draft-ietf-avtcore-srtp-aes-gcm-01.txt   draft-ietf-avtcore-srtp-aes-gcm-02.txt 
Network Working Group D. McGrew Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc. Internet Draft Cisco Systems, Inc.
Intended Status: Informational K.M. Igoe Intended Status: Informational K.M. Igoe
Expires: December 27, 2012 National Security Agency Expires: February 16, 2013 National Security Agency
June 25, 2012 August 15, 2012
AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP) AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-01 draft-ietf-avtcore-srtp-aes-gcm-02
Status of this Memo Status of this Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 27, 2012. This Internet-Draft will expire on February 16, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Abstract Abstract
This document defines how AES-GCM, AES-CCM, and other Authenticated This document defines how AES-GCM and AES-CCM Authenticated
Encryption with Associated Data (AEAD) algorithms can be used to Encryption with Associated Data algorithms can be used to provide
provide confidentiality and data authentication mechanisms in the confidentiality and data authentication in the SRTP protocol.
SRTP protocol.
Table of Contents Table of Contents
1. Introduction.....................................................3 1. Introduction.....................................................3
2. Conventions Used In This Document................................3 2. Conventions Used In This Document................................3
3. Overview of the SRTP/SRTCP Security Architecture.................4 3. Overview of the SRTP/SRTCP Security Architecture.................4
4. Terminology......................................................4 4. Terminology......................................................4
5. Generic AEAD Processing..........................................5 5. Generic AEAD Processing..........................................5
5.1. Types of Input Data.........................................5 5.1. Types of Input Data.........................................5
5.2. AEAD Invocation Inputs and Outputs..........................5 5.2. AEAD Invocation Inputs and Outputs..........................5
5.2.1. Encrypt Mode...........................................5 5.2.1. Encrypt Mode...........................................5
5.2.2. Decrypt Mode...........................................6 5.2.2. Decrypt Mode...........................................6
5.3. Handling of AEAD Authentication.............................6 5.3. Handling of AEAD Authentication.............................6
6. Counter Mode Encryption..........................................6 6. Counter Mode Encryption..........................................6
7. Unneeded SRTP/SRTCP Fields.......................................7 7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................7
7.1. SRTP/SRTCP Authentication Field.............................7 8. Unneeded SRTP/SRTCP Fields.......................................8
7.2. RTP Padding.................................................7 8.1. SRTP/SRTCP Authentication Field.............................8
8. AES-GCM/CCM processing for SRTP..................................7 8.2. RTP Padding.................................................8
8.1. SRTP IV formation for AES-GCM and AES-CCM...................7 9. AES-GCM/CCM processing for SRTP..................................8
8.2. Data Types in SRTP Packets..................................8 9.1. SRTP IV formation for AES-GCM and AES-CCM...................8
8.3. Prevention of SRTP IV Reuse.................................9 9.2. Data Types in SRTP Packets..................................9
9. AES-GCM/CCM Processing of SRTCP Compound Packets................10 9.3. Prevention of SRTP IV Reuse................................10
9.1. SRTCP IV formation for AES-GCM and AES-CCM.................10 10. AES-GCM/CCM Processing of SRTCP Compound Packets...............11
9.2. Data Types in Encrypted SRTCP Compound Packets.............10 10.1. SRTCP IV formation for AES-GCM and AES-CCM................11
9.3. Data Types in Unencrypted SRTCP Compound Packets...........12 10.2. Data Types in Encrypted SRTCP Compound Packets............12
9.4. Prevention of SRTCP IV Reuse...............................13 10.3. Data Types in Unencrypted SRTCP Compound Packets..........13
10. Constraints on AEAD for SRTP and SRTCP.........................13 10.4. Prevention of SRTCP IV Reuse..............................14
10.1. Generic AEAD Parameter Constraints........................13 11. Constraints on AEAD for SRTP and SRTCP.........................14
10.2. AES-GCM for SRTP/SRTCP....................................14 11.1. Generic AEAD Parameter Constraints........................15
10.3. AES-CCM for SRTP/SRTCP....................................14 11.2. AES-GCM for SRTP/SRTCP....................................15
11. Key Derivation Functions.......................................15 11.3. AES-CCM for SRTP/SRTCP....................................16
12. Security Considerations........................................15 12. Key Derivation Functions.......................................16
12.1. Handling of Security Critical Parameters..................15 13. Security Considerations........................................17
12.2. Size of the Authentication Tag............................15 13.1. Handling of Security Critical Parameters..................17
13. IANA Considerations............................................16 13.2. Size of the Authentication Tag............................17
13.1. SDES......................................................16 14. IANA Considerations............................................18
13.2. DTLS......................................................17 14.1. SDES......................................................18
13.3. MIKEY.....................................................19 14.2. DTLS......................................................19
14. Parameters for use with MIKEY..................................19 14.3. MIKEY.....................................................20
15. Acknowledgements...............................................20 14.4. AEAD registry.............................................21
16. References.....................................................21 15. Parameters for use with MIKEY..................................21
16.1. Normative References......................................21 16. Acknowledgements...............................................22
16.2. Informative References....................................22 17. References.....................................................23
17.1. Normative References......................................23
17.2. Informative References....................................24
1. Introduction 1. Introduction
The Secure Real-time Transport Protocol (SRTP) is a profile of the The Secure Real-time Transport Protocol (SRTP) is a profile of the
Real-time Transport Protocol (RTP), which can provide Real-time Transport Protocol (RTP), which can provide
confidentiality, message authentication, and replay protection to the confidentiality, message authentication, and replay protection to the
RTP traffic and to the control traffic for RTP, the Real-time RTP traffic and to the control traffic for RTP, the Real-time
Transport Control Protocol (RTCP). It is important to note that the Transport Control Protocol (RTCP). It is important to note that the
outgoing SRTP packets from a single endpoint may be originating from outgoing SRTP packets from a single endpoint may be originating from
several independent data sources. several independent data sources.
skipping to change at page 4, line 10 skipping to change at page 4, line 10
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Overview of the SRTP/SRTCP Security Architecture 3. Overview of the SRTP/SRTCP Security Architecture
SRTP/SRTCP security is based upon the following principles: SRTP/SRTCP security is based upon the following principles:
a) Both privacy and authentication are based upon the use of a) Both privacy and authentication are based upon the use of
symmetric algorithms. An AEAD algorithm such as AES-CCM and symmetric algorithms. An AEAD algorithm such as AES-CCM or
AES-GCM combines privacy and authentication into a single AES-GCM combines privacy and authentication into a single
process. process.
b) A secret master key is shared by all participating endpoints, b) A secret master key is shared by all participating endpoints,
both those originating SRTP/SRTCP packets and those receiving both those originating SRTP/SRTCP packets and those receiving
these packets. Any given master key MAY be used these packets. Any given master key MAY be used
simultaneously by several endpoints to originate SRTP/SRTCP simultaneously by several endpoints to originate SRTP/SRTCP
packets (as well one or more endpoints using this master key packets (as well one or more endpoints using this master key
to process inbound data). to process inbound data).
skipping to change at page 5, line 30 skipping to change at page 5, line 30
not encrypted. not encrypted.
