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Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 6188 Cisco Systems, Inc.
Category: Standards Track March 2011
ISSN: 2070-1721
The Use of AES-192 and AES-256 in Secure RTP
Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
with 192- and 256-bit keys within the Secure RTP (SRTP) protocol. It
details counter mode encryption for SRTP and Secure Realtime
Transport Control Protocol (SRTCP) and a new SRTP Key Derivation
Function (KDF) for AES-192 and AES-256.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6188.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
McGrew Standards Track [Page 1]
RFC 6188 SRTP AES-192 and AES-256 March 2011
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. AES-192 and AES-256 Encryption ..................................3
3. The AES_192_CM_PRF and AES_256_CM_PRF Key Derivation Functions ..4
3.1. Usage Requirements .........................................5
4. Crypto Suites ...................................................6
5. IANA Considerations .............................................9
6. Security Considerations .........................................9
7. Test Cases .....................................................10
7.1. AES-256-CM Test Cases .....................................10
7.2. AES_256_CM_PRF Test Cases .................................11
7.3. AES-192-CM Test Cases .....................................13
7.4. AES_192_CM_PRF Test Cases .................................13
8. Acknowledgements ...............................................15
9. References .....................................................15
9.1. Normative References ......................................15
9.2. Informative References ....................................15
McGrew Standards Track [Page 2]
RFC 6188 SRTP AES-192 and AES-256 March 2011
1. Introduction
This memo describes the use of the Advanced Encryption Standard (AES)
[FIPS197] with 192- and 256-bit keys within the Secure RTP (SRTP)
protocol [RFC3711]. Below, those block ciphers are referred to as
AES-192 and AES-256, respectively, and the use of AES with a 128-bit
key is referred to as AES-128. This document describes counter mode
encryption for SRTP and SRTCP and appropriate SRTP key derivation
functions for AES-192 and AES-256. It also defines new crypto suites
that use these new functions.
While AES-128 is widely regarded as more than adequately secure, some
users may be motivated to adopt AES-192 or AES-256 due to a perceived
need to pursue a highly conservative security strategy. For
instance, the Suite B profile requires AES-256 for the protection of
TOP SECRET information [suiteB]. (Note that while the AES-192 and
AES-256 encryption methods defined in this document use Suite B
algorithms, the crypto suites in this document use the HMAC-SHA-1
algorithm, which is not included in Suite B.) See Section 6 for more
discussion of security issues.
The crypto functions described in this document are an addition to,
and not a replacement for, the crypto functions defined in [RFC3711].
1.1. Conventions Used in This Document
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].
2. AES-192 and AES-256 Encryption
Section 4.1.1 of [RFC3711] defines AES counter mode encryption, which
it refers to as AES_CM. This definition applies to all of the AES
key sizes. In this note, AES-192 counter mode and AES-256 counter
mode and are denoted as AES_192_CM and AES_256_CM, respectively. In
both of these ciphers, the plaintext inputs to the block cipher are
formed as in AES_CM, and the block cipher outputs are processed as in
AES_CM. The only difference in the processing is that AES_192_CM
uses AES-192, and AES_256_CM uses AES-256. Both AES_192_CM and
AES_256_CM use a 112-bit salt as an input, as does AES_CM.
For the convenience of the reader, the structure of the counter
blocks in SRTP counter mode encryption is illustrated in Figure 1,
using the terminology from Section 4.1.1 of [RFC3711]. In this
diagram, the symbol (+) denotes the bitwise exclusive-or operation,
and the AES encrypt operation uses AES-128, AES-192, or AES-256 for
AES_CM, AES_192_CM, and AES_256_CM, respectively. The field labeled
McGrew Standards Track [Page 3]
RFC 6188 SRTP AES-192 and AES-256 March 2011
b_c contains a block counter, the value of which increments once for
each invocation of the "AES Encrypt" function. The SSRC field is
part of the RTP header [RFC3550].
one octet
<-->
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00|00|00| SSRC | packet index | b_c |---+
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ v
| salt (k_s) |00|00|->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
v
+-------------+
encryption key (k_e) -> | AES encrypt |
+-------------+
|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| keystream block |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES Counter Mode
3. The AES_192_CM_PRF and AES_256_CM_PRF Key Derivation Functions
Section 4.3.3 of [RFC3711] defines an AES counter mode key derivation
function, which it refers to as AES_CM PRF (and sometimes as AES-CM
PRF). (That specification uses the term PRF, or pseudo-random
function, interchangeably with the phrase "key derivation function".)
