The use of AES-192 and AES-256 in Secure RTP
draft-mcgrew-srtp-big-aes-01.txt

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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.

Table of Contents

1.  Introduction
    1.1.  Conventions Used In This Document
2.  AES-192 and AES-256 Encryption
3.  The AES_CM_192_PRF and AES_CM_256_PRF Key Derivation Functions
    3.1.  Usage Requirements
4.  Test Cases
5.  Crypto Suties
6.  IANA Considerations
7.  Security Considerations
8.  Open Questions
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Author's Address

1.  Introduction

This memo describes the use of the Advanced Encryption Standard (AES) [FIPS197] (, “The Advanced Encryption Standard (AES),” .) with 192 and 256 bit keys within the Secure RTP protocol [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.). 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] (, “Fact Sheet for NSA Suite B Cryptography,” .). Others may be motivated by a perceived need to purse a highly conservative security strategy; see Section 7 (Security Considerations) 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] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  AES-192 and AES-256 Encryption

Section 4.1.1 of [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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 (AES Counter Mode. ), using the terminology from Section 4.1.1 of [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) . 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                |<--+
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

3.  The AES_CM_192_PRF and AES_CM_256_PRF Key Derivation Functions

Section 4.3.3 of [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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 (The AES counter mode Key Derivation Function), using the terminology from Section 4.3.3 of [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.). 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                |<--+
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

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.

4.  Test Cases

In a future version of this document, this section will provide test cases that can be used to validate implementations.

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 [RFC4568] (Andreasen, F., Baugher, M., and D. Wing, “Session Description Protocol (SDP) Security Descriptions for Media Streams,” July 2006.)). Other SRTP key management methods that use the crypto functions defined in this document are encouraged to also use these crypto suite definitions.

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 (Crypto Suties).

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.

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.

9.  Acknowledgements

Thanks to Bob Bell for feedback and encouragement.

10.  References

10.1. Normative References

10.2. Informative References

Author's Address