Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc.
Intended Status: Informational January 26, 2011
Expires: July 30, 2011
AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avt-srtp-aes-gcm-01
Abstract
This document defines how AES-GCM, AES-CCM, and other Authenticated
Encryption with Associated Data (AEAD) algorithms, can be used to
provide confidentiality and data authentication mechanisms in the
SRTP protocol.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 16, 2011.
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document authors. All rights reserved.
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Table of Contents
1. Introduction.....................................................2
1.1. Conventions Used In This Document...........................3
1.2. AEAD processing for SRTP....................................4
1.2.1. AEAD Authentication versus SRTP Authentication.........5
1.2.2. Values used to form the Initialization Vector (IV).....5
1.2.3. SRTP IV formation for AES-GCM and AES-CCM..............6
1.2.4. SRTCP IV formation for AES-GCM and AES-CCM.............6
1.2.5. AEAD Processing of SRTP Packets........................7
1.2.6. AEAD Processing of SRTCP Packets.......................8
1.2.6.1. Encrypted SRTCP packets...........................8
1.2.6.2. Unencrypted SRTCP packets.........................9
2. AEAD parameters for SRTP and SRTCP...............................9
2.1. Generic AEAD Parameter Constraints.........................10
2.2. AES-GCM for SRTP/SRTCP.....................................11
2.3. AES-CCM for SRTP/SRTCP.....................................11
3. Security Considerations.........................................12
4. IANA Considerations.............................................13
5. Acknowledgements................................................14
6. References......................................................14
6.1. Normative References.......................................14
6.2. Informative References.....................................14
1. Introduction
The Secure Real-time Transport Protocol (SRTP) is a profile of the
Real-time Transport Protocol (RTP), which can provide
confidentiality, message authentication, and replay protection to the
RTP traffic and to the control traffic for RTP, the Real-time
Transport Control Protocol (RTCP).
SRTP/SRTCP assumes that both the sender and recipient have a shared
secret master key and a shared secret master salt. As described in
sections 4.3.1 and 4.3.3 of [RFC3711], a Key Derivation Function is
applied to these secret values to obtain separate encryption keys,
authentication keys and salting keys for SRTP and for SRTCP. (Note:
As will be explained below, AEAD SRTP/SRTCP does not make use of
these authentication keys.)
Authenticated encryption [BN00] is a form of encryption that, in
addition to providing confidentiality for the plaintext that is
encrypted, provides a way to check its integrity and authenticity.
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Authenticated Encryption with Associated Data, or AEAD [R02], adds
the ability to check the integrity and authenticity of some
Associated Data (AD), also called "additional authenticated data",
that is not encrypted. This specification makes use of the interface
to a generic AEAD algorithm as defined in [RFC5116].
The Advanced Encryption Standard (AES) is a block cipher that
provides a high level of security, and can accept different key
sizes. Two families of AEAD algorithm families, AES Galois/Counter
Mode (AES-GCM) and AES Cipher Block Chaining/Counter Mode (AES/CCM),
are based upon AES. This specification makes use of the AES versions
that use 128-bit and 256-bit keys, which we call AES-128 and AES-256,
respectively.
The Galois/Counter Mode (GCM) of operation and the Counter with CBC
MAC (CCM) mode are AEAD modes of operation for block ciphers. Both
use counter mode to encrypt the data, an operation that can be
efficiently pipelined. Further, GCM authentication uses operations
that are particularly well suited to efficient implementation in
hardware, making it especially appealing for high-speed
implementations, or for implementations in an efficient and compact
circuit. CCM is well suited for use in compact software
implementations. This specification uses GCM and CCM with both
AES-128 and AES-256.
In summary, this document defines how to use AEAD algorithms,
particularly AES-GCM and AES-CCM, to provide confidentiality and
message authentication within SRTP and SRTCP packets.
