draft-ietf-avtcore-srtp-aes-gcm-14.txt   draft-ietf-avtcore-srtp-aes-gcm-15.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: January 29, 2015 National Security Agency Expires: October 16, 2015 National Security Agency
July 28, 2014 April 14, 2015
AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP) AES-GCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-14 draft-ietf-avtcore-srtp-aes-gcm-15
Status of this Memo Status of this Memo
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Abstract Abstract
This document defines how AES-GCM and AES-CCM Authenticated This document defines how the AES-GCM Authenticated Encryption with
Encryption with Associated Data algorithms can be used to provide Associated Data family of algorithms can be used to provide
confidentiality and data authentication in the SRTP protocol. confidentiality and data authentication in the SRTP protocol. Note:
this is an intermediate draft, awaiting the inclusion of test
vectors. Care is being taken to ensure these test vectors will be
correct, always a desirable property.
Table of Contents Table of Contents
1. Introduction.....................................................3 1. Introduction.....................................................3
2. Conventions Used In This Document................................4 2. Conventions Used In This Document................................4
3. Overview of the SRTP/SRTCP AEAD security Architecture............4 3. Overview of the SRTP/SRTCP AEAD security Architecture............4
4. Terminology......................................................5 4. Terminology......................................................5
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..........................6 5.2. AEAD Invocation Inputs and Outputs..........................5
5.2.1. Encrypt Mode...........................................6 5.2.1. Encrypt Mode...........................................5
5.2.2. Decrypt Mode...........................................6 5.2.2. Decrypt Mode...........................................6
5.3. Handling of AEAD Authentication.............................7 5.3. Handling of AEAD Authentication.............................6
6. Counter Mode Encryption..........................................7 6. Counter Mode Encryption..........................................6
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12......................8 7. Unneeded SRTP/SRTCP Fields.......................................7
8. Unneeded SRTP/SRTCP Fields.......................................9 7.1. SRTP/SRTCP Authentication Field.............................7
8.1. SRTP/SRTCP Authentication Field.............................9 7.2. RTP Padding.................................................8
8.2. RTP Padding.................................................9 8. AES-GCM processing for SRTP......................................8
9. AES-GCM/CCM processing for SRTP.................................10 8.1. SRTP IV formation for AES-GCM...............................8
9.1. SRTP IV formation for AES-GCM and AES-CCM..................10 8.2. Data Types in SRTP Packets..................................8
9.2. Data Types in SRTP Packets.................................10 8.3. Handling Header Extensions.................................10
9.3. Handling Header Extensions.................................12 8.4. Prevention of SRTP IV Reuse................................11
9.4. Prevention of SRTP IV Reuse................................13 9. AES-GCM Processing of SRTCP Compound Packets....................12
10. AES-GCM/CCM Processing of SRTCP Compound Packets...............14 9.1. SRTCP IV formation for AES-GCM.............................12
10.1. SRTCP IV formation for AES-GCM and AES-CCM................14 9.2. Data Types in Encrypted SRTCP Compound Packets.............13
10.2. Data Types in Encrypted SRTCP Compound Packets............15 9.3. Data Types in Unencrypted SRTCP Compound Packets...........14
10.3. Data Types in Unencrypted SRTCP Compound Packets..........16 9.4. Prevention of SRTCP IV Reuse...............................15
10.4. Prevention of SRTCP IV Reuse..............................17 10. Constraints on AEAD for SRTP and SRTCP.........................15
11. Constraints on AEAD for SRTP and SRTCP.........................17 11. Key Derivation Functions.......................................16
12. Key Derivation Functions.......................................18 12. Summary of AES-GCM in SRTP/SRTCP...............................16
13. Summary of Algorithm Characteristics...........................18 13. Security Considerations........................................17
13.1. AES-GCM for SRTP/SRTCP....................................18 13.1. Handling of Security Critical Parameters..................18
13.2. AES-CCM for SRTP/SRTCP....................................20 13.2. Size of the Authentication Tag............................18
14. Security Considerations........................................23 14. IANA Considerations............................................19
14.1. Handling of Security Critical Parameters..................23 14.1. SDES......................................................19
14.2. Size of the Authentication Tag............................24 14.2. DTLS-SRTP.................................................20
15. IANA Considerations............................................25 14.3. MIKEY.....................................................21
15.1. SDES......................................................25 15. Parameters for use with MIKEY..................................21
15.2. DTLS-SRTP.................................................26 16. Acknowledgements...............................................22
15.3. MIKEY.....................................................29 17. References.....................................................23
15.4. AEAD registry.............................................29 17.1. Normative References......................................23
16. Parameters for use with MIKEY..................................29 17.2. Informative References....................................24
17. Acknowledgements...............................................30
18. References.....................................................31
18.1. Normative References......................................31
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.
skipping to change at page 3, line 26 skipping to change at page 3, line 26
addition to providing confidentiality for the plaintext that is addition to providing confidentiality for the plaintext that is
encrypted, provides a way to check its integrity and authenticity. encrypted, provides a way to check its integrity and authenticity.
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. AES Galois/Counter Mode (AES-GCM) [GCM] is a family of AEAD
Mode (AES-GCM) [GCM] and AES Counter with Cipher Block algorithms based upon AES. This specification makes use of the AES
Chaining-Message Authentication Code (AES-CCM) [RFC3610] are based versions that use 128-bit and 256-bit keys, which we call AES-128 and
upon AES. This specification makes use of the AES versions that use AES-256, respectively.
128-bit and 256-bit keys, which we call AES-128 and AES-256,
respectively.
Any AEAD algorithm provides an intrinsic authentication tag. In many Any AEAD algorithm provides an intrinsic authentication tag. In many
applications the authentication tag is truncated to less than full applications the authentication tag is truncated to less than full
length. When CCM is being used there are three allowed values for length. In this specification the authentication tag MUST be either
the length of the authentication tag. A CCM authentication tag MUST 8 octets or 16 octets in length, and the 8 byte authentication tag
be either 8 octets, 12 octets or 16 octets in length. But when GCM can only be used with AES-128. Thus when used in SRTP, GCM will have
is being used only two values are permitted. A GCM authentication three configurations:
tag MUST be either 12 octets or 16 octets in length. Thus CCM 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 CCM authentication tag (either 8, 12 or 16 octets), and
GCM will have four configurations reflecting two choices for the key
size and two choices for the length of the GCM authentication tag
(either 12 or 16 octets). The key size and the length of the
authentication tag are set when the session is initiated and SHOULD
NOT be altered.
