AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-10
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 | 2013-11-04 (Latest revision 2013-09-23) | ||
| 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) | ||
| Responsible AD | Alissa Cooper | ||
| Send notices to | avtcore-chairs@tools.ietf.org, draft-ietf-avtcore-srtp-aes-gcm@tools.ietf.org |
draft-ietf-avtcore-srtp-aes-gcm-10
Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc.
Intended Status: Standards Track K. Igoe
Expires: March 27, 2014 National Security Agency
September 23, 2013
AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-10
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 27, 2014.
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Igoe and McGrew Standards Track [Page 1]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
Abstract
This document defines how AES-GCM and AES-CCM Authenticated
Encryption with Associated Data algorithms can be used to provide
confidentiality and data authentication in the SRTP protocol.
Table of Contents
1. Introduction.....................................................3
2. Conventions Used In This Document................................4
3. Overview of the SRTP/SRTCP Security Architecture.................4
4. Terminology......................................................4
5. Generic AEAD Processing..........................................5
5.1. Types of Input Data.........................................5
5.2. AEAD Invocation Inputs and Outputs..........................6
5.2.1. Encrypt Mode...........................................6
5.2.2. Decrypt Mode...........................................6
5.3. Handling of AEAD Authentication.............................7
6. Counter Mode Encryption..........................................7
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................8
8. Unneeded SRTP/SRTCP Fields.......................................8
8.1. SRTP/SRTCP Authentication Field.............................9
8.2. RTP Padding.................................................9
9. AES-GCM/CCM processing for SRTP..................................9
9.1. SRTP IV formation for AES-GCM and AES-CCM...................9
9.2. Data Types in SRTP Packets.................................10
9.3. Handling Header Extensions.................................11
9.4. Prevention of SRTP IV Reuse................................12
10. AES-GCM/CCM Processing of SRTCP Compound Packets...............12
10.1. SRTCP IV formation for AES-GCM and AES-CCM................12
10.2. Data Types in Encrypted SRTCP Compound Packets............13
10.3. Data Types in Unencrypted SRTCP Compound Packets..........14
10.4. Prevention of SRTCP IV Reuse..............................15
11. Constraints on AEAD for SRTP and SRTCP.........................15
12. Key Derivation Functions.......................................16
13. Summary of Algorithm Characteristics...........................16
13.1. AES-GCM for SRTP/SRTCP....................................17
13.2. AES-CCM for SRTP/SRTCP....................................19
14. Security Considerations........................................22
14.1. Handling of Security Critical Parameters..................22
14.2. Size of the Authentication Tag............................22
15. IANA Considerations............................................23
15.1. SDES......................................................23
15.2. DTLS......................................................24
15.3. MIKEY.....................................................27
15.4. AEAD registry.............................................28
16. Parameters for use with MIKEY..................................28
17. Acknowledgements...............................................29
18. References.....................................................30
18.1. Normative References......................................30
18.2. Informative References....................................32
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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. Two families of AEAD algorithm families, AES Galois/Counter
Mode (AES-GCM) [GCM] and AES Counter with Cipher Block
Chaining-Message Authentication Code (AES-CCM) [RFC3610] are based
upon AES. This specification makes use of the AES versions that use
128-bit and 256-bit keys, which we call AES-128 and AES-256,
respectively.
Any AEAD algorithm provides an intrinsic authentication tag. In many
applications the authentication tag is truncated to less than full
length. This document only allows three values for the length of the
authentication tag: the length of the authentication tags MUST be
either 8 octets, 12 octets, or 16 octets in length. As with the size
of the key, the length of the authentication tag size is set when the
session is initiated and SHOULD NOT be altered. Thus each AEAD will
have a total of six configurations, reflecting the two choices for
key size (either 128 or 256 bits) and the three choices for the
length of the authentication tag (either 8, 12 or 16 octets).
The Galois/Counter Mode of operation (GCM) and the Counter with
Cipher Block Chaining-Message Authentication Code mode of operation
(CCM) are both AEAD modes of operation for block ciphers. Both use
counter mode to encrypt the data, an operation that can be
efficiently pipelined. Further, GCM authentication uses operations
that are particularly well suited to efficient implementation in
hardware, making it especially appealing for high-speed
implementations, or for implementations in an efficient and compact
circuit. CCM is well suited for use in compact software
implementations. This specification uses GCM and CCM with both
AES-128 and AES-256.
