[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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 01, 2016                       National Security Agency
                                                           June 30, 2015


          AES-GCM Authenticated Encryption in Secure RTP (SRTP)
                   draft-ietf-avtcore-srtp-aes-gcm-17


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts. The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 01, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.















Igoe and McGrew                 Standards Track                 [Page 1]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


Abstract

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


Table of Contents

   1. Introduction.....................................................4
   2. Conventions Used In This Document................................5
   3. Overview of the SRTP/SRTCP AEAD security Architecture............5
   4. Terminology......................................................5
   5. Generic AEAD Processing..........................................6
      5.1. Types of Input Data.........................................6
      5.2. AEAD Invocation Inputs and Outputs..........................6
         5.2.1. Encrypt Mode...........................................6
         5.2.2. Decrypt Mode...........................................7
      5.3. Handling of AEAD Authentication.............................7
   6. Counter Mode Encryption..........................................7
   7. Unneeded SRTP/SRTCP Fields.......................................8
      7.1. SRTP/SRTCP Authentication Field.............................8
      7.2. RTP Padding.................................................9
   8. AES-GCM processing for SRTP......................................9
      8.1. SRTP IV formation for AES-GCM...............................9
      8.2. Data Types in SRTP Packets..................................9
      8.3. Handling Header Extensions.................................11
      8.4. Prevention of SRTP IV Reuse................................12
   9. AES-GCM Processing of SRTCP Compound Packets....................13
      9.1. SRTCP IV formation for AES-GCM.............................13
      9.2. Data Types in Encrypted SRTCP Compound Packets.............14
      9.3. Data Types in Unencrypted SRTCP Compound Packets...........15
      9.4. Prevention of SRTCP IV Reuse...............................16
   10. Constraints on AEAD for SRTP and SRTCP.........................16
   11. Key Derivation Functions.......................................17
   12. Summary of AES-GCM in SRTP/SRTCP...............................17
   13. Security Considerations........................................18
      13.1. Handling of Security Critical Parameters..................18
      13.2. Size of the Authentication Tag............................19
   14. IANA Considerations............................................19
      14.1. SDES......................................................19
      14.2. DTLS-SRTP.................................................19
      14.3. MIKEY.....................................................20
   15. Parameters for use with MIKEY..................................21
   16. Some RTP Test Vectors..........................................21
      16.1. SRTP AEAD_AES_128_GCM.....................................22
         16.1.1. SRTP AEAD_AES_128_GCM Encryption.....................22
         16.1.2. SRTP AEAD_AES_128_GCM Decryption.....................25
         16.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging.........27
         16.1.4. SRTP AEAD_AES_128_GCM Tag Verification...............29
      16.2. SRTP AEAD_AES_256_GCM.....................................29
         16.2.1. SRTP AEAD_AES_256_GCM Encryption.....................30


Igoe and McGrew                 Standards Track                 [Page 2]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


         16.2.2. SRTP AEAD_AES_256_GCM Decryption.....................32
         16.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging.........34
         16.2.4. SRTP AEAD_AES_256_GCM Tag Verification...............36
   17. RTCP Test Vectors..............................................37
      17.1. SRTCP AEAD_AES_128_GCM Encrypt and Tag....................38
      17.2. SRTCP AEAD_AES_256_GCM Verify and Decryption..............39
      17.3. SRTCP AEAD_AES_128_GCM Tag Only...........................41
      17.4. SRTCP AEAD_AES_256_GCM Tag Verification...................42
   18. Acknowledgements...............................................43
   19. References.....................................................44
      19.1. Normative References......................................44
      19.2. Informative References....................................44










































Igoe and McGrew                 Standards Track                 [Page 3]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



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.  AES Galois/Counter Mode (AES-GCM) [GCM] is a family of AEAD
   algorithms 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.  In this specification the authentication tag MUST NOT be
   truncated.  The authentications tags MUST be a full 16 octets in
   length.  When used in SRTP/SRTCP AES-GCM will have two
   configurations:

       AEAD_AES_128_GCM       AES-128 with a 16 byte authentication tag
       AEAD_AES_256_GCM       AES-256 with a 16 byte authentication tag

   The key size is set when the session is initiated and SHOULD NOT be
   altered.

   The Galois/Counter Mode of operation (GCM) is an AEAD mode of
   operation for block ciphers.  GCM 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 implementations, or for implementations in an
   efficient and compact circuit.

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



Igoe and McGrew                 Standards Track                 [Page 4]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



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-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].  The master key MUST be at least as large as the
           encryption key derived from it.  Since AEAD algorithms such
           as AES-GCM combine encryption and authentication into a
           single process, AEAD algorithms do not make use of separate
           authentication keys.

       d)  Aside from making modifications to IANA registries to allow
           AES-GCM 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 is invoked 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 8.1).
           SRTCP combines the SSRC and 31-bit SRTCP index with the
           encryption salt to form a 12-octet IV (see section 9.1).


4. Terminology


Igoe and McGrew                 Standards Track                 [Page 5]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



   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 8.2, 9.2 and 9.3.


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


Igoe and McGrew                 Standards Track                 [Page 6]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


        Associated_Data             Octet string of variable length
        Plaintext                   Octet string of variable length

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

   (*): In AEAD the authentication tag in embedded in the cipher text.
   When GCM is being used the ciphertext consists of the encrypted plain
   text followed by the authentication tag.


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

      Outputs
        Plaintext                   Octet string, length =
                                      length(Ciphertext)-tag_length
        Validity_Flag               Boolean, TRUE if valid,
                                    FALSE otherwise




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

   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 key stream of octets, formed in blocks of
   16-octets each.  The first 16-octet block of key is saved for use in
   forming the authentication tag, and the remainder of the key stream
   is XORed to the plaintext to form cipher.  This key stream is formed


Igoe and McGrew                 Standards Track                 [Page 7]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


   one block at a time by inputting the concatenation of a 12-octet IV
   (see sections 8.1 and 9.1) with a 4-octet block to AES.  The
   pseudo-code below illustrates this process:



    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 )


   In theory this keystream generation process allows for the encryption
   of up to (2^36)-32 octets per invocation (i.e.  per packet), 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. Unneeded SRTP/SRTCP Fields

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


7.1. 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


Igoe and McGrew                 Standards Track                 [Page 8]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


      [RFC4771].  This document retains the Authentication Tag field
      primarily to preserve compatibility with these applications.


