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Versions: (draft-ietf-avt-srtp-aes-gcm) 00 01 02 03 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 7714

Network Working Group                                          D. McGrew
Internet Draft                                       Cisco Systems, Inc.
Intended Status: Standards Track                                 K. Igoe
Expires: January 29, 2015                       National Security Agency
                                                           July 28, 2014


    AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
                   draft-ietf-avtcore-srtp-aes-gcm-14


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   This Internet-Draft will expire on January 29, 2015.

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Abstract

   This document defines how AES-GCM and AES-CCM Authenticated
   Encryption with Associated Data algorithms can be used to provide
   confidentiality and data authentication in the SRTP protocol.


Table of Contents

   1. Introduction.....................................................3
   2. Conventions Used In This Document................................4
   3. Overview of the SRTP/SRTCP AEAD security Architecture............4
   4. Terminology......................................................5
   5. Generic AEAD Processing..........................................5
      5.1. Types of Input Data.........................................5
      5.2. AEAD Invocation Inputs and Outputs..........................6
         5.2.1. Encrypt Mode...........................................6
         5.2.2. Decrypt Mode...........................................6
      5.3. Handling of AEAD Authentication.............................7
   6. Counter Mode Encryption..........................................7
   7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................8
   8. Unneeded SRTP/SRTCP Fields.......................................9
      8.1. SRTP/SRTCP Authentication Field.............................9
      8.2. RTP Padding.................................................9
   9. AES-GCM/CCM processing for SRTP.................................10
      9.1. SRTP IV formation for AES-GCM and AES-CCM..................10
      9.2. Data Types in SRTP Packets.................................10
      9.3. Handling Header Extensions.................................12
      9.4. Prevention of SRTP IV Reuse................................13
   10. AES-GCM/CCM Processing of SRTCP Compound Packets...............14
      10.1. SRTCP IV formation for AES-GCM and AES-CCM................14
      10.2. Data Types in Encrypted SRTCP Compound Packets............15
      10.3. Data Types in Unencrypted SRTCP Compound Packets..........16
      10.4. Prevention of SRTCP IV Reuse..............................17
   11. Constraints on AEAD for SRTP and SRTCP.........................17
   12. Key Derivation Functions.......................................18
   13. Summary of Algorithm Characteristics...........................18
      13.1. AES-GCM for SRTP/SRTCP....................................18
      13.2. AES-CCM for SRTP/SRTCP....................................20
   14. Security Considerations........................................23
      14.1. Handling of Security Critical Parameters..................23
      14.2. Size of the Authentication Tag............................24
   15. IANA Considerations............................................25
      15.1. SDES......................................................25
      15.2. DTLS-SRTP.................................................26
      15.3. MIKEY.....................................................29
      15.4. AEAD registry.............................................29
   16. Parameters for use with MIKEY..................................29
   17. Acknowledgements...............................................30
   18. References.....................................................31
      18.1. Normative References......................................31
      18.2. Informative References....................................32


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1. Introduction

   The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile
   of the Real-time Transport Protocol (RTP) [RFC3550], 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).  It is important to note
   that the outgoing SRTP packets from a single endpoint may be
   originating from several independent data sources.

   Authenticated encryption [BN00] is a form of encryption that, in
   addition to providing confidentiality for the plaintext that is
   encrypted, provides a way to check its integrity and authenticity.
   Authenticated Encryption with Associated Data, or AEAD [R02], adds
   the ability to check the integrity and authenticity of some
   Associated Data (AD), also called "additional authenticated data",
   that is not encrypted.  This specification makes use of the interface
   to a generic AEAD algorithm as defined in [RFC5116].

   The Advanced Encryption Standard (AES) is a block cipher that
   provides a high level of security, and can accept different key
   sizes.  Two families of AEAD algorithm families, AES Galois/Counter
   Mode (AES-GCM) [GCM] and AES Counter with Cipher Block
   Chaining-Message Authentication Code (AES-CCM) [RFC3610] are based
   upon AES.  This specification makes use of the AES versions that use
   128-bit and 256-bit keys, which we call AES-128 and AES-256,
   respectively.

   Any AEAD algorithm provides an intrinsic authentication tag.  In many
   applications the authentication tag is truncated to less than full
   length.  When CCM is being used there are three allowed values for
   the length of the authentication tag.  A CCM authentication tag MUST
   be either 8 octets, 12 octets or 16 octets in length.  But when GCM
   is being used only two values are permitted.  A GCM authentication
   tag MUST be either 12 octets or 16 octets in length.  Thus CCM will
   have a total of six configurations, reflecting the two choices for
   key size (either 128 or 256 bits) and the three choices for the
   length of the CCM authentication tag (either 8, 12 or 16 octets), and
   GCM will have four configurations reflecting two choices for the key
   size and two choices for the length of the GCM authentication tag
   (either 12 or 16 octets).  The key size and the length of the
   authentication tag are set when the session is initiated and SHOULD
   NOT be altered.

   The Galois/Counter Mode of operation (GCM) and the Counter with
   Cipher Block Chaining-Message Authentication Code mode of operation
   (CCM) are both AEAD modes of operation for block ciphers.  Both use
   counter mode to encrypt the data, an operation that can be
   efficiently pipelined.  Further, GCM authentication uses operations
   that are particularly well suited to efficient implementation in
   hardware, making it especially appealing for high-speed


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   implementations, or for implementations in an efficient and compact
   circuit.  CCM is well suited for use in compact software
   implementations.  This specification uses GCM and CCM with both
   AES-128 and AES-256.

   In summary, this document defines how to use AEAD algorithms,
   particularly AES-GCM and AES-CCM, to provide confidentiality and
   message authentication within SRTP and SRTCP packets.


2. Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].