Plaintext Data that is to be both encrypted and Plaintext Data that is to be both encrypted and
authenticated. authenticated.
Raw Data Data that is to be neither encrypted nor Raw Data Data that is to be neither encrypted nor
authenticated. authenticated.
Which portions of SRTP/SRTCP packets that are to be treated as Which portions of SRTP/SRTCP packets that are to be treated as
associated data, which are to be treated as plaintext, and which are 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. 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. AEAD Invocation Inputs and Outputs
5.2.1. Encrypt Mode 5.2.1. Encrypt Mode
Inputs: Inputs:
Encryption_key Octet string, either 16 or 32 Encryption_key Octet string, either 16 or 32
octets long octets long
Initialization_Vector Octet string, 12 octets long Initialization_Vector Octet string, 12 octets long
Associated_Data Bit string of variable length Associated_Data Bit string of variable length
Plaintext Bit string of variable length Plaintext Bit string of variable length
Tag_Size_Flag (CCM only*) One Octet Tag_Size_Flag (CCM only*) One Octet
Outputs Outputs
Ciphertext Bit string, length = Ciphertext Bit string, length =
length(ciphertext)-tag_length length(plaintext)+tag_length
(*) For GCM, the algorithm choice determines the tag size. (*) For GCM, the algorithm choice determines the tag size.
AES-CCM uses a Tag_Size_Flag that has the hex value 5A if an 8-octet AES-CCM uses a Tag_Size_Flag that has the hex value 5A if an 8-octet
authentication tag is used, 6A if a 12-octet authentication tag is authentication tag is used, 6A if a 12-octet authentication tag is
used, and 7A if a 16-octet authentication tag is used. used, and 7A if a 16-octet authentication tag is used.
5.2.2. Decrypt Mode 5.2.2. Decrypt Mode
Inputs: Inputs:
Encryption_key Octet string, either 16 or 32 Encryption_key Octet string, either 16 or 32
octets long Octets long
Initialization_Vector octet string, 12 octets long Initialization_Vector Octet string, 12 octets long
Associated_Data Bit string of variable length Associated_Data Octet string of variable length
Ciphertext Bit string of variable length Ciphertext Octet string of variable length
Tag_Size_Flag (CCM only*) One Octet Tag_Size_Flag (CCM only*) One octet
Outputs Outputs
Plaintext Bit string, length = Plaintext Bit string, length =
length(ciphertext)-tag_length length(ciphertext)-tag_length
Validity_Flag Boolean, TRUE if valid, Validity_Flag Boolean, TRUE if valid,
FALSE otherwise FALSE otherwise
(*) For GCM, the algorithm choice determines the tag size. (*) For GCM, the algorithm choice determines the tag size.
AES-CCM uses a Tag_Size_Flag that has the hex value 5A if an 8-octet The Tag_Size_Flag used in AES-CCM has the hex value 5A if an 8-octet
authentication tag is used, 6A if a 12-octet authentication tag is authentication tag is used, 6A if a 12-octet authentication tag is
used, and 7A if a 16-octet authentication tag is used. used, and 7A if a 16-octet authentication tag is used.
5.3. Handling of AEAD Authentication 5.3. Handling of AEAD Authentication
AEAD requires that all incoming packets MUST pass AEAD authentication AEAD requires that all incoming packets MUST pass AEAD authentication
before any other action takes place. The ciphertext MUST NOT be before any other action takes place. Plaintext and associated data
decrypted until the AEAD tag has been validated. The associated data MUST NOT be released until the AEAD authentication tag has been
MUST NOT be released until the AEAD tag has been validated. 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 incoming data is to be Should the AEAD tag prove to be invalid, the packet in question is to
discarded and appropriate error flags raised. Local policy be discarded and a Validation Error flag raised. Local policy
determines how these flags are to be handled and are outside the determines how this flag is to be handled and is outside the scope of
scope of this document. this document.
6. Counter Mode Encryption 6. Counter Mode Encryption
Each outbound packet uses a 12 octet IV and encryption key to form a Each outbound packet uses a 12 octet IV and encryption key to form a
keystream of bits which is XORed to the plaintext to form cipher. keystream of bits which is XORed to the plaintext to form cipher.
Using the 12-octet IV and a 4-octet block counter, the keystream is When GCM is used, the concatenation of a 12-octet IV, with a 4-octet
formed in 16-octet blocks until it is at least as long as the block counter forms the input to AES. This is used to build a
plaintext, and any excess keystream bits are discarded. At the start key_stream as follows:
of each packet, the block counter is initialized to 0x0000 for
AES-CCM and to 0x0001 for AES-GCM. A key block is formed by
key_block = AES_ENC( IV || block_counter; key=Encryption_key ) def GCM_keystream( plaintext, IV, Encryption_key ):
assert len(plaintext) <= (2**36 - 32)
key_stream = ""
block_counter = 1
while len(key_stream) < len(plaintext):
key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key )
key_stream = key_stream || key_block
block_counter = block_counter + 1
key_stream = truncate( key_stream, len(plaintext) )
last_key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key )
return (key_stream, last_key_block )
and the block counter is incremented. This allows for a per packet The last_key_block is reserved to form the GMAC of the message by
keystream of length of up to 2^36 octets for AES-CCM and up to encrypting the GHASH of the message. It is not required for CCM.
2^36-16 octets for AES-GCM.
