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

Network Working Group                                          D. McGrew
Internet Draft                                       Cisco Systems, Inc.
Intended Status: Informational                                 K.M. Igoe
Expires: August 20, 2012                        National Security Agency
                                                       February 17, 2012


    AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
                   draft-ietf-avtcore-srtp-aes-gcm-00


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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.


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 versus SRTP/SRTCP Authentication..................5
         1.2.2. Values used to form the Initialization Vector (IV).....6
      1.3. SRTP IV formation for AES-GCM and AES-CCM...................6
      1.4. SRTCP IV formation for AES-GCM and AES-CCM..................7
      1.5. AEAD Processing of SRTP Packets.............................8
      1.6. AEAD Processing of SRTCP Packets............................8
         1.6.1. Encrypted SRTCP packets................................9
         1.6.2. Unencrypted SRTCP packets.............................10
      1.7. Initialization of the Counters.............................10
      1.8. Prevention of IV Reuse.....................................11
   2. AEAD parameters for SRTP and SRTCP..............................11
      2.1. Generic AEAD Parameter Constraints.........................11
      2.2. AES-GCM for SRTP/SRTCP.....................................12
      2.3. AES-CCM for SRTP/SRTCP.....................................13
      2.4. Key Derivation Functions...................................13
   3. Security Considerations.........................................14
      3.1. Handling of Security Critical Parameters...................14
      3.2. Size of the Authentication Tag.............................14
   4. IANA Considerations.............................................15
   5. Acknowledgements................................................16
   6. References......................................................17
      6.1. Normative References.......................................17
      6.2. Informative References.....................................18


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 master salt.  As described in sections
   4.3.1 and 4.3.3 of [RFC3711], a Key Derivation Function is applied to
   these 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


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   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.
   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 Counter with Cipher Block Chaining-Message
   Authentication Code (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 of operation (GCM) and the Counter with
   Cipher Block Chaining-Message Authentication Code mode of operation
   (CCM) are both AEAD modes of operation for block ciphers.  Both use
   counter mode to encrypt the data, an operation that can be
   efficiently pipelined.  Further, GCM authentication uses operations
   that are particularly well suited to efficient implementation in
   hardware, making it especially appealing for high-speed
   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 possession by each partcipant 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.  Similarly any mechanism for rekeying an
                      existing Ciper Contest is outside the scope of the
                      document.  The master key MUST be at least as
                      large as the encryption key.  The SRTP/SRTCP Key


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                      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
                      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.  See section 1.2 for details on what is
                      required of each instantiation.

      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.

   Each AEAD instantiation has its own key, a 48-bit zero-based packet
   counter that is incremented after that particular instantiation has
   been invoked to process an SRTP packet, and a 31-bit zero-based SRTCP
   index that is incremented each time an SRTCP packet is processed.  A
   32-bit Block Counter is incremented each time a block of key is
   produced and is reset (to zero for CCM and to one for GCM) at the
   start of each packet.  As we shall see in sections 1.3 and 1.4, the
   packet counter and SRTCP counter play a crucial role in the formation
   of each packet's IV.

   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).


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     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 3.

     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 3.
                             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
                             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 versus SRTP/SRTCP Authentication

   The reader is reminded that in addition to providing confidentiality
   for the plaintext that is encrypted, an AEAD algorithm also provides
   a mechanism that allows the intended recipient to check the data
   integrity and authenticity of the plaintext and associated data.  The
   AEAD authentication tag is incorporated into the Ciphertext field by
   RFC 5116, thus AEAD does not make use of the SRTP/SRTCP
   Authentication Tag fields defined in RFC 3711.  (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/SRTCP.  Additional SRTP/SRTCP
   authentication mechanisms SHOULD NOT be used with any AEAD algorithm
   and the optional SRTP/SRTCP Authentication Tags are NOT RECOMMENDED
   and SHOULD NOT be present.  Note that this contradicts section 3.4 of
   [RFC3711] which makes the use of the SRTCP Authentication field
   mandatory, but the presence of the AEAD authentication renders the
   older authentication methods redundant.

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


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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
                        each time an SRTP packet is sent.  As we shall
                        see below, the packet counter is used to insure
                        each SRTP packet gets a unique initialization
                        vector.

     Sequence Number    The 2-octet RTP Sequence Number (SEQ), found in
                        the RTP header.  The SEQ is just the 16 least
                        significant bits of the packet counter.

     Rollover Counter   A 4-octet Rollover Counter (ROC), maintained
                        independently by both sides of the link,
                        incremented each time the Sequence Number cycles
                        back to 0.  The ROC is just the 32 most
                        significant bits of the Packet Counter.

