Network Working Group D. McGrew
Internet-Draft Cisco Systems, Inc.
Expires: October 28, 2006 April 26, 2006
The use of AES-192 and AES-256 in Secure RTP
draft-mcgrew-srtp-big-aes-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
with 192 and 256 bit keys within the Secure RTP protocol. It defines
Counter Mode encryption for SRTP and SRTCP and a new SRTP Key
Derivation Function (KDF) for AES-192 and AES-256.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Conventions Used In This Document . . . . . . . . . . . . 3
2. AES-192 and AES-256 Encryption . . . . . . . . . . . . . . . 4
3. The AES_CM_192_PRF and AES_CM_256_PRF Key Derivation
Functions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Usage Requirements . . . . . . . . . . . . . . . . . . . . 6
4. Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Crypto Suties . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . 13
8. Open Questions . . . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1 Normative References . . . . . . . . . . . . . . . . . . 16
10.2 Informative References . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 17
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1. Introduction
This memo describes the use of the Advanced Encryption Standard (AES)
[FIPS197] with 192 and 256 bit keys within the Secure RTP 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 defines Counter Mode
encryption for SRTP and SRTCP and a new SRTP Key Derivation Function
for AES-192 and AES-256. It also defines new cryptosuites 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. One motivation
is conformance to the Suite B profile (which requires AES-256 for the
protection of TOP SECRET information) [suiteB]. Others may be
motivated by a perceived need to purse a highly conservative security
strategy; see Section 7 for more discussion of security issues.
The crypto functions defined 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].
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2. AES-192 and AES-256 Encryption
Section 4.1.1 of [RFC3711] defines AES-128 counter mode encryption,
which it refers to as AES_CM. AES-192 counter mode and AES-256
counter mode are defined in a similar manner, 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
b_c contains a block counter, the value of which increments once for
each invocation of the "AES Encrypt" function.
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.
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3. The AES_CM_192_PRF and AES_CM_256_PRF Key Derivation Functions
Section 4.3.3 of [RFC3711] defines AES-128 counter mode key
derivation function, which it refers to as "AES-CM PRF". (That
specification uses the term PRF, or pseudo-random function,
interchangeably with the term "key derivation function". ) The AES-
192 counter mode PRF and AES-256 counter mode PRF are defined in a
similar manner, and 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,
respectively. The field "LB" contains the 8-bit constant "label"
which is provided as an input to the key derivation function (and
which is distint for each 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.
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
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3.1 Usage Requirements
When AES_192_CM is used for encryption, AES_192_CM SHOULD be used as
the key derivation function, and AES_128_CM MUST NOT be used as the
key derivation function.
When AES_256_CM is used for encryption, AES_256_CM SHOULD be used as
the key derivation function. Both AES_128_CM and AES_192_CM MUST NOT
be used as the key derivation function.
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.
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4. Test Cases
In a future version of this document, this section will provide test
cases that can be used to validate implementations.
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5. Crypto Suties
This section defines SRTP crypto suites that use the ciphers and key
derivation functions defined in this document. These suites are
registered with IANA for use with the SDP Security Descriptions
attributes (Section 10.3.2.1 of [I-D.ietf-mmusic-sdescriptions]).
Other SRTP key management methods that use the crypto functions
defined in this document are encouraged to also use these crypto
suite definitions.
+---------------------------------+---------------------------------+
| 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 length | 160 bits |
| | |
| SRTP authentication tag length | 80 bits |
| | |
| SRTCP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| | |
| SRTCP authentication key length | 160 bits |
| | |
| SRTCP authentication tag length | 80 bits |
+---------------------------------+---------------------------------+
Table 1: The AES_CM_192_HMAC_SHA1_80 cryptosuite.
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+---------------------------------+---------------------------------+
| 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 length | 160 bits |
| | |
| SRTP authentication tag length | 32 bits |
| | |
| SRTCP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| | |
| SRTCP authentication key length | 160 bits |
| | |
| SRTCP authentication tag length | 80 bits |
+---------------------------------+---------------------------------+
Table 2: The AES_CM_192_HMAC_SHA1_32 cryptosuite.
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+---------------------------------+---------------------------------+
| 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 length | 160 bits |
| | |
| SRTP authentication tag length | 80 bits |
| | |
| SRTCP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| | |
| SRTCP authentication key length | 160 bits |
| | |
| SRTCP authentication tag length | 80 bits |
+---------------------------------+---------------------------------+
Table 3: The AES_CM_256_HMAC_SHA1_80 cryptosuite.
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+---------------------------------+---------------------------------+
| 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 length | 160 bits |
| | |
| SRTP authentication tag length | 32 bits |
| | |
| SRTCP authentication function | HMAC-SHA1 (Section 4.2.1 of |
| | [RFC3711]) |
| | |
| SRTCP authentication key length | 160 bits |
| | |
| SRTCP authentication tag length | 80 bits |
+---------------------------------+---------------------------------+
Table 4: The AES_CM_256_HMAC_SHA1_32 cryptosuite.
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6. IANA Considerations
IANA is expected to assign the following parameters for the SDP
Security Descriptions crypto suite attribute.
AES_CM_192_HMAC_SHA1_80
AES_CM_192_HMAC_SHA1_32
AES_CM_256_HMAC_SHA1_80
AES_CM_256_HMAC_SHA1_32
The cryptosuites are as defined in Section 5.
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7. 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 the state of the art of 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.
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.
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8. Open Questions
It may be desirable to eliminate AES-192 altogether, leaving users
with the simpler choice of using AES-128 or AES-256. This option
preserves the possibility of Suite B conformance. Given that the
incremental computational cost of AES-256 over AES-192 is only 16%,
and the additional key storage overhead is only 33%, this option
imposes only a minimal burden on implementations.
It may be desirable to use AES in the Chained Message Authentication
Code (CMAC) mode of operation [CMAC] in conjunction with the ciphers
defined in this document, with the CMAC key size matching the counter
mode key size. This mode of operation can be used as a replacement
for HMAC-SHA1, and that use would provide an authentication function
with security that is directly comparable to AES-192 and AES-256.
This mode of operation has some additional benefits and may be worth
considering for secure RTP.
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9. Acknowledgements
Thanks to Bob Bell for feedback and encouragement.
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10. References
10.1 Normative References
[FIPS197] "The Advanced Encryption Standard (AES)", FIPS-197 Federal
Information Processing Standard.
[I-D.ietf-mmusic-sdescriptions]
Andreasen, F., "Session Description Protocol Security
Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-12 (work in progress),
September 2005.
[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.
10.2 Informative References
[CMAC] "NIST Special Publication 800-38B", http://csrc.nist.gov/
CryptoToolkit/modes/800-38_Series_Publications/
SP800-38B.pdf.
[suiteB] "Fact Sheet for NSA Suite B Cryptography",
http://www.nsa.gov/ia/industry/crypto_suite_b.cfm.
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
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