





Internet Engineering Task Force (IETF)                         D. McGrew
Request for Comments: 7714                           Cisco Systems, Inc.
Category: Standards Track                                        K. Igoe
ISSN: 2070-1721                                 National Security Agency
                                                           December 2015


                    AES-GCM Authenticated Encryption
           in the Secure Real-time Transport Protocol (SRTP)

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 Secure Real-time
   Transport Protocol (SRTP).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7714.

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.






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Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................4
   3. Overview of the SRTP/SRTCP AEAD Security Architecture ...........4
   4. Terminology .....................................................5
   5. Generic AEAD Processing .........................................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 Tag 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 ................................10
      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 ..........16
      9.4. Prevention of SRTCP IV Reuse ..............................17
   10. Constraints on AEAD for SRTP and SRTCP ........................17
   11. Key Derivation Functions ......................................18
   12. Summary of AES-GCM in SRTP/SRTCP ..............................19
   13. Security Considerations .......................................20
      13.1. Handling of Security-Critical Parameters .................20
      13.2. Size of the Authentication Tag ...........................21
   14. IANA Considerations ...........................................21
      14.1. SDES .....................................................21
      14.2. DTLS-SRTP ................................................22
      14.3. MIKEY ....................................................23
   15. Parameters for Use with MIKEY .................................23
   16. Some RTP Test Vectors .........................................24
      16.1. SRTP AEAD_AES_128_GCM ....................................25
           16.1.1. SRTP AEAD_AES_128_GCM Encryption ..................25
           16.1.2. SRTP AEAD_AES_128_GCM Decryption ..................27
           16.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging ......29
           16.1.4. SRTP AEAD_AES_128_GCM Tag Verification ............30
      16.2. SRTP AEAD_AES_256_GCM ....................................31
           16.2.1. SRTP AEAD_AES_256_GCM Encryption ..................31
           16.2.2. SRTP AEAD_AES_256_GCM Decryption ..................33
           16.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging ......35
           16.2.4. SRTP AEAD_AES_256_GCM Tag Verification ............36



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   17. RTCP Test Vectors .............................................37
      17.1. SRTCP AEAD_AES_128_GCM Encryption and Tagging ............39
      17.2. SRTCP AEAD_AES_256_GCM Verification and Decryption .......41
      17.3. SRTCP AEAD_AES_128_GCM Tagging Only ......................43
      17.4. SRTCP AEAD_AES_256_GCM Tag Verification ..................44
   18. References ....................................................45
      18.1. Normative References .....................................45
      18.2. Informative References ...................................47
   Acknowledgements ..................................................48
   Authors' Addresses ................................................48

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"
   (AAD), 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-octet authentication tag
      AEAD_AES_256_GCM      AES-256 with a 16-octet authentication tag





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   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 uses 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.

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
         as one or more endpoints using this master key to process
         inbound data).

      c) A Key Derivation Function (KDF) 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.






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      d) Aside from making modifications to IANA registries to allow
         AES-GCM to work with Security Descriptions (SDES), Datagram
         Transport Layer Security for Secure RTP (DTLS-SRTP), and
         Multimedia Internet KEYing (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 Initialization
         Vector (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

   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.











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5.  Generic AEAD Processing

5.1.  Types of Input Data

      Associated Data: 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
        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
           ciphertext.  When GCM is being used, the ciphertext
           consists of the encrypted Plaintext followed by the
           authentication tag.















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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:

   o  a 16-octet first_key_block, which is used in forming the
      authentication tag, and

   o  a keystream of octets, formed in blocks of 16 octets each














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   The first 16-octet block of the key is saved for use in forming the
   authentication tag, and the remainder of the keystream is XORed to
   the Plaintext to form the cipher.  This keystream is formed 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 pseudocode
   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 [RFC3711], 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 Tag 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 tag field
   mandatory, but the presence of the AEAD authentication renders the
   older authentication methods redundant.




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

7.2.  RTP Padding

   AES-GCM does not require 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 IV 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.














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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), Contributing Source
                       (CSRC) count CC (4 bits), marker M (1 bit),
                       Payload Type PT (7 bits), sequence number
                       (16 bits), timestamp (32 bits), SSRC (32 bits),
                       optional CSRC identifiers (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 Master Key
                       Identifier (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





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   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 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 optional variable-length SRTP MKI
   and SRTP authentication tag fields.

        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 [RFC3550].  [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 Counter Mode (AES-CM) transform.  For the



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   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 it does provide integrity and authentication.