When CCM is used, the AES input is the concatenation of a 12-octet
IV, a 1-octet Tag_Size_Flag, and a 4-octet block counter. A
keystream is formed as follows:
def CCM_keystream( plaintext, IV, Tag_Size_Flag, Encryption_key ):
assert len(plaintext) <= 2**28
key_stream = ""
block_counter = 0
while len(key_stream)<len(plaintext):
key_block = AES_ENC( data=IV||Tag_Size_Flag||block_counter,
key=Encryption_key )
key_stream = key_stream || key_block
block_counter = block_counter + 1
key_stream = truncate( key_stream, len(plaintext) )
return key_stream
These keystream generation processes allows for a keystream of length
of up to 2^24 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 With any counter mode, if the same (IV, Encryption_key) pair is used
twice, precisely the same keystream is formed. As explained in twice, precisely the same keystream is formed. As explained in
section 9.1 of RFC 3711, this is a cryptographic disaster. For section 9.1 of RFC 3711, this is a cryptographic disaster. For
AES-GCM, the consequences of such a reuse are even worse than AES-GCM, the consequences of such a reuse are even worse than
explained in RFC 3711 because it would completely compromise the explained in RFC 3711 because it would completely compromise the
AES-GCM authentication mechanism. AES-GCM authentication mechanism.
7. Unneeded SRTP/SRTCP Fields 7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12
AEAD_AEC_128_CCM and AEAD_AEC_256_CCM are defined in [RFC5116] with
an authentication tag length of 16-octets. AEAD_AEC_128_CCM_8 and
AEAD_AEC_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_AEC_256_CCM_12 | 256 bits | 12 octets |
| AEAD_AEC_128_CCM_12 | 128 bits | 12 octets |
+=====================+===========+==============+
8. Unneeded SRTP/SRTCP Fields
AEAD counter mode encryption removes the need for certain existing AEAD counter mode encryption removes the need for certain existing
SRTP/SRTCP mechanisms. SRTP/SRTCP mechanisms.
7.1. SRTP/SRTCP Authentication Field 8.1. SRTP/SRTCP Authentication Field
The AEAD message authentication mechanism MUST be the primary message The AEAD message authentication mechanism MUST be the primary message
authentication mechanism for AEAD SRTP/SRTCP. Additional SRTP/SRTCP authentication mechanism for AEAD SRTP/SRTCP. Additional SRTP/SRTCP
authentication mechanisms SHOULD NOT be used with any AEAD algorithm authentication mechanisms SHOULD NOT be used with any AEAD algorithm
and the optional SRTP/SRTCP Authentication Tags are NOT RECOMMENDED and the optional SRTP/SRTCP Authentication Tags are NOT RECOMMENDED
and SHOULD NOT be present. Note that this contradicts section 3.4 of and SHOULD NOT be present. Note that this contradicts section 3.4 of
[RFC3711] which makes the use of the SRTCP Authentication field [RFC3711] which makes the use of the SRTCP Authentication field
mandatory, but the presence of the AEAD authentication renders the mandatory, but the presence of the AEAD authentication renders the
older authentication methods redundant. older authentication methods redundant.
Rationale. Some applications use the SRTP/SRTCP Authentication Rationale. Some applications use the SRTP/SRTCP Authentication
Tag as a means of conveying additional information, notably Tag as a means of conveying additional information, notably
[RFC4771]. This document retains the Authentication Tag field [RFC4771]. This document retains the Authentication Tag field
primarily to preserve compatibility with these applications. primarily to preserve compatibility with these applications.
7.2. RTP Padding 8.2. RTP Padding
Neither AES-GCM not AES-CCM requires that the data be padded out to a Neither AES-GCM not AES-CCM requires that the data be padded out to a
specific block size, reducing the need to ude the padding mechanism specific block size, reducing the need to use the padding mechanism
provided by RTP. It is RECOMENDED that the RTP padding mechanism not provided by RTP. It is RECOMENDED that the RTP padding mechanism not
be used unless it is necessary to disguise the length of the be used unless it is necessary to disguise the length of the
underlying plaintext. underlying plaintext.
8. AES-GCM/CCM processing for SRTP 9. AES-GCM/CCM processing for SRTP
8.1. SRTP IV formation for AES-GCM and AES-CCM
The 12 byte initialization vector used by both AES-GCM and AES-CCM 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 SRTP is formed by first concatenating 2-octets of zeroes, the 4-octet
SSRC, the 4-octer Rollover Counter (ROS) and the two octet sequence SSRC, the 4-octer Rollover Counter (ROC) and the two octet sequence
number SEQ. The resulting 12-octet value is then XORed to the number SEQ. The resulting 12-octet value is then XORed to the
12-octet salt to form the 12-octet IV. 12-octet salt to form the 12-octet IV.
0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1
+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC | ROC | SEQ |---+ |00|00| SSRC | ROC | SEQ |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
skipping to change at page 8, line 26 skipping to change at page 9, line 28
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+ | Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM and AES-CCM SRTP Figure 1: AES-GCM and AES-CCM SRTP
Initialization Vector formation. Initialization Vector formation.
Using the terminology of section 8.2.1. of [GCM], the first six Using the terminology of section 8.2.1. of [GCM], the first six
octets of the IV are the fixed field and the last six bytes are the octets of the IV are the fixed field and the last six octets are the
invocation field. invocation field.
8.2. Data Types in SRTP Packets 9.2. Data Types in SRTP Packets
All SRTP packets MUST be both authenticated and encrypted. The data All SRTP packets MUST be both authenticated and encrypted. The data
fields within the SRTP packets are broken into Associated Data, fields within the SRTP packets are broken into Associated Data,
Plaintext and Raw Data as follows (see figure 2): Plaintext and Raw Data as follows (see figure 2):
Associated Data The version (2 bits), padding flag (1 bit), Associated Data The version (2 bits), padding flag (1 bit),
extension flag (1 bit), CSRC count (4 bits), extension flag (1 bit), CSRC count (4 bits),
sequence number (16 bits), timestamp (32 bits), sequence number (16 bits), timestamp (32 bits),
SSRC (32 bits), optional contributing source SSRC (32 bits), optional contributing source
identifiers (CSRCs, 32 bits each), and optional identifiers (CSRCs, 32 bits each), and optional
RTP extension (32 bits). RTP extension (variable length).