The Galois/Counter Mode of operation (GCM) and the Counter with AEAD_AES_128_GCM_8 AES-128 with an 8 byte authentication tag
Cipher Block Chaining-Message Authentication Code mode of operation AEAD_AES_128_GCM AES-128 with a 16 byte authentication tag
(CCM) are both AEAD modes of operation for block ciphers. Both use AEAD_AES_256_GCM AES-256 with a 16 byte authentication tag
counter mode to encrypt the data, an operation that can be
efficiently pipelined. Further, GCM authentication uses operations
that are particularly well suited to efficient implementation in
hardware, making it especially appealing for high-speed
implementations, or for implementations in an efficient and compact
circuit. CCM is well suited for use in compact software
implementations. This specification uses GCM and CCM with both
AES-128 and AES-256.
In summary, this document defines how to use AEAD algorithms, The key size and the length of the authentication tag are set when
particularly AES-GCM and AES-CCM, to provide confidentiality and the session is initiated and SHOULD NOT be altered.
message authentication within SRTP and SRTCP packets.
The Galois/Counter Mode of operation (GCM) ia an AEAD mode of
operation for block ciphers. GCM use counter mode to encrypt the
data, an operation that can be efficiently pipelined. Further, GCM
authentication uses operations that are particularly well suited to
efficient implementation in hardware, making it especially appealing
for high-speed implementations, or for implementations in an
efficient and compact circuit.
In summary, this document defines how to use an AEAD algorithm,
particularly AES-GCM, to provide confidentiality and message
authentication within SRTP and SRTCP packets.
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", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
3. Overview of the SRTP/SRTCP AEAD security Architecture 3. Overview of the SRTP/SRTCP AEAD security Architecture
SRTP/SRTCP AEAD security is based upon the following principles: SRTP/SRTCP AEAD 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-GCM
AES-GCM combines privacy and authentication into a single 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).
c) A Key Derivation Function is applied to the shared master key c) A Key Derivation Function is applied to the shared master key
value to form separate encryption keys, authentication keys value to form separate encryption keys, authentication keys
and salting keys for SRTP and for SRTCP (a total of six and salting keys for SRTP and for SRTCP (a total of six
keys). This process is described in section 4.3 of keys). This process is described in section 4.3 of
[RFC3711]. Since AEAD algorithms such as AES-CCM and AES-GCM [RFC3711]. The master key MUST be at least as large as the
combine encryption and authentication into a single process, encryption key derived from it. Since AEAD algorithms such
AEAD algorithms do not make use of the authentication keys. as AES-GCM combine encryption and authentication into a
The master key MUST be at least as large as the encryption single process, AEAD algorithms do not make use of separate
key derived from it. authentication keys.
d) Aside from making modifications to IANA registries to allow d) Aside from making modifications to IANA registries to allow
AES-GCM and AES-CCM to work with SDES, DTLS-SRTP and MIKEY, AES-GCM to work with SDES, DTLS-SRTP and MIKEY, the details
the details of how the master key is established and shared of how the master key is established and shared between the
between the participants are outside the scope of this participants are outside the scope of this document.
document. Similarly any mechanism for rekeying an existing Similarly any mechanism for rekeying an existing session is
session is outside the scope of the document. outside the scope of the document.
e) Each time an instantiation of AES-GCM or AES-CCM is invoked e) Each time an instantiation of AES-GCM is invoked to encrypt
to encrypt and authenticate an SRTP or SRTCP data packet a and authenticate an SRTP or SRTCP data packet a new IV is
new IV is used. SRTP combines the 4-octet synchronization used. SRTP combines the 4-octet synchronization source
source (SSRC) identifier, the 4-octet rollover counter (ROC), (SSRC) identifier, the 4-octet rollover counter (ROC), and
and the 2-octet sequence number (SEQ) with the 12-octet the 2-octet sequence number (SEQ) with the 12-octet
encryption salt to form a 12-octet IV (see section 9.1). encryption salt to form a 12-octet IV (see section 8.1).
SRTCP combines the SSRC and 31-bit SRTCP index with the SRTCP combines the SSRC and 31-bit SRTCP index with the
encryption salt to form a 12-octet IV (see section 10.1). encryption salt to form a 12-octet IV (see section 9.1).
4. Terminology 4. Terminology
The following terms have very specific meanings in the context of The following terms have very specific meanings in the context of
this RFC: this RFC:
Instantiation: In AEAD, an instantiation is an (Encryption_key, Instantiation: In AEAD, an instantiation is an (Encryption_key,
salt) pair together with all of the data salt) pair together with all of the data
structures (for example, counters) needed for it structures (for example, counters) needed for it
to function properly. In SRTP/SRTCP, each to function properly. In SRTP/SRTCP, each
skipping to change at page 5, line 50 skipping to change at page 5, line 43
but not encrypted. but 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 9.2, 10.2 and to be treated as raw data are covered in sections 8.2, 9.2 and 9.3.
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 Octet string of variable length Associated_Data Octet string of variable length
Plaintext Octet string of variable length Plaintext Octet string of variable length
Tag_Size_Flag (CCM only*) One Octet
Outputs Outputs
Ciphertext Octet string, length = Ciphertext* Octet string, length =
length(Plaintext)+tag_length length(Plaintext)+tag_length
(*) CCM mode requires tag length to be explicitly input to (*): In AEAD the authentication tag in embedded in the cipher text.
the algorithm, whereas with GCM, the tag is simply truncated. When GCM is being used the ciphertext consists of the encrypted plain
For GCM, the algorithm choice determines the tag size. text followed by the authentication tag.
