Network Working Group D. McGrew Internet-Draft Cisco Systems, Inc. Intended status: Standards Track October 25, 2009 Expires: April 28, 2010 The use of AES-192 and AES-256 in Secure RTP draft-ietf-avt-srtp-big-aes-02.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 April 28, 2010. Copyright Notice Copyright (c) 2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. McGrew Expires April 28, 2010 [Page 1] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions Used In This Document . . . . . . . . . . . . 3 2. AES-192 and AES-256 Encryption . . . . . . . . . . . . . . . . 4 3. The AES_192_CM_PRF and AES_256_CM_PRF Key Derivation Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Usage Requirements . . . . . . . . . . . . . . . . . . . . 6 4. Crypto Suties . . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. AES-256-CM Test Cases . . . . . . . . . . . . . . . . . . 13 7.2. AES_256_CM_PRF Test Cases . . . . . . . . . . . . . . . . 13 7.3. AES-192-CM Test Cases . . . . . . . . . . . . . . . . . . 15 7.4. AES_192_CM_PRF Test Cases . . . . . . . . . . . . . . . . 15 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 9.1. Normative References . . . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20 McGrew Expires April 28, 2010 [Page 2] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 6 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 April 28, 2010 [Page 3] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 April 28, 2010 [Page 4] Internet-Draft SRTP AES-192 and AES-256 October 2009 3. The AES_192_CM_PRF and AES_256_CM_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 phrase "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 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. 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 April 28, 2010 [Page 5] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 April 28, 2010 [Page 6] Internet-Draft SRTP AES-192 and AES-256 October 2009 4. 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]). 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. +------------------------------+------------------------------------+ | 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 | | | | | McGrew Expires April 28, 2010 [Page 7] Internet-Draft SRTP AES-192 and AES-256 October 2009 | SRTCP authentication tag | 80 bits | | length | | +------------------------------+------------------------------------+ Table 1: The AES_192_CM_HMAC_SHA1_80 cryptosuite. +------------------------------+------------------------------------+ | 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 cryptosuite. McGrew Expires April 28, 2010 [Page 8] Internet-Draft SRTP AES-192 and AES-256 October 2009 +------------------------------+------------------------------------+ | 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 cryptosuite. McGrew Expires April 28, 2010 [Page 9] Internet-Draft SRTP AES-192 and AES-256 October 2009 +------------------------------+------------------------------------+ | 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 cryptosuite. McGrew Expires April 28, 2010 [Page 10] Internet-Draft SRTP AES-192 and AES-256 October 2009 5. IANA Considerations IANA is expected to assign the following parameters for the SDP Security Descriptions crypto suite attribute. AES_192_CM_HMAC_SHA1_80 AES_192_CM_HMAC_SHA1_32 AES_256_CM_HMAC_SHA1_80 AES_256_CM_HMAC_SHA1_32 The cryptosuites are as defined in Section 4. McGrew Expires April 28, 2010 [Page 11] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 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 April 28, 2010 [Page 12] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 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 McGrew Expires April 28, 2010 [Page 13] Internet-Draft SRTP AES-192 and AES-256 October 2009 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. 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. McGrew Expires April 28, 2010 [Page 14] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 four-byte length. The final auth key is shown below. auth key blocks AES-256 input blocks fd9c32d39ed5fbb5a9dc96b30818454d 3b04803de51ee7c86423ab5b78d20000 1313dc05 3b04803de51ee7c86423ab5b78d20001 auth key: fd9c32d39ed5fbb5a9dc96b30818454d1313dc05 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 McGrew Expires April 28, 2010 [Page 15] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 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. McGrew Expires April 28, 2010 [Page 16] Internet-Draft SRTP AES-192 and AES-256 October 2009 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) 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 McGrew Expires April 28, 2010 [Page 17] Internet-Draft SRTP AES-192 and AES-256 October 2009 8. Acknowledgements Thanks are due to John Mattsson for verifying the test cases in the document, and to Bob Bell for feedback and encouragement. McGrew Expires April 28, 2010 [Page 18] Internet-Draft SRTP AES-192 and AES-256 October 2009 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. [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 Expires April 28, 2010 [Page 19] Internet-Draft SRTP AES-192 and AES-256 October 2009 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 April 28, 2010 [Page 20]