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.  The following
   two scenarios illustrate 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 this
                          scenario implicitly assumes that either
                          (1) the outgoing RTP process is trusted to not
                          attempt to repeat a (ROC,SEQ) value or (2) the
                          encryption process ensures that 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
                          be generated or checked by either the SRTP
                          implementation or a module (e.g., the RTP
                          application) that can be considered equally
                          trustworthy.  While [RFC3711] allows the
                          detection of SSRC collisions after they
                          happen, SRTP using GCM with shared master keys
                          MUST prevent an 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



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                          identifiers that it has issued within the
                          lifetime of the current master key, and when a
                          new SSRC 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 exhaust their pools of SSRC
                          values.  Further, the identity of the entity
                          giving out SSRC values MUST be verified, and
                          the SSRC signaling MUST be integrity
                          protected.

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 IV used by AES-GCM SRTCP is formed by first
   concatenating 2 octets of zeroes, the 4-octet SSRC identifier,
   2 octets of zeroes, a single "0" bit, and the 31-bit SRTCP index.
   The resulting 12-octet value is then XORed to the 12-octet salt to
   form the 12-octet IV.

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

              Figure 4: SRTCP Initialization Vector Formation










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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
                   (The fields are defined in RFC 3550.)












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   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 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.



























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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














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   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 that 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
   index 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.















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   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

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

   For the sake of clarity, we specify three 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.  The AEAD_AES_128_GCM algorithm MUST use the (128-bit)
   AES_CM PRF KDF described in [RFC3711].  AEAD_AES_256_GCM MUST use the
   AES_256_CM_PRF KDF described in [RFC6188].












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12.  Summary of AES-GCM in SRTP/SRTCP

   For convenience, much of the information about the use of the 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-CM 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:

     +--------------------------------+------------------------------+
     | 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 2: The AEAD_AES_128_GCM Crypto Suite















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

                Table 3: 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 that 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 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 the 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 SRTP 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.



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13.2.  Size of the Authentication Tag

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

14.  IANA Considerations

14.1.  SDES

   "Session Description Protocol (SDP) Security Descriptions for Media
   Streams" [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 has registered the following crypto
   suites in the "SRTP Crypto Suite Registrations" subregistry of the
   "Session Description Protocol (SDP) Security Descriptions" registry.
   The ABNF [RFC5234] syntax is as follows:

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

























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14.2.  DTLS-SRTP

   DTLS-SRTP [RFC5764] defines DTLS-SRTP "SRTP protection profiles".
   These profiles 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, IANA has registered the following SRTP
   protection profiles:

         SRTP_AEAD_AES_128_GCM    = {0x00, 0x07}
         SRTP_AEAD_AES_256_GCM    = {0x00, 0x08}

   Below, we list the SRTP transform parameters for each of these
   protection profiles.  Unless separate parameters for SRTP 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.






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14.3.  MIKEY

   In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
   IANA maintains several subregistries under "Multimedia Internet
   KEYing (MIKEY) Payload Name Spaces".  Per this document, additions
   have been made to two of the MIKEY subregistries.

   In the "MIKEY Security Protocol Parameters" subregistry, the
   following has been added:

      Type | Meaning                         | Possible Values
      --------------------------------------------------------
        20 | 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 (Value 0)" subregistry (derived from
   Table 6.10.1.b of [RFC3830]), the following has been added:

        SRTP Encr. | Value | Default Session   |  Default Auth.
        Algorithm  |       | Encr. Key Length  |   Tag Length
      -----------------------------------------------------------
        AES-GCM    |    6  |    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 the AEAD authentication 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 4: Mapping MIKEY Parameters to AEAD Algorithms




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   Section 11 of this document restricts the choice of KDF for AEAD
   algorithms.  To enforce this restriction in MIKEY, we require that
   the SRTP Pseudorandom Function (PRF) has value AES-CM whenever an
   AEAD algorithm is used.  Note that, according to Section 6.10.1 of
   [RFC3830], the input key length of the KDF (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 KDF.

16.  Some RTP Test Vectors

   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.  At the time this document
   was written, the RTP payload type (1000000 binary = 64 decimal) was
   an unassigned value.

   As shown in Section 8.1, 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 ninth through 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
   third and fourth 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.







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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
       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 the 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





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   Compute the GMAC tag

     Process the AAD
           AAD word: 8040f17b8041f8d35501a0b200000000
       partial hash: bcfb3d1d0e6e3e78ba45403377dba11b

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

     Process the 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

   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
















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

     Process the AAD
           AAD word: 8040f17b8041f8d35501a0b200000000
       partial hash: bcfb3d1d0e6e3e78ba45403377dba11b

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

     Process the length word
        length word: 00000000000000600000000000000130
       partial hash: 1b964067078c408c4e442a8f015e5264








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RFC 7714                    AES-GCM for SRTP               December 2015


   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.

