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Versions: (draft-mcgrew-srtp-aes-gcm) 00 01 02 draft-ietf-avtcore-srtp-aes-gcm

Network Working Group                                          D. McGrew
Internet Draft                                       Cisco Systems, Inc.
Intended Status: Informational                          January 26, 2011
Expires: July 30, 2011


    AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
                      draft-ietf-avt-srtp-aes-gcm-01

Abstract

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

Status of this Memo

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

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

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

   This Internet-Draft will expire on July 16, 2011.

Copyright Notice

   Copyright (c) 2011 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.....................................................2
      1.1. Conventions Used In This Document...........................3
      1.2. AEAD processing for SRTP....................................4
         1.2.1. AEAD Authentication versus SRTP Authentication.........5
         1.2.2. Values used to form the Initialization Vector (IV).....5
         1.2.3. SRTP IV formation for AES-GCM and AES-CCM..............6
         1.2.4. SRTCP IV formation for AES-GCM and AES-CCM.............6
         1.2.5. AEAD Processing of SRTP Packets........................7
         1.2.6. AEAD Processing of SRTCP Packets.......................8
            1.2.6.1. Encrypted SRTCP packets...........................8
            1.2.6.2. Unencrypted SRTCP packets.........................9
   2. AEAD parameters for SRTP and SRTCP...............................9
      2.1. Generic AEAD Parameter Constraints.........................10
      2.2. AES-GCM for SRTP/SRTCP.....................................11
      2.3. AES-CCM for SRTP/SRTCP.....................................11
   3. Security Considerations.........................................12
   4. IANA Considerations.............................................13
   5. Acknowledgements................................................14
   6. References......................................................14
      6.1. Normative References.......................................14
      6.2. Informative References.....................................14






1. Introduction

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

   SRTP/SRTCP assumes that both the sender and recipient have a shared
   secret master key and a shared secret master salt.  As described in
   sections 4.3.1 and 4.3.3 of [RFC3711], a Key Derivation Function is
   applied to these secret values to obtain separate encryption keys,
   authentication keys and salting keys for SRTP and for SRTCP.  (Note:
   As will be explained below, AEAD SRTP/SRTCP does not make use of
   these authentication keys.)

   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.


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

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

   The Galois/Counter Mode (GCM) of operation and the Counter with CBC
   MAC (CCM) mode are AEAD modes of operation for block ciphers.  Both
   use counter mode to encrypt the data, an operation that can be
   efficiently pipelined.  Further, GCM authentication uses operations
   that are particularly well suited to efficient implementation in
   hardware, making it especially appealing for high-speed
   implementations, or for implementations in an efficient and compact
   circuit.  CCM is well suited for use in compact software
   implementations.  This specification uses GCM and CCM with both
   AES-128 and AES-256.

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


1.1. Conventions Used In This Document

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

      Crypto Context  For the purposes of this document a crypto context
                      is the outcome of any process which results in
                      authentication of each participant in the SRTP
                      session and in their possession of a shared secret
                      master key and a shared master salt.  Details of
                      how the maser key and master salt are established
                      are outside the scope of this document.  The
                      master key MUST be at least as large as the
                      encryption key.  The SRTP/SRTCP Key Derivation
                      Function (KDF) defined in [RFC3711] is applied to
                      the master key and master SALT to derive the
                      SRTP_encr_key, SRTCP_encr_key, SRTP_SALT, and
                      SRTCP_SALT.  Authentication keys are not used in
                      AEAD.

      Instantiation   Once keys have been established, an instance of
                      the AEAD algorithm is created using the


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                      appropriate key and salt.  In a point-to-point
                      scenario, each participant in the SRTP/SRTCP
                      session will need four instantiations of the AEAD
                      algorithm; one for inbound SRTP traffic, one for
                      outbound SRTP traffic source, one for inbound
                      SRTCP traffic, and one for outbound SRTCP traffic
                      source.

      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.

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


1.2. AEAD processing for SRTP

   We first define how to use a generic AEAD algorithm in SRTP, then we
   describe the specific use of the AES-128-GCM and AES-256-GCM
   algorithms.

   The use of an AEAD algorithm is defined by expressing the AEAD
   encryption algorithm inputs in terms of SRTP fields and data
   structures.  The AEAD encryption inputs are as follows:

     Key                     This input is the SRTP encryption key
                             (SRTP_encr_key) produced from the shared
                             secret master key using the key derivation
                             process.  (Note that the SRTP_auth_key is
                             not used).