7.2. RTP Padding

   AES-GCM does not 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.


8. AES-GCM processing for SRTP


8.1. SRTP IV formation for AES-GCM

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


8.2. Data Types in SRTP Packets

   All SRTP packets MUST be both authenticated and encrypted.  The data
   fields within the RTP 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


Igoe and McGrew                 Standards Track                 [Page 9]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


                       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.

        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 RTP 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.









Igoe and McGrew                Standards Track                 [Page 10]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


        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


8.3. 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
   algorithm, this keystream MUST be generated in the manner defined in
   [RFC6904] using the AES-CM transform.  For the AEAD_AES_256_GCM
   algorithm, 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.



Igoe and McGrew                Standards Track                 [Page 11]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



8.4. 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
                         (ROC,SEQ) value, or that the encryption process
                         ensures that the both the SEQ and ROC numbers
                         of the packets presented to it are always
                         incremented in the proper fashion.  This is
                         particularly important for GCM since using the
                         same (ROC,SEQ) value twice compromises the
                         authentication mechanism.  For GCM, the
                         (ROC,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.






Igoe and McGrew                Standards Track                 [Page 12]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



9. AES-GCM 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)


9.1. SRTCP IV formation for AES-GCM

   The 12-octet initialization vector used by AES-GCM 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 9 11
                +--+--+--+--+--+--+--+--+--+--+--+--+
                |00|00|    SSRC   |00|00|0+SRTCP Idx|---+
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                        |
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                |         Encryption Salt           |->(+)
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                        |
                +--+--+--+--+--+--+--+--+--+--+--+--+   |
                |       Initialization Vector       |<--+
                +--+--+--+--+--+--+--+--+--+--+--+--+

              Figure 4: SRTCP Initialization Vector formation























Igoe and McGrew                Standards Track                 [Page 13]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



9.2. 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


Igoe and McGrew                Standards Track                 [Page 14]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


                       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.


9.3. 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  |                  SRTCP MKI (optional) 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



Igoe and McGrew                Standards Track                 [Page 15]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


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


9.4. 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 8.4 also apply.


10. 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 one specific family of AEAD algorithms, namely
   AES-GCM.

   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


Igoe and McGrew                Standards Track                 [Page 16]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


   C_MAX      maximum ciphertext       GCM: MUST be <= 2^36-16 octets.
              length per invocation


   For sake of clarity we specify two additional parameters:

      AEAD Authentication Tag Length   MUST be 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               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.


11. 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.  AEAD_AES_128_GCM algorithm MUST use the (128-bit)
   AES_CM_PRF Key Derivation Function described in [RFC3711].
   AEAD_AES_256_GCM MUST use the AES_256_CM_PRF Key Derivation Function
   described in [RFC6188].


12. Summary of AES-GCM in SRTP/SRTCP

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

   The AES-GCM family of AEAD algorithms is 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:


      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]

                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.  Below we summarize parameters associated with
   these two GCM algorithms:





Igoe and McGrew                Standards Track                 [Page 17]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



     +--------------------------------+------------------------------+
     | 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




     +--------------------------------+------------------------------+
     | 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 4: The AEAD_AES_256_GCM Crypto Suite




13. Security Considerations


13.1. 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
        1.  The block counter is incremented after each block key has
        been produced, but it MUST NOT be allowed to exceed 2^32-1 for


Igoe and McGrew                Standards Track                 [Page 18]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


        GCM.  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 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 both the 31-bit
        SRTCP index and the 48-bit SRTP packet index (ROC||SEQ) MUST be
        saved in order to prevent IV reuse.
      - Processing MUST cease if either the 31-bit SRTCP index or the
        48-bit packet index ROC||SEQ cycles back to its initial value.
        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.


13.2. Size of the Authentication Tag

   We require that the AEAD authentication tag to 16 octets, effectively
   eliminating the risk of an adversary successfully introducing
   fraudulent data.  Though other protocols may allow the use of
   truncated authentication tags, the consenus of the authors and the
   working group is that risks associated with using truncated AES-GCM
   tags are deemed to be too high to allow the use of truncated
   authentication tags in STRP/SRTCP.


14. IANA Considerations


14.1. 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.

      srtp-crypto-suite-ext = "AEAD_AES_128_GCM"    /
                              "AEAD_AES_256_GCM"    /
                              srtp-crypto-suite-ext


14.2. 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:


Igoe and McGrew                Standards Track                 [Page 19]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015




         SRTP_AEAD_AES_128_GCM    = {TBD, TBD }
         SRTP_AEAD_AES_256_GCM    = {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.



   SRTP_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
   SRTP_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

   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
   the authentication tag field provided by SRTP/SRTCP.


14.3. MIKEY

   In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
   IANA maintains several subregistries 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:



Igoe and McGrew                Standards Track                 [Page 20]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


      Type | Meaning                         | Possible values
      --------------------------------------------------------
       TBD | AEAD authentication tag length  | 16 octets


   This list is, of course, intended for use with 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 addition:

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

   The encryption algorithm, session encryption key length, and AEAD
   authentication tag sizes received from MIKEY fully determine the AEAD
   algorithm to be used.  The exact mapping is described in section 15.


15. 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).


                           +------------+-------------+-------------+
                           | Encryption | Encryption  |  AEAD Auth  |
                           | Algorithm  | Key Length  |  Tag Length |
                           +============+=============+=============+
      AEAD_AES_128_GCM     |  AES-GCM   | 16 octets   | 16 octets   |
                           +------------+-------------+-------------+
      AEAD_AES_256_GCM     |  AES-GCM   | 32 octets   | 16 octets   |
                           +============+=============+=============+

             Table 6: Mapping MIKEY parameters to AEAD algorithm


   Section 11 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.