This key derivation function can be used with any AES key size. In
this note, the AES-192 counter mode PRF and AES-256 counter mode PRF
are denoted as AES_192_CM_PRF and AES_256_CM_PRF, respectively. In
both of these PRFs, the plaintext inputs to the block cipher are
formed as in the AES_CM PRF, and the block cipher outputs are
processed as in the AES_CM PRF. The only difference in the
processing is that AES_192_CM_PRF uses AES-192, and AES_256_CM_PRF
uses AES-256. Both AES_192_CM_PRF and AES_256_CM_PRF use a 112-bit
salt as an input, as does the AES_CM PRF.
For the convenience of the reader, the structure of the counter
blocks in SRTP counter mode key derivation is illustrated in
Figure 2, using the terminology from Section 4.3.3 of [RFC3711]. In
this diagram, the symbol (+) denotes the bitwise exclusive-or
operation, and the "AES Encrypt" operation uses AES-128, AES-192, or
AES-256 for the AES_CM PRF, AES_192_CM_PRF, and AES_256_CM_PRF,
McGrew Standards Track [Page 4]
RFC 6188 SRTP AES-192 and AES-256 March 2011
respectively. The field "LB" contains the 8-bit constant "label",
which is provided as an input to the key derivation function (and
which is distinct for each type of key generated by that function).
The field labeled b_c contains a block counter, the value of which
increments once for each invocation of the "AES Encrypt" function.
The DIV operation is defined in Section 4.3.1 of [RFC3711] as
follows. Let "a DIV t" denote integer division of a by t, rounded
down, and with the convention that "a DIV 0 = 0" for all a. We also
make the convention of treating "a DIV t" as a bit string of the same
length as a, and thus "a DIV t" will, in general, have leading zeros.
one octet
<-->
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00|00|00|00|00|00|LB| index DIV kdr | b_c |---+
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ v
| master salt |00|00|->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
v
+-------------+
master key -> | AES encrypt |
+-------------+
|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| output block |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2: The AES Counter Mode Key Derivation Function
3.1. Usage Requirements
When AES_192_CM is used for encryption, AES_192_CM_PRF SHOULD be used
as the key derivation function, and AES_128_CM_PRF MUST NOT be used
as the key derivation function.
When AES_256_CM is used for encryption, AES_256_CM_PRF SHOULD be used
as the key derivation function. Both AES_128_CM_PRF and
AES_192_CM_PRF MUST NOT be used as the key derivation function.
AES_256_CM_PRF MAY be used as the key derivation function when AES_CM
is used for encryption, and when AES_192_CM is used for encryption.
AES_192_CM_PRF MAY be used as the key derivation function when AES_CM
is used for encryption.
McGrew Standards Track [Page 5]
RFC 6188 SRTP AES-192 and AES-256 March 2011
Rationale: it is essential that the cryptographic strength of the
key derivation meets or exceeds that of the encryption method. It
is natural to use the same function for both encryption and key
derivation. However, it is not required to do so because it is
desirable to allow these ciphers to be used with alternative key
derivation functions that may be defined in the future.
4. Crypto Suites
This section defines SRTP crypto suites that use the ciphers and key
derivation functions defined in this document. The parameters in
these crypto suites are described in Section 8.2 of [RFC3711]. These
suites are registered with IANA for use with the SDP Security
Descriptions attributes (Section 10.3.2.1 of [RFC4568]). Other SRTP
key management methods that use the crypto functions defined in this
document are encouraged to also use these crypto suite definitions.
Rationale: the crypto suites use the same authentication function
that is mandatory to implement in SRTP, HMAC-SHA1 with a 160-bit
key. HMAC-SHA1 would accept larger key sizes, but when it is used
with keys larger than 160 bits, it does not provide resistance to
cryptanalysis greater than that security level, because it has
only 160 bits of internal state. By retaining 160-bit
authentication keys, the crypto suites in this note have more
compatibility with existing crypto suites and implementations of
them.
