draft-ietf-avtcore-srtp-aes-gcm-06.txt   draft-ietf-avtcore-srtp-aes-gcm-07.txt 
Network Working Group D. McGrew Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc. Internet Draft Cisco Systems, Inc.
Intended Status: Standards Track K. Igoe Intended Status: Standards Track K. Igoe
Expires: November 21, 2013 National Security Agency Expires: January 04, 2014 National Security Agency
May 20, 2013 July 03, 2013
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-06 draft-ietf-avtcore-srtp-aes-gcm-07
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 21, 2013. This Internet-Draft will expire on January 04, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
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skipping to change at page 2, line 14 skipping to change at page 2, line 14
Abstract Abstract
This document defines how AES-GCM and AES-CCM Authenticated This document defines how AES-GCM and AES-CCM Authenticated
Encryption with Associated Data algorithms can be used to provide Encryption with Associated Data algorithms can be used to provide
confidentiality and data authentication in the SRTP protocol. confidentiality and data authentication in the 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................................4
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..........................6
5.2.1. Encrypt Mode...........................................5 5.2.1. Encrypt Mode...........................................6
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.............................7
6. Counter Mode Encryption..........................................7 6. Counter Mode Encryption..........................................7
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................8 7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................8
8. Unneeded SRTP/SRTCP Fields.......................................8 8. Unneeded SRTP/SRTCP Fields.......................................8
8.1. SRTP/SRTCP Authentication Field.............................8 8.1. SRTP/SRTCP Authentication Field.............................9
8.2. RTP Padding.................................................9 8.2. RTP Padding.................................................9
9. AES-GCM/CCM processing for SRTP..................................9 9. AES-GCM/CCM processing for SRTP..................................9
9.1. SRTP IV formation for AES-GCM and AES-CCM...................9 9.1. SRTP IV formation for AES-GCM and AES-CCM...................9
9.2. Data Types in SRTP Packets..................................9 9.2. Data Types in SRTP Packets.................................10
9.3. Handling Header Extensions.................................11 9.3. Handling Header Extensions.................................11
9.4. Prevention of SRTP IV Reuse................................11 9.4. Prevention of SRTP IV Reuse................................12
10. AES-GCM/CCM Processing of SRTCP Compound Packets...............12 10. AES-GCM/CCM Processing of SRTCP Compound Packets...............12
10.1. SRTCP IV formation for AES-GCM and AES-CCM................12 10.1. SRTCP IV formation for AES-GCM and AES-CCM................12
10.2. Data Types in Encrypted SRTCP Compound Packets............12 10.2. Data Types in Encrypted SRTCP Compound Packets............13
10.3. Data Types in Unencrypted SRTCP Compound Packets..........13 10.3. Data Types in Unencrypted SRTCP Compound Packets..........14
10.4. Prevention of SRTCP IV Reuse..............................14 10.4. Prevention of SRTCP IV Reuse..............................15
11. Constraints on AEAD for SRTP and SRTCP.........................15 11. Constraints on AEAD for SRTP and SRTCP.........................15
11.1. Generic AEAD Parameter Constraints........................15 12. Key Derivation Functions.......................................16
11.2. AES-GCM for SRTP/SRTCP....................................16 13. Summary of Algorithm Characteristics...........................16
11.3. AES-CCM for SRTP/SRTCP....................................16 13.1. AES-GCM for SRTP/SRTCP....................................17
12. Key Derivation Functions.......................................17 13.2. AES-CCM for SRTP/SRTCP....................................19
13. Security Considerations........................................17 14. Security Considerations........................................22
13.1. Handling of Security Critical Parameters..................17 14.1. Handling of Security Critical Parameters..................22
13.2. Size of the Authentication Tag............................17 14.2. Size of the Authentication Tag............................22
14. IANA Considerations............................................18 15. IANA Considerations............................................23
14.1. SDES......................................................18 15.1. SDES......................................................23
14.2. DTLS......................................................19 15.2. DTLS......................................................24
14.3. MIKEY.....................................................22 15.3. MIKEY.....................................................27
14.4. AEAD registry.............................................22 15.4. AEAD registry.............................................28
15. Parameters for use with MIKEY..................................22 16. Parameters for use with MIKEY..................................28
16. Acknowledgements...............................................23 17. Acknowledgements...............................................29
17. References.....................................................24 18. References.....................................................30
17.1. Normative References......................................24 18.1. Normative References......................................30
17.2. Informative References....................................26 18.2. Informative References....................................32
1. Introduction 1. Introduction
The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile
of the Real-time Transport Protocol (RTP) [RFC3550], which can of the Real-time Transport Protocol (RTP) [RFC3550], which can
provide confidentiality, message authentication, and replay provide confidentiality, message authentication, and replay
protection to the RTP traffic and to the control traffic for RTP, the protection to the RTP traffic and to the control traffic for RTP, the
Real-time Transport Control Protocol (RTCP). It is important to note Real-time Transport Control Protocol (RTCP). It is important to note
that the outgoing SRTP packets from a single endpoint may be that the outgoing SRTP packets from a single endpoint may be
originating from several independent data sources. originating from several independent data sources.