Plaintext The RTP payload (variable length), RTP padding (if Plaintext The RTP payload (variable length), RTP padding (if
used, variable length), and RTP pad count ( if used, variable length), and RTP pad count ( if
used, 8 bits). used, 1 octet).
Raw Data The optional 32-bit SRTP MKI and the 32-bit SRTP Raw Data The optional 32-bit SRTP MKI and the 32-bit SRTP
authentication tag (whose use is NOT authentication tag (whose use is NOT
RECOMMENDED). RECOMMENDED).
0 1 2 3 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 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 |V=2|P|X| CC |M| Packet Type | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 9, line 35 skipping to change at page 10, line 35
R : authentication tag (NOT RECOMMENDED) : R : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated) P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only) A = Associated Data (to be authenticated only)
R = neither encrypted nor authenticated R = neither encrypted nor authenticated
Note: The RTP padding and RTP padding count fields are optional Note: The RTP padding and RTP padding count fields are optional
and are not recommended and are not recommended
Figure 2: AEAD inputs from an SRTP packet. Figure 2: AEAD inputs from an SRTP packet
8.3. Prevention of SRTP IV Reuse Since the AEAD cipher is larger that 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. Prevention of SRTP IV Reuse
In order to prevent IV reuse, we must ensure that the (ROC,SEQ,SSRC) 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 triple is never used twice with the same master key. There are two
phases to this issue. phases to this issue.
Counter Management A rekey MUST be performed to establish a new Counter Management A rekey MUST be performed to establish a new
master key before the (ROC,SEQ) pair cycles back master key before the (ROC,SEQ) pair cycles back
to its original value. to its original value.
SSRC Management The set of all SSRC values must be partitioned SSRC Management The set of all SSRC values must be partitioned
skipping to change at page 10, line 9 skipping to change at page 11, line 19
values from the pool it has been assigned. values from the pool it has been assigned.
Further, each endpoint MUST maintain a history Further, each endpoint MUST maintain a history
of outbound SSRC identifiers that it has issued of outbound SSRC identifiers that it has issued
within the lifetime of the current master key, within the lifetime of the current master key,
and when a new synchronization source requests and when a new synchronization source requests
an SSRC identifier it MUST NOT be given an an SSRC identifier it MUST NOT be given an
identifier that has been previously issued. A identifier that has been previously issued. A
rekey MUST be performed before its pool of SSRC rekey MUST be performed before its pool of SSRC
values is exhausted. values is exhausted.
9. AES-GCM/CCM Processing of SRTCP Compound Packets 10. AES-GCM/CCM Processing of SRTCP Compound Packets
All SRTCP compound packets MUST be authenticated, but unlike SRTP, All SRTCP compound packets MUST be authenticated, but unlike SRTP,
SRTCP packet encryption is optional. A sender can select which SRTCP packet encryption is optional. A sender can select which
packets to encrypt, and indicates this choice with a 1-bit encryption packets to encrypt, and indicates this choice with a 1-bit encryption
flag (located just before the 31-bit SRTCP index) flag (located just before the 31-bit SRTCP index)
9.1. SRTCP IV formation for AES-GCM and AES-CCM 10.1. SRTCP IV formation for AES-GCM and AES-CCM
The 12 byte initialization vector used by both AES-GCM and AES-CCM The 12 octet initialization vector used by both AES-GCM and AES-CCM
SRTCP is formed by first concatenating 2-octets of zeroes, the SRTCP is formed by first concatenating 2-octets of zeroes, the
4-octet Synchronization Source identifier (SSRC), 2-octets of zeroes, 4-octet Synchronization Source identifier (SSRC), 2-octets of zeroes,
a single zero bit, and the 31-bit SRTCP Index. The resulting 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 value is then XORed to the 12-octet salt to form the
12-octet IV. 12-octet IV.
0 1 2 3 4 5 6 7 8 9 10 11 0 1 2 3 4 5 6 7 8 9 10 11
+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC |00|00|0+SRTCP Idx|---+ |00|00| SSRC |00|00|0+SRTCP Idx|---+
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+) | Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+ | Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+
Figure 3: SRTCP Initialization Vector formation. Figure 3: SRTCP Initialization Vector formation
Using the terminology of section 8.2.1. of [GCM], the first eight Using the terminology of section 8.2.1. of [GCM], the first eight
octets of the IV are the fixed field and the last four bytes are the octets of the IV are the fixed field and the last four octets are the
invocation field. invocation field.
9.2. Data Types in Encrypted SRTCP Compound Packets 10.2. Data Types in Encrypted SRTCP Compound Packets
When the encryption flag is set to 1, the SRTCP packet is broken into When the encryption flag is set to 1, the SRTCP packet is broken into
plaintext, associated data, and raw (untouched) data as listed below plaintext, associated data, and raw (untouched) data as listed below
(see figure 4): (see figure 4):
Associated Data The packet version (2 bits), padding flag (1 bit), Associated Data The packet version (2 bits), padding flag (1 bit),
reception report count (5 bits), packet type (8 reception report count (5 bits), packet type (8
bits), length (2 octets), SSRC (4 octets), bits), length (2 octets), SSRC (4 octets),
encryption flag (1 bit) and SRTCP index (31 encryption flag (1 bit) and SRTCP index (31
bits). bits).
Raw Data The 32-bit optional SRTCP MKI index and 32-bit Raw Data The 32-bit optional SRTCP MKI index and 32-bit
SRTCP authentication tag (whose use is NOT SRTCP authentication tag (whose use is NOT
RECOMMENDED). RECOMMENDED).
Plaintext All other data. Plaintext All other data.
Note that the plaintext comes in one contiguous field. Since the 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 cipher is larger than the plaintext by exactly the length of the
AEAD authentication tag, the corresponding STRCP encrypted packet AEAD authentication tag, the corresponding SRTCP encrypted packet
replaces the plaintext field with a slightly larger field containing replaces the plaintext field with a slightly larger field containing
the cipher. 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.
0 1 2 3 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 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 |V=2|P| RC | Packet Type | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) of Sender | A | synchronization source (SSRC) of Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | sender info | P | sender info |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 12, line 5 skipping to change at page 13, line 41
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : authentication tag (NOT RECOMMENDED) : R : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated) P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only) A = Associated Data (to be authenticated only)
R = neither encrypted nor authenticated R = neither encrypted nor authenticated
Figure 4: AEAD SRTCP inputs when encryption flag = 1. Figure 4: AEAD SRTCP inputs when encryption flag = 1.