In both CCM and GCM, the algorithm negotiation selects what tag size
is to be used. In GCM, the authentication tag is simply truncated to
the appropriate length, but CCM requires that the tag length be an
explicitly input to the algorithm as the Tag_Size_Field. For the
three tag lengths allowed for CCM in this document the corresponding
Tag_Size_Flag values are as follows:
Tag Length | Tag_Size_Flag (hex)
-----------------------------------
8 octets | 5A
12 octets | 6A
16 octets | 7A
Once an SRTP/SRTCP session has been initiated the length of the tag
is a fixed value and MUST NOT 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
Outputs Outputs
Plaintext Octet string, length = Plaintext Octet 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.
As mentioned in section 5.2.1, in SRTP/SRTCP CCM supports three tag
lengths (8 octets, 12 octets and 16 octets) while GCM only supports
two tag sizes (12 octets and 16 octets).
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 the ciphertext MUST NOT be decrypted until the validated. Further the ciphertext MUST NOT be decrypted until the
AEAD tag has been validated. 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
be discarded and a Validation Error flag raised. Local policy be discarded and a Validation Error flag raised. Local policy
determines how this flag is to be handled and is outside the scope of determines how this flag is to be handled and is outside the scope of
this document. this document.
6. Counter Mode Encryption 6. Counter Mode Encryption
In both GCM and CCM, each outbound packet uses a 12-octet IV and an Each outbound packet uses a 12-octet IV and an encryption key to form
encryption key to form two outputs, a 16-octet first_key_block which two outputs, a 16-octet first_key_block which is used in forming the
is used in forming the authentication tag and a keystream of octets authentication tag and a key stream of octets, formed in blocks of
which is XORed to the plaintext to form cipher. 16-octets each. The first 16-octet block of key is saved for use in
forming the authentication tag, and the of remainder of the key
When GCM is used, the concatenation of a 12-octet IV (see sections stream is XORed to the plaintext to form cipher. This key stream is
9.1 and 10.1) with a 4-octet block counter forms the input to AES. formed one block at a time by inputting the concatenation of a
This is used to build a key_stream as follows: 12-octet IV (see sections 8.1 and 9.1) with a 4-octet block to AES.
The pseudo-code below illustrates this process:
def GCM_keystream( Plaintext_len, IV, Encryption_key ): def GCM_keystream( Plaintext_len, IV, Encryption_key ):
assert Plaintext_len <= (2**36) - 32 ## measured in octets assert Plaintext_len <= (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) < Plaintext_len: while len(key_stream) < Plaintext_len:
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, Plaintext_len ) key_stream = truncate( key_stream, Plaintext_len )
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 theory this keystream generation process allows for the encryption
the concatenation of a 1-octet flag, a 12-octet IV, and a 3-octet of up to (2^36)-32 octets per invocation (i.e. per packet), far
block counter. Note that in this application the flag octet will
always have the value 0x02 (see section 2.3 of [RFC3610]). A
(first_key_block, key_stream) pair is formed as follows:
def CCM_keystream( Plaintext_len, IV, Encryption_key ):
assert Plaintext_len <= (2**24)-1 ## measured in octets
key_stream = ""
block_counter = 0
first_key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key )
while len(key_stream)<Plaintext_len:
block_counter = block_counter + 1
key_block = AES_ENC( data=0x02||IV||block_counter,
key=Encryption_key )
key_stream = key_stream || key_block
key_stream = truncate( key_stream, Plaintext_len )
return (first_key_block, key_stream )
In theory these keystream generation processes allow for each packet
to use s keystream of length up to (2^24)-1 octets per invocation for
AES-CCM and up to (2^36)-32 octets per invocation for AES-GCM, far
longer than is actually required. longer than is actually required.
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 GCM section 9.1 of RFC 3711, this is a cryptographic disaster. For GCM
the consequences are even worse since such a reuse compromises GCM's the consequences are even worse since such a reuse compromises GCM's
integrity mechanism not only for the current packet stream but for integrity mechanism not only for the current packet stream but for
all future uses of the current encryption_key. all future uses of the current encryption_key.
7. AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12 7. Unneeded SRTP/SRTCP Fields
AEAD_AES_128_CCM and AEAD_AES_256_CCM are defined in [RFC5116] with
an authentication tag length of 16-octets. AEAD_AES_128_CCM_8 and
AEAD_AES_256_CCM_8 are defined in [RFC6655] with an authentication
tag length of 8-octets. We require two new variants,
AEAD_AES_128_CCM_12 and AEAD_AES_256_CCM_12, with 12-octet
authentication tags. In each case the authentication tag is formed
by taking the 12 most significant octets (in network order) of the
AEAD_AES_128/256_CCM authentication tag:
+=====================+===========+==============+
| Name | Key Size | tag size (t) |
+=====================+===========+==============+
| AEAD_AES_256_CCM_12 | 256 bits | 12 octets |
| AEAD_AES_128_CCM_12 | 128 bits | 12 octets |
+=====================+===========+==============+
8. Unneeded SRTP/SRTCP Fields
AEAD counter mode encryption removes the need for certain existing AEAD counter mode encryption removes the need for certain existing
SRTP/SRTCP mechanisms. SRTP/SRTCP mechanisms.
8.1. SRTP/SRTCP Authentication Field 7.1. 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.
8.2. RTP Padding 7.2. RTP Padding
Neither AES-GCM nor AES-CCM requires that the data be padded out to a AES-GCM does not requires that the data be padded out to a specific
specific block size, reducing the need to use the padding mechanism block size, reducing the need to use the padding mechanism provided
provided by RTP. It is RECOMMENDED that the RTP padding mechanism by RTP. It is RECOMMENDED that the RTP padding mechanism not be used
not be used unless it is necessary to disguise the length of the unless it is necessary to disguise the length of the underlying
underlying plaintext. plaintext.
9. AES-GCM/CCM processing for SRTP 8. AES-GCM processing for SRTP
9.1. SRTP IV formation for AES-GCM and AES-CCM 8.1. SRTP IV formation for AES-GCM
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 |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+) | Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+ | +--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+ | Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM and AES-CCM SRTP Figure 1: AES-GCM SRTP Initialization
Initialization Vector formation. Vector formation.