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Authenticated Encryption with Associated Data, or AEAD [R02], adds Authenticated Encryption with Associated Data, or AEAD [R02], adds
the ability to check the integrity and authenticity of some the ability to check the integrity and authenticity of some
Associated Data (AD), also called "additional authenticated data", Associated Data (AD), also called "additional authenticated data",
that is not encrypted. This specification makes use of the interface that is not encrypted. This specification makes use of the interface
to a generic AEAD algorithm as defined in [RFC5116]. to a generic AEAD algorithm as defined in [RFC5116].
The Advanced Encryption Standard (AES) is a block cipher that The Advanced Encryption Standard (AES) is a block cipher that
provides a high level of security, and can accept different key provides a high level of security, and can accept different key
sizes. Two families of AEAD algorithm families, AES Galois/Counter sizes. Two families of AEAD algorithm families, AES Galois/Counter
Mode (AES-GCM) [GCM] and AES Counter with Cipher Block Mode (AES-GCM) [GCM] and AES Counter with Cipher Block
Chaining-Message Authentication Code (AES-CCM) [RFC3610], are based Chaining-Message Authentication Code (AES-CCM) [RFC3610] are based
upon AES. This specification makes use of the AES versions that use 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, 128-bit and 256-bit keys, which we call AES-128 and AES-256,
respectively. 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 The Galois/Counter Mode of operation (GCM) and the Counter with
Cipher Block Chaining-Message Authentication Code mode of operation Cipher Block Chaining-Message Authentication Code mode of operation
(CCM) are both AEAD modes of operation for block ciphers. Both use (CCM) are both AEAD modes of operation for block ciphers. Both use
counter mode to encrypt the data, an operation that can be counter mode to encrypt the data, an operation that can be
efficiently pipelined. Further, GCM authentication uses operations efficiently pipelined. Further, GCM authentication uses operations
that are particularly well suited to efficient implementation in that are particularly well suited to efficient implementation in
hardware, making it especially appealing for high-speed hardware, making it especially appealing for high-speed
implementations, or for implementations in an efficient and compact implementations, or for implementations in an efficient and compact
circuit. CCM is well suited for use in compact software circuit. CCM is well suited for use in compact software
implementations. This specification uses GCM and CCM with both implementations. This specification uses GCM and CCM with both
AES-128 and AES-256. AES-128 and AES-256.
In summary, this document defines how to use AEAD algorithms, In summary, this document defines how to use AEAD algorithms,
particularly AES-GCM and AES-CCM, to provide confidentiality and particularly AES-GCM and AES-CCM, to provide confidentiality and
message authentication within SRTP and SRTCP packets. message authentication within SRTP and SRTCP packets.
2. Conventions Used In This Document 2. Conventions Used In This Document
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", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this 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 or 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.
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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(Plaintext)+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 As defined in [RFC3610], AES-CCM authentication uses a Tag_Size_Flag
authentication tag is used, 6A if a 12-octet authentication tag is to specify the length of the intrinsic authentication tag provided by
used, and 7A if a 16-octet authentication tag is used. This flag AES-CCM authentication. For the three tag lengths allowed in this
refers to the size of the intrinsic authentication tag provided by document the corresponding Tag_Size_Flag values are as follows:
the AEAD algorithm.
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 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 Octet string of variable length Associated_Data Octet string of variable length
Ciphertext Octet string of variable length Ciphertext Octet string of variable length
Tag_Size_Flag (CCM only*) One octet Tag_Size_Flag (CCM only*) One octet
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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 Octet string of variable length Associated_Data Octet string of variable length
Ciphertext Octet 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.
The Tag_Size_Flag, used in AES-CCM authentication, has the hex value As mentioned in section 5.2.1, only three tag lengths are supported
5A if an 8-octet authentication tag is used, 6A if a 12-octet for use in SRTP/SRTCP, namely 8 octetes, 12 octets and 16 octets.
authentication tag is used, and 7A if a 16-octet authentication tag
is used. This flag refers to the size of the intrinsic
authentication tag provided by the AEAD algorithm.