   Decrypt the 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 tagged packet:
        47616c6c 69612065 7374206f 6d6e6973
        20646976 69736120 696e2070 61727465
        73207472 6573





















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RFC 7714                    AES-GCM for SRTP               December 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 the GMAC tag

     Process the 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 the 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





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RFC 7714                    AES-GCM for SRTP               December 2015


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

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

     Process the 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 the length word
        length word: 00000000000001900000000000000000
       partial hash: b04200c26b81c98af55cc2eafccd1cbc



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RFC 7714                    AES-GCM for SRTP               December 2015


   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

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

















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RFC 7714                    AES-GCM for SRTP               December 2015


   Encrypt the 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 the GMAC tag

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

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

     Process the 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







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RFC 7714                    AES-GCM for SRTP               December 2015


   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

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

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






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RFC 7714                    AES-GCM for SRTP               December 2015


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

     Process the 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
     Received tag verified.

   Decrypt the 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 tagged packet:
        47616c6c 69612065 7374206f 6d6e6973
        20646976 69736120 696e2070 61727465
        73207472 6573









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RFC 7714                    AES-GCM for SRTP               December 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 the GMAC tag

     Process the 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 the 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




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RFC 7714                    AES-GCM for SRTP               December 2015


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

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

     Process the 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






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RFC 7714                    AES-GCM for SRTP               December 2015


     Process the 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.

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 fifth through
   eighth octets of the RTP packet), another two padding octets, and the
   31-bit SRTCP index, right-justified in a 32-bit = 4-octet field with
   a single "0" bit prepended 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 prepended 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.







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RFC 7714                    AES-GCM for SRTP               December 2015


   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 a cipher that consists
   solely of the 8-octet or 16-octet authentication tag.  The tagged
   SRTCP consists of the following fields in the order listed below:

   - All of the AAD, except for the ESRTCP 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.






















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RFC 7714                    AES-GCM for SRTP               December 2015


17.1.  SRTCP AEAD_AES_128_GCM Encryption and Tagging

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




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   Compute the GMAC tag

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

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

     Process the 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 the ESRTCP word with the E-flag set
        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










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RFC 7714                    AES-GCM for SRTP               December 2015


17.2.  SRTCP AEAD_AES_256_GCM Verification and Decryption

   Key size = 256 bits
   Tag size =  16 octets

     Process the length word

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

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

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



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     Process the 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
     Received tag verified.

   Decrypt the 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
















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RFC 7714                    AES-GCM for SRTP               December 2015


17.3.  SRTCP AEAD_AES_128_GCM Tagging 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 the GMAC tag

     Process the 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 the 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





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RFC 7714                    AES-GCM for SRTP               December 2015


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






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RFC 7714                    AES-GCM for SRTP               December 2015


     Process the 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 the 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.  References

18.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.




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RFC 7714                    AES-GCM for SRTP               December 2015


   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              DOI 10.17487/RFC3830, August 2004,
              <http://www.rfc-editor.org/info/rfc3830>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <http://www.rfc-editor.org/info/rfc4568>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [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,
              DOI 10.17487/RFC5764, May 2010,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC6188]  McGrew, D., "The Use of AES-192 and AES-256 in Secure
              RTP", RFC 6188, DOI 10.17487/RFC6188, March 2011,
              <http://www.rfc-editor.org/info/rfc6188>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,
              <http://www.rfc-editor.org/info/rfc6904>.


















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RFC 7714                    AES-GCM for SRTP               December 2015


18.2.  Informative References

   [BN00]     Bellare, M. and C. Namprempre, "Authenticated Encryption:
              Relations among notions and analysis of the generic
              composition paradigm", Proceedings of ASIACRYPT 2000,
              Springer-Verlag, LNCS 1976, pp. 531-545,
              DOI 10.1007/3-540-44448-3_41,
              <http://www-cse.ucsd.edu/users/mihir/papers/oem.html>.

   [GCM]      Dworkin, M., "NIST Special Publication 800-38D:
              Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC", U.S. National
              Institute of Standards and Technology, November 2007,
              <http://csrc.nist.gov/publications/nistpubs/
              800-38D/SP-800-38D.pdf>.

   [R02]      Rogaway, P., "Authenticated-Encryption with Associated-
              Data", ACM Conference on Computer and Communications
              Security (CCS'02), pp. 98-107, ACM Press,
              DOI 10.1145/586110.586125, September 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,
              DOI 10.17487/RFC4771, January 2007,
              <http://www.rfc-editor.org/info/rfc4771>.
























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RFC 7714                    AES-GCM for SRTP               December 2015


Acknowledgements

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

Authors' Addresses

   David A. McGrew
   Cisco Systems, Inc.
   510 McCarthy Blvd.
   Milpitas, CA  95035
   United States
   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: mythicalkevin@yahoo.com

























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