3. Overview of the SRTP/SRTCP AEAD security Architecture

   SRTP/SRTCP AEAD security is based upon the following principles:

       a)  Both privacy and authentication are based upon the use of
           symmetric algorithms.  An AEAD algorithm such as AES-CCM or
           AES-GCM combines privacy and authentication into a single
           process.

       b)  A secret master key is shared by all participating endpoints,
           both those originating SRTP/SRTCP packets and those receiving
           these packets.  Any given master key MAY be used
           simultaneously by several endpoints to originate SRTP/SRTCP
           packets (as well one or more endpoints using this master key
           to process inbound data).

       c)  A Key Derivation Function is applied to the shared master key
           value to form separate encryption keys, authentication keys
           and salting keys for SRTP and for SRTCP (a total of six
           keys).  This process is described in section 4.3 of
           [RFC3711].  Since AEAD algorithms such as AES-CCM and AES-GCM
           combine encryption and authentication into a single process,
           AEAD algorithms do not make use of the authentication keys.
           The master key MUST be at least as large as the encryption
           key derived from it.

       d)  Aside from making modifications to IANA registries to allow
           AES-GCM and AES-CCM to work with SDES, DTLS-SRTP and MIKEY,
           the details of how the master key is established and shared
           between the participants are outside the scope of this
           document.  Similarly any mechanism for rekeying an existing
           session is outside the scope of the document.

       e)  Each time an instantiation of AES-GCM or AES-CCM is invoked


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           to encrypt and authenticate an SRTP or SRTCP data packet a
           new IV is used.  SRTP combines the 4-octet synchronization
           source (SSRC) identifier, the 4-octet rollover counter (ROC),
           and the 2-octet sequence number (SEQ) with the 12-octet
           encryption salt to form a 12-octet IV (see section 9.1).
           SRTCP combines the SSRC and 31-bit SRTCP index with the
           encryption salt to form a 12-octet IV (see section 10.1).


4. Terminology

   The following terms have very specific meanings in the context of
   this RFC:

      Instantiation:   In AEAD, an instantiation is an (Encryption_key,
                       salt) pair together with all of the data
                       structures (for example, counters) needed for it
                       to function properly.  In SRTP/SRTCP, each
                       endpoint will need two instantiations of the AEAD
                       algorithm for each master key in its possession,
                       one instantiation for SRTP traffic and one
                       instantiation for SRTCP traffic.

      Invocation:      SRTP/SRTCP data streams are broken into packets.
                       Each packet is processed by a single invocation
                       of the appropriate instantiation of the AEAD
                       algorithm.

   In many applications, each endpoint will have one master key for
   processing outbound data but may have one or more separate master
   keys for processing inbound data.


5. Generic AEAD Processing


5.1. Types of Input Data

     Associated Data:        This is data that is to be authenticated
                             but not encrypted.

     Plaintext:              Data that is to be both encrypted and
                             authenticated.

     Raw Data:               Data that is to be neither encrypted nor
                             authenticated.

   Which portions of SRTP/SRTCP packets that are to be treated as
   associated data, which are to be treated as plaintext, and which are
   to be treated as raw data are covered in sections 9.2, 10.2 and
   10.3.



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5.2. AEAD Invocation Inputs and Outputs


5.2.1. Encrypt Mode


      Inputs:
        Encryption_key              Octet string, either 16 or 32
                                    octets long
        Initialization_Vector       Octet string, 12 octets long
        Associated_Data             Octet string of variable length
        Plaintext                   Octet string of variable length
        Tag_Size_Flag (CCM only*)   One Octet

      Outputs
        Ciphertext                  Octet string, length =
                                     length(Plaintext)+tag_length

     (*) CCM mode requires tag length to be explicitly input to
         the algorithm, whereas with GCM, the tag is simply truncated.
         For GCM, the algorithm choice determines the tag size.

   In both CCM and GCM, the algorithm negotiation selects what tag size
   is to be used.  In GCM, the authentication tag is simply truncated to
   the appropriate length, but CCM requires that the tag length be an
   explicitly input to the algorithm as the Tag_Size_Field.  For the
   three tag lengths allowed for CCM in this document the corresponding
   Tag_Size_Flag values are as follows:

          Tag Length   |  Tag_Size_Flag (hex)
          -----------------------------------
            8 octets   |        5A
           12 octets   |        6A
           16 octets   |        7A

   Once an SRTP/SRTCP session has been initiated the length of the tag
   is a fixed value and MUST NOT be altered.


5.2.2. Decrypt Mode

      Inputs:
        Encryption_key              Octet string, either 16 or 32
                                    octets long
        Initialization_Vector       Octet string, 12 octets long
        Associated_Data             Octet string of variable length
        Ciphertext                  Octet string of variable length
        Tag_Size_Flag (CCM only*)   One octet

      Outputs
        Plaintext                   Octet string, length =


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                                      length(Ciphertext)-tag_length
        Validity_Flag               Boolean, TRUE if valid,
                                    FALSE otherwise

      (*) For GCM, the algorithm choice determines the tag size.


   As mentioned in section 5.2.1, in SRTP/SRTCP CCM supports three tag
   lengths (8 octets, 12 octets and 16 octets) while GCM only supports
   two tag sizes (12 octets and 16 octets).


5.3. Handling of AEAD Authentication

   AEAD requires that all incoming packets MUST pass AEAD authentication
   before any other action takes place.  Plaintext and associated data
   MUST NOT be released until the AEAD authentication tag has been
   validated.  Further the ciphertext MUST NOT be decrypted until the
   AEAD tag has been validated.

   Should the AEAD tag prove to be invalid, the packet in question is to
   be discarded and a Validation Error flag raised.  Local policy
   determines how this flag is to be handled and is outside the scope of
   this document.


6. Counter Mode Encryption

   In both GCM and CCM, each outbound packet uses a 12-octet IV and an
   encryption key to form two outputs, a 16-octet first_key_block which
   is used in forming the authentication tag and a keystream of octets
   which is XORed to the plaintext to form cipher.

   When GCM is used, the concatenation of a 12-octet IV (see sections
   9.1 and 10.1) with a 4-octet block counter forms the input to AES.
   This is used to build a key_stream as follows:



    def GCM_keystream( Plaintext_len, IV, Encryption_key ):
        assert Plaintext_len  <= (2**36) - 32 ## measured in octets
        key_stream = ""
        block_counter = 1
        first_key_block = AES_ENC( data=IV||block_counter,
                                   key=Encryption_key        )
        while len(key_stream) < Plaintext_len:
            block_counter = block_counter + 1
            key_block = AES_ENC( data=IV||block_counter,
                                 key=Encryption_key        )
            key_stream  = key_stream || key_block
        key_stream = truncate( key_stream, Plaintext_len )
        return (first_key_block, key_stream )


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   In AES-CCM counter mode encryption, the AES data input consists of
   the concatenation of a 1-octet flag, a 12-octet IV, and a 3-octet
   block counter.  Note that in this application the flag octet will
   always have the value 0x02 (see section 2.3 of [RFC3610]).  A
   (first_key_block, key_stream) pair is formed as follows:


    def CCM_keystream( Plaintext_len, IV, Encryption_key ):
        assert Plaintext_len <= (2**24)-1  ## measured in octets
        key_stream = ""
        block_counter = 0
        first_key_block = AES_ENC( data=0x02||IV||block_counter,
                                   key=Encryption_key            )
        while len(key_stream)<Plaintext_len:
            block_counter = block_counter + 1
            key_block = AES_ENC( data=0x02||IV||block_counter,
                                 key=Encryption_key            )
            key_stream  = key_stream || key_block
        key_stream = truncate( key_stream, Plaintext_len )
        return (first_key_block, key_stream )


   In theory these keystream generation processes allow for each packet
   to use s keystream of length up to (2^24)-1 octets per invocation for
   AES-CCM and up to (2^36)-32 octets per invocation for AES-GCM, far
   longer than is actually required.