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In summary, this document defines how to use AEAD algorithms,
particularly AES-GCM and AES-CCM, to provide confidentiality and
message authentication within SRTP and SRTCP packets.
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 Security Architecture
SRTP/SRTCP 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-CCM or
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 sections 4.3.1 and 4.3.3
of [RFC3711]. Since AEAD algorithms such as AES-CCM and
AES-GCM combine encryption and authentication into a single
process, AEAD algorithms do not make use of the
authentication keys. The master key MUST be at least as
large as the encryption key derived from it.
d) Each time an instantiation of AES-GCM or AES-CCM 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 9.1).
SRTCP combines the SSRC and 31-bit SRTCP index with the
encryption salt to form a 12-octet IV (see section 10.1).
4. Terminology
The following terms have very specific meanings in the context of
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this RFC:
Crypto Context: For the purposes of this document, a crypto
context is the outcome of any process which
results in authentication of each endpoint in the
SRTP session and possession by each endpoint of a
shared secret master key. Various encryption
keys, authentication keys and salts are derived
from the master key. Aside from making
modifications to IANA registries to allow AES-GCM
and AES-CCM to work with SDES, DTLS and MIKEY,
the details of how the master key is established
are outside the scope of this document.
Similarly any mechanism for rekeying an existing
Cipher Context is outside the scope of the
document.
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
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to be treated as raw data are covered in sections 9.2, 10.2 and
10.3.
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 Bit string of variable length
Plaintext Bit string of variable length
Tag_Size_Flag (CCM only*) One Octet
Outputs
Ciphertext Bit string, length =
length(Plaintext)+tag_length
(*) For GCM, the algorithm choice determines the tag size.
As defined in [RFC3610], AES-CCM authentication uses a Tag_Size_Flag
to specify the length of the intrinsic authentication tag provided by
AES-CCM authentication. For the three tag lengths allowed in this
document the corresponding Tag_Size_Flag values are as follows:
Tag Length | Tag_Size_Flag (hex)
----------------------------------
8 bytes | 5A
12 bytes | 6A
16 bytes | 7A
Once an SRTP/SRTCP session has been initiated the length of the tag
is a fixed value and cannot be altered.
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
Tag_Size_Flag (CCM only*) One octet
Outputs
Plaintext Bit string, length =
length(Ciphertext)-tag_length
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Validity_Flag Boolean, TRUE if valid,
FALSE otherwise
(*) For GCM, the algorithm choice determines the tag size.
As mentioned in section 5.2.1, only three tag lengths are supported
for use in SRTP/SRTCP, namely 8 octets, 12 octets and 16 octets.
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, when GCM is being used, 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
In both GCM and CCM, 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 keystream of octets
which is XORed to the plaintext to form cipher.
When GCM is used, the concatenation of a 12-octet IV (see sections
9.1 and 10.1)with a 4-octet block counter forms the input to AES.
This is used to build a key_stream as follows:
def GCM_keystream( Plaintext, IV, Encryption_key ):
assert len(plaintext) <= (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) < len(Plaintext):
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, len(Plaintext) )
return (first_key_block, key_stream )
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In AES-CCM counter mode encryption, the AES data input consists of
the concatenation of a 1-octet flag, a 12-octet IV, and a 3-octet
block counter. Note that in this application the flag octet will
always have the value 0x02 (see section 2.3 of [RFC3610]). A
(first_key_block, key_stream) pair is formed as follows:
def CCM_keystream( Plaintext, IV, Encryption_key ):
assert len(Plaintext) <= (2**28)-16 ## measured in octets
key_stream = ""
block_counter = 0
first_key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key )
while len(key_stream)<len(Plaintext):
block_counter = block_counter + 1
key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key )
key_stream = key_stream || key_block
key_stream = truncate( key_stream, len(Plaintext) )
return (first_key_block, key_stream )
These keystream generation processes allow for a keystream of length
up to (2^28)-16 octets for AES-CCM and up to (2^36)-32 octets for
AES-GCM.
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.