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. Plaintext and associated data before any other action takes place. Plaintext and associated data
MUST NOT be released until the AEAD authentication tag has been MUST NOT be released until the AEAD authentication tag has been
validated. Further, when GCM is being used, the ciphertext MUST NOT validated. Further, when GCM is being used, the ciphertext MUST NOT
be decrypted until the AEAD tag has been validated. be decrypted until the AEAD tag has been validated.
Should the AEAD tag prove to be invalid, the packet in question is to Should the AEAD tag prove to be invalid, the packet in question is to
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In both GCM and CCM, each outbound packet uses a 12-octet IV and an 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 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 is used in forming the authentication tag and a keystream of octets
which is XORed to the plaintext to form cipher. which is XORed to the plaintext to form cipher.
When GCM is used, the concatenation of a 12-octet IV (see sections 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. 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: This is used to build a key_stream as follows:
def GCM_keystream( Plaintext, IV, Encryption_key ): def GCM_keystream( Plaintext, IV, Encryption_key ):
assert len(plaintext) <= (2**36) - 32 assert len(plaintext) <= (2**36) - 32 ## measured in octets
key_stream = "" key_stream = ""
block_counter = 1 block_counter = 1
first_key_block = AES_ENC( data=IV||block_counter, first_key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key ) key=Encryption_key )
while len(key_stream) < len(Plaintext): while len(key_stream) < len(Plaintext):
block_counter = block_counter + 1 block_counter = block_counter + 1
key_block = AES_ENC( data=IV||block_counter, key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key ) key=Encryption_key )
key_stream = key_stream || key_block key_stream = key_stream || key_block
key_stream = truncate( key_stream, len(Plaintext) ) key_stream = truncate( key_stream, len(Plaintext) )
return (first_key_block, key_stream ) return (first_key_block, key_stream )
In AES-CCM counter mode encryption, the AES data input consists of 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 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 block counter. Note that in this application the flag octet will
always have the value 0x02 (see section 2.3 of [RFC3610]). A always have the value 0x02 (see section 2.3 of [RFC3610]). A
(first_key_block, keystream) pair is formed as follows: (first_key_block, key_stream) pair is formed as follows:
def CCM_keystream( Plaintext, IV, Encryption_key ): def CCM_keystream( Plaintext, IV, Encryption_key ):
assert len(Plaintext) <= (2**28)-16 assert len(Plaintext) <= (2**32)-16 ## measured in octets
key_stream = "" key_stream = ""
block_counter = 0 block_counter = 0
first_key_block = AES_ENC( data=0x02||IV||block_counter, first_key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key ) key=Encryption_key )
while len(key_stream)<len(Plaintext): while len(key_stream)<len(Plaintext):
block_counter = block_counter + 1 block_counter = block_counter + 1
key_block = AES_ENC( data=0x02||IV||block_counter, key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key ) key=Encryption_key )
key_stream = key_stream || key_block key_stream = key_stream || key_block
key_stream = truncate( key_stream, len(Plaintext) ) key_stream = truncate( key_stream, len(Plaintext) )
return (first_key_block, key_stream ) return (first_key_block, key_stream )
These keystream generation processes allows for a keystream of length These keystream generation processes allow for a keystream of length
of up to (2^24)-16 octets for AES-CCM and up to (2^36)-32 octets for up to (2^32)-16 octets for AES-CCM and up to (2^36)-32 octets for
AES-GCM. 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.
AES-GCM, the consequences of such a reuse are even worse than
explained in RFC 3711 because it would completely compromise the
AES-GCM authentication mechanism.
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12 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 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 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 AEAD_AES_256_CCM_8 are defined in [RFC6655] with an authentication
tag length of 8-octets. We require two new variants, tag length of 8-octets. We require two new variants,
AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12, with 12-octet AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12, with 12-octet
authentication tags. In each case the authentication tag is formed authentication tags. In each case the authentication tag is formed
by taking the 12 most significant octets (in network order) of the by taking the 12 most significant octets (in network order) of the
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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.
8.2. RTP Padding 8.2. RTP Padding
Neither AES-GCM not AES-CCM requires that the data be padded out to a Neither AES-GCM nor AES-CCM requires that the data be padded out to a
specific block size, reducing the need to use the padding mechanism specific block size, reducing the need to use the padding mechanism
provided by RTP. It is RECOMMENDED that the RTP 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 not be used unless it is necessary to disguise the length of the
underlying plaintext. underlying plaintext.