McGrew Standards Track [Page 6]
RFC 6188 SRTP AES-192 and AES-256 March 2011
+------------------------------+------------------------------------+
| Parameter | Value |
+------------------------------+------------------------------------+
| Master key length | 192 bits |
| Master salt length | 112 bits |
| Key Derivation Function | AES_192_CM_PRF (Section 3) |
| Default key lifetime | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AES_192_CM (Section 2) |
| SRTP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| SRTP authentication key | 160 bits |
| length | |
| SRTP authentication tag | 80 bits |
| length | |
| SRTCP authentication | HMAC-SHA1 (Section 4.2.1 of |
| function | [RFC3711]) |
| SRTCP authentication key | 160 bits |
| length | |
| SRTCP authentication tag | 80 bits |
| length | |
+------------------------------+------------------------------------+
Table 1: The AES_192_CM_HMAC_SHA1_80 Crypto Suite
+------------------------------+------------------------------------+
| Parameter | Value |
+------------------------------+------------------------------------+
| Master key length | 192 bits |
| Master salt length | 112 bits |
| Key Derivation Function | AES_192_CM_PRF (Section 3) |
| Default key lifetime | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AES_192_CM (Section 2) |
| SRTP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| SRTP authentication key | 160 bits |
| length | |
| SRTP authentication tag | 32 bits |
| length | |
| SRTCP authentication | HMAC-SHA1 (Section 4.2.1 of |
| function | [RFC3711]) |
| SRTCP authentication key | 160 bits |
| length | |
| SRTCP authentication tag | 80 bits |
| length | |
+------------------------------+------------------------------------+
Table 2: The AES_192_CM_HMAC_SHA1_32 Crypto Suite
McGrew Standards Track [Page 7]
RFC 6188 SRTP AES-192 and AES-256 March 2011
+------------------------------+------------------------------------+
| Parameter | Value |
+------------------------------+------------------------------------+
| Master key length | 256 bits |
| Master salt length | 112 bits |
| Key Derivation Function | AES_256_CM_PRF (Section 3) |
| Default key lifetime | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AES_256_CM (Section 2) |
| SRTP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| SRTP authentication key | 160 bits |
| length | |
| SRTP authentication tag | 80 bits |
| length | |
| SRTCP authentication | HMAC-SHA1 (Section 4.2.1 of |
| function | [RFC3711]) |
| SRTCP authentication key | 160 bits |
| length | |
| SRTCP authentication tag | 80 bits |
| length | |
+------------------------------+------------------------------------+
Table 3: The AES_256_CM_HMAC_SHA1_80 Crypto Suite
+------------------------------+------------------------------------+
| Parameter | Value |
+------------------------------+------------------------------------+
| Master key length | 256 bits |
| Master salt length | 112 bits |
| Key Derivation Function | AES_256_CM_PRF (Section 3) |
| Default key lifetime | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AES_256_CM (Section 2) |
| SRTP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| SRTP authentication key | 160 bits |
| length | |
| SRTP authentication tag | 32 bits |
| length | |
| SRTCP authentication | HMAC-SHA1 (Section 4.2.1 of |
| function | [RFC3711]) |
| SRTCP authentication key | 160 bits |
| length | |
| SRTCP authentication tag | 80 bits |
| length | |
+------------------------------+------------------------------------+
Table 4: The AES_256_CM_HMAC_SHA1_32 Crypto Suite
McGrew Standards Track [Page 8]
RFC 6188 SRTP AES-192 and AES-256 March 2011
5. IANA Considerations
IANA has assigned the following parameters in the Session Description
Protocol (SDP) Security Descriptions registry.
+-------------------------+-----------+
| Crypto Suite Name | Reference |
+-------------------------+-----------+
| AES_192_CM_HMAC_SHA1_80 | [RFC6188] |
| AES_192_CM_HMAC_SHA1_32 | [RFC6188] |
| AES_256_CM_HMAC_SHA1_80 | [RFC6188] |
| AES_256_CM_HMAC_SHA1_32 | [RFC6188] |
+-------------------------+-----------+
6. Security Considerations
AES-128 provides a level of security that is widely regarded as being
more than sufficient for providing confidentiality. It is believed
that the economic cost of breaking AES-128 is significantly higher
than the cost of more direct approaches to violating system security,
e.g., theft, bribery, wiretapping, and other forms of malfeasance.