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12
AEAD_AES_128_CCM and AEAD_AES_256_CCM are defined in [RFC5116] with
an authentication tag length of 16-octets. AEAD_AES_128_CCM_8 and
AEAD_AES_256_CCM_8 are defined in [RFC6655] with an authentication
tag length of 8-octets. We require two new variants,
AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12, with 12-octet
authentication tags. In each case the authentication tag is formed
by taking the 12 most significant octets (in network order) of the
AEAD_AES_128/256_CCM authentication tag:
+=====================+===========+==============+
| Name | Key Size | tag size (t) |
+=====================+===========+==============+
| AEAD_AES_256_CCM_12 | 256 bits | 12 octets |
| AEAD_AES_128_CCM_12 | 128 bits | 12 octets |
+=====================+===========+==============+
8. Unneeded SRTP/SRTCP Fields
AEAD counter mode encryption removes the need for certain existing
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SRTP/SRTCP mechanisms.
8.1. 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
Tag as a means of conveying additional information, notably
[RFC4771]. This document retains the Authentication Tag field
primarily to preserve compatibility with these applications.
8.2. RTP Padding
Neither AES-GCM nor AES-CCM 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.
9. AES-GCM/CCM processing for SRTP
9.1. SRTP IV formation for AES-GCM and AES-CCM
The 12 octet initialization vector used by both AES-GCM and AES-CCM
SRTP is formed by first concatenating 2-octets of zeroes, the 4-octet
SSRC, the 4-octer 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.
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0 0 0 0 0 0 0 0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC | ROC | SEQ |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM and AES-CCM SRTP
Initialization Vector formation.
9.2. 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 (2 bits), padding flag (1 bit),
extension flag (1 bit), CSRC count (4 bits),
sequence number (16 bits), timestamp (32 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 32-bit SRTP MKI and the 32-bit SRTP
authentication tag (whose use is NOT
RECOMMENDED).
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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| Packet Type | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | contributing source (CSRC) identifiers (optional) |
A | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | RTP extension (OPTIONAL) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | payload ... |
P | +-------------------------------+
P | | RTP padding | RTP pad count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : SRTP MKI (optional) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
R = neither encrypted nor authenticated
Note: The RTP padding and RTP padding count fields are optional
and are not recommended
Figure 2: AEAD inputs from an SRTP packet
Since the AEAD cipher 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 SRTP MKI and SRTP
authentication tag fields.
9.3. 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
the AEAD_AES_128_CCM algorithms, this keystream MUST be generated in
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the manner defined in [RFC6904] using the AES_128_CM transform. For
the AEAD_AES_256_GCM and the AEAD_AES_256_CCM algorithms, 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 Additional Authenticated Data (AAD). Thus the AEAD
algorithm does not provide any additional privacy for the header
extensions, but does provide integrity and authentication.
9.4. 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.
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.
10. AES-GCM/CCM 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)
10.1. SRTCP IV formation for AES-GCM and AES-CCM
The 12 octet initialization vector used by both AES-GCM and AES-CCM
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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 10 11
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC |00|00|0+SRTCP Idx|---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 3: SRTCP Initialization Vector formation
10.2. Data Types in Encrypted SRTCP Compound Packets
When the encryption flag is set to 1, the SRTCP packet is broken into
plaintext, associated data, and raw (untouched) data as listed below
(see figure 4):
Associated Data: The packet version (2 bits), padding flag (1
bit), reception report count (5 bits), packet
type (8 bits), length (2 octets), SSRC (4
octets), encryption flag (1 bit) and SRTCP index
(31 bits).
Raw Data: The 32-bit optional SRTCP MKI index and 32-bit
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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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 : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
R = neither encrypted nor authenticated
Figure 4: AEAD SRTCP inputs when encryption flag = 1.
10.3. Data Types in Unencrypted SRTCP Compound Packets
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 5):
Plaintext: None.
Raw Data: The 32-bit optional SRTCP MKI index and 32-bit
SRTCP authentication tag (whose use is NOT
RECOMMENDED).
Associated Data: All other data.
Even though there is no plaintext in this RTCP packet, AEAD
encryption returns a cipher field which is precisely the length of
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the AEAD authentication tag. This cipher is to be placed before the
Encryption flag and the SRTCP index in the authenticated SRTCP
packet.