     Associated Data         This is data that is to be authenticated
                             but not encrypted.  In SRTP, the associated
                             data consists of the entire RTP header,
                             including the list of CSRC identifiers (if
                             present) and the RTP header extension (if
                             present), as shown in Figure 2.

     Plaintext               Data that is to be both encrypted and
                             authenticated.  In SRTP this consists of
                             the RTP payload, the RTP padding and the
                             RTP pad count fields (if the latter two
                             fields are present) as shown in Figure 2.
                             The padding service provided by RTP is not
                             needed by the AEAD encryption algorithm, so
                             the RTP padding and RTP pad count fields
                             SHOULD be omitted.

     Initialization Vector   Each SRTP/SRTCP packet has its own 12-octet


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                             initialization vector (IV).  Construction
                             of this IV is covered in more detail
                             below.

   The AEAD encryption algorithm accepts these four inputs and returns a
   Ciphertext field.


1.2.1. AEAD Authentication versus SRTP Authentication

   The reader is reminded that in addition to providing confidentiality
   for the plaintext that is encrypted, an AEAD algorithm also provides
   a way to check the data integrity and authenticity of the plaintext
   and associated data.  The AEAD integrity check is incorporated into
   the Ciphertext field by RFC 5116, thus AEAD does not make use of the
   optional SRTP Authentication Tag field.  (Note that this means that
   the cipher text will be longer than the plain text by precisely the
   length of the AEAD authentication tag.)

   The AEAD message authentication mechanism MUST be the primary message
   authentication mechanism for AEAD SRTP.  Additional SRTP
   authentication mechanisms SHOULD NOT be used with any AEAD algorithm
   and the optional SRTP Authentication Tag SHOULD NOT be present.

      Rationale.  Some applications use the 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.


1.2.2. Values used to form the Initialization Vector (IV)

   The initialization vector for an SRTP packet is formed from the:

     SSRC               The 4-octet Synchronization Source identifier
                        (SSRC), found in the RTP header.

     Packet Counter     Each AEAD instantiation MUST maintain a 6 octet
                        zero-based packet counter which is incremented
                        after a given instantiation has been invoked to
                        process a packet of data.  The packet counter is
                        closely related to the invocation field
                        discussed in NIST Special Publication 800 38-D
                        [GCM], "Recommendation for Block Cipher Modes of
                        Operation: Galois/Counter Mode (GCM) and GMAC".
                        As we shall see below, the packet counter is
                        used to insure each packet gets a unique
                        initialization vector.

     Sequence Number    The 2-octet RTP Sequence Number (SEQ), found in
                        the RTP header.  SEQ is just the two least
                        significant bytes of the packet counter.


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     Rollover Counter   A 4-octet Rollover Counter (ROC), maintained by
                        both sides of the link.  The ROC is just the 4
                        most significant octets of the packet counter.

     SALT               A 12-octet SRTP session encryption salt produced
                        by the SRTP Key Derivation Function (KDF).


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

   AES-GCM and AES-CCM SRTP use a 12 byte inititialition vector which is
   formed as follows.  A 12-octet string is formed by concatenating a
   2-octets of zeroes, the 4-octet SSRC, and the the 6-byte invocation
   counter.  The resulting string is bitwise exclusive-ored with the
   12-octet salt to form the 12-octet IV

               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   |  Packet_Counter |---+
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |         Encryption Salt           |->(+)
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |       Initialization Vector       |<--+
             +--+--+--+--+--+--+--+--+--+--+--+--+

           Figure 1: AES-GCM and AES-CCM SRTP
                     Initialization Vector formation.
   Using the terminology of section 8.2.1. of [GCM], the first six
   octets of the IV are the fixed field and the last six bytes are the
   invocation field.


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

   The initialization vector for an SRTCP packet is formed from the
   4-octet Synchronization Source identifier (SSRC), 31-bit SRTCP Index
   (packed zero-filled, right justified into a 4-octet field), and a
   12-octet SRTP session encryption salt produced by the SRTP Key
   Derivation Function (KDF) as described in [RFC3711].  (The 31-bit
   SRTCP index serves as the invocation counter.) First a 12-octet
   string is formed by concatenating in order 2-octets of zeroes, the
   4-octet SSRC, 2 more zero octets, and the 4-octet SRTCP index.  The
   resulting 12-octet string is bitwise exclusive-ored into salt; the
   output of that process is the IV.  The process is illustrated in
   Figure 3.  The IV is always exactly 12 octets in length.