   With any counter mode, if the same (IV, Encryption_key) pair is used
   twice, precisely the same keystream is formed.  As explained in
   section 9.1 of RFC 3711, this is a cryptographic disaster.  For GCM
   the consequences are even worse since such a reuse compromises GCM's
   integrity mechanism not only for the current packet stream but for
   all future uses of the current encryption_key.


7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12

   AEAD_AES_128_CCM and AEAD_AES_256_CCM are defined in [RFC5116] with
   an authentication tag length of 16-octets.  AEAD_AES_128_CCM_8 and
   AEAD_AES_256_CCM_8 are defined in [RFC6655] with an authentication
   tag length of 8-octets.  We require two new variants,
   AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12, with 12-octet
   authentication tags.  In each case the authentication tag is formed
   by taking the 12 most significant octets (in network order) of the
   AEAD_AES_128/256_CCM authentication tag:







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         +=====================+===========+==============+
         |        Name         | Key Size  | tag size (t) |
         +=====================+===========+==============+
         | AEAD_AES_256_CCM_12 | 256 bits  | 12 octets    |
         | AEAD_AES_128_CCM_12 | 128 bits  | 12 octets    |
         +=====================+===========+==============+

8. Unneeded SRTP/SRTCP Fields

   AEAD counter mode encryption removes the need for certain existing
   SRTP/SRTCP mechanisms.


8.1. SRTP/SRTCP Authentication Field

   The AEAD message authentication mechanism MUST be the primary message
   authentication mechanism for AEAD SRTP/SRTCP.  Additional SRTP/SRTCP
   authentication mechanisms SHOULD NOT be used with any AEAD algorithm
   and the optional SRTP/SRTCP Authentication Tags are NOT RECOMMENDED
   and SHOULD NOT be present.  Note that this contradicts section 3.4 of
   [RFC3711] which makes the use of the SRTCP Authentication field
   mandatory, but the presence of the AEAD authentication renders the
   older authentication methods redundant.

      Rationale.  Some applications use the SRTP/SRTCP Authentication
      Tag as a means of conveying additional information, notably
      [RFC4771].  This document retains the Authentication Tag field
      primarily to preserve compatibility with these applications.


8.2. RTP Padding

   Neither AES-GCM nor AES-CCM requires that the data be padded out to a
   specific block size, reducing the need to use the padding mechanism
   provided by RTP.  It is RECOMMENDED that the RTP padding mechanism
   not be used unless it is necessary to disguise the length of the
   underlying plaintext.

















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9. AES-GCM/CCM processing for SRTP


9.1. SRTP IV formation for AES-GCM and AES-CCM

               0  0  0  0  0  0  0  0  0  0  1  1
               0  1  2  3  4  5  6  7  8  9  0  1
             +--+--+--+--+--+--+--+--+--+--+--+--+
             |00|00|    SSRC   |     ROC   | SEQ |---+
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |         Encryption Salt           |->(+)
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |       Initialization Vector       |<--+
             +--+--+--+--+--+--+--+--+--+--+--+--+

           Figure 1: AES-GCM and AES-CCM SRTP
                     Initialization Vector formation.
   The 12 octet initialization vector used by both AES-GCM and AES-CCM
   SRTP is formed by first concatenating 2-octets of zeroes, the 4-octet
   SSRC, the 4-octet Rollover Counter (ROC) and the two octet sequence
   number SEQ.  The resulting 12-octet value is then XORed to the
   12-octet salt to form the 12-octet IV.


9.2. Data Types in SRTP Packets

   All SRTP packets MUST be both authenticated and encrypted.  The data
   fields within the SRTP packets are broken into Associated Data,
   Plaintext and Raw Data as follows (see Figure 2):

     Associated Data:  The version V (2 bits), padding flag P (1 bit),
                       extension flag X (1 bit), CSRC count CC (4 bits),
                       marker M (1 bit), the Payload Type PT (7 bits),
                       the sequence number (16 bits), timestamp (32
                       bits), SSRC (32 bits), optional contributing
                       source identifiers (CSRCs, 32 bits each), and
                       optional RTP extension (variable length).

     Plaintext:        The RTP payload (variable length), RTP padding
                       (if used, variable length), and RTP pad count (
                       if used, 1 octet).

     Raw Data:         The optional variable length SRTP MKI and SRTP
                       authentication tag (whose use is NOT
                       RECOMMENDED).  These fields are appended after
                       encryption has been performed.



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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |V=2|P|X|  CC   |M|     PT      |       sequence number         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                           timestamp                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |           synchronization source (SSRC) identifier            |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |      contributing source (CSRC) identifiers (optional)        |
    A  |                               ....                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                   RTP extension (OPTIONAL)                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                          payload  ...                         |
    P  |                               +-------------------------------+
    P  |                               | RTP padding   | RTP pad count |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                P = Plaintext (to be encrypted and authenticated)
                A = Associated Data (to be authenticated only)

      Figure 2: Structure of an SRTP packet before Authenticated
                Encryption

   Since the AEAD ciphertext is larger than the plaintext by exactly the
   length of the AEAD authentication tag, the corresponding SRTP
   encrypted packet replaces the plaintext field by a slightly larger
   field containing the cipher.  Even if the plaintext field is empty,
   AEAD encryption must still be performed, with the resulting cipher
   consisting solely of the authentication tag.  This tag is to be
   placed immediately before the optional variable length SRTP MKI and
   SRTP authentication tag fields.





