9.3. Data Types in Unencrypted SRTCP Compound Packets 10.3. Data Types in Unencrypted SRTCP Compound Packets
When the encryption flag is set to 0, the SRTCP compound packet is When the encryption flag is set to 0, the SRTCP compound packet is
broken into plaintext, associated data, and raw (untouched) data as broken into plaintext, associated data, and raw (untouched) data as
follows (see figure 5): follows (see figure 5):
Plaintext None. Plaintext None.
Raw Data The 32-bit optional SRTCP MKI index and 32-bit Raw Data The 32-bit optional SRTCP MKI index and 32-bit
SRTCP authentication tag (whose use is NOT SRTCP authentication tag (whose use is NOT
RECOMMENDED). RECOMMENDED).
skipping to change at page 13, line 5 skipping to change at page 14, line 41
A |0| SRTCP index | A |0| SRTCP index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R | SRTCP MKI (optional)index | R | SRTCP MKI (optional)index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : authentication tag (NOT RECOMMENDED) : R : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A = Associated Data (to be authenticated only) A = Associated Data (to be authenticated only)
R = neither encrypted nor authenticated R = neither encrypted nor authenticated
Figure 5: AEAD SRTCP inputs when encryption flag = 0. Figure 5: AEAD SRTCP inputs when encryption flag = 0
9.4. Prevention of SRTCP IV Reuse 10.4. Prevention of SRTCP IV Reuse
A new master key MUST be established before the 31-bit SRTCP index A new master key MUST be established before the 31-bit SRTCP index
cycles back to its original value. Ideally a rekey performed should cycles back to its original value. Ideally, a rekey performed should
be performed and a new master key in place and well before the SRTCP be performed and a new master key put in place well before the SRTCP
index overflows. index overflows.
The comments on SSRC management in section 8.3 also apply. The comments on SSRC management in section 9.3 also apply.
10. Constraints on AEAD for SRTP and SRTCP 11. Constraints on AEAD for SRTP and SRTCP
In general, any AEAD algorithm can accept inputs with varying In general, any AEAD algorithm can accept inputs with varying
lengths, but each algorithm can accept only a limited range of lengths, but each algorithm can accept only a limited range of
lengths for a specific parameter. In this section, we describe the lengths for a specific parameter. In this section, we describe the
constraints on the parameter lengths that any AEAD algorithm must constraints on the parameter lengths that any AEAD algorithm must
support to be used in AEAD-SRTP. Additionally we specify a complete support to be used in AEAD-SRTP. Additionally, we specify a complete
parameter set for two specific AEAD algorithms, namely AES-GCM and parameter set for two specific AEAD algorithms, namely AES-GCM and
AES-CCM. AES-CCM.
10.1. Generic AEAD Parameter Constraints 11.1. Generic AEAD Parameter Constraints
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three
constraints listed below: constraints listed below:
PARAMETER Meaning Value PARAMETER Meaning Value
A_MAX maximum additional MUST be at least 12 octets A_MAX maximum additional MUST be at least 12 octets
authenticated data authenticated data
length length
N_MIN minimum nonce (IV) MUST be no more than 12 octets N_MIN minimum nonce (IV) MUST be no more than 12 octets
length length
N_MAX maximum nonce (IV) MUST be at least 12 octets N_MAX maximum nonce (IV) MUST be at least 12 octets
length length
C_MAX maximum ciphertext MUST be at most 2^36-16 octets C_MAX maximum ciphertext GCM: MUST be at most 2^36-16 octets
length per invocation C_max values less than 2232 length per invocation CCM: MUST be at most 2^24+16 octets
are discouraged C_MAX values less than 2232
are discouraged
The upper bound on C_MAX are based on purely cryptographic The upper bound on C_MAX are based on purely cryptographic
considerations. The lower bound on C_MAX is obtained by subtracting considerations. The lower bound on C_MAX is obtained by subtracting
away a 20-octet IP header, 8-octet UDP header, and 12-octet RTP away a 20-octet IP header, 8-octet UDP header, and 12-octet RTP
header from the maximum transmission unit (MTU) of 2272. header from the maximum transmission unit (MTU) of 2272.
For sake of clarity we specify two additional parameters: For sake of clarity we specify two additional parameters:
Authentication Tag Length MUST be either 8, 12, or 16 Authentication Tag Length MUST be either 8, 12, or 16
octets octets
Maximum number of invocations MUST be at most 2^48 for SRTP Maximum number of invocations MUST be at most 2^48 for SRTP
for a given instantiation MUST be at most 2^31 for SRTCP for a given instantiation MUST be at most 2^31 for SRTCP
Block Counter size MUST be 32 bits Block Counter size MUST be 24 bits for CCM,
MUST be 32 bits for GCM
The reader is reminded that the plaintext is shorter than the The reader is reminded that the ciphertext is longer than the
ciphertext by exactly the length of the AEAD authentication tag. plaintext by exactly the length of the AEAD authentication tag.
10.2. AES-GCM for SRTP/SRTCP 11.2. AES-GCM for SRTP/SRTCP
AES-GCM is a family of AEAD algorithms built around the AES block AES-GCM is a family of AEAD algorithms built around the AES block
cipher algorithm. AES-GCM uses AES counter mode for encryption and cipher algorithm. AES-GCM uses AES counter mode for encryption and
Galois Message Authentication Code (GMAC) for authentication. A Galois Message Authentication Code (GMAC) for authentication. A
detailed description of the AES-GCM family can be found in detailed description of the AES-GCM family can be found in
[RFC5116]. The following members of the AES-GCM family may be used [RFC5116]. The following members of the AES-GCM family may be used
with SRTP/SRTCP: with SRTP/SRTCP:
Table 1: AES-GCM algorithms for SRTP/SRTCP Table 1: AES-GCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference Name Key Size Auth. Tag Size Reference
================================================================ ================================================================
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116] AEAD_AES_128_GCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_GCM 32 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_128_GCM_8 16 octets 8 octets [RFC5282]
AEAD_AES_256_GCM_8 32 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_128_GCM_12 16 octets 12 octets [RFC5282]
AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282] AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282]
Any implementation of AES-GCM SRTP SHOULD support both 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 AEAD_AES_128_GCM_8 and AEAD_AES_256_GCM_8, and it MAY support the
four other variants shown table 1. four other variants shown in table 1.