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
Figure 5: AEAD SRTCP inputs when encryption flag = 0
10.4. 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 performed should
be performed and a new master key put in place well before the SRTCP
index overflows.
The comments on SSRC management in section 9.4 also apply.
11. Constraints on AEAD for SRTP and SRTCP
In general, any AEAD algorithm can accept inputs with varying
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lengths, but each algorithm can accept only a limited range of
lengths for a specific parameter. In this section, we describe the
constraints on the parameter lengths that any AEAD algorithm must
support to be used in AEAD-SRTP. Additionally, we specify a complete
parameter set for two specific AEAD algorithms, namely AES-GCM and
AES-CCM.
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three
constraints listed below:
PARAMETER Meaning Value
A_MAX maximum additional MUST be at least 12 octets.
authenticated data
length
N_MIN minimum nonce (IV) MUST be 12 octets.
length
N_MAX maximum nonce (IV) MUST be 12 octets.
length
C_MAX maximum ciphertext GCM: MUST be <= 2^36-16 octets.
length per invocation CCM: MUST be <= 2^28-16 octets.
The values for C_MAX are based on purely cryptographic
considerations.
For sake of clarity we specify two additional parameters:
AEAD Authentication Tag Length MUST be either 8, 12, or 16
octets
Maximum number of invocations MUST be at most 2^48 for SRTP
for a given instantiation MUST be at most 2^31 for SRTCP
Block Counter size MUST be 24 bits for CCM,
MUST be 32 bits for GCM
The reader is reminded that the ciphertext is longer than the
plaintext by exactly the length of the AEAD authentication tag.
12. 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 the AEAD_AES_128_GCM algorithms and the
AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key
Derivation Function described in [RFC3711]. Both the
AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST
use the AES_256_CM_PRF Key Derivation Function described in [RFC6188]
.
13. Summary of Algorithm Characteristics
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For convenience, much of the information about the use of AES-GCM and
AES-CCM algorithms in SRTP is collected in the tables contained in
this section.
13.1. AES-GCM for SRTP/SRTCP
AES-GCM is a family of AEAD algorithms built around the AES block
cipher algorithm. AES-GCM uses AES counter mode for encryption and
Galois Message Authentication Code (GMAC) for authentication. A
detailed description of the AES-GCM family can be found in
[RFC5116]. The following members of the AES-GCM family may be used
with SRTP/SRTCP:
Table 1: AES-GCM algorithms for SRTP/SRTCP
Name Key Size AEAD Tag Size Reference
================================================================
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_GCM 32 octets 16 octets [RFC5116]
AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282]
AEAD_AES_256_GCM_8 32 octets 8 octets [RFC5282]
AEAD_AES_128_GCM_12 16 octets 12 octets [RFC5282]
AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282]
Any implementation of AES-GCM SRTP SHOULD support both
AEAD_AES_128_GCM_8 and AEAD_AES_256_GCM_8, and it MAY support the
four other variants shown in table 1. Below we summarize parameters
associated with these six GCM algorithms:
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 2: The AEAD_AES_128_GCM_8 Crypto Suite
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+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 3: The AEAD_AES_128_GCM_12 Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 4: 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] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 5: The AEAD_AES_256_GCM_8 Crypto Suite
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+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 6: The AEAD_AES_256_GCM_12 Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_GCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 7: The AEAD_AES_256_GCM Crypto Suite
13.2. AES-CCM for SRTP/SRTCP
AES-CCM is another family of AEAD algorithms built around the AES
block cipher algorithm. AES-CCM uses AES counter mode for encryption
and AES Cipher Block Chaining Message Authentication Code (CBC MAC)
for authentication. A detailed description of the AES-CCM family can
be found in [RFC5116]. Four of the six CCM algorithms used in this
document are defined in previous RFCs, while two, AEAD_AES_128_CCM_12
and AEAD_AES_256_CCM_12, are defined in section 7 of this document.