The 12 octet initialization vector used by both AES-GCM and AES-CCM The 12 octet initialization vector used by AES-GCM SRTP is formed by
SRTP is formed by first concatenating 2-octets of zeroes, the 4-octet first concatenating 2-octets of zeroes, the 4-octet SSRC, the 4-octet
SSRC, the 4-octet Rollover Counter (ROC) and the two octet sequence Rollover Counter (ROC) and the two octet sequence number SEQ. The
number SEQ. The resulting 12-octet value is then XORed to the resulting 12-octet value is then XORed to the 12-octet salt to form
12-octet salt to form the 12-octet IV. the 12-octet IV.
9.2. Data Types in SRTP Packets 8.2. 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 V (2 bits), padding flag P (1 bit), Associated Data: The version V (2 bits), padding flag P (1 bit),
extension flag X (1 bit), CSRC count CC (4 bits), extension flag X (1 bit), CSRC count CC (4 bits),
marker M (1 bit), the Payload Type PT (7 bits), marker M (1 bit), the Payload Type PT (7 bits),
the sequence number (16 bits), timestamp (32 the sequence number (16 bits), timestamp (32
bits), SSRC (32 bits), optional contributing bits), SSRC (32 bits), optional contributing
skipping to change at page 12, line 36 skipping to change at page 10, line 36
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C = Ciphertext (encrypted and authenticated) C = Ciphertext (encrypted and authenticated)
A = Associated Data (authenticated only) A = Associated Data (authenticated only)
R = neither encrypted nor authenticated, added R = neither encrypted nor authenticated, added
after authenticated encryption completed after authenticated encryption completed
Figure 3: Structure of an SRTP packet after Authenticated Figure 3: Structure of an SRTP packet after Authenticated
Encryption Encryption
9.3. Handling Header Extensions 8.3. Handling Header Extensions
RTP header extensions were first defined in RFC 3550. RFC 6904 RTP header extensions were first defined in RFC 3550. RFC 6904
[RFC6904] describes how these header extensions are to be encrypted [RFC6904] describes how these header extensions are to be encrypted
in SRTP. in SRTP.
When RFC 6904 is in use, a separate keystream is generated to encrypt When RFC 6904 is in use, a separate keystream is generated to encrypt
selected RTP header extension elements. For the AEAD_AES_128_GCM and selected RTP header extension elements. For the AEAD_AES_128_GCM and
the AEAD_AES_128_CCM algorithms, this keystream MUST be generated in AEAD_AES_128_GCM_8 algorithms, this keystream MUST be generated in
the manner defined in [RFC6904] using the AES_128_CM transform. For the manner defined in [RFC6904] using the AES_128_CM transform. For
the AEAD_AES_256_GCM and the AEAD_AES_256_CCM algorithms, the the AEAD_AES_256_GCM algorithm, the keystream MUST be generated in
keystream MUST be generated in the manner defined for the AES_256_CM the manner defined for the AES_256_CM transform. The originator must
transform. The originator must perform any required header extension perform any required header extension encryption before the AEAD
encryption before the AEAD algorithm is invoked. algorithm is invoked.
As with the other fields contained within the RTP header, both As with the other fields contained within the RTP header, both
encrypted and unencrypted header extensions are to be treated by the encrypted and unencrypted header extensions are to be treated by the
AEAD algorithm as Associated Data (AD). Thus the AEAD algorithm does AEAD algorithm as Associated Data (AD). Thus the AEAD algorithm does
not provide any additional privacy for the header extensions, but not provide any additional privacy for the header extensions, but
does provide integrity and authentication. does provide integrity and authentication.
9.4. Prevention of SRTP IV Reuse 8.4. 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 master key before the (ROC,SEQ) pair cycles
back to its original value. Note that back to its original value. Note that
implicitly assumes that either the outgoing RTP implicitly assumes that either the outgoing RTP
process is trusted to not attempt to repeat a process is trusted to not attempt to repeat a
SEQ value, or that the encryption process (ROC,SEQ) value, or that the encryption process
ensures that the SEQ number of the packets ensures that the both the SEQ and ROC numbers
presented to it are always incremented in the of the packets presented to it are always
proper fashion. This is particularly important incremented in the proper fashion. This is
for GCM since using the same SEQ value twice particularly important for GCM since using the
compromises the authentication mechanism. For same (ROC,SEQ) value twice compromises the
GCM, the SEQ and SSRC values used MUST either authentication mechanism. For GCM, the
be generated or checked by the SRTP (ROC,SEQ) and SSRC values used MUST either be
generated or checked by the SRTP
implementation, or by a module (e.g. the RTP implementation, or by a module (e.g. the RTP
application) that can be considered equally application) that can be considered equally
trusted as the SRTP implementation. While trusted as the SRTP implementation. While
[RFC3711] allows detecting SSRC collisions [RFC3711] allows detecting SSRC collisions
after they happen, SRTP using GCM with shared after they happen, SRTP using GCM with shared
master keys MUST prevent SSRC collision from master keys MUST prevent SSRC collision from
happening even once. happening even once.
SSRC Management: For a given master key, the set of all SSRC SSRC Management: For a given master key, the set of all SSRC
values used with that master key must be values used with that master key must be
skipping to change at page 14, line 5 skipping to change at page 12, line 5
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 any of the rekey MUST be performed before any of the
originating endpoints using that master key originating endpoints using that master key
exhausts its pool of SSRC values. Further, the exhausts its pool of SSRC values. Further, the
identity of the entity giving out SSRC values identity of the entity giving out SSRC values
MUST be verified, and the SSRC signaling MUST MUST be verified, and the SSRC signaling MUST
be integrity protected. be integrity protected.