16. Some RTP Test Vectors


Igoe and McGrew                Standards Track                 [Page 21]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



   The examples in this section are all based upon the same RTP packet


            8040f17b 8041f8d3 5501a0b2 47616c6c
            69612065 7374206f 6d6e6973 20646976
            69736120 696e2070 61727465 73207472
            6573

   consisting of a 12 octet header (8040f17b 8041f8d3 5501a0b2) and a 38
   octet payload (47616c6c 69612065 7374206f 6d6e6973 20646976 69736120
   696e2070 61727465 73207472 6573) which is just the ASCII string
   "Gallia est omnis divisa in partes tres".  The salt used (51756964
   2070726f 2071756f) comes from the ASCII string "Quid pro quo".  The
   16 octet (128 bit) key is 00 01 02 ...  0f and the 32 octet (256 bit)
   key is 00 01 02 ...  1f.  The RTP payload type (1000000 binary = 64
   decimal) was at the time this document was written an unassigned
   value.

   As shown in section 8.1, the IV is formed XORing two 12-octet
   values.  The first 12-octet value is formed by concatenating two zero
   octets, the 4-octet SSRC (found in the 9th thru 12th octets of the
   packet), the 4-octet rollover counter ROC maintained at each end of
   the link, and the 2-octet sequence number SEQ (found in the 3rd and
   4th octets of the packet).  The second 12-octet value is the salt, a
   value that is held constant at least until the key is changed.

             | Pad |   SSRC    |    ROC    | SEQ |
              00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt   51 75 69 64 20 70 72 6f 20 71 75 6f
              ------------------------------------
         IV   51 75 3c 65 80 c2 72 6f 20 71 84 14

   All of the RTP examples use this IV.


16.1. SRTP AEAD_AES_128_GCM


16.1.1. SRTP AEAD_AES_128_GCM Encryption

     Encrypting the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f


Igoe and McGrew                Standards Track                 [Page 22]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 8040f17b 8041f8d3 5501a0b2
      PT: 47616c6c 69612065 7374206f 6d6e6973
          20646976 69736120 696e2070 61727465
          73207472 6573
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: c6a13b37878f5b826f4f8162a1c8d879

     Encrypt plaintext
       block # 0
         IV||blk_cntr: 51753c6580c2726f2071841400000002
            key_block: b5 2c 8f cf 92 55 fe 09 df ce a6 73 f0 10 22 b9
          plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
         cipher_block: f2 4d e3 a3 fb 34 de 6c ac ba 86 1c 9d 7e 4b ca
       block # 1
         IV||blk_cntr: 51753c6580c2726f2071841400000003
            key_block: 9e 07 52 a3 64 5a 2f 4f 2b cb d4 0a 30 b5 a5 fe
          plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
         cipher_block: be 63 3b d5 0d 29 4e 6f 42 a5 f4 7a 51 c7 d1 9b
       block # 2
         IV||blk_cntr: 51753c6580c2726f2071841400000004
            key_block: 45 fe 4e ad ed 40 0a 5d 1a f3 63 f9 0c e1 49 3b
          plain_block: 73 20 74 72 65 73
         cipher_block: 36 de 3a df 88 33

     Cipher before tag appended
          f24de3a3 fb34de6c acba861c 9d7e4bca
          be633bd5 0d294e6f 42a5f47a 51c7d19b
          36de3adf 8833

     Compute GMAC tag

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b200000000
         partial hash: bcfb3d1d0e6e3e78ba45403377dba11b

       Process Cipher
          Cipher word: f24de3a3fb34de6cacba861c9d7e4bca
         partial hash: 0ebc0abe1b15b32fedd2b07888c1ef61
          Cipher word: be633bd50d294e6f42a5f47a51c7d19b
         partial hash: 438e5797011ea860585709a2899f4685
          Cipher word: 36de3adf883300000000000000000000
         partial hash: 336fb643310d7bac2aeaa76247f6036d

       Process Length Word
          Length word: 00000000000000600000000000000130
         partial hash: 1b964067078c408c4e442a8f015e5264

     Turn GHASH into GMAC
                GHASH: 1b 96 40 67 07 8c 40 8c 4e 44 2a 8f 01 5e 52 64


Igoe and McGrew                Standards Track                 [Page 23]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


                   K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
            full GMAC: 89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ce

     Cipher with tag
          f24de3a3 fb34de6c acba861c 9d7e4bca
          be633bd5 0d294e6f 42a5f47a 51c7d19b
          36de3adf 8833899d 7f27beb1 6a9152cf
          765ee439 0cce

     Encrypted and Tagged packet:
          8040f17b 8041f8d3 5501a0b2 f24de3a3
          fb34de6c acba861c 9d7e4bca be633bd5
          0d294e6f 42a5f47a 51c7d19b 36de3adf
          8833899d 7f27beb1 6a9152cf 765ee439
          0cce







































Igoe and McGrew                Standards Track                 [Page 24]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.1.2. SRTP AEAD_AES_128_GCM Decryption

     Decrypting the following packet:

          8040f17b 8041f8d3 5501a0b2 f24de3a3
          fb34de6c acba861c 9d7e4bca be633bd5
          0d294e6f 42a5f47a 51c7d19b 36de3adf
          8833899d 7f27beb1 6a9152cf 765ee439
          0cce

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 8040f17b 8041f8d3 5501a0b2
      CT: f24de3a3 fb34de6c acba861c 9d7e4bca
          be633bd5 0d294e6f 42a5f47a 51c7d19b
          36de3adf 8833899d 7f27beb1 6a9152cf
          765ee439 0cce
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: c6a13b37878f5b826f4f8162a1c8d879

     Verify received tag  89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ce

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b200000000
         partial hash: bcfb3d1d0e6e3e78ba45403377dba11b

       Process Cipher
          Cipher word: f24de3a3fb34de6cacba861c9d7e4bca
         partial hash: 0ebc0abe1b15b32fedd2b07888c1ef61
          Cipher word: be633bd50d294e6f42a5f47a51c7d19b
         partial hash: 438e5797011ea860585709a2899f4685
          Cipher word: 36de3adf883300000000000000000000
         partial hash: 336fb643310d7bac2aeaa76247f6036d

       Process Length Word
          Length word: 00000000000000600000000000000130
         partial hash: 1b964067078c408c4e442a8f015e5264

     Turn GHASH into GMAC
                GHASH: 1b 96 40 67 07 8c 40 8c 4e 44 2a 8f 01 5e 52 64
                   K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
            full GMAC: 89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ce

          Received tag = 899d7f27 beb16a91 52cf765e e4390cce
          Computed tag = 899d7f27 beb16a91 52cf765e e4390cce
       Received tag verified.