10.3. AES-CCM for SRTP/SRTCP 11.3. AES-CCM for SRTP/SRTCP
AES-CCM is another family of AEAD algorithms built around the AES AES-CCM is another family of AEAD algorithms built around the AES
block cipher algorithm. AES-CCM uses AES counter mode for encryption block cipher algorithm. AES-CCM uses AES counter mode for encryption
and AES Cipher Block Chaining Message Authentication Code (CBC MAC) and AES Cipher Block Chaining Message Authentication Code (CBC MAC)
for authentication. A detailed description of the AES-CCM family can for authentication. A detailed description of the AES-CCM family can
be found in [RFC5116]. The following members of the AES-CCM family be found in [RFC5116]. Four of the six CCM algorithms used in this
may be used with SRTP/SRTCP: 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 2: AES-CCM algorithms for SRTP/SRTCP Table 2: AES-CCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference Name Key Size Auth. Tag Size Reference
================================================================ ================================================================
AEAD_AES_128_CCM 16 octets 8, 12 or 16 octets [RFC5116] AEAD_AES_128_CCM 128 bits 16 octets [RFC5116]
AEAD_AES_256_CCM 32 octets 8, 12 or 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]
Any implementation of AES-CCM SRTP/SRTCP SHOULD support both Any implementation of AES-CCM SRTP/SRTCP SHOULD support both
AEAD_AES_128_CCM and AEAD_AES_256_CCM. AEAD_AES_128_CCM and AEAD_AES_256_CCM.
AES-CCM uses a flag octet that conveys information about the length AES-CCM uses a flag octet that conveys information about the length
of the authentication tag, length of the block counter, and presence of the authentication tag, length of the block counter, and presence
of additional authenticated data. For AES-CCM in SRTP/SRTCP, the of additional authenticated data. For AES-CCM in SRTP/SRTCP, the
flag octet has the hex value 5A if an 8-octet authentication tag is flag octet has the hex value 5A if an 8-octet authentication tag is
used, 6A if a 12-octet authentication tag is used, and 7A if a used, 6A if a 12-octet authentication tag is used, and 7A if a
16-octet authentication tag is used. The flag octet is one of the 16-octet authentication tag is used. The flag octet is one of the
inputs to AES during the counter mode encryption of the plaintext. inputs to AES during the counter mode encryption of the plaintext.
11. Key Derivation Functions 12. Key Derivation Functions
A Key Derivation Function (KDF) is used to derive all of the required A Key Derivation Function (KDF) is used to derive all of the required
encryption and authentication keys from a secret value shared by the encryption and authentication keys from a secret value shared by the
endpoints. Both the AEAD_AES_128_GCM algorithms and 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 AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key
Derivation Function described in [RFC3711]. Both the Derivation Function described in [RFC3711]. Both the
AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST
use the AES_256_CM_PRF Key Derivation Function described in [RFC use the AES_256_CM_PRF Key Derivation Function described in [RFC
6188]. 6188].
12. Security Considerations 13. Security Considerations
12.1. Handling of Security Critical Parameters 13.1. Handling of Security Critical Parameters
As with any security process, the implementer must take care to As with any security process, the implementer must take care to
ensure cryptographically sensitive parameters are properly handled. ensure cryptographically sensitive parameters are properly handled.
Many of these recommendations hold for all SRTP cryptographic Many of these recommendations hold for all SRTP cryptographic
algorithms, but we include them here to emphasize their importance. algorithms, but we include them here to emphasize their importance.
- If the master salt is to be kept secret, it MUST be properly - If the master salt is to be kept secret, it MUST be properly
erased when no longer needed. erased when no longer needed.
- The secret master key and all keys derived from it MUST be kept - The secret master key and all keys derived from it MUST be kept
secret. All keys MUST be properly erased when no longer secret. All keys MUST be properly erased when no longer
needed. needed.
- At the start of each packet, the block counter MUST be reset (to - 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 0 for CCM, to 1 for GCM). The block counter is incremented
after each block key has been produced, but it MUST NOT be after each block key has been produced, but it MUST NOT be
allowed to exceed 2^32-1. allowed to exceed 2^32 for GCM and 2^24 for CCM.
- Each time a rekey occurs, the initial values of the SRTCP index - Each time a rekey occurs, the initial values of the SRTCP index
and the values of all the SEQ counters MUST be saved. and the values of all the SEQ counters MUST be saved.
- Processing MUST cease if the 48-bit Packet Counter or the 31-bit - Processing MUST cease if the 48-bit Packet Counter or the 31-bit
SRTCP index cycles back to its initial value. Processing MUST SRTCP index cycles back to its initial value. Processing MUST
NOT resume until a new SRTP/SRTCP session has been established NOT resume until a new SRTP/SRTCP session has been established
using a new SRTP master key. Ideally, a rekey should be done using a new SRTP master key. Ideally, a rekey should be done
well before either of these counters cycle. well before either of these counters cycle.
12.2. Size of the Authentication Tag 13.2. Size of the Authentication Tag
We require that the AEAD authentication tag must be at least 8 We require that the AEAD authentication tag must be at least 8
octets, significantly reducing the probability of an adversary octets, significantly reducing the probability of an adversary
successfully introducing fraudulent data. The goal of an successfully introducing fraudulent data. The goal of an
authentication tag is to minimize the probability of a successful authentication tag is to minimize the probability of a successful
forgery occurring anywhere in the network we are attempting to forgery occurring anywhere in the network we are attempting to
defend. There are three relevant factors: how low we wish the defend. There are three relevant factors: how low we wish the
probability of successful forgery to be (prob_success), how many probability of successful forgery to be (prob_success), how many
attempts the adversary can make (N_tries) and the size of the attempts the adversary can make (N_tries) and the size of the
authentication tag in bits (N_tag_bits). Then authentication tag in bits (N_tag_bits). Then
skipping to change at page 16, line 41 skipping to change at page 18, line 32
| 8 | 2^34 tries | 2^44 tries | 2^54 tries | | 8 | 2^34 tries | 2^44 tries | 2^54 tries |
|==================+============+=============+=============| |==================+============+=============+=============|
| 12 | 2^66 tries | 2^76 tries | 2^86 tries | | 12 | 2^66 tries | 2^76 tries | 2^86 tries |
|==================+============+=============+=============| |==================+============+=============+=============|
| 16 | 2^98 tries | 2^108 tries | 2^118 tries | | 16 | 2^98 tries | 2^108 tries | 2^118 tries |
+=================+============+=============+==============+ +=================+============+=============+==============+
Table 3: Probability of a compromise occurring for a given Table 3: Probability of a compromise occurring for a given
number of forgery attempts and tag size. number of forgery attempts and tag size.