1.1. Conventions Used In This Document
The following terms have very specific meanings in the context of
this RFC:
Crypto Context For the purposes of this document a crypto context
is the outcome of any process which results in
authentication of each participant in the SRTP
session and in their possession of a shared secret
master key and a shared master salt. Details of
how the maser key and master salt are established
are outside the scope of this document. The
master key MUST be at least as large as the
encryption key. The SRTP/SRTCP Key Derivation
Function (KDF) defined in [RFC3711] is applied to
the master key and master SALT to derive the
SRTP_encr_key, SRTCP_encr_key, SRTP_SALT, and
SRTCP_SALT. Authentication keys are not used in
AEAD.
Instantiation Once keys have been established, an instance of
the AEAD algorithm is created using the
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appropriate key and salt. In a point-to-point
scenario, each participant in the SRTP/SRTCP
session will need four instantiations of the AEAD
algorithm; one for inbound SRTP traffic, one for
outbound SRTP traffic source, one for inbound
SRTCP traffic, and one for outbound SRTCP traffic
source.
Invocation SRTP/SRTCP data streams are broken into packets.
Each packet is processed by a single invocation of
the appropriate instantiation of the AEAD
algorithm.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. AEAD processing for SRTP
We first define how to use a generic AEAD algorithm in SRTP, then we
describe the specific use of the AES-128-GCM and AES-256-GCM
algorithms.
The use of an AEAD algorithm is defined by expressing the AEAD
encryption algorithm inputs in terms of SRTP fields and data
structures. The AEAD encryption inputs are as follows:
Key This input is the SRTP encryption key
(SRTP_encr_key) produced from the shared
secret master key using the key derivation
process. (Note that the SRTP_auth_key is
not used).
Associated Data This is data that is to be authenticated
but not encrypted. In SRTP, the associated
data consists of the entire RTP header,
including the list of CSRC identifiers (if
present) and the RTP header extension (if
present), as shown in Figure 2.
Plaintext Data that is to be both encrypted and
authenticated. In SRTP this consists of
the RTP payload, the RTP padding and the
RTP pad count fields (if the latter two
fields are present) as shown in Figure 2.
The padding service provided by RTP is not
needed by the AEAD encryption algorithm, so
the RTP padding and RTP pad count fields
SHOULD be omitted.
Initialization Vector Each SRTP/SRTCP packet has its own 12-octet
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initialization vector (IV). Construction
of this IV is covered in more detail
below.
The AEAD encryption algorithm accepts these four inputs and returns a
Ciphertext field.
1.2.1. AEAD Authentication versus SRTP Authentication
The reader is reminded that in addition to providing confidentiality
for the plaintext that is encrypted, an AEAD algorithm also provides
a way to check the data integrity and authenticity of the plaintext
and associated data. The AEAD integrity check is incorporated into
the Ciphertext field by RFC 5116, thus AEAD does not make use of the
optional SRTP Authentication Tag field. (Note that this means that
the cipher text will be longer than the plain text by precisely the
length of the AEAD authentication tag.)
The AEAD message authentication mechanism MUST be the primary message
authentication mechanism for AEAD SRTP. Additional SRTP
authentication mechanisms SHOULD NOT be used with any AEAD algorithm
and the optional SRTP Authentication Tag SHOULD NOT be present.
Rationale. Some applications use the Authentication Tag as a
means of conveying additional information, notably [RFC4771].
This document retains the Authentication Tag field primarily to
preserve compatibility with these applications.
1.2.2. Values used to form the Initialization Vector (IV)
The initialization vector for an SRTP packet is formed from the:
SSRC The 4-octet Synchronization Source identifier
(SSRC), found in the RTP header.
Packet Counter Each AEAD instantiation MUST maintain a 6 octet
zero-based packet counter which is incremented
after a given instantiation has been invoked to
process a packet of data. The packet counter is
closely related to the invocation field
discussed in NIST Special Publication 800 38-D
[GCM], "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC".
As we shall see below, the packet counter is
used to insure each packet gets a unique
initialization vector.