Igoe and McGrew                Standards Track                 [Page 25]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



     Decrypt cipher
       block # 0
         IV||blk_cntr: 51753c6580c2726f2071841400000002
            key_block: b5 2c 8f cf 92 55 fe 09 df ce a6 73 f0 10 22 b9
         cipher_block: f2 4d e3 a3 fb 34 de 6c ac ba 86 1c 9d 7e 4b ca
          plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
       block # 1
         IV||blk_cntr: 51753c6580c2726f2071841400000003
            key_block: 9e 07 52 a3 64 5a 2f 4f 2b cb d4 0a 30 b5 a5 fe
         cipher_block: be 63 3b d5 0d 29 4e 6f 42 a5 f4 7a 51 c7 d1 9b
          plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
       block # 2
         IV||blk_cntr: 51753c6580c2726f2071841400000004
            key_block: 45 fe 4e ad ed 40 0a 5d 1a f3 63 f9 0c e1 49 3b
         cipher_block: 36 de 3a df 88 33
          plain_block: 73 20 74 72 65 73

     Verified and Taged packet:
          47616c6c 69612065 7374206f 6d6e6973
          20646976 69736120 696e2070 61727465
          73207472 6573
































Igoe and McGrew                Standards Track                 [Page 26]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging

     Tagging the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: c6a13b37878f5b826f4f8162a1c8d879

     Compute GMAC tag

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b247616c6c
         partial hash: 79f41fea34a474a77609d8925e9f2b22
             AAD word: 696120657374206f6d6e697320646976
         partial hash: 84093a2f85abf17ab37d3ce2f706138f
             AAD word: 69736120696e20706172746573207472
         partial hash: ab2760fee24e6dec754739d8059cd144
             AAD word: 65730000000000000000000000000000
         partial hash: e84f3c55d287fc561c41d09a8aada4be

       Process Length Word
          Length word: 00000000000001900000000000000000
         partial hash: b04200c26b81c98af55cc2eafccd1cbc

     Turn GHASH into GMAC
                GHASH: b0 42 00 c2 6b 81 c9 8a f5 5c c2 ea fc cd 1c bc
                   K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
            full GMAC: 22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16

     Cipher with tag
          22493f82 d2bce397 e9d79e3b 19aa4216

     Tagged Packet:
          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472


Igoe and McGrew                Standards Track                 [Page 27]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


          65732249 3f82d2bc e397e9d7 9e3b19aa
          4216




















































Igoe and McGrew                Standards Track                 [Page 28]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.1.4. SRTP AEAD_AES_128_GCM Tag Verification

     Verifying the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          65732249 3f82d2bc e397e9d7 9e3b19aa
          4216

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573
      CT: 22493f82 d2bce397 e9d79e3b 19aa4216
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: c6a13b37878f5b826f4f8162a1c8d879

     Verify received tag  22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b247616c6c
         partial hash: 79f41fea34a474a77609d8925e9f2b22
             AAD word: 696120657374206f6d6e697320646976
         partial hash: 84093a2f85abf17ab37d3ce2f706138f
             AAD word: 69736120696e20706172746573207472
         partial hash: ab2760fee24e6dec754739d8059cd144
             AAD word: 65730000000000000000000000000000
         partial hash: e84f3c55d287fc561c41d09a8aada4be

       Process Length Word
          Length word: 00000000000001900000000000000000
         partial hash: b04200c26b81c98af55cc2eafccd1cbc

     Turn GHASH into GMAC
                GHASH: b0 42 00 c2 6b 81 c9 8a f5 5c c2 ea fc cd 1c bc
                   K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
            full GMAC: 22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16

          Received tag = 22493f82 d2bce397 e9d79e3b 19aa4216
          Computed tag = 22493f82 d2bce397 e9d79e3b 19aa4216
       Received tag verified.

16.2. SRTP AEAD_AES_256_GCM


Igoe and McGrew                Standards Track                 [Page 29]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015




16.2.1. SRTP AEAD_AES_256_GCM Encryption

     Encrypting the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 8040f17b 8041f8d3 5501a0b2
      PT: 47616c6c 69612065 7374206f 6d6e6973
          20646976 69736120 696e2070 61727465
          73207472 6573
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: f29000b62a499fd0a9f39a6add2e7780

     Encrypt plaintext
       block # 0
         IV||blk_cntr: 51753c6580c2726f2071841400000002
            key_block: 75 d0 b2 14 c1 43 de 77 9c eb 58 95 5e 40 5a d9
          plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
         cipher_block: 32 b1 de 78 a8 22 fe 12 ef 9f 78 fa 33 2e 33 aa
       block # 1
         IV||blk_cntr: 51753c6580c2726f2071841400000003
            key_block: 91 e4 7b 4e f3 2b 83 d3 dc 65 0a 72 17 8d da 6a
          plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
         cipher_block: b1 80 12 38 9a 58 e2 f3 b5 0b 2a 02 76 ff ae 0f
       block # 2
         IV||blk_cntr: 51753c6580c2726f2071841400000004
            key_block: 68 86 43 eb dd 08 07 98 16 3a 16 d5 e5 04 f6 3a
          plain_block: 73 20 74 72 65 73
         cipher_block: 1b a6 37 99 b8 7b