13. IANA Considerations 14. IANA Considerations
13.1. SDES 14.1. SDES
Security description [RFC 4568] defines SRTP "crypto suites"; a Security description [RFC 4568] defines SRTP "crypto suites"; a
crypto suite corresponds to a particular AEAD algorithm in SRTP. In crypto suite corresponds to a particular AEAD algorithm in SRTP. In
order to allow SDP to signal the use of the algorithms defined in order to allow SDP to signal the use of the algorithms defined in
this document, IANA will register the following crypto suites into this document, IANA will register the following crypto suites into
the subregistry for SRTP crypto suites under the SRTP transport of the subregistry for SRTP crypto suites under the SRTP transport of
the SDP Security Descriptions: the SDP Security Descriptions:
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" / srtp-crypto-suite-ext = "AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" / "AEAD_AES_256_GCM" /
"AEAD_AES_128_GCM_8" / "AEAD_AES_128_GCM_8" /
"AEAD_AES_256_GCM_8" / "AEAD_AES_256_GCM_8" /
"AEAD_AES_128_GCM_12" / "AEAD_AES_128_GCM_12" /
"AEAD_AES_256_GCM_12" / "AEAD_AES_256_GCM_12" /
"AEAD_AES_128_CCM" / "AEAD_AES_128_CCM" /
"AEAD_AES_256_CCM" / "AEAD_AES_256_CCM" /
srtp-crypto-suite-ext srtp-crypto-suite-ext
13.2. DTLS 14.2. DTLS
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile"; it DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile"; it
also corresponds to the use of an AEAD algorithm in SRTP. In order also corresponds to the use of an AEAD algorithm in SRTP. In order
to allow the use of the algorithms defined in this document in to allow the use of the algorithms defined in this document in
DTLS-SRTP, we request IANA register the following SRTP Protection DTLS-SRTP, we request IANA register the following SRTP Protection
Profiles: Profiles:
AEAD_AES_128_CCM AEAD_AES_128_CCM
cipher: AES_128_CCM cipher: AES_128_CCM
cipher_key_length: 128 bits cipher_key_length: 128 bits
skipping to change at page 18, line 12 skipping to change at page 19, line 55
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets at most 2^48 SRTP packets
AEAD_AES_256_CCM_12 AEAD_AES_256_CCM_12
cipher: AES_256_CCM cipher: AES_256_CCM
cipher_key_length: 256 bits cipher_key_length: 256 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets at most 2^48 SRTP packets
AEAD_AES_128_CCM AEAD_AES_128_GCM
cipher: AES_128_CCM cipher: AES_128_GCM
cipher_key_length: 128 bits cipher_key_length: 128 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets at most 2^48 SRTP packets
AEAD_AES_256_CCM AEAD_AES_256_GCM
cipher: AES_256_CCM cipher: AES_256_GCM
cipher_key_length: 256 bits cipher_key_length: 256 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets at most 2^48 SRTP packets
AEAD_AES_128_GCM_8 AEAD_AES_128_GCM_8
cipher: AES_128_GCM cipher: AES_128_GCM
cipher_key_length: 128 bits cipher_key_length: 128 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
skipping to change at page 19, line 8 skipping to change at page 20, line 50
cipher_key_length: 256 bits cipher_key_length: 256 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
maximum lifetime: at most 2^31 SRTCP packets and maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets at most 2^48 SRTP packets
Note that these SRTP Protection Profiles do not specify an Note that these SRTP Protection Profiles do not specify an
auth_function, auth_key_length, or auth_tag_length because all of auth_function, auth_key_length, or auth_tag_length because all of
these profiles use AEAD algorithms, and thus do not use a separate these profiles use AEAD algorithms, and thus do not use a separate
auth_function, auth_key, or auth_tag. auth_function, auth_key, or auth_tag.
13.3. MIKEY 14.3. MIKEY
In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830], In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
IANA maintains several Payload Name Spaces under Multimedia Internet IANA maintains several Payload Name Spaces under Multimedia Internet
KEYing (MIKEY). This document requires dditions to two of the lists KEYing (MIKEY). This document requires additions to two of the lists
maintained under MIKEY Security Protocol Parameters. maintained under MIKEY Security Protocol Parameters.
On the SRTP policy Type/Value list (derived from Table 6.10.1.a of On the SRTP policy Type/Value list (derived from Table 6.10.1.a of
[RFC3830]) we request the following addition: [RFC3830]) we request the following addition:
Type | Meaning | Possible values Type | Meaning | Possible values
---------------------------------------------------------------- ----------------------------------------------------------------
TBD | AEAD authentication tag length | 8, 12, or 16 (in octets) TBD | AEAD authentication tag length | 8, 12, or 16 (in octets)
On the Encryption Algorithm List (derived from Table 6.10.1.b of On the Encryption Algorithm List (derived from Table 6.10.1.b of
[RFC3830]) we request the following additions: [RFC3830]) we request the following additions:
SRTP encr alg. | Value | Default Session Encr. Key Length SRTP encr alg. | Value | Default Session Encr. Key Length
----------------------------------------------------------- -----------------------------------------------------------
AES-CCM | TBD | 16 octets AES-CCM | TBD | 16 octets
AES-GCM | TBD | 16 octets AES-GCM | TBD | 16 octets
The SRTP encryption algorithm, session encryption key length, and The SRTP encryption algorithm, session encryption key length, and
AEAD authentication tag values received from MIKEY fully determine AEAD authentication tag values received from MIKEY fully determine
the AEAD algorithm (e.g., AEAD_AES_256_GCM_8). The exact mapping is the AEAD algorithm (e.g., AEAD_AES_256_GCM_8). The exact mapping is
described in section 14. described in section 15.