10. AES-GCM/CCM Processing of SRTCP Compound Packets 9. AES-GCM 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)
10.1. SRTCP IV formation for AES-GCM and AES-CCM 9.1. SRTCP IV formation for AES-GCM
The 12-octet initialization vector used by both AES-GCM and AES-CCM The 12-octet initialization vector used by AES-GCM SRTCP is formed by
SRTCP is formed by first concatenating 2-octets of zeroes, the first concatenating 2-octets of zeroes, the 4-octet Synchronization
4-octet Synchronization Source identifier (SSRC), 2-octets of zeroes, Source identifier (SSRC), 2-octets of zeroes, a single zero bit, and
a single zero bit, and the 31-bit SRTCP Index. The resulting the 31-bit SRTCP Index. The resulting 12-octet value is then XORed
12-octet value is then XORed to the 12-octet salt to form the 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 9 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 4: SRTCP Initialization Vector formation Figure 4: SRTCP Initialization Vector formation
10.2. Data Types in Encrypted SRTCP Compound Packets 9.2. Data Types in Encrypted SRTCP Compound Packets
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 16, line 17 skipping to change at page 14, line 17
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 SRTCP 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. Even if the plaintext field is empty, AEAD encryption the cipher. Even if the plaintext field is empty, AEAD encryption
must still be performed, with the resulting cipher consisting solely must still be performed, with the resulting cipher consisting solely
of the authentication tag. This tag is to be placed immediately of the authentication tag. This tag is to be placed immediately
before the encryption flag and SRTCP index. before the encryption flag and SRTCP index.
10.3. Data Types in Unencrypted SRTCP Compound Packets 9.3. Data Types in Unencrypted SRTCP Compound Packets
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | sender info : A | sender info :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 16, line 44 skipping to change at page 14, line 44
A |V=2|P| SC | Packet Type | length | A |V=2|P| SC | Packet Type | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | SSRC/CSRC_1 | A | SSRC/CSRC_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | SDES items : A | SDES items :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | ... : A | ... :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A |0| SRTCP index | A |0| SRTCP 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, added after R = neither encrypted nor authenticated, added after
encryption encryption
Figure 6: AEAD SRTCP inputs when encryption flag = 0 Figure 6: AEAD SRTCP inputs when encryption flag = 0
skipping to change at page 17, line 15 skipping to change at page 15, line 17
follows (see figure 6): follows (see figure 6):
Plaintext: None. Plaintext: None.
Raw Data: The variable length optional SRTCP MKI and SRTCP Raw Data: The variable length optional SRTCP MKI and SRTCP
authentication tag (whose use is NOT authentication tag (whose use is NOT
RECOMMENDED). RECOMMENDED).
Associated Data: All other data. Associated Data: All other data.
Even though there is no plaintext in this RTCP packet, AEAD Even though there is no ciphertext in this RTCP packet, AEAD
encryption returns a cipher field which is precisely the length of encryption returns a cipher field which is precisely the length of
the AEAD authentication tag. This cipher is to be placed before the the AEAD authentication tag. This cipher is to be placed before the
Encryption flag and the SRTCP index in the authenticated SRTCP Encryption flag and the SRTCP index in the authenticated SRTCP
packet. packet.
10.4. Prevention of SRTCP IV Reuse 9.4. 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 should be cycles back to its original value. Ideally, a rekey should be
performed and a new master key put in place well before the SRTCP performed and a new master key put in place well before the SRTCP
cycles back to the starting value. cycles back to the starting value.
The comments on SSRC management in section 9.4 also apply. The comments on SSRC management in section 8.4 also apply.
11. Constraints on AEAD for SRTP and SRTCP 10. 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 one specific fasmily of AEAD algorithms, namely
AES-CCM. AES-GCM.
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the five All AEAD algorithms used with SRTP/SRTCP MUST satisfy the five
constraints listed below: constraints listed below:
PARAMETER Meaning Value PARAMETER Meaning Value
A_MAX maximum associated MUST be at least 12 octets. A_MAX maximum associated MUST be at least 12 octets.
data length data 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
P_MAX maximum plaintext GCM: MUST be <= 2^36-32 octets. P_MAX maximum plaintext GCM: MUST be <= 2^36-32 octets.
length per invocation CCM: MUST be <= 2^24-1 octets. length per invocation
C_MAX maximum ciphertext GCM: MUST be <= 2^36-16 octets.
length per invocation CCM: MUST be <= 2^24+15 octets.
For GCM the value of P_MAX is based on purely cryptographic C_MAX maximum ciphertext GCM: MUST be <= 2^36-16 octets.
considerations. CCM requires the length of the plaintext, measured length per invocation
in octets, must fit in a 24-bit field. Hence P_MAX is 2^24-1..
For sake of clarity we specify two additional parameters: For sake of clarity we specify two additional parameters:
AEAD Authentication Tag Length CCM: MUST be 8, 12, or 16 octets, AEAD Authentication Tag Length MUST be 8 or 16 octets,
GCM: MUST be 12 or 16 octets.
Maximum number of invocations SRTP: MUST be at most 2^48, Maximum number of invocations SRTP: MUST be at most 2^48,
for a given instantiation SRTCP: MUST be at most 2^31. for a given instantiation SRTCP: MUST be at most 2^31.
Block Counter size CCM: MUST be 24 bits, Block Counter size GCM: MUST be 32 bits.
GCM: MUST be 32 bits.
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.
12. Key Derivation Functions 11. 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 AEAD_AES_128_GCM and AEAD_AES_128_GCM_8 algorithms
AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key MUST use the (128-bit) AES_CM_PRF Key Derivation Function described
Derivation Function described in [RFC3711]. Both the in [RFC3711]. AEAD_AES_256_GCM MUST use the AES_256_CM_PRF Key
AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST Derivation Function described in [RFC6188].
use the AES_256_CM_PRF Key Derivation Function described in
[RFC6188].
13. Summary of Algorithm Characteristics 12. Summary of AES-GCM in SRTP/SRTCP
For convenience, much of the information about the use of AES-GCM and For convenience, much of the information about the use of AES-GCM
AES-CCM algorithms in SRTP is collected in the tables contained in family of algorithms in SRTP is collected in the tables contained in
this section. this section.