9. AES-GCM/CCM processing for SRTP 9. AES-GCM/CCM processing for SRTP
9.1. SRTP IV formation for 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 The 12 octet initialization vector used by both AES-GCM and AES-CCM
skipping to change at page 15, line 4 skipping to change at page 15, line 44
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
10.4. 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 put in place 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 9.3 also apply. The comments on SSRC management in section 9.4 also apply.
11. 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.
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 12 octets. N_MIN minimum nonce (IV) MUST be 12 octets.
length length
N_MAX maximum nonce (IV) MUST be 12 octets. N_MAX maximum nonce (IV) MUST be 12 octets.
length length
C_MAX maximum ciphertext GCM: MUST be at most 2^36-16 C_MAX maximum ciphertext GCM: MUST be <= 2^36-16 octets.
octets. length per invocation CCM: MUST be <= 2^24-16 octets.
length per invocation CCM: MUST be at most 2^24+16
octets.
The values for C_MAX are based on purely cryptographic The values for C_MAX are based on purely cryptographic
considerations. considerations.
For sake of clarity we specify two additional parameters: For sake of clarity we specify two additional parameters:
AEAD Authentication Tag Length MUST be either 8, 12, or 16 AEAD 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 24 bits for CCM, Block Counter size MUST be 24 bits for CCM,
MUST be 32 bits for GCM MUST be 32 bits for GCM
The reader is reminded that the ciphertext is longer than the The reader is reminded that the ciphertext is longer than the
plaintext by exactly the length of the AEAD authentication tag. plaintext by exactly the length of the AEAD authentication tag.
11.2. AES-GCM for SRTP/SRTCP 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
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 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 AEAD Tag Size Reference Name Key Size AEAD 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 in table 1. four other variants shown in table 1. Below we summarize parameters
associated with these six GCM algorithms:
11.3. AES-CCM for SRTP/SRTCP +--------------------------------+------------------------------+
| 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
+--------------------------------+------------------------------+
| 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
+--------------------------------+------------------------------+
| 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 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]. Four of the six CCM algorithms used in this 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 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. and AEAD_AES_256_CCM_12, are defined in section 7 of this document.
Table 2: AES-CCM algorithms for SRTP/SRTCP Table 8: AES-CCM algorithms for SRTP/SRTCP
Name Key Size AEAD Tag Size Reference Name Key Size AEAD Tag Size Reference
================================================================ ================================================================
AEAD_AES_128_CCM 128 bits 16 octets [RFC5116] AEAD_AES_128_CCM 128 bits 16 octets [RFC5116]
AEAD_AES_256_CCM 256 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_128_CCM_12 128 bits 12 octets see section 7
AEAD_AES_256_CCM_12 256 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_128_CCM_8 128 bits 8 octets [RFC6655]
AEAD_AES_256_CCM_8 256 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_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, In addition to the flag octet used in counter mode encryption,
AES-CCM authentications also uses a flag octet that conveys AES-CCM authentications also uses a flag octet that conveys
information about the length of the authentication tag, length of the information about the length of the authentication tag, length of the
block counter, and presence of additional authenticated data (see block counter, and presence of additional authenticated data (see
section 2.2 of [RFC3610]). For AES-CCM in SRTP/SRTCP, the flag octet 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, 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 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 16-octet AEAD authentication tag is used. The flag octet is one of
the inputs to AES during the counter mode encryption of the the inputs to AES during the counter mode encryption of the
plaintext. plaintext.
12. Key Derivation Functions +--------------------------------+------------------------------+
| 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 |
+--------------------------------+------------------------------+
A Key Derivation Function (KDF) is used to derive all of the required Table 9: The AEAD_AES_128_CCM_8 Crypto Suite
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 [RFC
6188].
13. Security Considerations +--------------------------------+------------------------------+
| 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 |
+--------------------------------+------------------------------+
13.1. Handling of Security Critical Parameters Table 10: The AEAD_AES_128_CCM_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_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
+--------------------------------+------------------------------+
| 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 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
skipping to change at page 17, line 49 skipping to change at page 22, line 45
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 for GCM and 2^24 for CCM. 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.
13.2. Size of the Authentication Tag 14.2. 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 18, line 41 skipping to change at page 23, line 39
|==================+=============+=============+============| |==================+=============+=============+============|
| 4 | 2^2 tries | 2^12 tries | 2^22 tries | | 4 | 2^2 tries | 2^12 tries | 2^22 tries |
|==================+============+=============+=============| |==================+============+=============+=============|
| 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 15: Probability of a compromise occurring for a given
number of forgery attempts and tag size. number of forgery attempts and tag size.
