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RFC 7714

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

Pages: 48
Proposed Standard
Errata
Part 1 of 2 – Pages 1 to 21
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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.


(page 21 continued on part 2)

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