Table 8: AES-CCM algorithms for SRTP/SRTCP
Name Key Size AEAD Tag Size Reference
================================================================
AEAD_AES_128_CCM 128 bits 16 octets [RFC5116]
AEAD_AES_256_CCM 256 bits 16 octets [RFC5116]
AEAD_AES_128_CCM_12 128 bits 12 octets see section 7
AEAD_AES_256_CCM_12 256 bits 12 octets see section 7
AEAD_AES_128_CCM_8 128 bits 8 octets [RFC6655]
AEAD_AES_256_CCM_8 256 bits 8 octets [RFC6655]
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Any implementation of AES-CCM SRTP/SRTCP SHOULD support both
AEAD_AES_128_CCM_8 and AEAD_AES_256_CCM_8, and MAY support the other
four variants.
In addition to the flag octet used in counter mode encryption,
AES-CCM authentications also uses a flag octet that conveys
information about the length of the authentication tag, length of the
block counter, and presence of additional authenticated data (see
section 2.2 of [RFC3610]). For AES-CCM in SRTP/SRTCP, the flag octet
has the hex value 5A if an 8-octet AEAD authentication tag is used,
6A if a 12-octet AEAD authentication tag is used, and 7A if a
16-octet AEAD authentication tag is used. The flag octet is one of
the inputs to AES during the counter mode encryption of the
plaintext.
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 9: The AEAD_AES_128_CCM_8 Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 10: The AEAD_AES_128_CCM_12 Crypto Suite
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+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 11: The AEAD_AES_128_CCM Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 12: The AEAD_AES_256_CCM_8 Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 13: The AEAD_AES_256_CCM_12 Crypto Suite
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+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Default key lifetime (SRTP) | 2^48 packets |
| Default key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_CCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 14: The AEAD_AES_256_CCM Crypto Suite
14. Security Considerations
14.1. 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
0 for CCM, to 1 for GCM). The block counter is incremented
after each block key has been produced, but it MUST NOT be
allowed to exceed 2^32 for GCM and 2^24 for CCM.
- Each time a rekey occurs, the initial values of the SRTCP index
and the values of all the SEQ counters MUST be saved.
- Processing MUST cease if the 48-bit Packet Counter or the 31-bit
SRTCP index cycles back to its initial value. Processing MUST
NOT resume until a new SRTP/SRTCP session has been established
using a new SRTP master key. Ideally, a rekey should be done
well before either of these counters cycle.
14.2. Size of the Authentication Tag
We require that the AEAD authentication tag must be at least 8
octets, significantly reducing the probability of an adversary
successfully introducing fraudulent data. The goal of an
authentication tag is to minimize the probability of a successful
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Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
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.
Suppose an adversary wishes to introduce a forged or altered packet
into a target network by randomly selecting an authentication value
until by chance they hit a valid authentication tag. The table below
summarizes the relationship between the number of forged packets the
adversary has tried, the size of the authentication tag, and the
probability of a compromise occurring (i.e. at least one of the
attempted forgeries having a valid authentication tag). The reader
is reminded that the forgery attempts can be made over the entire
network, not just a single link, and that frequently changing the key
does not decrease the probability of a compromise occurring.
+==================+========================================+
| Authentication | Probability of a Compromise Occurring |
| Tag | for a given number of forgery attempts |
| Size |------------+-------------+-------------|
| (octets) | prob=2^-30 | prob=2^-20 | prob=2^-10 |
|==================+=============+=============+============|
| 4 | 2^2 tries | 2^12 tries | 2^22 tries |
|==================+============+=============+=============|
| 8 | 2^34 tries | 2^44 tries | 2^54 tries |
|==================+============+=============+=============|
| 12 | 2^66 tries | 2^76 tries | 2^86 tries |
|==================+============+=============+=============|
| 16 | 2^98 tries | 2^108 tries | 2^118 tries |
+=================+============+=============+==============+
Table 15: Probability of a compromise occurring for a given
number of forgery attempts and tag size.
15. IANA Considerations
15.1. SDES
SDP Security Descriptions [RFC4568] defines SRTP "crypto suites". A
crypto suite corresponds to a particular AEAD algorithm in SRTP. In
order to allow SDP 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) Parameters" registry.