     Cipher before tag appended
          32b1de78 a822fe12 ef9f78fa 332e33aa
          b1801238 9a58e2f3 b50b2a02 76ffae0f
          1ba63799 b87b

     Compute GMAC tag

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b200000000


Igoe and McGrew                Standards Track                 [Page 30]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


         partial hash: 0154dcb75485b71880e1957c877351bd

       Process Cipher
          Cipher word: 32b1de78a822fe12ef9f78fa332e33aa
         partial hash: c3f07db9a8b9cb4345eb07f793d322d2
          Cipher word: b18012389a58e2f3b50b2a0276ffae0f
         partial hash: 6d1e66fe32eb32ecd8906ceab09db996
          Cipher word: 1ba63799b87b00000000000000000000
         partial hash: b3d1d2f1fa3b366619bc42cd2eedafee

       Process Length Word
          Length word: 00000000000000600000000000000130
         partial hash: 7debf5fa1fac3bd318d5e1a7ee401091

     Turn GHASH into GMAC
                GHASH: 7d eb f5 fa 1f ac 3b d3 18 d5 e1 a7 ee 40 10 91
                   K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
            full GMAC: 7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13

     Cipher with tag
          32b1de78 a822fe12 ef9f78fa 332e33aa
          b1801238 9a58e2f3 b50b2a02 76ffae0f
          1ba63799 b87b7aa3 db36dfff d6b0f9bb
          7878d7a7 6c13

     Encrypted and Tagged packet:
          8040f17b 8041f8d3 5501a0b2 32b1de78
          a822fe12 ef9f78fa 332e33aa b1801238
          9a58e2f3 b50b2a02 76ffae0f 1ba63799
          b87b7aa3 db36dfff d6b0f9bb 7878d7a7
          6c13























Igoe and McGrew                Standards Track                 [Page 31]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.2.2. SRTP AEAD_AES_256_GCM Decryption

     Decrypting the following packet:

          8040f17b 8041f8d3 5501a0b2 32b1de78
          a822fe12 ef9f78fa 332e33aa b1801238
          9a58e2f3 b50b2a02 76ffae0f 1ba63799
          b87b7aa3 db36dfff d6b0f9bb 7878d7a7
          6c13

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 8040f17b 8041f8d3 5501a0b2
      CT: 32b1de78 a822fe12 ef9f78fa 332e33aa
          b1801238 9a58e2f3 b50b2a02 76ffae0f
          1ba63799 b87b7aa3 db36dfff d6b0f9bb
          7878d7a7 6c13
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: f29000b62a499fd0a9f39a6add2e7780

     Verify received tag  7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b200000000
         partial hash: 0154dcb75485b71880e1957c877351bd

       Process Cipher
          Cipher word: 32b1de78a822fe12ef9f78fa332e33aa
         partial hash: c3f07db9a8b9cb4345eb07f793d322d2
          Cipher word: b18012389a58e2f3b50b2a0276ffae0f
         partial hash: 6d1e66fe32eb32ecd8906ceab09db996
          Cipher word: 1ba63799b87b00000000000000000000
         partial hash: b3d1d2f1fa3b366619bc42cd2eedafee

       Process Length Word
          Length word: 00000000000000600000000000000130
         partial hash: 7debf5fa1fac3bd318d5e1a7ee401091

     Turn GHASH into GMAC
                GHASH: 7d eb f5 fa 1f ac 3b d3 18 d5 e1 a7 ee 40 10 91
                   K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
            full GMAC: 7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13

          Received tag = 7aa3db36 dfffd6b0 f9bb7878 d7a76c13
          Computed tag = 7aa3db36 dfffd6b0 f9bb7878 d7a76c13


Igoe and McGrew                Standards Track                 [Page 32]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


       Received tag verified.

     Decrypt cipher
       block # 0
         IV||blk_cntr: 51753c6580c2726f2071841400000002
            key_block: 75 d0 b2 14 c1 43 de 77 9c eb 58 95 5e 40 5a d9
         cipher_block: 32 b1 de 78 a8 22 fe 12 ef 9f 78 fa 33 2e 33 aa
          plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
       block # 1
         IV||blk_cntr: 51753c6580c2726f2071841400000003
            key_block: 91 e4 7b 4e f3 2b 83 d3 dc 65 0a 72 17 8d da 6a
         cipher_block: b1 80 12 38 9a 58 e2 f3 b5 0b 2a 02 76 ff ae 0f
          plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
       block # 2
         IV||blk_cntr: 51753c6580c2726f2071841400000004
            key_block: 68 86 43 eb dd 08 07 98 16 3a 16 d5 e5 04 f6 3a
         cipher_block: 1b a6 37 99 b8 7b
          plain_block: 73 20 74 72 65 73

     Verified and Taged packet:
          47616c6c 69612065 7374206f 6d6e6973
          20646976 69736120 696e2070 61727465
          73207472 6573































Igoe and McGrew                Standards Track                 [Page 33]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging

     Tagging the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: f29000b62a499fd0a9f39a6add2e7780

     Compute GMAC tag

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b247616c6c
         partial hash: c059753e6763791762ca630d8ef97714
             AAD word: 696120657374206f6d6e697320646976
         partial hash: a4e3401e712900dc4f1d2303bc4b2675
             AAD word: 69736120696e20706172746573207472
         partial hash: 1c8c1af883de0d67878f379a19c65987
             AAD word: 65730000000000000000000000000000
         partial hash: 958462781aa8e8feacce6d93b54472ac

       Process Length Word
          Length word: 00000000000001900000000000000000
         partial hash: af2efb5dcfdb9900e7127721fdb56956

     Turn GHASH into GMAC
                GHASH: af 2e fb 5d cf db 99 00 e7 12 77 21 fd b5 69 56
                   K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
            full GMAC: a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4

     Cipher with tag
          a866d591 0f887463 067ceefe c45215d4

     Tagged Packet:
          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976


Igoe and McGrew                Standards Track                 [Page 34]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