Sequence Number The 2-octet RTP Sequence Number (SEQ), found in
the RTP header. SEQ is just the two least
significant bytes of the packet counter.
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Rollover Counter A 4-octet Rollover Counter (ROC), maintained by
both sides of the link. The ROC is just the 4
most significant octets of the packet counter.
SALT A 12-octet SRTP session encryption salt produced
by the SRTP Key Derivation Function (KDF).
1.2.3. SRTP IV formation for AES-GCM and AES-CCM
AES-GCM and AES-CCM SRTP use a 12 byte inititialition vector which is
formed as follows. A 12-octet string is formed by concatenating a
2-octets of zeroes, the 4-octet SSRC, and the the 6-byte invocation
counter. The resulting string is bitwise exclusive-ored with the
12-octet salt to form the 12-octet IV
0 0 0 0 0 0 0 0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC | Packet_Counter |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM and AES-CCM SRTP
Initialization Vector formation.
Using the terminology of section 8.2.1. of [GCM], the first six
octets of the IV are the fixed field and the last six bytes are the
invocation field.
1.2.4. SRTCP IV formation for AES-GCM and AES-CCM
The initialization vector for an SRTCP packet is formed from the
4-octet Synchronization Source identifier (SSRC), 31-bit SRTCP Index
(packed zero-filled, right justified into a 4-octet field), and a
12-octet SRTP session encryption salt produced by the SRTP Key
Derivation Function (KDF) as described in [RFC3711]. (The 31-bit
SRTCP index serves as the invocation counter.) First a 12-octet
string is formed by concatenating in order 2-octets of zeroes, the
4-octet SSRC, 2 more zero octets, and the 4-octet SRTCP index. The
resulting 12-octet string is bitwise exclusive-ored into salt; the
output of that process is the IV. The process is illustrated in
Figure 3. The IV is always exactly 12 octets in length.
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0 0 0 0 0 0 0 0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC |00|00|SRTCP Index|---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2: SRTCP Initialization Vector formation.
Using the terminology of section 8.2.1. of [GCM], the first eight
octets of the IV are the fixed field and the last four bytes are the
invocation field.
1.2.5. AEAD Processing of SRTP Packets
All SRTP packets MUST be authenticated and encrypted. Figure 3 below
shows which fields of AEAD SRTP packet are to be treated as
plaintext, which are to be treated as additional authenticated data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P|X| CC |M| Packet Type | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | contributing source (CSRC) identifiers (optional) |
A | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | RTP extension (OPTIONAL) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | payload ... |
P | +-------------------------------+
P | | RTP padding | RTP pad count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
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Note: The RTP padding and RP padding count fields are optional
and are not recommended
Figure 3: AEAD inputs from an SRTP packet.
1.2.6. AEAD Processing of SRTCP Packets
All SRTCP packets MUST be authenticated, but unlike SRTP, SRTCP
packet encryption is optional. A sender can select which packets to
encrypt, and indicates this choice with a 1-bit encryption flag
(located in the leftmost bit of the 32-bit word that contains the
SRTCP index)
1.2.6.1. Encrypted SRTCP packets
When the encryption flag is set to 1, the first 8-octets, the
encryption flag and SRTCP index are treated as AAD and eight octets
and the encryption flag are treated as plaintext. Figure 4 below
shows how fields of an RTCP packet are to be treated when the
encryption flag is set to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| RC | Packet Type | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) of Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | sender info |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | report block 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | report block 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P |V=2|P| SC | Packet Type | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
P | SSRC/CSRC_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | SDES items |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
P | ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A |1| SRTCP index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X | SRTCP MKI (optional)index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
Figure 4: AEAD SRTCP inputs when encryption flag = 1.
1.2.6.2. Unencrypted SRTCP packets
When the encryption flag is set to 0, all of the data up to and
including the SRTCP index is treated as AAD. Figure 5 shows how the
fields of an RTCP packet are to be treated when the encryption flag
is set to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| RC | Packet Type | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) of Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | sender info |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | report block 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | report block 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| SC | Packet Type | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | SSRC/CSRC_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | SDES items |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A |0| SRTCP index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X | SRTCP MKI (optional)index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
Figure 5: AEAD SRTCP inputs when encryption flag = 0.
