AES-GCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-15
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7714.
|
|
|---|---|---|---|
| Authors | David McGrew , Kevin Igoe | ||
| Last updated | 2015-04-14 | ||
| Replaces | draft-ietf-avt-srtp-aes-gcm | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Magnus Westerlund | ||
| Shepherd write-up | Show Last changed 2013-09-24 | ||
| IESG | IESG state | Became RFC 7714 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date |
(None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Ben Campbell | ||
| Send notices to | avtcore-chairs@ietf.org, draft-ietf-avtcore-srtp-aes-gcm@ietf.org, "Magnus Westerlund" <magnus.westerlund@ericsson.com> | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-avtcore-srtp-aes-gcm-15
Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc.
Intended Status: Standards Track K. Igoe
Expires: October 16, 2015 National Security Agency
April 14, 2015
AES-GCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-15
Status of this Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 16, 2015.
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
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described in the Simplified BSD License.
Igoe and McGrew Standards Track [Page 1]
Internet Draft AES-GCM for SRTP Apr 14, 2015
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 SRTP protocol. Note:
this is an intermediate draft, awaiting the inclusion of test
vectors. Care is being taken to ensure these test vectors will be
correct, always a desirable property.
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..........................................5
5.1. Types of Input Data.........................................5
5.2. AEAD Invocation Inputs and Outputs..........................5
5.2.1. Encrypt Mode...........................................5
5.2.2. Decrypt Mode...........................................6
5.3. Handling of AEAD Authentication.............................6
6. Counter Mode Encryption..........................................6
7. Unneeded SRTP/SRTCP Fields.......................................7
7.1. SRTP/SRTCP Authentication Field.............................7
7.2. RTP Padding.................................................8
8. AES-GCM processing for SRTP......................................8
8.1. SRTP IV formation for AES-GCM...............................8
8.2. Data Types in SRTP Packets..................................8
8.3. Handling Header Extensions.................................10
8.4. Prevention of SRTP IV Reuse................................11
9. AES-GCM Processing of SRTCP Compound Packets....................12
9.1. SRTCP IV formation for AES-GCM.............................12
9.2. Data Types in Encrypted SRTCP Compound Packets.............13
9.3. Data Types in Unencrypted SRTCP Compound Packets...........14
9.4. Prevention of SRTCP IV Reuse...............................15
10. Constraints on AEAD for SRTP and SRTCP.........................15
11. Key Derivation Functions.......................................16
12. Summary of AES-GCM in SRTP/SRTCP...............................16
13. Security Considerations........................................17
13.1. Handling of Security Critical Parameters..................18
13.2. Size of the Authentication Tag............................18
14. IANA Considerations............................................19
14.1. SDES......................................................19
14.2. DTLS-SRTP.................................................20
14.3. MIKEY.....................................................21
15. Parameters for use with MIKEY..................................21
16. Acknowledgements...............................................22
17. References.....................................................23
17.1. Normative References......................................23
17.2. Informative References....................................24
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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",
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 be either
8 octets or 16 octets in length, and the 8 byte authentication tag
can only be used with AES-128. Thus when used in SRTP, GCM will have
three configurations:
AEAD_AES_128_GCM_8 AES-128 with an 8 byte authentication tag
AEAD_AES_128_GCM AES-128 with a 16 byte authentication tag
AEAD_AES_256_GCM AES-256 with a 16 byte authentication tag
The key size and the length of the authentication tag are set when
the session is initiated and SHOULD NOT be altered.
The Galois/Counter Mode of operation (GCM) ia an AEAD mode of
operation for block ciphers. GCM use counter mode to encrypt the
data, an operation that can be efficiently pipelined. Further, GCM
authentication uses operations that are particularly well suited to
efficient implementation in hardware, making it especially appealing
for high-speed implementations, or for implementations in an
efficient and compact circuit.
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.
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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 one or more endpoints using this master key
to process inbound data).
c) A Key Derivation Function 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.
d) Aside from making modifications to IANA registries to allow
AES-GCM to work with SDES, DTLS-SRTP and 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 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).