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               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   |00|00|SRTCP Index|---+
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |         Encryption Salt           |->(+)
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
                                                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+   |
             |       Initialization Vector       |<--+
             +--+--+--+--+--+--+--+--+--+--+--+--+

           Figure 2: SRTCP Initialization Vector formation.

   Using the terminology of section 8.2.1. of [GCM], the first eight
   octets of the IV are the fixed field and the last four bytes are the
   invocation field.


1.2.5. AEAD Processing of SRTP Packets

   All SRTP packets MUST be authenticated and encrypted.  Figure 3 below
   shows which fields of AEAD SRTP packet are to be treated as
   plaintext, which are to be treated as additional authenticated data.

        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| Packet Type |       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 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    X  :              authentication tag (NOT RECOMMENDED)             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                P = Plaintext (to be encrypted and authenticated)
                A = Associated Data (to be authenticated only)
                X = neither encrypted nor authenticated



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        Note: The RTP padding and RP padding count fields are optional
              and are not recommended

    Figure 3: AEAD inputs from an SRTP packet.


1.2.6. AEAD Processing of SRTCP Packets

   All SRTCP 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 in the leftmost bit of the 32-bit word that contains the
   SRTCP index)


1.2.6.1. Encrypted SRTCP packets

   When the encryption flag is set to 1, the first 8-octets, the
   encryption flag and SRTCP index are treated as AAD and eight octets
   and the encryption flag are treated as plaintext.  Figure 4 below
   shows how fields of an RTCP packet are to be treated when the
   encryption flag is set to 1.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    X  |                   SRTCP MKI (optional)index                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    X  :              authentication tag (NOT RECOMMENDED)             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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                P = Plaintext (to be encrypted and authenticated)
                A = Associated Data (to be authenticated only)
                X = neither encrypted nor authenticated

    Figure 4: AEAD SRTCP inputs when encryption flag = 1.

1.2.6.2. Unencrypted SRTCP packets

   When the encryption flag is set to 0, all of the data up to and
   including the SRTCP index is treated as AAD.  Figure 5 shows how the
   fields of an RTCP packet are to be treated when the encryption flag
   is set to 0.

        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                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    X  |                   SRTCP MKI (optional)index                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    X  :              authentication tag (NOT RECOMMENDED)             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                A = Associated Data (to be authenticated only)
                X = neither encrypted nor authenticated

    Figure 5: AEAD SRTCP inputs when encryption flag = 0.


2. AEAD parameters for SRTP and SRTCP



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


2.1. Generic AEAD Parameter Constraints

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

      PARAMETER  Meaning                 Value

      A_MAX      maximum additional       MUST be at least 12 octets
                 authenticated data
                 length
      N_MIN      minimum nonce (IV)       MUST be no more than 12 octets
                 length
      N_MAX      maximum nonce (IV)       MUST be at least 12 octets
                 length
      C_MAX      maximum ciphertext       MUST be at most 2^16-40 octets
                 length per invocation    SHOULD be at least 2232


   The upper bound on C_MAX is obtained by subtracting away a 20-octet
   IP header, an 8-octet UDP header, and a 12-octet RTP header out of
   the largest possible IP packet, the total length of which is 2^16
   octets.

   Similarly the lower bound on C_MAX is based on the maximum
   transmission unit (MTU) of 2272 octets in IEEE 802.11.  Because many
   RTP applications use very short payloads (for example, the G.729
   codec used in VoIP can be as short as 20 octets), implementations
   that only support a maximum ciphertext length smaller than 2232
   octets are permitted under this RFC.  However, in the interest of
   maximizing interoperability between various AEAD implementations, the
   use of C_MAX values less than 2232 is discouraged.

   For sake of clarity we specify two additional parameters:

      Authentication Tag Length         MUST be either 8, 12, or 16
                                             octets
      Maximum number of invocations     MUST be at most 2^48 for SRTP
         for a given instantiation      MUST be at most 2^31 for SRTCP

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




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2.2. AES-GCM for SRTP/SRTCP

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

             Table 1: AES-GCM algorithms for SRTP/SRTCP
      Name                 Key Size      Auth. Tag Size     Reference
      ================================================================
      AEAD_AES_128_GCM     16 octets     16 octets          [RFC5116]
      AEAD_AES_256_GCM     32 octets     16 octets          [RFC5116]
      AEAD_AES_128_GCM_8   16 octets     8 octets           [RFC5282]
      AEAD_AES_256_GCM_8   32 octets     8 octets           [RFC5282]
      AEAD_AES_128_GCM_12  16 octets     12 octets          [RFC5282]
      AEAD_AES_256_GCM_12  32 octets     12 octets          [RFC5282]


   Any implementation of AES-GCM SRTP MUST support both
   AEAD-AES-128-GCM-8 and AEAD-AES-256-GCM-8, ant it MAY support the
   four other variants shown in the table.