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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |V=2|P|X|  CC   |M|     PT      |       sequence number         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                           timestamp                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |           synchronization source (SSRC) identifier            |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |      contributing source (CSRC) identifiers (optional)        |
    A  |                               ....                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                   RTP extension (OPTIONAL)                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    C  |                             cipher                            |
    C  |                               ...                             |
    C  |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    R  :                     SRTP MKI (OPTIONAL)                       :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    R  :           SRTP authentication tag (NOT RECOMMENDED)           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                C = Ciphertext (encrypted and authenticated)
                A = Associated Data (authenticated only)
                R = neither encrypted nor authenticated, added
                    after authenticated encryption completed

      Figure 3: Structure of an SRTP packet after Authenticated
                Encryption


9.3. Handling Header Extensions

   RTP header extensions were first defined in RFC 3550.  RFC 6904
   [RFC6904] describes how these header extensions are to be encrypted
   in SRTP.

   When RFC 6904 is in use, a separate keystream is generated to encrypt
   selected RTP header extension elements.  For the AEAD_AES_128_GCM and
   the AEAD_AES_128_CCM algorithms, this keystream MUST be generated in
   the manner defined in [RFC6904] using the AES_128_CM transform.  For
   the AEAD_AES_256_GCM and the AEAD_AES_256_CCM algorithms, the
   keystream MUST be generated in the manner defined for the AES_256_CM
   transform.  The originator must perform any required header extension
   encryption before the AEAD algorithm is invoked.

   As with the other fields contained within the RTP header, both
   encrypted and unencrypted header extensions are to be treated by the
   AEAD algorithm as Associated Data (AD).  Thus the AEAD algorithm does
   not provide any additional privacy for the header extensions, but
   does provide integrity and authentication.


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9.4. Prevention of SRTP IV Reuse

   In order to prevent IV reuse, we must ensure that the (ROC,SEQ,SSRC)
   triple is never used twice with the same master key.  There are two
   phases to this issue.

     Counter Management: A rekey MUST be performed to establish a new
                         master key before the (ROC,SEQ) pair cycles
                         back to its original value.  Note that
                         implicitly assumes that either the outgoing RTP
                         process is trusted to not attempt to repeat a
                         SEQ value, or that the encryption process
                         ensures that the SEQ number of the packets
                         presented to it are always incremented in the
                         proper fashion.  This is particularly important
                         for GCM since using the same SEQ value twice
                         compromises the authentication mechanism.  For
                         GCM, the SEQ and SSRC values used MUST either
                         be generated or checked by the SRTP
                         implementation, or by a module (e.g.  the RTP
                         application) that can be considered equally
                         trusted as the SRTP implementation.  While
                         [RFC3711] allows detecting SSRC collisions
                         after they happen, SRTP using GCM with shared
                         master keys MUST prevent SSRC collision from
                         happening even once.

     SSRC Management:    For a given master key, the set of all SSRC
                         values used with that master key must be
                         partitioned into disjoint pools, one pool for
                         each endpoint using that master key to
                         originate outbound data.  Each such originating
                         endpoint MUST only issue SSRC values from the
                         pool it has been assigned.  Further, each
                         originating endpoint MUST maintain a history of
                         outbound SSRC identifiers that it has issued
                         within the lifetime of the current master key,
                         and when a new synchronization source requests
                         an SSRC identifier it MUST NOT be given an
                         identifier that has been previously issued.  A
                         rekey MUST be performed before any of the
                         originating endpoints using that master key
                         exhausts its pool of SSRC values.  Further, the
                         identity of the entity giving out SSRC values
                         MUST be verified, and the SSRC signaling MUST
                         be integrity protected.






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10. AES-GCM/CCM Processing of SRTCP Compound Packets

   All SRTCP compound packets MUST be authenticated, but unlike SRTP,
   SRTCP packet encryption is optional.  A sender can select which
   packets to encrypt, and indicates this choice with a 1-bit encryption
   flag (located just before the 31-bit SRTCP index)


10.1. SRTCP IV formation for AES-GCM and AES-CCM

   The 12-octet initialization vector used by both AES-GCM and AES-CCM
   SRTCP is formed by first concatenating 2-octets of zeroes, the
   4-octet Synchronization Source identifier (SSRC), 2-octets of zeroes,
   a single zero bit, and the 31-bit SRTCP Index.  The resulting
   12-octet value is then XORed to the 12-octet salt to form the
   12-octet IV.

                  0  1  2  3  4  5  6  7  8  9 10 11
                +--+--+--+--+--+--+--+--+--+--+--+--+
                |00|00|    SSRC   |00|00|0+SRTCP Idx|---+
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                        |
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                |         Encryption Salt           |->(+)
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                        |
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                |       Initialization Vector       |<--+
                +--+--+--+--+--+--+--+--+--+--+--+--+

              Figure 4: SRTCP Initialization Vector formation






















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10.2. Data Types in Encrypted SRTCP Compound Packets

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |V=2|P|   RC    |  Packet Type  |            length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |           synchronization source (SSRC) of Sender             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                         sender info                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                        report block 1                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                        report block 2                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                              ...                              :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |V=2|P|   SC    |  Packet Type  |              length           |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    P  |                          SSRC/CSRC_1                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    P  |                           SDES items                          :
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    P  |                              ...                              :
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |1|                         SRTCP index                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    R  |                  SRTCP MKI (optional) index                   :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    R  :           SRTCP authentication tag (NOT RECOMMENDED)          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                P = Plaintext (to be encrypted and authenticated)
                A = Associated Data (to be authenticated only)
                R = neither encrypted nor authenticated, added after
                    encryption

    Figure 5: AEAD SRTCP inputs when encryption flag = 1.

   When the encryption flag is set to 1, the SRTCP packet is broken into
   plaintext, associated data, and raw (untouched) data (as shown above
   in figure 5):

     Associated Data:  The packet version V (2 bits), padding flag P (1
                       bit), reception report count RC (5 bits), packet
                       type (8 bits), length (2 octets), SSRC (4
                       octets), encryption flag (1 bit) and SRTCP index
                       (31 bits).

     Raw Data:         The optional variable length SRTCP MKI and SRTCP
                       authentication tag (whose use is NOT


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                       RECOMMENDED).

     Plaintext:        All other data.

   Note that the plaintext comes in one contiguous field.  Since the
   AEAD cipher is larger than the plaintext by exactly the length of the
   AEAD authentication tag, the corresponding SRTCP encrypted packet
   replaces the plaintext field with a slightly larger field containing
   the cipher.  Even if the plaintext field is empty, AEAD encryption
   must still be performed, with the resulting cipher consisting solely
   of the authentication tag.  This tag is to be placed immediately
   before the encryption flag and SRTCP index.