Future advances in state-of-the art cryptanalysis could eliminate
this confidence in AES-128, and motivate the use of AES-192 or AES-
256. AES-192 is regarded as being secure even against some
adversaries for which breaking AES-128 may be feasible. Similarly,
AES-256 is regarded as being secure even against some adversaries for
which it may be feasible to break AES-192. The availability of the
larger key size versions of AES provides a fallback plan in case of
unanticipated cryptanalytic results.
It is conjectured that AES-256 provides adequate security even
against adversaries that possess the ability to construct a quantum
computer that works on 256 or more quantum bits. No such computer is
known to exist; its feasibility is an area of active speculation and
research.
Despite the apparent sufficiency of AES-128, some users are
interested in the larger AES key sizes. For some applications, the
40% increase in computational cost for AES-256 over AES-128 is a
worthwhile bargain when traded for the security advantages outlined
above. These applications include those with a perceived need for
very high security, e.g., due to a desire for very long-term
confidentiality.
AES-256 (as it is used in this note) provides the highest level of
security, and it SHOULD be used whenever the highest possible
security is desired. AES-192 provides a middle ground between the
McGrew Standards Track [Page 9]
RFC 6188 SRTP AES-192 and AES-256 March 2011
128-bit and 256-bit versions of AES, and it MAY be used when security
higher than that of AES-128 is desired. In this note, AES-192 and
AES-256 are used with keys that are generated via a strong pseudo-
random source, and thus the related-key attacks that have been
described in the theoretical literature are not applicable.
As with any cipher, the conjectured security level of AES may change
over time. The considerations in this section reflect the best
knowledge available at the time of publication of this document.
It is desirable that AES_192_CM and AES_192_CM_PRF be used with an
authentication function that uses a 192-bit key, and that AES_256_CM
and AES_256_CM_PRF be used with an authentication function that uses
a 256-bit key. However, this desire is not regarded as security
critical. Cryptographic authentication is resilient against future
advances in cryptanalysis, since the opportunity for a forgery attack
against a session closes when that session closes. For this reason,
this note defines new ciphers, but not new authentication functions.
7. Test Cases
The test cases in this section are based on Appendix B of [RFC3711].
7.1. AES-256-CM Test Cases
Keystream segment length: 1044512 octets (65282 AES blocks)
Session Key: 57f82fe3613fd170a85ec93c40b1f092
2ec4cb0dc025b58272147cc438944a98
Rollover Counter: 00000000
Sequence Number: 0000
SSRC: 00000000
Session Salt: f0f1f2f3f4f5f6f7f8f9fafbfcfd0000 (already shifted)
Offset: f0f1f2f3f4f5f6f7f8f9fafbfcfd0000
Counter Keystream
f0f1f2f3f4f5f6f7f8f9fafbfcfd0000 92bdd28a93c3f52511c677d08b5515a4
f0f1f2f3f4f5f6f7f8f9fafbfcfd0001 9da71b2378a854f67050756ded165bac
f0f1f2f3f4f5f6f7f8f9fafbfcfd0002 63c4868b7096d88421b563b8c94c9a31
... ...
f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff cea518c90fd91ced9cbb18c078a54711
f0f1f2f3f4f5f6f7f8f9fafbfcfdff00 3dbc4814f4da5f00a08772b63c6a046d
f0f1f2f3f4f5f6f7f8f9fafbfcfdff01 6eb246913062a16891433e97dd01a57f
McGrew Standards Track [Page 10]
RFC 6188 SRTP AES-192 and AES-256 March 2011
7.2. AES_256_CM_PRF Test Cases
This section provides test data for the AES_256_CM_PRF key derivation
function, which uses AES-256 in counter mode. In the following, we
walk through the initial key derivation for the AES-256 counter mode
cipher, which requires a 32-octet session encryption key and a 14-
octet session salt, and the HMAC-SHA1 authentication function, which
requires a 20-octet session authentication key. These values are
called the cipher key, the cipher salt, and the auth key in the
following. Since this is the initial key derivation and the key
derivation rate is equal to zero, the value of (index DIV
key_derivation_rate) is zero (actually, a six-octet string of zeros).
