQ), which
      SRTP extracts from the RTP packet header, the ROC is maintained by
      SRTP as described in Section 3.3.1.

      We define the index of the SRTP packet corresponding to a given
      ROC and RTP sequence number to be the 48-bit quantity

            i = 2^16 * ROC + SEQ.

   *  for the receiver only, a 16-bit sequence number s_l, which can be
      thought of as the highest received RTP sequence number (see
      Section 3.3.1 for its handling), which SHOULD be authenticated
      since message authentication is RECOMMENDED,

   *  an identifier for the encryption algorithm, i.e., the cipher and
      its mode of operation,

   *  an identifier for the message authentication algorithm,

   *  a replay list, maintained by the receiver only (when
      authentication and replay protection are provided), containing
      indices of recently received and authenticated SRTP packets,

   *  an MKI indicator (0/1) as to whether an MKI is present in SRTP and
      SRTCP packets,

   *  if the MKI indicator is set to one, the length (in octets) of the
      MKI field, and (for the sender) the actual value of the currently
      active MKI (the value of the MKI indicator and length MUST be kept
      fixed for the lifetime of the context),

   *  the master key(s), which MUST be random and kept secret,

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   *  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.

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   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

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   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

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      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.

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   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).

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   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.

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   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.

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   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.

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   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

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      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.

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   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.



(page 19 continued on part 2)

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