13.1. AES-GCM for SRTP/SRTCP The AES-GCM 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:
Name Key Size AEAD Tag Size Reference Name Key Size AEAD Tag Size Reference
================================================================ ================================================================
AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282]
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116] AEAD_AES_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_12 16 octets 12 octets [RFC5282]
AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282]
Table 1: AES-GCM algorithms for SRTP/SRTCP Table 1: AES-GCM algorithms for SRTP/SRTCP
Any implementation of AES-GCM SRTP MUST support both AEAD_AES_128_GCM Any implementation of AES-GCM SRTP MUST support both AEAD_AES_128_GCM
and AEAD_AES_256_GCM (the versions with 16 octet AEAD authentication and AEAD_AES_256_GCM (the versions with 16 octet AEAD authentication
tags), and it MAY support the four other variants shown in table 1. tags), and it MAY support AEAD_AES_128_GCM_8. Below we summarize
Below we summarize parameters associated with these four GCM parameters associated with these three GCM algorithms:
algorithms:
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Parameter | Value | | Parameter | Value |
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Master key length | 128 bits | | Master key length | 128 bits |
| Master salt length | 96 bits | | Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] | | Key Derivation Function | AES_CM_PRF [RFC3711] |
| Maximum key lifetime (SRTP) | 2^48 packets | | Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets | | Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_GCM_12 | | Cipher (for SRTP and SRTCP) | AEAD_AES_128_GCM_8 |
| AEAD authentication tag length | 96 bits | | AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
Table 2: The AEAD_AES_128_GCM_12 Crypto Suite Table 2: The AEAD_AES_128_GCM_8 Crypto Suite
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Parameter | Value | | Parameter | Value |
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Master key length | 128 bits | | Master key length | 128 bits |
| Master salt length | 96 bits | | Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] | | Key Derivation Function | AES_CM_PRF [RFC3711] |
| Maximum key lifetime (SRTP) | 2^48 packets | | Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets | | Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_GCM | | Cipher (for SRTP and SRTCP) | AEAD_AES_128_GCM |
skipping to change at page 20, line 13 skipping to change at page 17, line 41
Table 3: The AEAD_AES_128_GCM Crypto Suite Table 3: The AEAD_AES_128_GCM Crypto Suite
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Parameter | Value | | Parameter | Value |
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
| Master key length | 256 bits | | Master key length | 256 bits |
| Master salt length | 96 bits | | Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] | | Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Maximum key lifetime (SRTP) | 2^48 packets | | Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets | | Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_GCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 4: 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] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_GCM | | Cipher (for SRTP and SRTCP) | AEAD_AES_256_GCM |
| AEAD authentication tag length | 128 bits | | AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+ +--------------------------------+------------------------------+
Table 5: The AEAD_AES_256_GCM Crypto Suite Table 4: The AEAD_AES_256_GCM Crypto Suite
13.2. AES-CCM for SRTP/SRTCP
AES-CCM is another family of AEAD algorithms built around the AES
block cipher algorithm. AES-CCM uses AES counter mode for encryption
and AES Cipher Block Chaining Message Authentication Code (CBC-MAC)
for authentication. A detailed description of the AES-CCM family can
be found in [RFC5116]. Four of the six CCM algorithms used in this
document are defined in previous RFCs, while two, AEAD_AES_128_CCM_12
and AEAD_AES_256_CCM_12, are defined in section 7 of this document.
Any implementation of AES-CCM SRTP/SRTCP MUST support both
AEAD_AES_128_CCM and AEAD_AES_256_CCM (the versions with 16 octet
AEAD authentication tags), and MAY support the other four variants.
Name Key Size AEAD Tag Size Reference
================================================================
AEAD_AES_128_CCM 128 bits 16 octets [RFC5116]
AEAD_AES_256_CCM 256 bits 16 octets [RFC5116]
AEAD_AES_128_CCM_12 128 bits 12 octets see section 7
AEAD_AES_256_CCM_12 256 bits 12 octets see section 7
AEAD_AES_128_CCM_8 128 bits 8 octets [RFC6655]
AEAD_AES_256_CCM_8 256 bits 8 octets [RFC6655]
Table 6: AES-CCM algorithms for SRTP/SRTCP
In addition to the flag octet used in counter mode encryption,
AES-CCM authentications also uses a flag octet that conveys
information about the length of the authentication tag, length of the
block counter, and presence of additional authenticated data (see
section 2.2 of [RFC3610]). For AES-CCM in SRTP/SRTCP, the flag octet
has the hex value 5A if an 8-octet AEAD authentication tag is used,
6A if a 12-octet AEAD authentication tag is used, and 7A if a
16-octet AEAD authentication tag is used. The flag octet is one of
the inputs to AES during the counter mode encryption of the
plaintext.
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_CCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 7: The AEAD_AES_128_CCM_8 Crypto Suite
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM_PRF [RFC3711] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_CCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 8: 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] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_CCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 9: 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] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_CCM_8 |
| AEAD authentication tag length | 64 bits |
+--------------------------------+------------------------------+
Table 10: 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] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_CCM_12 |
| AEAD authentication tag length | 96 bits |
+--------------------------------+------------------------------+
Table 11: 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] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_CCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 12: The AEAD_AES_256_CCM Crypto Suite
14. Security Considerations
14.1. Handling of Security Critical Parameters 13. Security Considerations
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 1. The block counter is incremented after each block key has
after each block key has been produced, but it MUST NOT be been produced, but it MUST NOT be allowed to exceed 2^32-1 for
allowed to exceed 2^32-1 for GCM and 2^24-1 for CCM. Note that GCM. Note that even though the block counter is reset at the
even though the block counter is reset at the start of each start of each packet, IV uniqueness is ensured by the inclusion
packet, IV uniqueness is ensured by the inclusion of of SSRC/ROC/SEQ or SRTCP Index in the IV. (The reader is
SSRC/ROC/SEQ or SRTCP Index in the IV. (The reader is reminded reminded that the first block of key produced is reserved for
that in both GCM and CCM the first block of key produced is use in authenticating the packet and is not used to encrypt
reserved for use in authenticating the packet and is not used to plaintext.)
encrypt plaintext.) - Each time a rekey occurs, the initial values of both the 31-bit
- Each time a rekey occurs, the initial values of the SRTCP index SRTCP index and the 48-bit SRTP packet index (ROC||SEQ) MUST be
and the SRTP packet indices MUST be saved in order to prevent IV saved in order to prevent IV reuse.
reuse. - Processing MUST cease if either the 31-bit SRTCP index or the
- Processing MUST cease if the 31-bit SRTCP index or any of the 48-bit packet index ROC||SEQ cycles back their initial values .