10.3. Data Types in Unencrypted SRTCP Compound Packets

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |V=2|P|   RC    |  Packet Type  |            length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |           synchronization source (SSRC) of Sender             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                         sender info                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                        report block 1                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                        report block 2                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                              ...                              :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |V=2|P|   SC    |  Packet Type  |              length           |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |                          SSRC/CSRC_1                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    A  |                           SDES items                          :
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |                              ...                              :
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    A  |0|                         SRTCP index                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    R  :              authentication tag (NOT RECOMMENDED)             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                A = Associated Data (to be authenticated only)
                R = neither encrypted nor authenticated, added after
                    encryption

    Figure 6: AEAD SRTCP inputs when encryption flag = 0

   When the encryption flag is set to 0, the SRTCP compound packet is


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   broken into plaintext, associated data, and raw (untouched) data as
   follows (see figure 6):

     Plaintext:        None.

     Raw Data:         The variable length optional SRTCP MKI and SRTCP
                       authentication tag (whose use is NOT
                       RECOMMENDED).

     Associated Data:  All other data.

   Even though there is no plaintext in this RTCP packet, AEAD
   encryption returns a cipher field which is precisely the length of
   the AEAD authentication tag.  This cipher is to be placed before the
   Encryption flag and the SRTCP index in the authenticated SRTCP
   packet.


10.4. Prevention of SRTCP IV Reuse

   A new master key MUST be established before the 31-bit SRTCP index
   cycles back to its original value.  Ideally, a rekey should be
   performed and a new master key put in place well before the SRTCP
   cycles back to the starting value.

   The comments on SSRC management in section 9.4 also apply.


11. Constraints on AEAD for SRTP and SRTCP

   In general, any AEAD algorithm can accept inputs with varying
   lengths, but each algorithm can accept only a limited range of
   lengths for a specific parameter.  In this section, we describe the
   constraints on the parameter lengths that any AEAD algorithm must
   support to be used in AEAD-SRTP.  Additionally, we specify a complete
   parameter set for two specific AEAD algorithms, namely AES-GCM and
   AES-CCM.

   All AEAD algorithms used with SRTP/SRTCP MUST satisfy the five
   constraints listed below:

   PARAMETER  Meaning                  Value

   A_MAX      maximum associated       MUST be at least 12 octets.
              data length
   N_MIN      minimum nonce (IV)       MUST be 12 octets.
              length
   N_MAX      maximum nonce (IV)       MUST be 12 octets.
              length
   P_MAX      maximum plaintext        GCM: MUST be <= 2^36-32 octets.
              length per invocation    CCM: MUST be <= 2^24-1 octets.
   C_MAX      maximum ciphertext       GCM: MUST be <= 2^36-16 octets.


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              length per invocation    CCM: MUST be <= 2^24+15 octets.


   For GCM the value of P_MAX is based on purely cryptographic
   considerations.  CCM requires the length of the plaintext, measured
   in octets, must fit in a 24-bit field.  Hence P_MAX is 2^24-1..

   For sake of clarity we specify two additional parameters:

      AEAD Authentication Tag Length   CCM: MUST be 8, 12, or 16 octets,
                                       GCM: MUST be 12 or 16 octets.
      Maximum number of invocations    SRTP: MUST be at most 2^48,
         for a given instantiation     SRTCP: MUST be at most 2^31.
      Block Counter size               CCM: MUST be 24 bits,
                                       GCM: MUST be 32 bits.

   The reader is reminded that the ciphertext is longer than the
   plaintext by exactly the length of the AEAD authentication tag.


12. Key Derivation Functions

   A Key Derivation Function (KDF) is used to derive all of the required
   encryption and authentication keys from a secret value shared by the
   endpoints.  Both the AEAD_AES_128_GCM algorithms and the
   AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key
   Derivation Function described in [RFC3711].  Both the
   AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST
   use the AES_256_CM_PRF Key Derivation Function described in
   [RFC6188].


13. Summary of Algorithm Characteristics

   For convenience, much of the information about the use of AES-GCM and
   AES-CCM algorithms in SRTP is collected in the tables contained in
   this section.


13.1. AES-GCM for SRTP/SRTCP

   AES-GCM is a family of AEAD algorithms built around the AES block
   cipher algorithm.  AES-GCM uses AES counter mode for encryption and
   Galois Message Authentication Code (GMAC) for authentication.  A
   detailed description of the AES-GCM family can be found in
   [RFC5116].  The following members of the AES-GCM family may be used
   with SRTP/SRTCP:







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      Name                 Key Size      AEAD Tag Size      Reference
      ================================================================
      AEAD_AES_128_GCM     16 octets     16 octets          [RFC5116]
      AEAD_AES_256_GCM     32 octets     16 octets          [RFC5116]
      AEAD_AES_128_GCM_12  16 octets     12 octets          [RFC5282]
      AEAD_AES_256_GCM_12  32 octets     12 octets          [RFC5282]

                Table 1: AES-GCM algorithms for SRTP/SRTCP

   Any implementation of AES-GCM SRTP MUST support both AEAD_AES_128_GCM
   and AEAD_AES_256_GCM (the versions with 16 octet AEAD authentication
   tags), and it MAY support the four other variants shown in table 1.
   Below we summarize parameters associated with these four GCM
   algorithms:


     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 128 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_CM_PRF [RFC3711]         |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_128_GCM_12          |
     | AEAD authentication tag length | 96 bits                      |
     +--------------------------------+------------------------------+

                Table 2: The AEAD_AES_128_GCM_12 Crypto Suite



     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 128 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_CM_PRF [RFC3711]         |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_128_GCM             |
     | AEAD authentication tag length | 128 bits                     |
     +--------------------------------+------------------------------+

                Table 3: The AEAD_AES_128_GCM Crypto Suite








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     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 256 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_256_CM_PRF [RFC6188]     |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_256_GCM_12          |
     | AEAD authentication tag length | 96 bits                      |
     +--------------------------------+------------------------------+

                Table 4: The AEAD_AES_256_GCM_12 Crypto Suite



     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 256 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_256_CM_PRF [RFC6188]     |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_256_GCM             |
     | AEAD authentication tag length | 128 bits                     |
     +--------------------------------+------------------------------+
                Table 5: The AEAD_AES_256_GCM Crypto Suite




13.2. AES-CCM for SRTP/SRTCP

   AES-CCM is another family of AEAD algorithms built around the AES
   block cipher algorithm.  AES-CCM uses AES counter mode for encryption
   and AES Cipher Block Chaining Message Authentication Code (CBC-MAC)
   for authentication.  A detailed description of the AES-CCM family can
   be found in [RFC5116].  Four of the six CCM algorithms used in this
   document are defined in previous RFCs, while two, AEAD_AES_128_CCM_12
   and AEAD_AES_256_CCM_12, are defined in section 7 of this document.

