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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 28, 2006. 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. McGrew Expires October 28, 2006 [Page 1] Internet-Draft SRTP AES-192 and AES-256 April 2006 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 McGrew Expires October 28, 2006 [Page 2] Internet-Draft SRTP AES-192 and AES-256 April 2006 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]. McGrew Expires October 28, 2006 [Page 3] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 4] Internet-Draft SRTP AES-192 and AES-256 April 2006 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 McGrew Expires October 28, 2006 [Page 5] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 6] Internet-Draft SRTP AES-192 and AES-256 April 2006 4. Test Cases In a future version of this document, this section will provide test cases that can be used to validate implementations. McGrew Expires October 28, 2006 [Page 7] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 8] Internet-Draft SRTP AES-192 and AES-256 April 2006 +---------------------------------+---------------------------------+ | 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. McGrew Expires October 28, 2006 [Page 9] Internet-Draft SRTP AES-192 and AES-256 April 2006 +---------------------------------+---------------------------------+ | 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. McGrew Expires October 28, 2006 [Page 10] Internet-Draft SRTP AES-192 and AES-256 April 2006 +---------------------------------+---------------------------------+ | 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. McGrew Expires October 28, 2006 [Page 11] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 12] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 13] Internet-Draft SRTP AES-192 and AES-256 April 2006 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. McGrew Expires October 28, 2006 [Page 14] Internet-Draft SRTP AES-192 and AES-256 April 2006 9. Acknowledgements Thanks to Bob Bell for feedback and encouragement. McGrew Expires October 28, 2006 [Page 15] Internet-Draft SRTP AES-192 and AES-256 April 2006 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 McGrew Expires October 28, 2006 [Page 16] Internet-Draft SRTP AES-192 and AES-256 April 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. McGrew Expires October 28, 2006 [Page 17]