          69736120 696e2070 61727465 73207472
          6573a866 d5910f88 7463067c eefec452
          15d4



















































Igoe and McGrew                Standards Track                 [Page 35]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



16.2.4. SRTP AEAD_AES_256_GCM Tag Verification

     Verifying the following packet:

          8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573a866 d5910f88 7463067c eefec452
          15d4

     Form the IV
            | Pad |   SSRC    |    ROC    | SEQ |
             00 00 55 01 a0 b2 00 00 00 00 f1 7b
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
          69612065 7374206f 6d6e6973 20646976
          69736120 696e2070 61727465 73207472
          6573
      CT: a866d591 0f887463 067ceefe c45215d4
      IV:  51 75 3c 65 80 c2 72 6f 20 71 84 14
       H: f29000b62a499fd0a9f39a6add2e7780

     Verify received tag  a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4

       Process AAD
             AAD word: 8040f17b8041f8d35501a0b247616c6c
         partial hash: c059753e6763791762ca630d8ef97714
             AAD word: 696120657374206f6d6e697320646976
         partial hash: a4e3401e712900dc4f1d2303bc4b2675
             AAD word: 69736120696e20706172746573207472
         partial hash: 1c8c1af883de0d67878f379a19c65987
             AAD word: 65730000000000000000000000000000
         partial hash: 958462781aa8e8feacce6d93b54472ac

       Process Length Word
          Length word: 00000000000001900000000000000000
         partial hash: af2efb5dcfdb9900e7127721fdb56956

     Turn GHASH into GMAC
                GHASH: af 2e fb 5d cf db 99 00 e7 12 77 21 fd b5 69 56
                   K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
            full GMAC: a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4

          Received tag = a866d591 0f887463 067ceefe c45215d4
          Computed tag = a866d591 0f887463 067ceefe c45215d4
       Received tag verified.



Igoe and McGrew                Standards Track                 [Page 36]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


17. RTCP Test Vectors

   The examples in this section are all based upon the same RTCP packet:


            81c8000e 4d617273 4e545031 4e545031
            52545020 0000042a 0000eb98 4c756e61
            deadbeef deadbeef deadbeef deadbeef
            deadbeef


   with 32-bit SRTCP index 000005d4.

   As shown in section 9.1, the IV is formed by XORing two 12-octet
   values.  The first 12-octet value is formed by concatenating two zero
   octets, the 4-octet SSRC (found in the 5th thru 8th octets of the RTP
   packet), another 2 padding octets and the 31-bit SRTCP index, right
   justified in a 32-bit = 4-octet field with a single "0" bit
   pre-pended as padding.  An example of SRTCP IV formation is shown
   below:

             | Pad |   SSRC    | Pad |  0+SRTCP  |
              00 00 4d 61 72 73 00 00 00 00 05 d4
       salt   51 75 69 64 20 70 72 6f 20 71 75 6f
              ------------------------------------
         IV   51 75 24 05 52 03 72 6f 20 71 70 bb

   In an SRTCP packet a 1-bit encryption flag is pre-pended to the
   31-bit SRTCP index to form a 32-bit value we shall call the ESRTCP
   word.  The E flag is one if the SRTCP packet has been encrypted and
   zero if it has been tagged but not encrypted.  Note that the ESRTCP
   field is only present in an SRTCP packet, not in an RTCP packet.  The
   full ESRTCP word is part of the AAD.

   When encrypting and tagging an RTCP packet (E flag = 1), the SRTCP
   packet consists of the following fields in the following order:

         - The first 8 octets of the RTCP packet (part of the AAD).
         - The cipher.
         - The ESRTCP word (the final part of the AAD).
         - Any raw data that might have been appended to the end of the
           original RTCP packet.

   Recall that AEAD treats the authentication tag as an integral part of
   the cipher, and in fact the authentication tag is the last 8 or 16
   octets of the cipher.

   The reader is reminded that when the RTCP packet is to be tagged but
   not encrypted (E flag = 0), GCM will produce cipher that consists
   solely of the 8 or 16 byte authentication tag.  The tagged SRTCP
   consists of the following fields in the order listed below:



Igoe and McGrew                Standards Track                 [Page 37]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


         - All of the AAD save for the ECSRTP word.
         - The cipher (= the authentication tag).
         - The ESRTCP word (the final part of the AAD).
         - Any raw data that might have been appended to the end of the
           original RTCP packet.


17.1. SRTCP AEAD_AES_128_GCM Encrypt and Tag

     Encrypting the following packet:

          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef

     Key size = 128 bits
     Tag size =  16 octets

     Form the IV
            | Pad |   SSRC    | Pad |   SRTCP   |
             00 00 4d 61 72 73 00 00 00 00 05 d4
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 24 05 52 03 72 6f 20 71 70 bb

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 81c8000d 4d617273 800005d4
      PT: 4e545031 4e545032 52545020 0000042a
          0000e930 4c756e61 deadbeef deadbeef
          deadbeef deadbeef deadbeef
      IV:  51 75 24 05 52 03 72 6f 20 71 70 bb
       H: c6a13b37878f5b826f4f8162a1c8d879

     Encrypt plaintext
       block # 0
         IV||blk_cntr: 517524055203726f207170bb00000002
            key_block: 2d bd 18 b4 92 8e e6 4e f5 73 87 46 2f 6b 7a b3
          plain_block: 4e 54 50 31 4e 54 50 32 52 54 50 20 00 00 04 2a
         cipher_block: 63 e9 48 85 dc da b6 7c a7 27 d7 66 2f 6b 7e 99
       block # 1
         IV||blk_cntr: 517524055203726f207170bb00000003
            key_block: 7f f5 29 c7 20 73 9d 4c 18 db 1b 1e ad a0 d1 35
          plain_block: 00 00 e9 30 4c 75 6e 61 de ad be ef de ad be ef
         cipher_block: 7f f5 c0 f7 6c 06 f3 2d c6 76 a5 f1 73 0d 6f da
       block # 2
         IV||blk_cntr: 517524055203726f207170bb00000004
            key_block: 92 4d 25 a9 58 9d 83 02 d5 14 99 b4 e0 14 78 15
          plain_block: de ad be ef de ad be ef de ad be ef
         cipher_block: 4c e0 9b 46 86 30 3d ed 0b b9 27 5b