In the following, we shorten key_derivation_rate to kdr.
The inputs to the key derivation function are the 32-octet master key
and the 14-octet master salt:
master key: f0f04914b513f2763a1b1fa130f10e29
98f6f6e43e4309d1e622a0e332b9f1b6
master salt: 3b04803de51ee7c96423ab5b78d2
We first show how the cipher key is generated. The input block for
AES-256-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption key label 0x00 with (index DIV kdr),
then padding on the right with two null octets (which implements the
multiply-by-2^16 operation, see Section 4.3.3 of RFC 3711). The
resulting value is then AES-256-CM-encrypted using the master key to
get the cipher key.
index DIV kdr: 000000000000
label: 00
master salt: 3b04803de51ee7c96423ab5b78d2
-----------------------------------------------
xor: 3b04803de51ee7c96423ab5b78d2 (x, PRF input)
x*2^16: 3b04803de51ee7c96423ab5b78d20000 (AES-256-CM input)
x*2^16 + 1: 3b04803de51ee7c96423ab5b78d20001 (2nd AES input)
cipher key: 5ba1064e30ec51613cad926c5a28ef73 (1st AES output)
1ec7fb397f70a960653caf06554cd8c4 (2nd AES output)
Next, we show how the cipher salt is generated. The input block for
AES-256-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption salt label. That value is padded and
encrypted as above.
McGrew Standards Track [Page 11]
RFC 6188 SRTP AES-192 and AES-256 March 2011
index DIV kdr: 000000000000
label: 02
master salt: 3b04803de51ee7c96423ab5b78d2
----------------------------------------------
xor: 3b04803de51ee7cb6423ab5b78d2 (x, PRF input)
x*2^16: 3b04803de51ee7cb6423ab5b78d20000 (AES-256-CM input)
fa31791685ca444a9e07c6c64e93ae6b (AES-256 ouptut)
cipher salt: fa31791685ca444a9e07c6c64e93
We now show how the auth key is generated. The input block for AES-
256-CM is generated as above, but using the authentication key label.
index DIV kdr: 000000000000
label: 01
master salt: 3b04803de51ee7c96423ab5b78d2
-----------------------------------------------
xor: 3b04803de51ee7c86423ab5b78d2 (x, PRF input)
x*2^16: 3b04803de51ee7c86423ab5b78d20000 (AES-256-CM in)
Below, the AES-256 output blocks that form the auth key are shown
on the left, while the corresponding AES-256 input blocks are shown
on the right. Note that the final AES-256 output is truncated to a
4-byte length. The final auth key is shown below.
auth key blocks AES-256 input blocks
fd9c32d39ed5fbb5a9dc96b30818454d 3b04803de51ee7c86423ab5b78d20000
1313dc05 3b04803de51ee7c86423ab5b78d20001
auth key: fd9c32d39ed5fbb5a9dc96b30818454d1313dc05
McGrew Standards Track [Page 12]
RFC 6188 SRTP AES-192 and AES-256 March 2011
7.3. AES-192-CM Test Cases
Keystream segment length: 1044512 octets (65282 AES blocks)
Session Key: eab234764e517b2d3d160d587d8c8621
9740f65f99b6bcf7
Rollover Counter: 00000000
Sequence Number: 0000
SSRC: 00000000
Session Salt: f0f1f2f3f4f5f6f7f8f9fafbfcfd0000 (already shifted)
Offset: f0f1f2f3f4f5f6f7f8f9fafbfcfd0000
Counter Keystream
f0f1f2f3f4f5f6f7f8f9fafbfcfd0000 35096cba4610028dc1b57503804ce37c
f0f1f2f3f4f5f6f7f8f9fafbfcfd0001 5de986291dcce161d5165ec4568f5c9a
f0f1f2f3f4f5f6f7f8f9fafbfcfd0002 474a40c77894bc17180202272a4c264d
... ...