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Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" /
"AEAD_AES_128_GCM_8" /
"AEAD_AES_256_GCM_8" /
"AEAD_AES_128_GCM_12" /
"AEAD_AES_256_GCM_12" /
"AEAD_AES_128_CCM" /
"AEAD_AES_256_CCM" /
"AEAD_AES_128_CCM_8" /
"AEAD_AES_256_CCM_8" /
"AEAD_AES_128_CCM_12" /
"AEAD_AES_256_CCM_12" /
srtp-crypto-suite-ext
15.2. DTLS
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_256_GCM = {TBD, TBD }
AEAD_AES_128_GCM_8 = {TBD, TBD }
AEAD_AES_256_GCM_8 = {TBD, TBD }
AEAD_AES_128_GCM_12 = {TBD, TBD }
AEAD_AES_256_GCM_12 = {TBD, TBD }
AEAD_AES_128_CCM = {TBD, TBD }
AEAD_AES_256_CCM = {TBD, TBD }
AEAD_AES_128_CCM_8 = {TBD, TBD }
AEAD_AES_256_CCM_8 = {TBD, TBD }
AEAD_AES_128_CCM_12 = {TBD, TBD }
AEAD_AES_256_CCM_12 = {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_CCM
cipher: AES_128_CCM
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
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Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
AEAD_AES_256_CCM
cipher: AES_256_CCM
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
AEAD_AES_128_CCM_8
cipher: AES_128_CCM
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_CCM_8
cipher: AES_256_CCM
cipher_key_length: 256 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_128_CCM_12
cipher: AES_128_CCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 12 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_CCM_12
cipher: AES_256_CCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 12 octets
auth_function: NULL
auth_key_length: N/A
Igoe and McGrew Standards Track [Page 25]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
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_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
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_8
cipher: AES_256_GCM
cipher_key_length: 256 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_128_GCM_12
cipher: AES_128_GCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
Igoe and McGrew Standards Track [Page 26]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
aead_auth_tag_length: 12 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_12
cipher: AES_256_GCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 12 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.
15.3. 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. lists maintained under "MIKEY
Security Protocol Parameters".
In the "MIKEY Security Protocol Parameters" subregistry we request
the following addition:
Type | Meaning | Possible values
----------------------------------------------------------------
TBD | AEAD authentication tag length | 8, 12, or 16 (in octets)
In the "Encryption Algorithm" subregistry (derived from Table
6.10.1.b of [RFC3830]) we request the following additions:
SRTP encr alg. | Value | Default Session Encr. Key Length
-----------------------------------------------------------
AES-CCM | TBD | 16 octets
AES-GCM | TBD | 16 octets
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Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
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.
15.4. AEAD registry
We request that IANA make the following additions to the IANA
"Authenticated Encryption with Associated Data (AEAD) Parameters"
page's registry for "AEAD Algorithms":
AEAD_AES_128_CCM_12 = TBD
AEAD_AES_256_CCM_12 = TBD
16. 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 AEAD Auth. tag length).
Igoe and McGrew Standards Track [Page 28]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
+------------+-------------+-------------+
| Encryption | Encryption | AEAD Auth. |
| Algorithm | Key Length | Tag Length |
+============+=============+=============+
AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_128_CCM | AES-CCM | 16 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_128_GCM_12 | AES-GCM | 16 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_128_CCM_12 | AES-CCM | 16 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_128_GCM_8 | AES-GCM | 16 octets | 8 octets |
+------------+-------------+-------------+
AEAD_AES_128_CCM_8 | AES-CCM | 16 octets | 8 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM | AES-CCM | 32 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM_12 | AES-GCM | 32 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM_12 | AES-CCM | 32 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM_8 | AES-GCM | 32 octets | 8 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM_8 | AES-CCM | 32 octets | 8 octets |
+============+=============+=============+
Table 16: Mapping MIKEY parameters to AEAD algorithm
Section 12 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 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.
17. Acknowledgements
The authors would like to thank Michael Peck, Michael Torla, Qin Wu,
Magnus Westerland, Oscar Ohllson, Woo-Hwan Kim and many other
reviewers who provided valuable comments on earlier drafts of this
document.
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Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
18. References
18.1. 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.
[RFC3610] Whiting,D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, March 2004.
[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.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013.
, January 2013.
Igoe and McGrew Standards Track [Page 30]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013.
Igoe and McGrew Standards Track [Page 31]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
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 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.
Igoe and McGrew Standards Track [Page 32]
Internet Draft AES-GCM and AES-CCM for SRTP Sep 23, 2013
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).
Igoe and McGrew Standards Track [Page 33]