     Cipher before tag appended
          63e94885 dcdab67c a727d766 2f6b7e99


Igoe and McGrew                Standards Track                 [Page 38]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


          7ff5c0f7 6c06f32d c676a5f1 730d6fda
          4ce09b46 86303ded 0bb9275b

     Compute GMAC tag

       Process AAD
             AAD word: 81c8000d4d617273800005d400000000
         partial hash: 085d6eb166c555aa62982f630430ec6e

       Process Cipher
          Cipher word: 63e94885dcdab67ca727d7662f6b7e99
         partial hash: 8c9221be93466d68bbb16fa0d42b0187
          Cipher word: 7ff5c0f76c06f32dc676a5f1730d6fda
         partial hash: 221ebb044ec9fd0bf116d7780f198792
          Cipher word: 4ce09b4686303ded0bb9275b00000000
         partial hash: 50f70b9ca110ab312dce212657328dae

       Process Length Word
          Length word: 00000000000000600000000000000160
         partial hash: 7296107c9716534371dfc1a30c5ffeb5

     Turn GHASH into GMAC
                GHASH: 72 96 10 7c 97 16 53 43 71 df c1 a3 0c 5f fe b5
                   K0: ba dc b4 24 01 d9 1e 6c b4 74 39 d1 49 86 14 6b
            full GMAC: c8 4a a4 58 96 cf 4d 2f c5 ab f8 72 45 d9 ea de

     Cipher with tag
          63e94885 dcdab67c a727d766 2f6b7e99
          7ff5c0f7 6c06f32d c676a5f1 730d6fda
          4ce09b46 86303ded 0bb9275b c84aa458
          96cf4d2f c5abf872 45d9eade

     Append ESRTCP word with Eflag set.
     Cext:63e94885 dcdab67c a727d766 2f6b7e99
          7ff5c0f7 6c06f32d c676a5f1 730d6fda
          4ce09b46 86303ded 0bb9275b c84aa458
          96cf4d2f c5abf872 45d9eade 800005d4

     Encrypted and Tagged packet:
          81c8000d 4d617273 63e94885 dcdab67c
          a727d766 2f6b7e99 7ff5c0f7 6c06f32d
          c676a5f1 730d6fda 4ce09b46 86303ded
          0bb9275b c84aa458 96cf4d2f c5abf872
          45d9eade 800005d4

17.2. SRTCP AEAD_AES_256_GCM Verify and Decryption

     Key size = 256 bits
     Tag size =  16 octets


       Process Length Word


Igoe and McGrew                Standards Track                 [Page 39]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


     Decrypting the following packet:

          81c8000d 4d617273 d50ae4d1 f5ce5d30
          4ba297e4 7d470c28 2c3ece5d bffe0a50
          a2eaa5c1 110555be 8415f658 c61de047
          6f1b6fad 1d1eb30c 4446839f 57ff6f6c
          b26ac3be 800005d4

     Key size = 256 bits
     Key size =  16 octets

     Form the IV
            | Pad |   SSRC    | Pad |   SRTCP   |
             00 00 4d 61 72 73 00 00 00 00 05 d4
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 24 05 52 03 72 6f 20 71 70 bb

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 81c8000d 4d617273 800005d4
      CT: d50ae4d1 f5ce5d30 4ba297e4 7d470c28
          2c3ece5d bffe0a50 a2eaa5c1 110555be
          8415f658 c61de047 6f1b6fad 1d1eb30c
          4446839f 57ff6f6c b26ac3be
      IV:  51 75 24 05 52 03 72 6f 20 71 70 bb
       H: f29000b62a499fd0a9f39a6add2e7780

     Verify received tag  1d 1e b3 0c 44 46 83 9f 57 ff 6f 6c b2 6a c3 be

       Process AAD
             AAD word: 81c8000d4d617273800005d400000000
         partial hash: 3ae5afd36dead5280b18950400176b5b

       Process Cipher
          Cipher word: d50ae4d1f5ce5d304ba297e47d470c28
         partial hash: e90fab7546f6940781227227ac926ebe
          Cipher word: 2c3ece5dbffe0a50a2eaa5c1110555be
         partial hash: 9b236807d8b2dab07583adce367aa88f
          Cipher word: 8415f658c61de0476f1b6fad00000000
         partial hash: e69313f423a75e3e0b7eb93321700e86

       Process Length Word
          Length word: 00000000000000600000000000000160
         partial hash: 3a284af2616fdf505faf37eec39fbc8b

     Turn GHASH into GMAC
                GHASH: 3a 28 4a f2 61 6f df 50 5f af 37 ee c3 9f bc 8b
                   K0: 27 36 f9 fe 25 29 5c cf 08 50 58 82 71 f5 7f 35
            full GMAC: 1d 1e b3 0c 44 46 83 9f 57 ff 6f 6c b2 6a c3 be

          Received tag = 1d1eb30c 4446839f 57ff6f6c b26ac3be
          Computed tag = 1d1eb30c 4446839f 57ff6f6c b26ac3be


Igoe and McGrew                Standards Track                 [Page 40]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


       Received tag verified.