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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.
5. Generic AEAD Processing
5.1. Types of Input Data
Associated Data: This is 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
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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 cipher text.
When GCM is being used the ciphertext consists of the encrypted plain
text followed by the authentication tag.
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, a 16-octet first_key_block which is used in forming the
authentication tag and a key stream of octets, formed in blocks of
16-octets each. The first 16-octet block of key is saved for use in
forming the authentication tag, and the of remainder of the key
Igoe and McGrew Standards Track [Page 6]
Internet Draft AES-GCM for SRTP Apr 14, 2015
stream is XORed to the plaintext to form cipher. This key stream 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 pseudo-code 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 RFC 3711, 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 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 field
mandatory, but the presence of the AEAD authentication renders the
older authentication methods redundant.
Rationale. Some applications use the SRTP/SRTCP Authentication
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Internet Draft AES-GCM for SRTP Apr 14, 2015
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 requires 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 initialization vector 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 two octet sequence number SEQ. The
resulting 12-octet value is then XORed to the 12-octet salt to form
the 12-octet IV.
8.2. Data Types in SRTP Packets
All SRTP packets MUST be both authenticated and encrypted. The data
fields within the SRTP 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), CSRC count CC (4 bits),
marker M (1 bit), the Payload Type PT (7 bits),
the sequence number (16 bits), timestamp (32
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Internet Draft AES-GCM for SRTP Apr 14, 2015
bits), SSRC (32 bits), optional contributing
source identifiers (CSRCs, 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 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 SRTP packet before Authenticated
Encryption
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 by 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.
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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 RFC 3550. RFC 6904
[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 and
AEAD_AES_128_GCM_8 algorithms, this keystream MUST be generated in
the manner defined in [RFC6904] using the AES_128_CM transform. For
the 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
does provide integrity and authentication.
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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. There are two
phases to 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
implicitly assumes that either the outgoing RTP
process is trusted to not attempt to repeat a
(ROC,SEQ) value, or that the encryption process
ensures that the 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 either be
generated or checked by the SRTP
implementation, or by a module (e.g. the RTP
application) that can be considered equally
trusted as the SRTP implementation. While
[RFC3711] allows detecting SSRC collisions
after they happen, SRTP using GCM with shared
master keys MUST prevent 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 identifiers that it has issued
within the lifetime of the current master key,
and when a new synchronization source 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
exhausts its pool of SSRC values. Further, the
identity of the entity giving out SSRC values
MUST be verified, and the SSRC signaling MUST
be integrity protected.
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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 initialization vector used by AES-GCM SRTCP is formed by
first concatenating 2-octets of zeroes, the 4-octet Synchronization
Source identifier (SSRC), 2-octets of zeroes, a single zero 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 9 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.
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
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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.
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 which 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
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 fasmily of AEAD algorithms, namely
AES-GCM.
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
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C_MAX maximum ciphertext GCM: MUST be <= 2^36-16 octets.
length per invocation
For sake of clarity we specify two additional parameters:
AEAD Authentication Tag Length MUST be 8 or 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. Both AEAD_AES_128_GCM and AEAD_AES_128_GCM_8 algorithms
MUST use the (128-bit) AES_CM_PRF Key Derivation Function described
in [RFC3711]. AEAD_AES_256_GCM MUST use the AES_256_CM_PRF Key
Derivation Function described in [RFC6188].
12. Summary of AES-GCM in SRTP/SRTCP
For convenience, much of the information about the use of AES-GCM
family of algorithms in SRTP is collected in the tables contained in
this section.