14. IANA Considerations 15. IANA Considerations
14.1. SDES 15.1. SDES
Security description [RFC4568] defines SRTP "crypto suites"; a crypto Session description [RFC4568] defines SRTP "crypto suites". A crypto
suite corresponds to a particular AEAD algorithm in SRTP. In order suite corresponds to a particular AEAD algorithm in SRTP. In order
to allow SDP to signal the use of the algorithms defined in this to allow SDP to signal the use of the algorithms defined in this
document, IANA will register the following crypto suites into the 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 Session Description Protocol
SDP Security Descriptions: (SDP) Parameters:
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" /
"AEAD_AES_128_CCM_8" /
"AEAD_AES_256_CCM_8" /
"AEAD_AES_128_CCM_12" /
"AEAD_AES_256_CCM_12" /
srtp-crypto-suite-ext srtp-crypto-suite-ext
14.2. DTLS 15.2. DTLS
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile"; it DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile".
also corresponds to the use of an AEAD algorithm in SRTP. In order These also correspond to the use of an AEAD algorithm in SRTP. In
to allow the use of the algorithms defined in this document in order 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_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 AEAD_AES_128_CCM
cipher: AES_128_CCM cipher: AES_128_CCM
cipher_key_length: 128 bits cipher_key_length: 128 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets aead_auth_tag_length: 16 octets
auth_function: NULL auth_function: NULL
auth_key_length: N/A auth_key_length: N/A
auth_tag_length: N/A auth_tag_length: N/A
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
skipping to change at page 22, line 10 skipping to change at page 27, line 30
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. The term aead_auth_tag_length auth_function, auth_key, or auth_tag. The term aead_auth_tag_length
is used to emphasize that this refers to the authentication tag is used to emphasize that this refers to the authentication tag
provided by the AEAD algorithm and that this tag is not located in provided by the AEAD algorithm and that this tag is not located in
the authentication tag field provided by SRTP/SRTCP. the authentication tag field provided by SRTP/SRTCP.
14.3. MIKEY 15.3. 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 additions 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
skipping to change at page 22, line 35 skipping to change at page 28, line 6
[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 15. described in section 16.
14.4. AEAD registry 15.4. AEAD registry
We request that IANA make the following additions to the AEAD We request that IANA make the following additions to the AEAD
registry: registry:
AEAD_AES_128_CCM_12 = TBD AEAD_AES_128_CCM_12 = TBD
AEAD_AES_256_CCM_12 = TBD AEAD_AES_256_CCM_12 = TBD
15. Parameters for use with MIKEY 16. 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 octets | 16 octets | AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
skipping to change at page 23, line 34 skipping to change at page 28, line 51
+------------+-------------+-------------+ +------------+-------------+-------------+
AEAD_AES_256_GCM_12 | AES-GCM | 32 octets | 12 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_CCM_12 | AES-CCM | 32 octets | 12 octets |
+------------+-------------+-------------+ +------------+-------------+-------------+
AEAD_AES_256_GCM_8 | AES-GCM | 32 octets | 8 octets | AEAD_AES_256_GCM_8 | AES-GCM | 32 octets | 8 octets |
+------------+-------------+-------------+ +------------+-------------+-------------+
AEAD_AES_256_CCM_8 | AES-CCM | 32 octets | 8 octets | AEAD_AES_256_CCM_8 | AES-CCM | 32 octets | 8 octets |
+============+=============+=============+ +============+=============+=============+
Table 4: Mapping MIKEY parameters to AEAD algorithm Table 16: Mapping MIKEY parameters to AEAD algorithm
Section 12 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.
16. Acknowledgements 17. 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.
17. References 18. References
17.1. Normative References 18.1. Normative References
[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://
csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf. csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
skipping to change at page 24, line 47 skipping to change at page 30, line 47
[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, Exchange version 2 (IKEv2) Protocol", RFC 5282,
August 2008. 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.
[RFC6655] McGrwe, D. and D. Bailey, "AES-CCM Cipher Suites for [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. Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure [RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013. Real-Time Transport Protocol (SRTP)", January 2013.
, January 2013. , January 2013.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure [RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", January 2013. Real-Time Transport Protocol (SRTP)", January 2013.
17.2. Informative References 18.2. 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.
[R02] Rogaway, P., "Authenticated encryption with Associated- [R02] Rogaway, P., "Authenticated encryption with Associated-
Data", ACM Conference on Computer and Communication Data", ACM Conference on Computer and Communication
Security (CCS'02), pp. 98-107, ACM Press, Security (CCS'02), pp. 98-107, ACM Press,
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