Internet Engineering Task Force (IETF) D. McGrew

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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RFC 7714 AES-GCM for SRTP December 2015

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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RFC 7714 AES-GCM for SRTP December 2015

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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RFC 7714 AES-GCM for SRTP December 2015

When the Encryption flag is set to 0, the SRTCP compound packet is
broken into Plaintext, Associated Data, and Raw (untouched) Data, as
follows (see Figure 6):

Plaintext: None.

Raw Data: The variable-length optional SRTCP MKI and SRTCP
authentication tag (whose use is
NOT RECOMMENDED).

Associated Data: All other data.

Even though there is no ciphertext in this RTCP packet, AEAD
encryption returns a cipher field 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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RFC 7714 AES-GCM for SRTP December 2015

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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RFC 7714 AES-GCM for SRTP December 2015

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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RFC 7714 AES-GCM for SRTP December 2015

+--------------------------------+------------------------------+
| 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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RFC 7714 AES-GCM for SRTP December 2015

13.2. Size of the Authentication Tag

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

14. IANA Considerations

14.1. SDES

"Session Description Protocol (SDP) Security Descriptions for Media
Streams" [RFC4568] defines SRTP "crypto suites". A crypto suite
corresponds to a particular AEAD algorithm in SRTP. In order to
allow security descriptions to signal the use of the algorithms
defined in this document, IANA has registered the following crypto
suites in the "SRTP Crypto Suite Registrations" subregistry of the
"Session Description Protocol (SDP) Security Descriptions" registry.
The ABNF [RFC5234] syntax is as follows:

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

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RFC 7714 AES-GCM for SRTP December 2015

14.2. DTLS-SRTP

DTLS-SRTP [RFC5764] defines DTLS-SRTP "SRTP protection profiles".
These profiles also correspond to the use of an AEAD algorithm in
SRTP. In order to allow the use of the algorithms defined in this
document in DTLS-SRTP, IANA has registered the following SRTP
protection profiles:

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

Below, we list the SRTP transform parameters for each of these
protection profiles. Unless separate parameters for SRTP and SRTCP
are explicitly listed, these parameters apply to both SRTP and SRTCP.

SRTP_AEAD_AES_128_GCM
cipher: AES_128_GCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets

SRTP_AEAD_AES_256_GCM
cipher: AES_256_GCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets

Note that these SRTP protection profiles do not specify an
auth_function, auth_key_length, or auth_tag_length, because all
of these profiles use AEAD algorithms and thus do not use a
separate auth_function, auth_key, or auth_tag. The term
"aead_auth_tag_length" is used to emphasize that this refers to
the authentication tag provided by the AEAD algorithm and that
this tag is not located in the authentication tag field provided by
SRTP/SRTCP.

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RFC 7714 AES-GCM for SRTP December 2015

14.3. MIKEY

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

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

Type | Meaning | Possible Values
--------------------------------------------------------
20 | AEAD authentication tag length | 16 octets

This list is, of course, intended for use with GCM. It is
conceivable that new AEAD algorithms introduced at some point in the
future may require a different set of authentication tag lengths.

In the "Encryption algorithm (Value 0)" subregistry (derived from
Table 6.10.1.b of [RFC3830]), the following has been added:

SRTP Encr. | Value | Default Session | Default Auth.
Algorithm | | Encr. Key Length | Tag Length
-----------------------------------------------------------
AES-GCM | 6 | 16 octets | 16 octets

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

15. Parameters for Use with MIKEY

MIKEY specifies the algorithm family separately from the key length
(which is specified by the Session Encryption key length) and the
authentication tag length (specified by the AEAD authentication tag
length).

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

Table 4: Mapping MIKEY Parameters to AEAD Algorithms

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RFC 7714 AES-GCM for SRTP December 2015

Section 11 of this document restricts the choice of KDF for AEAD
algorithms. To enforce this restriction in MIKEY, we require that
the SRTP Pseudorandom Function (PRF) has value AES-CM whenever an
AEAD algorithm is used. Note that, according to Section 6.10.1 of
[RFC3830], the input key length of the KDF (i.e., the SRTP master key
length) is always equal to the session encryption key length. This
means, for example, that AEAD_AES_256_GCM will use AES_256_CM_PRF as
the KDF.

16. Some RTP Test Vectors

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

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

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

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

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

All of the RTP examples use this IV.

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RFC 7714 AES-GCM for SRTP December 2015

16.1. SRTP AEAD_AES_128_GCM

16.1.1. SRTP AEAD_AES_128_GCM Encryption

Encrypting the following packet:

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

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

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

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

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

McGrew & Igoe Standards Track [Page 25]

RFC 7714 AES-GCM for SRTP December 2015

Compute the GMAC tag

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

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

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

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

16.1.2. SRTP AEAD_AES_128_GCM Decryption

Decrypting the following packet:

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

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

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

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

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

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

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

16.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging

Tagging the following packet:

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

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

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

Compute the GMAC tag

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

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

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

Cipher with tag
22493f82 d2bce397 e9d79e3b 19aa4216

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RFC 7714 AES-GCM for SRTP December 2015

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

16.1.4. SRTP AEAD_AES_128_GCM Tag Verification

Verifying the following packet:

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

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

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

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

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

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

McGrew & Igoe Standards Track [Page 30]

RFC 7714 AES-GCM for SRTP December 2015

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

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

16.2. SRTP AEAD_AES_256_GCM

16.2.1. SRTP AEAD_AES_256_GCM Encryption

Encrypting the following packet:

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

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

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

McGrew & Igoe Standards Track [Page 31]