2. AEAD parameters for SRTP and SRTCP
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In general, any AEAD algorithm can accept inputs with varying
lengths, but each algorithm can accept only a limited range of
lengths for a specific parameter. In this section, we describe the
constraints on the parameter lengths that any AEAD algorithm must
support to be used in AEAD-SRTP. Additionally we specify a complete
parameter set for two specific AEAD algorithms, namely AES-GCM and
AES-CCM.
2.1. Generic AEAD Parameter Constraints
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three
constraints listed below:
PARAMETER Meaning Value
A_MAX maximum additional MUST be at least 12 octets
authenticated data
length
N_MIN minimum nonce (IV) MUST be no more than 12 octets
length
N_MAX maximum nonce (IV) MUST be at least 12 octets
length
C_MAX maximum ciphertext MUST be at most 2^16-40 octets
length per invocation SHOULD be at least 2232
The upper bound on C_MAX is obtained by subtracting away a 20-octet
IP header, an 8-octet UDP header, and a 12-octet RTP header out of
the largest possible IP packet, the total length of which is 2^16
octets.
Similarly the lower bound on C_MAX is based on the maximum
transmission unit (MTU) of 2272 octets in IEEE 802.11. Because many
RTP applications use very short payloads (for example, the G.729
codec used in VoIP can be as short as 20 octets), implementations
that only support a maximum ciphertext length smaller than 2232
octets are permitted under this RFC. However, in the interest of
maximizing interoperability between various AEAD implementations, the
use of C_MAX values less than 2232 is discouraged.
For sake of clarity we specify two additional parameters:
Authentication Tag Length MUST be either 8, 12, or 16
octets
Maximum number of invocations MUST be at most 2^48 for SRTP
for a given instantiation MUST be at most 2^31 for SRTCP
The reader is reminded that the plaintext is shorter than the
ciphertext by exactly the length of the AEAD authentication tag.
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2.2. AES-GCM for SRTP/SRTCP
AES-GCM is a family of AEAD algorithms built around the AES block
cipher algorithm. AES-GCM uses AES counter mode for encryption and
Galois Message Authentication Code (GMAC) for authentication. A
detailed description of the AES-GCM family can be found in
[RFC5116]. The following members of the AES-GCM family may be used
with SRTP/SRTCP:
Table 1: AES-GCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference
================================================================
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_GCM 32 octets 16 octets [RFC5116]
AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282]
AEAD_AES_256_GCM_8 32 octets 8 octets [RFC5282]
AEAD_AES_128_GCM_12 16 octets 12 octets [RFC5282]
AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282]
Any implementation of AES-GCM SRTP MUST support both
AEAD-AES-128-GCM-8 and AEAD-AES-256-GCM-8, ant it MAY support the
four other variants shown in the table.
In addition to the invocation counter used in the formation of IVs,
each instantiation of AES-GCM has a block counter which is
incremented each time AES is called to produce a 16-octet output
block. The block counter is reset to "1" each time AES-GCM is
invoked.
2.3. AES-CCM for SRTP/SRTCP
AES-CCM is another family of AEAD algorithms built around the AES
block cipher algorithm. AES-GCM 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]. The following members of the AES-CCM family
may be used with SRTP/SRTCP:
Table 2: AES-CCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference
================================================================
AEAD_AES_128_CCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_CCM 32 octets 16 octets [RFC5116]
Any implementation of AES-CCM SRTP/SRTCP MUST support both
AEAD-AES-128-CCM and AEAD-AES-256-CCM.
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In addition to the invocation counter used in the formation of IVs,
each instantiation of AES-CCM has a block counter which is
incremented each time AES is called to produce a 16-octet output
block. The block counter is reset to "0" each time AES-CCM is
invoked.