   Any implementation of AES-CCM SRTP/SRTCP MUST support both
   AEAD_AES_128_CCM and AEAD_AES_256_CCM (the versions with 16 octet
   AEAD authentication tags), and MAY support the other four variants.








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      Name                 Key Size    AEAD Tag Size    Reference
      ================================================================
      AEAD_AES_128_CCM     128 bits    16 octets        [RFC5116]
      AEAD_AES_256_CCM     256 bits    16 octets        [RFC5116]
      AEAD_AES_128_CCM_12  128 bits    12 octets        see section 7
      AEAD_AES_256_CCM_12  256 bits    12 octets        see section 7
      AEAD_AES_128_CCM_8   128 bits     8 octets        [RFC6655]
      AEAD_AES_256_CCM_8   256 bits     8 octets        [RFC6655]

             Table 6: AES-CCM algorithms for SRTP/SRTCP

   In addition to the flag octet used in counter mode encryption,
   AES-CCM authentications also uses a flag octet that conveys
   information about the length of the authentication tag, length of the
   block counter, and presence of additional authenticated data (see
   section 2.2 of [RFC3610]).  For AES-CCM in SRTP/SRTCP, the flag octet
   has the hex value 5A if an 8-octet AEAD authentication tag is used,
   6A if a 12-octet AEAD authentication tag is used, and 7A if a
   16-octet AEAD authentication tag is used.  The flag octet is one of
   the inputs to AES during the counter mode encryption of the
   plaintext.


     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 128 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_CM_PRF [RFC3711]         |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_128_CCM_8           |
     | AEAD authentication tag length | 64 bits                      |
     +--------------------------------+------------------------------+

                Table 7: The AEAD_AES_128_CCM_8 Crypto Suite

















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     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 128 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_CM_PRF [RFC3711]         |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_128_CCM_12          |
     | AEAD authentication tag length | 96 bits                      |
     +--------------------------------+------------------------------+

                Table 8: The AEAD_AES_128_CCM_12 Crypto Suite



     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 128 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_CM_PRF [RFC3711]         |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_128_CCM             |
     | AEAD authentication tag length | 128 bits                     |
     +--------------------------------+------------------------------+

                Table 9: The AEAD_AES_128_CCM Crypto Suite




     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 256 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_256_CM_PRF [RFC6188]     |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_256_CCM_8           |
     | AEAD authentication tag length | 64 bits                      |
     +--------------------------------+------------------------------+

                Table 10: The AEAD_AES_256_CCM_8 Crypto Suite







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     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 256 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_256_CM_PRF [RFC6188]     |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_256_CCM_12          |
     | AEAD authentication tag length | 96 bits                      |
     +--------------------------------+------------------------------+

                Table 11: The AEAD_AES_256_CCM_12 Crypto Suite



     +--------------------------------+------------------------------+
     | Parameter                      | Value                        |
     +--------------------------------+------------------------------+
     | Master key length              | 256 bits                     |
     | Master salt length             | 96 bits                      |
     | Key Derivation Function        | AES_256_CM_PRF [RFC6188]     |
     | Maximum key lifetime (SRTP)    | 2^48 packets                 |
     | Maximum key lifetime (SRTCP)   | 2^31 packets                 |
     | Cipher (for SRTP and SRTCP)    | AEAD_AES_256_CCM             |
     | AEAD authentication tag length | 128 bits                     |
     +--------------------------------+------------------------------+

                Table 12: The AEAD_AES_256_CCM Crypto Suite




14. Security Considerations


14.1. Handling of Security Critical Parameters

   As with any security process, the implementer must take care to
   ensure cryptographically sensitive parameters are properly handled.
   Many of these recommendations hold for all SRTP cryptographic
   algorithms, but we include them here to emphasize their importance.

      - If the master salt is to be kept secret, it MUST be properly
        erased when no longer needed.
      - The secret master key and all keys derived from it MUST be kept
        secret.  All keys MUST be properly erased when no longer
        needed.
      - At the start of each packet, the block counter MUST be reset (to
        0 for CCM, to 1 for GCM).  The block counter is incremented
        after each block key has been produced, but it MUST NOT be


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        allowed to exceed 2^32-1 for GCM and 2^24-1 for CCM.  Note that
        even though the block counter is reset at the start of each
        packet, IV uniqueness is ensured by the inclusion of
        SSRC/ROC/SEQ or SRTCP Index in the IV.  (The reader is reminded
        that in both GCM and CCM the first block of key produced is
        reserved for use in authenticating the packet and is not used to
        encrypt plaintext.)
      - Each time a rekey occurs, the initial values of the SRTCP index
        and the SRTP packet indices MUST be saved in order to prevent IV
        reuse.
      - Processing MUST cease if the 31-bit SRTCP index or any of the
        48-bit packet indices cycle back their initial values .
        Processing MUST NOT resume until a new SRTP/SRTCP session has
        been established using a new SRTP master key.  Ideally, a rekey
        should be done well before any of these counters cycle.


14.2. Size of the Authentication Tag

   We require that the AEAD authentication tag must be at least 8
   octets, significantly reducing the probability of an adversary
   successfully introducing fraudulent data.  The goal of an
   authentication tag is to reduce the probability of a successful
   forgery occurring anywhere in the network we are attempting to
   defend.  There are three relevant factors: how low we wish the
   probability of successful forgery to be (prob_success), how many
   attempts the adversary can make (N_tries) and the size of the
   authentication tag in bits (N_tag_bits).  Then

           prob_success <= expected number of successes
                        = N_tries * 2^-N_tag_bits.

   When the expected number of successes is much less than one, the
   probability of success is well approximated by the expected number of
   successes.

   Suppose an adversary wishes to introduce a forged or altered packet
   into a target network by randomly selecting an authentication value
   until by chance they hit a valid authentication tag.  The table below
   summarizes the relationship between the number of forged packets the
   adversary has tried, the size of the authentication tag, and the
   probability of a compromise occurring (i.e.  at least one of the
   attempted forgeries having a valid authentication tag).  The reader
   is reminded that the forgery attempts can be made over the entire
   network, not just a single link, and that frequently changing the key
   does not decrease the probability of a compromise occurring.