14. Parameters for use with MIKEY 14.4. AEAD registry
We request that IANA make the following additions to the AEAD
registry:
AEAD_AES_128_CCM_12 = TBD
AEAD_AES_256_CCM_12 = TBD
15. Parameters for use with MIKEY
MIKEY specifies the algorithm family separately from the key length MIKEY specifies the algorithm family separately from the key length
(which is specified by the Session Encryption key length ) and the (which is specified by the Session Encryption key length ) and the
authentication tag length (specified by AEAD Auth. tag length). authentication tag length (specified by AEAD Auth. tag length).
+------------+-------------+---------------+ +------------+-------------+-------------+
| Encryption | Encryption | AEAD Auth. | | Encryption | Encryption | AEAD Auth. |
| Algorithm | Key Length | Tag Length | | Algorithm | Key Length | Tag Length |
+============+=============+===============+ +============+=============+=============+
AEAD_AES_128_GCM | AES-GCM | 16 | 16 | AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
+------------+-------------+---------------+ +------------+-------------+-------------+
AEAD_AES_128_CCM | AES-CCM | 16 | 16 | AEAD_AES_128_CCM | AES-CCM | 16 octets | 16 octets |
+------------+-------------+---------------+ +------------+-------------+-------------+
AEAD_AES_128_GCM_12 | AES-GCM | 16 | 12 | AEAD_AES_128_GCM_12 | AES-GCM | 16 octets | 12 octets |
+------------+-------------+---------------+ +------------+-------------+-------------+
AEAD_AES_128_CCM_12 | AES-CCM | 16 | 12 | AEAD_AES_128_CCM_12 | AES-CCM | 16 octets | 12 octets |
+------------+-------------+---------------+ +------------+-------------+-------------+
AEAD_AES_128_GCM_8 | AES-GCM | 16 | 8 |
+------------+-------------+---------------+ AEAD_AES_128_GCM_8 | AES-GCM | 16 octets | 8 octets |
AEAD_AES_128_CCM_8 | AES-CCM | 16 | 8 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_128_CCM_8 | AES-CCM | 16 octets | 8 octets |
AEAD_AES_256_GCM | AES-GCM | 32 | 16 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets |
AEAD_AES_256_CCM | AES-CCM | 16 | 16 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_256_CCM | AES-CCM | 32 octets | 16 octets |
AEAD_AES_256_GCM_12 | AES-GCM | 32 | 12 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_256_GCM_12 | AES-GCM | 32 octets | 12 octets |
AEAD_AES_256_CCM_12 | AES-CCM | 16 | 12 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_256_CCM_12 | AES-CCM | 32 octets | 12 octets |
AEAD_AES_256_GCM_8 | AES-GCM | 32 | 8 | +------------+-------------+-------------+
+------------+-------------+---------------+ AEAD_AES_256_GCM_8 | AES-GCM | 32 octets | 8 octets |
AEAD_AES_256_CCM_8 | AES-CCM | 16 | 8 | +------------+-------------+-------------+
+============+=============+===============+ AEAD_AES_256_CCM_8 | AES-CCM | 32 octets | 8 octets |
+============+=============+=============+
Table 4: Mapping MIKEY parameters to AEAD algorithm Table 4: Mapping MIKEY parameters to AEAD algorithm
Section 11 in this document restricts the choice of Key Derivation Section 12 in this document restricts the choice of Key Derivation
Function for AEAD algorithms. To enforce this restriction in MIKEY, Function for AEAD algorithms. To enforce this restriction in MIKEY,
we require that the SRTP PRF has value AES-CM whenever an AEAD 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 algorithm is used. Note that, according to Section 6.10.1 in
[RFC3830], the key length of the Key Derivation Function (i.e. the [RFC3830], the key length of the Key Derivation Function (i.e. the
SRTP master key length) is always equal to the session encryption key SRTP master key length) is always equal to the session encryption key
length. This means, for example, that AEAD_AES_256_GCM will use length. This means, for example, that AEAD_AES_256_GCM will use
AES_256_CM_PRF as the Key Derivation Function. AES_256_CM_PRF as the Key Derivation Function.
15. Acknowledgements 16. Acknowledgements
The authors would like to thank Michael Peck, Michael Torla, Qin Wu, The authors would like to thank Michael Peck, Michael Torla, Qin Wu,
Magnus Westerland, Oscar Ohllson and many other reviewers who Magnus Westerland, Oscar Ohllson and many other reviewers who
provided valuable comments on earlier drafts of this document. provided valuable comments on earlier drafts of this document.
16. References 17. References
16.1. Normative References 17.1. Normative References
[CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM [CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM
Mode for Authentication and Confidentiality", U.S. Mode for Authentication and Confidentiality", U.S.
National Institute of Standards and Technology http:// National Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf. csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf.
[GCM] Dworkin, M., "NIST Special Publication 800-38D: [GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation: Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", U.S. National Galois/Counter Mode (GCM) and GMAC.", U.S. National
Institute of Standards and Technology http:// Institute of Standards and Technology http://
skipping to change at page 21, line 48 skipping to change at page 23, line 48
[RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption [RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008. Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC6188] McGrew,D.,"The Use of AES-192 and AES-256 in Secure RTP" [RFC6188] McGrew,D.,"The Use of AES-192 and AES-256 in Secure RTP"
RFC 6811, March 2011 RFC 6188, March 2011
16.2. Informative References [RFC6655] McGrew,D. and D. Bailey,"AES-CCM Cipher Suites for Transport
Layer Security (TLS)", July 2012
17.2. Informative References
[BN00] Bellare, M. and C. Namprempre, "Authenticated encryption: [BN00] Bellare, M. and C. Namprempre, "Authenticated encryption:
Relations among notions and analysis of the generic Relations among notions and analysis of the generic
composition paradigm", Proceedings of ASIACRYPT 2000, composition paradigm", Proceedings of ASIACRYPT 2000,
Springer-Verlag, LNCS 1976, pp. 531-545 http:// Springer-Verlag, LNCS 1976, pp. 531-545 http://
www-cse.ucsd.edu/users/mihir/papers/oem.html. www-cse.ucsd.edu/users/mihir/papers/oem.html.
[BOYD] Boyd, C. and A. Mathuria, "Protocols for Authentication [BOYD] Boyd, C. and A. Mathuria, "Protocols for Authentication
and Key Establishment", Springer, 2003 . and Key Establishment", Springer, 2003 .
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