f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff d108d1a31a00bad6367ec23eb044b415
f0f1f2f3f4f5f6f7f8f9fafbfcfdff00 c8f57129fdeb970b59f917b257662d4c
f0f1f2f3f4f5f6f7f8f9fafbfcfdff01 a5dab625811034e8cebdfeb6dc158dd3
7.4. AES_192_CM_PRF Test Cases
This section provides test data for the AES_192_CM_PRF key derivation
function, which uses AES-192 in counter mode. In the following, we
walk through the initial key derivation for the AES-192 counter mode
cipher, which requires a 24-octet session encryption key and a 14-
octet session salt, and the HMAC-SHA1 authentication function, which
requires a 20-octet session authentication key. These values are
called the cipher key, the cipher salt, and the auth key in the
following. Since this is the initial key derivation and the key
derivation rate is equal to zero, the value of (index DIV
key_derivation_rate) is zero (actually, a six-octet string of zeros).
In the following, we shorten key_derivation_rate to kdr.
The inputs to the key derivation function are the 24-octet master key
and the 14-octet master salt:
master key: 73edc66c4fa15776fb57f9505c171365
50ffda71f3e8e5f1
master salt: c8522f3acd4ce86d5add78edbb11
We first show how the cipher key is generated. The input block for
AES-192-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption key label 0x00 with (index DIV kdr),
then padding on the right with two null octets (which implements the
McGrew Standards Track [Page 13]
RFC 6188 SRTP AES-192 and AES-256 March 2011
multiply-by-2^16 operation, see Section 4.3.3 of RFC 3711). The
resulting value is then AES-192-CM encrypted using the master key to
get the cipher key.
index DIV kdr: 000000000000
label: 00
master salt: c8522f3acd4ce86d5add78edbb11
-----------------------------------------------
xor: c8522f3acd4ce86d5add78edbb11 (x, PRF input)
x*2^16: c8522f3acd4ce86d5add78edbb110000 (AES-192-CM input)
x*2^16 + 1: c8522f3acd4ce86d5add78edbb110001 (2nd AES input)
cipher key: 31874736a8f1143870c26e4857d8a5b2 (1st AES output)
c4a354407faadabb (2nd AES output)
Next, we show how the cipher salt is generated. The input block for
AES-192-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption salt label. That value is padded and
encrypted as above.
index DIV kdr: 000000000000
label: 02
master salt: c8522f3acd4ce86d5add78edbb11
----------------------------------------------
xor: c8522f3acd4ce86f5add78edbb11 (x, PRF input)
x*2^16: c8522f3acd4ce86f5add78edbb110000 (AES-192-CM input)
2372b82d639b6d8503a47adc0a6c2590 (AES-192 ouptut)
cipher salt: 2372b82d639b6d8503a47adc0a6c
We now show how the auth key is generated. The input block for AES-
192-CM is generated as above, but using the authentication key label.
index DIV kdr: 000000000000
label: 01
master salt: c8522f3acd4ce86d5add78edbb11
-----------------------------------------------
xor: c8522f3acd4ce86c5add78edbb11 (x, PRF input)
x*2^16: c8522f3acd4ce86c5add78edbb110000 (AES-192-CM in)
McGrew Standards Track [Page 14]
RFC 6188 SRTP AES-192 and AES-256 March 2011
Below, the AES-192 output blocks that form the auth key are shown
on the left, while the corresponding AES-192 input blocks are shown
on the right. Note that the final AES-192 output is truncated to a
four-byte length. The final auth key is shown below.
auth key blocks AES-192 input blocks
355b10973cd95b9eacf4061c7e1a7151 c8522f3acd4ce86c5add78edbb110000
e7cfbfcb c8522f3acd4ce86c5add78edbb110001
auth key: 355b10973cd95b9eacf4061c7e1a7151e7cfbfcb
8. Acknowledgements
Thanks are due to John Mattsson for verifying the test cases in the
document and providing comments, to Bob Bell for feedback and
encouragement, and to Richard Barnes and Hilarie Orman for
constructive review.
9. References
9.1. Normative References
[FIPS197] "The Advanced Encryption Standard (AES)", FIPS-197 Federal
Information Processing Standard.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
9.2. Informative References
[suiteB] "Suite B Cryptography", http://www.nsa.gov/ia/programs/
suiteb_cryptography/index.shtml.
McGrew Standards Track [Page 15]
RFC 6188 SRTP AES-192 and AES-256 March 2011
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
McGrew Standards Track [Page 16]