The AES-GCM family of AEAD algorithms built around the AES block
cipher algorithm. AES-GCM uses AES counter mode for encryption and
Galois Message Authentication Code (GMAC) for authentication. A
detailed description of the AES-GCM family can be found in
[RFC5116]. The following members of the AES-GCM family may be used
with SRTP/SRTCP:
Name Key Size AEAD Tag Size Reference
================================================================
AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282]
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 (the versions with 16 octet AEAD authentication
tags), and it MAY support AEAD_AES_128_GCM_8. Below we summarize
parameters associated with these three GCM algorithms:
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+--------------------------------+------------------------------+
| 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_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 2: The AEAD_AES_128_GCM_8 Crypto Suite
+--------------------------------+------------------------------+
| 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 3: The AEAD_AES_128_GCM Crypto Suite
+--------------------------------+------------------------------+
| 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 4: The AEAD_AES_256_GCM Crypto Suite
13. Security Considerations
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13.1. Handling of Security Critical Parameters
As with any security process, the implementer must take care to
ensure cryptographically sensitive parameters are properly handled.
Many of these recommendations hold for all SRTP cryptographic
algorithms, but we include them here to emphasize their importance.
- If the master salt is to be kept secret, it MUST be properly
erased when no longer needed.
- The secret master key and all keys derived from it MUST be kept
secret. All keys MUST be properly erased when no longer
needed.
- 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 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 packet index ROC||SEQ cycles back their initial values .
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.
13.2. Size of the Authentication Tag
We require that the AEAD authentication tag must be at least 8
octets, significantly reducing the probability of an adversary
successfully introducing fraudulent data. The goal of an
authentication tag is to reduce the probability of a successful
forgery occurring anywhere in the network we are attempting to
defend. There are three relevant factors: how low we wish the
probability of successful forgery to be (prob_success), how many
attempts the adversary can make (N_tries) and the size of the
authentication tag in bits (N_tag_bits). Then
prob_success <= expected number of successes
= N_tries * 2^-N_tag_bits.
When the expected number of successes is much less than one, the
probability of success is well approximated by the expected number of
successes.
Suppose an adversary wishes to introduce a forged or altered packet
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into a target network by randomly selecting an authentication value
until by chance they hit a valid authentication tag. The table below
summarizes the relationship between the number of forged packets the
adversary has tried, the size of the authentication tag, and the
probability of a compromise occurring (i.e. at least one of the
attempted forgeries having a valid authentication tag). The reader
is reminded that the forgery attempts can be made over the entire
network, not just a single link, and that frequently changing the key
does not decrease the probability of a compromise occurring.
It should be noted that the cryptographic properties of the GHASH
algorithm used in GCM reduces the effective authentication tag size
(in bits) by the log base 2 of the of blocks of encrypted and/or
authenticated data in a packet. In practice an SRTP payload will be
less than 2^16 bytes, because of the 16-bit IPv4 and UDP length
fields. The exception to this case is IPv6 jumbograms [RFC2675],
which is unlikely to be used for RTP-based multimedia traffic
[RFC3711]. This corresponds to 2^12 blocks of data, so the effective
GCM authentication tag size is reduced by at most 12 bits.
+===========+=============+========================================+
| Auth. Tag | Effective | Number of Forgery Attempts |
| Size | Tag Size | Needed to Achieve a Given |
| (bytes) | (bits) | Probability of Success |
|-----------+-------------+------------+-------------+-------------|
| | prob=2^-30 | prob=2^-20 | prob=2^-10 |
|===========+=============+=============+============+=============|
| 4 | 20 (GCM) | 1 try | 1 try | 2^10 tries |
|===========+=============+============+=============+=============|
| 8 | 52 (GCM) | 2^22 tries | 2^32 tries | 2^42 tries |
|===========+=============+============+=============+=============|
| 12 | 84 (GCM) | 2^54 tries | 2^64 tries | 2^74 tries |
|===========+=============+============+=============+=============|
| 16 | 116 (GCM) | 2^86 tries | 2^96 tries | 2^106 tries |
|===========+=============+============+=============+=============|
Table 5: Number of forgery attempts needed to achieve a given
probability of success for various tag sizes.
14. IANA Considerations
14.1. SDES
SDP Security Descriptions [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 will register the following
crypto suites into the "SRTP Crypto Suite Registrations" subregistry
of the "Session Description Protocol (SDP) Security Descriptions"
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registry.
srtp-crypto-suite-ext = "AEAD_AES_128_GCM_8" /
"AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" /
srtp-crypto-suite-ext
14.2. DTLS-SRTP
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile".