48-bit packet indices cycle back their initial values .
Processing MUST NOT resume until a new SRTP/SRTCP session has Processing MUST NOT resume until a new SRTP/SRTCP session has
been established using a new SRTP master key. Ideally, a rekey been established using a new SRTP master key. Ideally, a rekey
should be done well before any of these counters cycle. should be done well before any of these counters cycle.
14.2. 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 reduce the probability of a successful authentication tag is to reduce 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 25, line 10 skipping to change at page 19, line 25
algorithm used in GCM reduces the effective authentication tag size algorithm used in GCM reduces the effective authentication tag size
(in bits) by the log base 2 of the of blocks of encrypted and/or (in bits) by the log base 2 of the of blocks of encrypted and/or
authenticated data in a packet. In practice an SRTP payload will be authenticated data in a packet. In practice an SRTP payload will be
less than 2^16 bytes, because of the 16-bit IPv4 and UDP length less than 2^16 bytes, because of the 16-bit IPv4 and UDP length
fields. The exception to this case is IPv6 jumbograms [RFC2675], fields. The exception to this case is IPv6 jumbograms [RFC2675],
which is unlikely to be used for RTP-based multimedia traffic which is unlikely to be used for RTP-based multimedia traffic
[RFC3711]. This corresponds to 2^12 blocks of data, so the effective [RFC3711]. This corresponds to 2^12 blocks of data, so the effective
GCM authentication tag size is reduced by at most 12 bits. GCM authentication tag size is reduced by at most 12 bits.
+===========+=============+========================================+ +===========+=============+========================================+
| Auth. Tag | Eff. Tag | Number of Forgery Attempts | | Auth. Tag | Effective | Number of Forgery Attempts |
| Size | Tag Size | Needed to Achieve a Given | | Size | Tag Size | Needed to Achieve a Given |
| (bytes) | (bits) | Probability of Success | | (bytes) | (bits) | Probability of Success |
|-----------+-------------+------------+-------------+-------------| |-----------+-------------+------------+-------------+-------------|
| | prob=2^-30 | prob=2^-20 | prob=2^-10 | | | prob=2^-30 | prob=2^-20 | prob=2^-10 |
|===========+=============+=============+============+=============| |===========+=============+=============+============+=============|
| | 32 (CCM) | 2^2 tries | 2^12 tries | 2^22 tries | | 4 | 20 (GCM) | 1 try | 1 try | 2^10 tries |
| 4 +-------------+------------+-------------+-------------|
| | 20 (GCM) | 1 try | 1 try | 2^10 tries |
|===========+=============+============+=============+=============| |===========+=============+============+=============+=============|
| | 64 (CCM) | 2^34 tries | 2^44 tries | 2^54 tries | | 8 | 52 (GCM) | 2^22 tries | 2^32 tries | 2^42 tries |
| 8 +-------------+------------+-------------+-------------|
| | 52 (GCM) | 2^22 tries | 2^32 tries | 2^42 tries |
|===========+=============+============+=============+=============| |===========+=============+============+=============+=============|
| | 96 (CCM) | 2^66 tries | 2^76 tries | 2^86 tries | | 12 | 84 (GCM) | 2^54 tries | 2^64 tries | 2^74 tries |
| 12 +-------------+------------+-------------+-------------|
| | 84 (GCM) | 2^54 tries | 2^64 tries | 2^74 tries |
|===========+=============+============+=============+=============| |===========+=============+============+=============+=============|
| | 128 (CCM) | 2^98 tries | 2^108 tries | 2^118 tries | | 16 | 116 (GCM) | 2^86 tries | 2^96 tries | 2^106 tries |
| 16 +-------------+------------+-------------+-------------|
| | 116 (GCM) | 2^86 tries | 2^96 tries | 2^106 tries |
|===========+=============+============+=============+=============| |===========+=============+============+=============+=============|
Table 13: Number of forgery attempts needed to achieve a given Table 5: Number of forgery attempts needed to achieve a given
probability of success for various tag sizes. probability of success for various tag sizes.
15. IANA Considerations 14. IANA Considerations
15.1. SDES 14.1. SDES
SDP Security Descriptions [RFC4568] defines SRTP "crypto suites". A SDP Security Descriptions [RFC4568] 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 Security Descriptions to signal the use of the order to allow Security Descriptions to signal the use of the
algorithms defined in this document, IANA will register the following algorithms defined in this document, IANA will register the following
crypto suites into the "SRTP Crypto Suite Registrations" subregistry crypto suites into the "SRTP Crypto Suite Registrations" subregistry
of the "Session Description Protocol (SDP) Security Descriptions" of the "Session Description Protocol (SDP) Security Descriptions"
registry. registry.
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" / srtp-crypto-suite-ext = "AEAD_AES_128_GCM_8" /
"AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" / "AEAD_AES_256_GCM" /
"AEAD_AES_128_GCM_12" /
"AEAD_AES_256_GCM_12" /
"AEAD_AES_128_CCM" /
"AEAD_AES_256_CCM" /
"AEAD_AES_128_CCM_8" /
"AEAD_AES_256_CCM_8" /
"AEAD_AES_128_CCM_12" /
"AEAD_AES_256_CCM_12" /
srtp-crypto-suite-ext srtp-crypto-suite-ext
15.2. DTLS-SRTP 14.2. DTLS-SRTP
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile". DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile".
These also correspond to the use of an AEAD algorithm in SRTP. In These also correspond to the use of an AEAD algorithm in SRTP. In
order to allow the use of the algorithms defined in this document in 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_128_GCM = {TBD, TBD }
AEAD_AES_128_GCM_8 = {TBD, TBD }
AEAD_AES_256_GCM = {TBD, TBD } AEAD_AES_256_GCM = {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 Below we list the SRTP transform parameters for each of these
protection profile. Unless separate parameters for SRTCP and SRTCP protection profile. Unless separate parameters for SRTCP and SRTCP
are explicitly listed, these parameters apply to both SRTP and are explicitly listed, these parameters apply to both SRTP and
SRTCP. SRTCP.