   It should be noted that the cryptographic properties of the GHASH
   algorithm used in GCM reduces the effective authentication tag size
   (in bits) by the log base 2 of the of blocks of encrypted and/or
   authenticated data in a packet.  In practice an SRTP payload will be
   less than 2^16 bytes, because of the 16-bit IPv4 and UDP length


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   fields.  The exception to this case is IPv6 jumbograms [RFC2675],
   which is unlikely to be used for RTP-based multimedia traffic
   [RFC3711].  This corresponds to 2^12 blocks of data, so the effective
   GCM authentication tag size is reduced by at most 12 bits.



    +===========+=============+========================================+
    | Auth. Tag |  Eff. Tag   |      Number of Forgery Attempts        |
    |   Size    |   Tag Size  |      Needed to Achieve a Given         |
    |  (bytes)  |     (bits)  |        Probability of Success          |
    |-----------+-------------+------------+-------------+-------------|
    |                         | prob=2^-30 | prob=2^-20  | prob=2^-10  |
    |===========+=============+=============+============+=============|
    |           |  32 (CCM)   |  2^2 tries |  2^12 tries |  2^22 tries |
    |    4      +-------------+------------+-------------+-------------|
    |           |  20 (GCM)   |   1 try    |   1 try     |  2^10 tries |
    |===========+=============+============+=============+=============|
    |           |  64 (CCM)   | 2^34 tries |  2^44 tries |  2^54 tries |
    |    8      +-------------+------------+-------------+-------------|
    |           |  52 (GCM)   | 2^22 tries |  2^32 tries |  2^42 tries |
    |===========+=============+============+=============+=============|
    |           |  96 (CCM)   | 2^66 tries |  2^76 tries |  2^86 tries |
    |   12      +-------------+------------+-------------+-------------|
    |           |  84 (GCM)   | 2^54 tries |  2^64 tries |  2^74 tries |
    |===========+=============+============+=============+=============|
    |           | 128 (CCM)   | 2^98 tries | 2^108 tries | 2^118 tries |
    |   16      +-------------+------------+-------------+-------------|
    |           | 116 (GCM)   | 2^86 tries |  2^96 tries | 2^106 tries |
    |===========+=============+============+=============+=============|

     Table 13: Number of forgery attempts needed to achieve a given
               probability of success for various tag sizes.

15. IANA Considerations


15.1. SDES

   SDP Security Descriptions [RFC4568] defines SRTP "crypto suites".  A
   crypto suite corresponds to a particular AEAD algorithm in SRTP.  In
   order to allow Security Descriptions to signal the use of the
   algorithms defined in this document, IANA will register the following
   crypto suites into the "SRTP Crypto Suite Registrations" subregistry
   of the "Session Description Protocol (SDP) Security Descriptions"
   registry.








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      srtp-crypto-suite-ext = "AEAD_AES_128_GCM"    /
                              "AEAD_AES_256_GCM"    /
                              "AEAD_AES_128_GCM_12" /
                              "AEAD_AES_256_GCM_12" /
                              "AEAD_AES_128_CCM"    /
                              "AEAD_AES_256_CCM"    /
                              "AEAD_AES_128_CCM_8"  /
                              "AEAD_AES_256_CCM_8"  /
                              "AEAD_AES_128_CCM_12" /
                              "AEAD_AES_256_CCM_12" /
                              srtp-crypto-suite-ext


15.2. DTLS-SRTP

   DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile".
   These also correspond to the use of an AEAD algorithm in SRTP.  In
   order to allow the use of the algorithms defined in this document in
   DTLS-SRTP, we request IANA register the following SRTP Protection
   Profiles:


         AEAD_AES_128_GCM    = {TBD, TBD }
         AEAD_AES_256_GCM    = {TBD, TBD }
         AEAD_AES_128_GCM_12 = {TBD, TBD }
         AEAD_AES_256_GCM_12 = {TBD, TBD }
         AEAD_AES_128_CCM    = {TBD, TBD }
         AEAD_AES_256_CCM    = {TBD, TBD }
         AEAD_AES_128_CCM_8  = {TBD, TBD }
         AEAD_AES_256_CCM_8  = {TBD, TBD }
         AEAD_AES_128_CCM_12 = {TBD, TBD }
         AEAD_AES_256_CCM_12 = {TBD, TBD }

   Below we list the SRTP transform parameters for each of these
   protection profile.  Unless separate parameters for SRTCP and SRTCP
   are explicitly listed, these parameters apply to both SRTP and
   SRTCP.

   AEAD_AES_128_CCM
        cipher:                 AES_128_CCM
        cipher_key_length:      128 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   16 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_256_CCM
        cipher:                 AES_256_CCM
        cipher_key_length:      256 bits


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        cipher_salt_length:     96 bits
        aead_auth_tag_length:   16 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_128_CCM_8
        cipher:                 AES_128_CCM
        cipher_key_length:      128 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   8 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_256_CCM_8
        cipher:                 AES_256_CCM
        cipher_key_length:      256 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   8 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_128_CCM_12
        cipher:                 AES_128_CCM
        cipher_key_length:      128 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   12 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_256_CCM_12
        cipher:                 AES_256_CCM
        cipher_key_length:      256 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   12 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets



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   AEAD_AES_128_GCM
        cipher:                 AES_128_GCM
        cipher_key_length:      128 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   16 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_256_GCM
        cipher:                 AES_256_GCM
        cipher_key_length:      256 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   16 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets


   AEAD_AES_128_GCM_12
        cipher:                 AES_128_GCM
        cipher_key_length:      128 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   12 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets

   AEAD_AES_256_GCM_12
        cipher:                 AES_256_GCM
        cipher_key_length:      256 bits
        cipher_salt_length:     96 bits
        aead_auth_tag_length:   12 octets
        auth_function:          NULL
        auth_key_length:        N/A
        auth_tag_length:        N/A
        maximum lifetime:       at most 2^31 SRTCP packets and
                                at most 2^48 SRTP packets


   Note that these SRTP Protection Profiles do not specify an
   auth_function, auth_key_length, or auth_tag_length because all of
   these profiles use AEAD algorithms, and thus do not use a separate
   auth_function, auth_key, or auth_tag.  The term aead_auth_tag_length
   is used to emphasize that this refers to the authentication tag
   provided by the AEAD algorithm and that this tag is not located in


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   the authentication tag field provided by SRTP/SRTCP.