These 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, we request IANA register the following SRTP Protection
Profiles:
AEAD_AES_128_GCM = {TBD, TBD }
AEAD_AES_128_GCM_8 = {TBD, TBD }
AEAD_AES_256_GCM = {TBD, TBD }
Below we list the SRTP transform parameters for each of these
protection profile. Unless separate parameters for SRTCP and SRTCP
are explicitly listed, these parameters apply to both SRTP and
SRTCP.
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
AEAD_AES_128_GCM_8
cipher: AES_128_GCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 8 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
AEAD_AES_256_GCM
cipher: AES_256_GCM
cipher_key_length: 256 bits
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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.
14.3. MIKEY
In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
IANA maintains several subregitries under "Multimedia Internet KEYing
(MIKEY) Payload Name Spaces". This document requires additions to
two of the MIKEY subregistries.
In the "MIKEY Security Protocol Parameters" subregistry we request
the following addition:
Type | Meaning | Possible values
----------------------------------------------------------------
TBD | AEAD authentication tag length | 8 octets or 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" subregistry (derived from Table
6.10.1.b of [RFC3830]) we request the following addition:
SRTP encr | Value | Default Session | Default Auth.
Algorithm | | Encr. Key Length | Tag Length
-----------------------------------------------------------
AES-GCM | TBD | 16 octets | 16 octets
The SRTP encryption algorithm, session encryption key length, and
AEAD authentication tag values received from MIKEY fully determine
the AEAD algorithm (e.g., AEAD_AES_256_GCM_8). The exact mapping is
described in section 16.
15. Parameters for use with MIKEY
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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 AEAD Auth. tag length).
+------------+-------------+-------------+
| Encryption | Encryption | AEAD Auth. |
| Algorithm | Key Length | Tag Length |
+============+=============+=============+
AEAD_AES_128_GCM_8 | AES-GCM | 16 octets | 8 octets |
+------------+-------------+-------------+
AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets |
+============+=============+=============+
Table 6: Mapping MIKEY parameters to AEAD algorithm
Section 11 in this document restricts the choice of Key Derivation
Function for AEAD algorithms. To enforce this restriction in MIKEY,
we require that the SRTP PRF has value AES-CM whenever an AEAD
algorithm is used. Note that, according to Section 6.10.1 in
[RFC3830], the input key length of the Key Derivation Function (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 Key Derivation Function.
16. Acknowledgements
The authors would like to thank Michael Peck, Michael Torla, Qin Wu,
Magnus Westerland, Oscar Ohllson, Woo-Hwan Kim, John Mattsson,
Richard Barnes, John Mattisson, Morris Dworkin, Stehen Farrell and
many other reviewers who provided valuable comments on earlier drafts
of this document.
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17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Casner, S., Frederick, R., and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC 3550,
July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, September 2003.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,and
Norrman, K, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4568] Andreasen, F., Baugher, M., and D.Wing, "Session
Description Protocol (SDP): Security Descriptions for
Media Streams", RFC 4568, July 2006.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption with Associated Data", RFC 5116,
January 2008.
[RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282,
August 2008.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC6188] D. McGrew, "The Use of AES-192 and AES-256 in Secure
RTP", RFC 6188, March 2011.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013.
, January 2013.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013.
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17.2. Informative References
[BN00] Bellare, M. and C. Namprempre, "Authenticated encryption:
Relations among notions and analysis of the generic
composition paradigm", Proceedings of ASIACRYPT 2000,
Springer-Verlag, LNCS 1976, pp. 531-545 http://
www-cse.ucsd.edu/users/mihir/papers/oem.html.
[GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", U.S. National
Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.
[R02] Rogaway, P., "Authenticated encryption with Associated-
Data", ACM Conference on Computer and Communication
Security (CCS'02), pp. 98-107, ACM Press,
2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
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Author's Address
David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 525 8651
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
Kevin M. Igoe
NSA/CSS Commercial Solutions Center
National Security Agency
EMail: kmigoe@nsa.gov
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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