     SRTCP index        The SRTCP index is a 31-bit counter that plays
                        the same role for SRTCP packets that the Packet
                        Counter does for SRTP packets.  Unlike the
                        Packet Counter, the SRTCP index is explicitly
                        included in each STRCP packet.  The sender MUST
                        increment the SRTCP index by one after each SRTP
                        packet is sent.

     SALT               A 12-octet SRTP session encryption salt produced
                        by the SRTP Key Derivation Function (KDF) (see
                        section 2.4).

   The reader is reminded that both SRTP and SRTCP allow packets to
   arrive out of order, presenting the receiver with a synchronization
   problem.  The 31-bit SRTCP index is contained in the unencrypted (but
   authenticated) portion of the SRTCP header, allowing the recipient to
   read the SRTCP index directly from the header.  But only the low 16
   bits of the SRTP Packet counter are contained in the SRTP header (in
   the sequence number field).  Section 3.3.1 of [RFC3711] explains in
   great detail how the 16-bit sequence number and 32-bit Rollover
   Counter are to be used to recover the 48-bit Packet Counter.


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

   AES-GCM and AES-CCM SRTP use a 12 byte initialization vector which is


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   formed as follows.  A 12-octet string is formed by concatenating
   2-octets of zeroes, the 4-octet SSRC, and the 6-octet Packet
   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.


1.4. 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 SRTCP session encryption salt produced by the SRTP Key
   Derivation Function (KDF).

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

           Figure 2: SRTCP Initialization Vector formation.

   As shown if figure 2, a 12-octet string is formed by concatenating in
   order 2-octets of zeroes, the 4-octet SSRC, 2 more zero octets, and


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   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
   IV is always exactly 12 octets in length.  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.




1.5. 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
   and 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

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

    Figure 3: AEAD inputs from an SRTP packet.


1.6. 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


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   SRTCP index)


1.6.1. Encrypted SRTCP packets

   When the encryption flag is set to 1, the first 8-octets, the
   encryption flag and 31-bit SRTCP index MUST be treated as AAD.  The
   remaining data MUST be treated as plaintext, and hence is to be both
   encrypted and AEAD authenticated, save for the optional SRTCP MKI
   index and optional SRTCP authentication tag, which MUST be neither
   encrypted nor AEAD authenticated.  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)             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                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.





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1.6.2. 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.


1.7. Initialization of the Counters

   When an AEAD Crypto Context is first established, both the SRTCP
   index and the rollover counter are set to zero.  The Sequence Number
   is set to a value passed to it by RTP.  When the context is rekeyed
   these counters keep their current values and are not reset to zero.
   These conventions assist in making a seamless transition from the old
   key (if any) to the new key despite of the fact that packets are


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   allowed to arrive out of order.

   As mentioned in section 1.1, AES-GCM and AES-CCM both use a Block
   Counter which is reset at the start of each packet.  For AES-CCM it
   is reset to 0 and for AES-GCM it is reset to 1.


1.8. Prevention of IV Reuse

   For a given key it is critical that the IV not repeat.  This reduces
   to the problem of insuring neither the Packet Counter nor the SRTCP
   index do not repeat before the AEAD instantiation is rekeyed.

   Processing MUST cease if either the 48-bit Packet Counter or the
   31-bit SRTCP index cycles back to their initial value.  Processing
   MUST NOT resume until a new SRTP/SRTCP session has been established
   using a new shared secret master key and shares master salt.
   Ideally, a rekey should be done well before either of these counters
   cycle.


2. AEAD parameters for SRTP and SRTCP

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


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


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   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.


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 SHOULD support both
   AEAD_AES_128_GCM_8 and AEAD_AES_256_GCM_8, and it MAY support the
   four other variants shown in the table.

   In addition to the Packet 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 to process
   a new packet.  The 128-bit concatentation of the IV and the block


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   counter is input to AES and the output is used as a block of key that
   is XORed to the next block of data to be encrypted/decypted.

                                               1   1   1   1   1   1
       0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
     ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    |                  initialization                |     block     |
    |                      vector                    |    counter    |
     ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

      Figure 6: AES Inputs for Counter Mode Encryption

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 SHOULD support both
   AEAD_AES_128_CCM and AEAD_AES_256_CCM.

      In addition to the Packet 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 to process a new packet.  As with AES-GCM, the 128-bit
      concatentation of the IV abd the block counter is input to AES to
      produce a block of key that is XORed to the next block of data to
      be encrypted/decypted.

   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.