AES-CCM 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. For AES-CCM in SRTP/SRTCP, the
flag octet has the hex value 5A if an 8-octet authentication tag is
used, 6A if a 12-octet authentication tag is used, and 7A if a
16-octet authentication tag is used. The flag octet is one of the
inputs to AES during the counter mode encryption of the plaintext.
3. Security Considerations
We require that the AEAD authentication tag must be at least 8
octets, significantly reducing the probability of an adversary
successfully introducing fraudulent data. The goal of an
authentication tag is to minimize the probability of a successful
forgery occurring anywhere in the network we are attempting to
defend. There are three relevant factors: how low we wish the
probability of successful forgery to be (prob_success), how many
attempts the adversary can make (N_tries) and the size of the
authentication tag in bits (N_tag_bits). Then
prob_success < expected number of successes
= N_tries * 2^-N_tag_bits.
The table below summarizes the relationship between the
authentication tag size, the probability of success, and the maximum
numbers of forgery attempts that can be permitted on our network.
|==================+========================================|
| Authentication | Probability any Successful Forgeries |
| Tag Size |-------------+-------------+------------|
| (octets) | 2^-10 | 2^-20 | 2^-30 |
|==================+=============+=============+============|
| 4 | 2^22 tries | 2^12 tries | 2^2 tries |
|==================+=============+=============+============|
| 8 | 2^54 tries | 2^44 tries | 2^34 tries |
|==================+=============+=============+============|
| 12 | 2^86 tries | 2^76 tries | 2^66 tries |
|==================+=============+=============+============|
| 16 | 2^118 tries | 2^108 tries | 2^98 tries |
|==================+=============+=============+============|
Table 1: Maximum allowable number of forgery attempts for
a given tag size and probability of success.
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4. IANA Considerations
RFC 4568 defines SRTP "crypto suites"; a crypto suite corresponds to
a particular AEAD algorithm in SRTP. In order to allow SDP to signal
the use of the algorithms defined in this document, IANA will
register the following crypto suites into the subregistry for SRTP
crypto suites under the SRTP transport of the SDP Security
Descriptions:
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" /
"AEAD_AES_128_GCM_8" /
"AEAD_AES_256_GCM_8" /
"AEAD_AES_128_GCM_12" /
"AEAD_AES_256_GCM_12" /
"AEAD_AES_128_CCM" /
"AEAD_AES_256_CCM" /
srtp-crypto-suite-ext
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile",
which corresponds to the use of an AEAD algorithm in SRTP. In
order to allow the use of the algorithms defined in this document
in DTLS-SRTP, IANA will also register the following SRTP Protection
Profiles:
SRTP_AEAD_AES_128_GCM
SRTP_AEAD_AES_256_GCM
SRTP_AEAD_AES_128_GCM_8
SRTP_AEAD_AES_256_GCM_8
SRTP_AEAD_AES_128_GCM_12
SRTP_AEAD_AES_256_GCM_12
SRTP_AEAD_AES_128_CCM
SRTP_AEAD_AES_256_CCM
5. Acknowledgements
The author would like to thank Kevin Igoe and many other reviewers
who provided valuable comments on earlier drafts of this document.
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6. References
6.1. Normative References
[GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", U.S. National
Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption with Associated Data", RFC 5116,
January 2008.
[RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key Exchange
version 2 (IKEv2) Protocol", RFC 5282, August 2008.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
6.2. Informative References
[BN00] Bellare, M. and C. Namprempre, "Authenticated encryption:
Relations among notions and analysis of the generic
composition paradigm", Proceedings of ASIACRYPT 2000,
Springer-Verlag, LNCS 1976, pp. 531-545 http://
www-cse.ucsd.edu/users/mihir/papers/oem.html.
[R02] Rogaway, P., "Authenticated encryption with Associated-
Data", ACM Conference on Computer and Communication
Security (CCS'02), pp. 98-107, ACM Press,
2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
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Author's Address
David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 525 8651
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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