RFC 7714 AES-GCM for SRTP December 2015

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

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

Compute the GMAC tag

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

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

Process the length word
length word: 00000000000000600000000000000130
partial hash: 7debf5fa1fac3bd318d5e1a7ee401091

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

McGrew & Igoe Standards Track [Page 32]

RFC 7714 AES-GCM for SRTP December 2015

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

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

16.2.2. SRTP AEAD_AES_256_GCM Decryption

Decrypting the following packet:

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

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

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

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

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

McGrew & Igoe Standards Track [Page 33]

RFC 7714 AES-GCM for SRTP December 2015

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

Process the length word
length word: 00000000000000600000000000000130
partial hash: 7debf5fa1fac3bd318d5e1a7ee401091

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

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

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

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

McGrew & Igoe Standards Track [Page 34]

RFC 7714 AES-GCM for SRTP December 2015

16.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging

Tagging the following packet:

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

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

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

Compute the GMAC tag

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

Process the length word
length word: 00000000000001900000000000000000
partial hash: af2efb5dcfdb9900e7127721fdb56956

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

Cipher with tag
a866d591 0f887463 067ceefe c45215d4

McGrew & Igoe Standards Track [Page 35]

RFC 7714 AES-GCM for SRTP December 2015

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

16.2.4. SRTP AEAD_AES_256_GCM Tag Verification

Verifying the following packet:

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

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

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

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

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

McGrew & Igoe Standards Track [Page 36]

RFC 7714 AES-GCM for SRTP December 2015

Process the length word
length word: 00000000000001900000000000000000
partial hash: af2efb5dcfdb9900e7127721fdb56956

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

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

17. RTCP Test Vectors

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

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

with 32-bit SRTCP index 000005d4.

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

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

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

McGrew & Igoe Standards Track [Page 37]

RFC 7714 AES-GCM for SRTP December 2015

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

- The first 8 octets of the RTCP packet (part of the AAD).

- The cipher.

- The ESRTCP word (the final part of the AAD).

- Any Raw Data that might have been appended to the end of the
original RTCP packet.

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

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

- All of the AAD, except for the ESRTCP word.

- The cipher (= the authentication tag).

- The ESRTCP word (the final part of the AAD).

- Any Raw Data that might have been appended to the end of the
original RTCP packet.

McGrew & Igoe Standards Track [Page 38]

RFC 7714 AES-GCM for SRTP December 2015

17.1. SRTCP AEAD_AES_128_GCM Encryption and Tagging

Encrypting the following packet:

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

Key size = 128 bits
Tag size = 16 octets

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

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

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

Cipher before tag appended
63e94885 dcdab67c a727d766 2f6b7e99
7ff5c0f7 6c06f32d c676a5f1 730d6fda
4ce09b46 86303ded 0bb9275b

McGrew & Igoe Standards Track [Page 39]

RFC 7714 AES-GCM for SRTP December 2015

Compute the GMAC tag

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

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

Process the length word
length word: 00000000000000600000000000000160
partial hash: 7296107c9716534371dfc1a30c5ffeb5

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

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

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

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

McGrew & Igoe Standards Track [Page 40]

RFC 7714 AES-GCM for SRTP December 2015

17.2. SRTCP AEAD_AES_256_GCM Verification and Decryption

Key size = 256 bits
Tag size = 16 octets

Process the length word

Decrypting the following packet:

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

Key size = 256 bits
Key size = 16 octets

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

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

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

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

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

McGrew & Igoe Standards Track [Page 41]

RFC 7714 AES-GCM for SRTP December 2015

Process the length word
length word: 00000000000000600000000000000160
partial hash: 3a284af2616fdf505faf37eec39fbc8b

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

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

17.3. SRTCP AEAD_AES_128_GCM Tagging Only

Tagging the following packet:

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

Key size = 128 bits
Tag size = 16 octets

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

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

Compute the GMAC tag

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

Process the length word
length word: 00000000000001c00000000000000000
partial hash: 3ec16d4c3c0e90a59e91be415bd976d8

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

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RFC 7714 AES-GCM for SRTP December 2015

Cipher with tag
841dd968 3dd78ec9 2ae58790 125f62b3

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

17.4. SRTCP AEAD_AES_256_GCM Tag Verification

Key size = 256 bits
Tag size = 16 octets

Process the length word
Verifying the following packet:

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

Key size = 256 bits
Key size = 16 octets

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

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

Process the length word
length word: 00000000000001c00000000000000000
partial hash: b6edb305dbc7065887fb1b119cd36acb

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

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

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

18. References

18.1. Normative References

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

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

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

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RFC 7714 AES-GCM for SRTP December 2015

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

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

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

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

[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<http://www.rfc-editor.org/info/rfc5764>.

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

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

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RFC 7714 AES-GCM for SRTP December 2015

18.2. Informative References

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

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

[R02] Rogaway, P., "Authenticated-Encryption with Associated-
Data", ACM Conference on Computer and Communications
Security (CCS'02), pp. 98-107, ACM Press,
DOI 10.1145/586110.586125, September 2002,
<http://www.cs.ucdavis.edu/~rogaway/papers/ad.html>.

[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure
Real-time Transport Protocol (SRTP)", RFC 4771,
DOI 10.17487/RFC4771, January 2007,
<http://www.rfc-editor.org/info/rfc4771>.

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RFC 7714 AES-GCM for SRTP December 2015

Acknowledgements

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

Authors' Addresses

David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
United States
Phone: (408) 525 8651

Email: [email protected]
URI: http://www.mindspring.com/~dmcgrew/dam.htm

Kevin M. Igoe
NSA/CSS Commercial Solutions Center
National Security Agency

Email: [email protected]

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