15.3. MIKEY

   In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
   IANA maintains several subregitries under "Multimedia Internet KEYing
   (MIKEY) Payload Name Spaces".  This document requires additions to
   two of the MIKEY subregistries.

   In the "MIKEY Security Protocol Parameters" subregistry we request
   the following addition:

      Type | Meaning                         | Possible values
      ----------------------------------------------------------------
       TBD | AEAD authentication tag length  | 8, 12, or 16 (in octets)


   This list is, of course, intended for use with CM and GCM.  It is
   conceivable that new AEAD algorithms introduced at some point in the
   future may require a different set of Authentication tag lengths.

   In the "Encryption Algorithm" subregistry (derived from Table
   6.10.1.b of [RFC3830]) we request the following additions:

         SRTP encr  | Value | Default Session   |  Default Auth.
         Algorithm  |       | Encr. Key Length  |   Tag Length
       -----------------------------------------------------------
         AES-CCM    |  TBD  |    16 octets      |  16 octets
         AES-GCM    |  TBD  |    16 octets      |  16 octets

   The SRTP encryption algorithm, session encryption key length, and
   AEAD authentication tag values received from MIKEY fully determine
   the AEAD algorithm (e.g., AEAD_AES_256_GCM_8).  The exact mapping is
   described in section 16.


15.4. AEAD registry

   We request that IANA make the following additions to the IANA
   "Authenticated Encryption with Associated Data (AEAD) Parameters"
   page's registry for "AEAD Algorithms":

                 AEAD_AES_128_CCM_12     = TBD
                 AEAD_AES_256_CCM_12     = TBD

16. Parameters for use with MIKEY

   MIKEY specifies the algorithm family separately from the key length
   (which is specified by the Session Encryption key length) and the
   authentication tag length (specified by AEAD Auth.  tag length).



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                           +------------+-------------+-------------+
                           | Encryption | Encryption  |  AEAD Auth. |
                           | Algorithm  | Key Length  |  Tag Length |
                           +============+=============+=============+
      AEAD_AES_128_GCM     |  AES-GCM   | 16 octets   | 16 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_128_CCM     |  AES-CCM   | 16 octets   | 16 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_128_GCM_12  |  AES-GCM   | 16 octets   | 12 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_128_CCM_12  |  AES-CCM   | 16 octets   | 12 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_128_CCM_8   |  AES-CCM   | 16 octets   |  8 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_GCM     |  AES-GCM   | 32 octets   | 16 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_CCM     |  AES-CCM   | 32 octets   | 16 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_GCM_12  |  AES-GCM   | 32 octets   | 12 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_CCM_12  |  AES-CCM   | 32 octets   | 12 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_CCM_8   |  AES-CCM   | 32 octets   |  8 octets   |
                           +============+=============+=============+

             Table 14: Mapping MIKEY parameters to AEAD algorithm


   Section 12 in this document restricts the choice of Key Derivation
   Function for AEAD algorithms.  To enforce this restriction in MIKEY,
   we require that the SRTP PRF has value AES-CM whenever an AEAD
   algorithm is used.  Note that, according to Section 6.10.1 in
   [RFC3830], the input key length of the Key Derivation Function (i.e.
   the SRTP master key length) is always equal to the session encryption
   key length.  This means, for example, that AEAD_AES_256_GCM will use
   AES_256_CM_PRF as the Key Derivation Function.


17. Acknowledgements

   The authors would like to thank Michael Peck, Michael Torla, Qin Wu,
   Magnus Westerland, Oscar Ohllson, Woo-Hwan Kim, John Mattsson,
   Richard Barnes, John Mattisson, Morris Dworkin and many other
   reviewers who provided valuable comments on earlier drafts of this
   document.








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18. References


18.1. Normative References


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Casner, S., Frederick, R., and V. Jacobson, "RTP: A
              Transport Protocol for Real-Time Applications", RFC 3550,
              July 2003.

   [RFC3610]  Whiting,D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, March 2004.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
              K. Norrman, "The Secure Real-time Transport Protocol
              (SRTP)", RFC 3711, September 2003.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,and
              Norrman, K, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4568]  Andreasen, F., Baugher, M., and D.Wing, "Session
              Description Protocol (SDP): Security Descriptions for
              Media Streams", RFC 4568, July 2006.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for
              Authenticated Encryption with Associated Data", RFC 5116,
              January 2008.

   [RFC5282]  McGrew, D. and D. Black, "Using Authenticated Encryption
              Algorithms with the Encrypted Payload of the Internet Key
              Exchange version 2 (IKEv2) Protocol", RFC 5282,
              August 2008.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [RFC6188]  D. McGrew, "The Use of AES-192 and AES-256 in Secure
              RTP", RFC 6188, March 2011.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.

   [RFC6904]  J. Lennox, "Encryption of Header Extensions in the Secure
              Real-Time Transport Protocol (SRTP)", January 2013.

, January 2013.


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   [RFC6904]  J. Lennox, "Encryption of Header Extensions in the Secure
              Real-Time Transport Protocol (SRTP)", January 2013.



18.2. Informative References


   [BN00]     Bellare, M. and C. Namprempre, "Authenticated encryption:
              Relations among notions and analysis of the generic
              composition paradigm", Proceedings of ASIACRYPT 2000,
              Springer-Verlag, LNCS 1976, pp. 531-545 http://
              www-cse.ucsd.edu/users/mihir/papers/oem.html.

   [GCM]      Dworkin, M., "NIST Special Publication 800-38D:
              Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC.", U.S. National
              Institute of Standards and Technology http://
              csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.

   [R02]      Rogaway, P., "Authenticated encryption with Associated-
              Data", ACM Conference on Computer and Communication
              Security (CCS'02), pp. 98-107, ACM Press,
              2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771, January 2007.





















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   Author's Address

      David A. McGrew
      Cisco Systems, Inc.
      510 McCarthy Blvd.
      Milpitas, CA  95035
      US
      Phone: (408) 525 8651
      Email: mcgrew@cisco.com
      URI:   http://www.mindspring.com/~dmcgrew/dam.htm


      Kevin M. Igoe
      NSA/CSS Commercial Solutions Center
      National Security Agency
      EMail: kmigoe@nsa.gov


Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).
































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