AEAD_AES_128_CCM
cipher: AES_128_CCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_256_CCM
cipher: AES_256_CCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_128_CCM_8
cipher: AES_128_CCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 8 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_256_CCM_8
cipher: AES_256_CCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 8 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_128_CCM_12
cipher: AES_128_CCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 12 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_256_CCM_12
cipher: AES_256_CCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 12 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_128_GCM AEAD_AES_128_GCM
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
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
AEAD_AES_128_GCM_8
AEAD_AES_256_GCM
cipher: AES_256_GCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
AEAD_AES_128_GCM_12
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
aead_auth_tag_length: 12 octets aead_auth_tag_length: 8 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
AEAD_AES_256_GCM_12 AEAD_AES_256_GCM
cipher: AES_256_GCM cipher: AES_256_GCM
cipher_key_length: 256 bits cipher_key_length: 256 bits
cipher_salt_length: 96 bits cipher_salt_length: 96 bits
aead_auth_tag_length: 12 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
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.
15.3. MIKEY 14.3. MIKEY
In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830], In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830],
IANA maintains several subregitries under "Multimedia Internet KEYing IANA maintains several subregitries under "Multimedia Internet KEYing
(MIKEY) Payload Name Spaces". This document requires additions to (MIKEY) Payload Name Spaces". This document requires additions to
two of the MIKEY subregistries. two of the MIKEY subregistries.
In the "MIKEY Security Protocol Parameters" subregistry we request In the "MIKEY Security Protocol Parameters" subregistry we request
the following addition: 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 octets or 16 octets
This list is, of course, intended for use with CM and GCM. It is This list is, of course, intended for use with GCM. It is
conceivable that new AEAD algorithms introduced at some point in the conceivable that new AEAD algorithms introduced at some point in the
future may require a different set of Authentication tag lengths. future may require a different set of Authentication tag lengths.
In the "Encryption Algorithm" subregistry (derived from Table In the "Encryption Algorithm" subregistry (derived from Table
6.10.1.b of [RFC3830]) we request the following additions: 6.10.1.b of [RFC3830]) we request the following addition:
SRTP encr | Value | Default Session | Default Auth. SRTP encr | Value | Default Session | Default Auth.
Algorithm | | Encr. Key Length | Tag Length Algorithm | | Encr. Key Length | Tag Length
----------------------------------------------------------- -----------------------------------------------------------
AES-CCM | TBD | 16 octets | 16 octets
AES-GCM | TBD | 16 octets | 16 octets AES-GCM | TBD | 16 octets | 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 16. described in section 16.
15.4. AEAD registry 15. Parameters for use with MIKEY
We request that IANA make the following additions to the IANA
"Authenticated Encryption with Associated Data (AEAD) Parameters"
page's registry for "AEAD Algorithms":
AEAD_AES_128_CCM_12 = TBD
AEAD_AES_256_CCM_12 = TBD
16. Parameters for use with MIKEY
MIKEY specifies the algorithm family separately from the key length 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_8 | AES-GCM | 16 octets | 8 octets |
+------------+-------------+-------------+
AEAD_AES_128_CCM | AES-CCM | 16 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_128_GCM_12 | AES-GCM | 16 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_128_CCM_12 | AES-CCM | 16 octets | 12 octets |
+------------+-------------+-------------+ +------------+-------------+-------------+
AEAD_AES_128_CCM_8 | AES-CCM | 16 octets | 8 octets | AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
+------------+-------------+-------------+ +------------+-------------+-------------+
AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets | AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM | AES-CCM | 32 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM_12 | AES-GCM | 32 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM_12 | AES-CCM | 32 octets | 12 octets |
+------------+-------------+-------------+
AEAD_AES_256_CCM_8 | AES-CCM | 32 octets | 8 octets |
+============+=============+=============+ +============+=============+=============+
Table 14: Mapping MIKEY parameters to AEAD algorithm Table 6: Mapping MIKEY parameters to AEAD algorithm
Section 12 in this document restricts the choice of Key Derivation Section 11 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 input key length of the Key Derivation Function (i.e. [RFC3830], the input key length of the Key Derivation Function (i.e.
the SRTP master key length) is always equal to the session encryption the SRTP master key length) is always equal to the session encryption
key length. This means, for example, that AEAD_AES_256_GCM will use key 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.
17. 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, Woo-Hwan Kim, John Mattsson, Magnus Westerland, Oscar Ohllson, Woo-Hwan Kim, John Mattsson,
Richard Barnes, John Mattisson, Morris Dworkin and many other Richard Barnes, John Mattisson, Morris Dworkin, Stehen Farrell and
reviewers who provided valuable comments on earlier drafts of this many other reviewers who provided valuable comments on earlier drafts
document. of this document.
18. References 17. References
18.1. Normative References 17.1. Normative References
[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.
[RFC3550] Casner, S., Frederick, R., and V. Jacobson, "RTP: A [RFC3550] Casner, S., Frederick, R., and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC 3550, Transport Protocol for Real-Time Applications", RFC 3550,
July 2003. July 2003.
[RFC3610] Whiting,D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, March 2004.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, September 2003. (SRTP)", RFC 3711, September 2003.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,and [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,and
Norrman, K, "MIKEY: Multimedia Internet KEYing", RFC 3830, Norrman, K, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004. August 2004.
[RFC4568] Andreasen, F., Baugher, M., and D.Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D.Wing, "Session
Description Protocol (SDP): Security Descriptions for Description Protocol (SDP): Security Descriptions for
skipping to change at page 31, line 47 skipping to change at page 23, line 44
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.
[RFC6188] D. McGrew, "The Use of AES-192 and AES-256 in Secure [RFC6188] D. McGrew, "The Use of AES-192 and AES-256 in Secure
RTP", RFC 6188, March 2011. RTP", RFC 6188, March 2011.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure [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.
18.2. Informative References 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.
[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
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