2.4. 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


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   two endpoints.  Both the AEAD_AES_128_GCM algorithms and the
   AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key
   Derivation Function described in [RFC3711].  Both the
   AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST
   use the AES_256_CM_PRF Key Derivation Function described in [RFC
   6188].


3. Security Considerations


3.1. Handling of Security Critical Parameters

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

      - If the master salt is to be kept secret it MUST be properly
        erased when no longer needed.
      - The secret master key and all keys derived from it MUST be kept
        secret.  All keys MUST be properly erased when no longer
        needed.
      - Packets that fail the authentication check SHOULD be silently
        discarded.
      - The sender MUST increment the Packet Counter after each SRTP
        packet is processed.
      - The sender MUST increment the SRTCP index after each SRTCP
        packet is processed.
      - At the start of each packet the block counter MUST be reset (to
        0 for CCM, to 1 for GCM).  The block counter is incremented
        after each block key has been produced, but it MUST NOT be
        allowed to exceed 2^32-1.
      - Each time a rekey occurs the initial values of the invocation
        counter and SRTCP index MUST be saved.
      - Processing MUST cease if the 48-bit Packet Counter or the 31-bit
        SRTCP index 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 either of these counters cycle.


3.2. Size of the Authentication Tag

   We require that the AEAD authentication tag must be at least 8
   octets, significantly reducing the probability of an adversary
   successfully introducing fraudulent data.  The goal of an
   authentication tag is to 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


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   authentication tag in bits (N_tag_bits).  Then

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

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


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

       Table 1: Probability of a compromise occurring for a given
                number of forgery attempts and tag size.


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"    /


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                              srtp-crypto-suite-ext

   DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile"; it
   also 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 authors would like to thank Michael Peck, Michael Torla, Qin Wu,
   and many other reviewers who provided valuable comments on earlier
   drafts of this document.






























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6. References


6.1. Normative References


   [CCM]      Dworkin, M., "NIST Special Publication 800-38C: The CCM
              Mode for Authentication and Confidentiality", U.S.
              National Institute of Standards and Technology http://
              csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf.

   [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.

   [RFC4658]  Andreasen, F., Baugher, M., and D.Wing, "Session
              Description Protocol (SDP): Security Descriptions for
              Media Streams", RFC 4568, July 2006.

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

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

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

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

   [RFC6188]  McGrew,D.,"The Use of AES-192 and AES-256 in Secure RTP"
              RFC 6811, March 2011







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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.

   [BOYD]     Boyd, C. and A. Mathuria, "Protocols for Authentication
              and Key Establishment", Springer, 2003 .

   [CMAC]     "NIST Special Publication 800-38B",  http://csrc.nist.gov/
              CryptoToolkit/modes/800-38_Series_Publications/
              SP800-38B.pdf.

   [EEM04]    Bellare, M., Namprempre, C., and T. Kohno, "Breaking and
              provably repairing the SSH authenticated encryption
              scheme: A case study of the Encode-then-Encrypt-and-MAC
              paradigm", ACM Transactions on Information and System Secu
              rity, http://www-cse.ucsd.edu/users/tkohno/papers/
              TISSEC04/.

   [GR05]     Garfinkel, T. and M. Rosenblum, "When Virtual is Harder
              than Real: Security Challenges in Virtual Machine Based
              Computing Environments", Proceedings of the 10th Workshop
              on Hot Topics in Operating Systems http://
              www.stanford.edu/~talg/papers/HOTOS05/
              virtual-harder-hotos05.pdf.

   [J02]      Jonsson, J., "On the Security of CTR + CBC-MAC",
              Proceedings of the 9th Annual Workshop on Selected Areas
              on Cryptography, http://csrc.nist.gov/CryptoToolkit/modes/
              proposedmodes/ccm/ccm-ad1.pdf, 2002.

   [MODES]    Dworkin, M., "NIST Special Publication 800-38:
              Recommendation for Block Cipher Modes of Operation", U.S.
              National Institute of Standards and Technology http://
              csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.

   [MV04]     McGrew, D. and J. Viega, "The Security and Performance of
              the Galois/Counter Mode (GCM)", Proceedings of INDOCRYPT
              '04, http://eprint.iacr.org/2004/193, December 2004.

   [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.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.


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   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)",
              RFC 4106, June 2005.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4309]  Housley, R., "Using Advanced Encryption Standard (AES) CCM
              Mode with IPsec Encapsulating Security Payload (ESP)",
              RFC 4309, December 2005.

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

































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   Author's Address

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


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



Acknowledgement

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































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