     Decrypt cipher
       block # 0
         IV||blk_cntr: 517524055203726f207170bb00000002
            key_block: 9b 5e b4 e0 bb 9a 0d 02 19 f6 c7 c4 7d 47 08 02
         cipher_block: d5 0a e4 d1 f5 ce 5d 30 4b a2 97 e4 7d 47 0c 28
          plain_block: 4e 54 50 31 4e 54 50 32 52 54 50 20 00 00 04 2a
       block # 1
         IV||blk_cntr: 517524055203726f207170bb00000003
            key_block: 2c 3e 27 6d f3 8b 64 31 7c 47 1b 2e cf a8 eb 51
         cipher_block: 2c 3e ce 5d bf fe 0a 50 a2 ea a5 c1 11 05 55 be
          plain_block: 00 00 e9 30 4c 75 6e 61 de ad be ef de ad be ef
       block # 2
         IV||blk_cntr: 517524055203726f207170bb00000004
            key_block: 5a b8 48 b7 18 b0 5e a8 b1 b6 d1 42 3b 74 39 55
         cipher_block: 84 15 f6 58 c6 1d e0 47 6f 1b 6f ad
          plain_block: de ad be ef de ad be ef de ad be ef

     Verified and Decrypted packet:
          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef

17.3. SRTCP AEAD_AES_128_GCM Tag Only

     Tagging the following packet:

          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef

     Key size = 128 bits
     Tag size =  16 octets

     Form the IV
            | Pad |   SSRC    | Pad |   SRTCP   |
             00 00 4d 61 72 73 00 00 00 00 05 d4
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
         IV: 51 75 24 05 52 03 72 6f 20 71 70 bb

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
     AAD: 81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef 000005d4
      IV:  51 75 24 05 52 03 72 6f 20 71 70 bb
       H: c6a13b37878f5b826f4f8162a1c8d879

     Compute GMAC tag


Igoe and McGrew                Standards Track                 [Page 41]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



       Process AAD
             AAD word: 81c8000d4d6172734e5450314e545032
         partial hash: f8dbbe278e06afe17fb4fb2e67f0a22e
             AAD word: 525450200000042a0000e9304c756e61
         partial hash: 6ccd900dfd0eb292f68f8a410d0648ec
             AAD word: deadbeefdeadbeefdeadbeefdeadbeef
         partial hash: 6a14be0ea384c6b746235ba955a57ff5
             AAD word: deadbeef000005d40000000000000000
         partial hash: cc81f14905670a1e37f8bc81a91997cd

       Process Length Word
          Length word: 00000000000001c00000000000000000
         partial hash: 3ec16d4c3c0e90a59e91be415bd976d8

     Turn GHASH into GMAC
                GHASH: 3e c1 6d 4c 3c 0e 90 a5 9e 91 be 41 5b d9 76 d8
                   K0: ba dc b4 24 01 d9 1e 6c b4 74 39 d1 49 86 14 6b
            full GMAC: 84 1d d9 68 3d d7 8e c9 2a e5 87 90 12 5f 62 b3

     Cipher with tag
          841dd968 3dd78ec9 2ae58790 125f62b3

     Tagged Packet:
          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef 841dd968 3dd78ec9 2ae58790
          125f62b3 000005d4

17.4. SRTCP AEAD_AES_256_GCM Tag Verification

     Key size = 256 bits
     Tag size =  16 octets


       Process Length Word
     Verifying the following packet:

          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef 91db4afb feee5a97 8fab4393
          ed2615fe 000005d4

     Key size = 256 bits
     Key size =  16 octets

     Form the IV
            | Pad |   SSRC    | Pad |   SRTCP   |
             00 00 4d 61 72 73 00 00 00 00 05 d4
       salt: 51 75 69 64 20 70 72 6f 20 71 75 6f


Igoe and McGrew                Standards Track                 [Page 42]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


         IV: 51 75 24 05 52 03 72 6f 20 71 70 bb

     Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
          10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
     AAD: 81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef 000005d4
      CT: 91db4afb feee5a97 8fab4393 ed2615fe
      IV:  51 75 24 05 52 03 72 6f 20 71 70 bb
       H: f29000b62a499fd0a9f39a6add2e7780

     Verify received tag  91 db 4a fb fe ee 5a 97 8f ab 43 93 ed 26 15 fe

       Process AAD
             AAD word: 81c8000d4d6172734e5450314e545032
         partial hash: 7bc665c71676a5a5f663b3229af4b85c
             AAD word: 525450200000042a0000e9304c756e61
         partial hash: 34ed77752703ab7d69f44237910e3bc0
             AAD word: deadbeefdeadbeefdeadbeefdeadbeef
         partial hash: 74a59f1a99282344d64ab1c8a2be6cf8
             AAD word: deadbeef000005d40000000000000000
         partial hash: 126335c0baa7ab1b79416ceeb9f7a518

       Process Length Word
          Length word: 00000000000001c00000000000000000
         partial hash: b6edb305dbc7065887fb1b119cd36acb

     Turn GHASH into GMAC
                GHASH: b6 ed b3 05 db c7 06 58 87 fb 1b 11 9c d3 6a cb
                   K0: 27 36 f9 fe 25 29 5c cf 08 50 58 82 71 f5 7f 35
            full GMAC: 91 db 4a fb fe ee 5a 97 8f ab 43 93 ed 26 15 fe

          Received tag = 91db4afb feee5a97 8fab4393 ed2615fe
          Computed tag = 91db4afb feee5a97 8fab4393 ed2615fe
       Received tag verified.

     Verified Packet:
          81c8000d 4d617273 4e545031 4e545032
          52545020 0000042a 0000e930 4c756e61
          deadbeef deadbeef deadbeef deadbeef
          deadbeef

18. Acknowledgements

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




Igoe and McGrew                Standards Track                 [Page 43]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015



19. References


19.1. Normative References


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

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

   [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.

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




19.2. Informative References


   [BN00]     Bellare, M. and C. Namprempre, "Authenticated encryption:


Igoe and McGrew                Standards Track                 [Page 44]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


              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.

   [Ferg]     Ferguson, N., "Authentication weaknesses in GCM", http://
              csrc.nist.gov/groups/ST/toolkit/BCM/documents/comments/
                          CWC-GCM/Ferguson2.pdf. May 2005.

   [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.


   [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.






























Igoe and McGrew                Standards Track                 [Page 45]


Internet Draft               AES-GCM for SRTP               Jun 30, 2015


   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).
































Igoe and McGrew                Standards Track                 [Page 46]


Html markup produced by rfcmarkup 1.122, available from https://tools.ietf.org/tools/rfcmarkup/