   In addition to the invocation counter used in the formation of IVs,
   each instantiation of AES-GCM has a block counter which is
   incremented each time AES is called to produce a 16-octet output
   block.  The block counter is reset to "1" each time AES-GCM is
   invoked.

2.3. AES-CCM for SRTP/SRTCP

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

          Table 2: AES-CCM algorithms for SRTP/SRTCP
   Name                 Key Size      Auth. Tag Size     Reference
   ================================================================
   AEAD_AES_128_CCM     16 octets     16 octets          [RFC5116]
   AEAD_AES_256_CCM     32 octets     16 octets          [RFC5116]

   Any implementation of AES-CCM SRTP/SRTCP MUST support both
   AEAD-AES-128-CCM and AEAD-AES-256-CCM.







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   In addition to the invocation counter used in the formation of IVs,
   each instantiation of AES-CCM has a block counter which is
   incremented each time AES is called to produce a 16-octet output
   block.  The block counter is reset to "0" each time AES-CCM is
   invoked.

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

3. Security Considerations

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

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

   The table below summarizes the relationship between the
   authentication tag size, the probability of success, and the maximum
   numbers of forgery attempts that can be permitted on our network.


      |==================+========================================|
      |  Authentication  |  Probability any Successful Forgeries  |
      |    Tag Size      |-------------+-------------+------------|
      |    (octets)      |    2^-10    |   2^-20     |   2^-30    |
      |==================+=============+=============+============|
      |        4         |  2^22 tries |  2^12 tries |  2^2 tries |
      |==================+=============+=============+============|
      |        8         |  2^54 tries |  2^44 tries | 2^34 tries |
      |==================+=============+=============+============|
      |       12         |  2^86 tries |  2^76 tries | 2^66 tries |
      |==================+=============+=============+============|
      |       16         | 2^118 tries | 2^108 tries | 2^98 tries |
      |==================+=============+=============+============|

       Table 1: Maximum allowable number of forgery attempts for
                a given tag size and probability of success.




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4. IANA Considerations

   RFC 4568 defines SRTP "crypto suites"; a crypto suite corresponds to
   a particular AEAD algorithm in SRTP.  In order to allow SDP to signal
   the use of the algorithms defined in this document, IANA will
   register the following crypto suites into the subregistry for SRTP
   crypto suites under the SRTP transport of the SDP Security
   Descriptions:

      srtp-crypto-suite-ext = "AEAD_AES_128_GCM"    /
                              "AEAD_AES_256_GCM"    /
                              "AEAD_AES_128_GCM_8"  /
                              "AEAD_AES_256_GCM_8"  /
                              "AEAD_AES_128_GCM_12" /
                              "AEAD_AES_256_GCM_12" /
                              "AEAD_AES_128_CCM"    /
                              "AEAD_AES_256_CCM"    /
                              srtp-crypto-suite-ext

   DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile",
   which corresponds 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 will also register the following SRTP Protection
   Profiles:

        SRTP_AEAD_AES_128_GCM
        SRTP_AEAD_AES_256_GCM
        SRTP_AEAD_AES_128_GCM_8
        SRTP_AEAD_AES_256_GCM_8
        SRTP_AEAD_AES_128_GCM_12
        SRTP_AEAD_AES_256_GCM_12
        SRTP_AEAD_AES_128_CCM
        SRTP_AEAD_AES_256_CCM


5. Acknowledgements

   The author would like to thank Kevin Igoe and many other reviewers
   who provided valuable comments on earlier drafts of this document.















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6. References


6.1. Normative References

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

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

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
              K. Norrman, "The Secure Real-time Transport Protocol
              (SRTP)", RFC 3711, March 2004.

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

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

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


6.2. Informative References

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

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

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







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   Author's Address

      David A. McGrew
      Cisco Systems, Inc.
      510 McCarthy Blvd.
      Milpitas, CA  95035
      US

      Phone: (408) 525 8651
      Email: mcgrew@cisco.com
      URI:   http://www.mindspring.com/~dmcgrew/dam.htm




Acknowledgement

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



































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