AVT Working Group S. Yoon Internet Draft J. Kim Expires: May 2009 H. Park H. Jeong Y. Won Korea Information Security Agency November 18, 2008 The SEED Cipher Algorithm and Its Use with the Secure Real-time Transport Protocol (SRTP) draft-ietf-avt-seed-srtp-07 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 May 18, 2009. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract This document describes the use of the SEED block cipher algorithm in the Secure Real-time Transport Protocol (SRTP) for providing confidentiality for the Real-time Transport Protocol (RTP) traffic Yoon, et al. Expires November 18, 2009 [Page 1] Internet-Draft SEED-SRTP November 5, 2008 and for the control traffic for RTP, the Real-time Transport Control Protocol (RTCP). Table of Contents 1. Introduction..................................................3 1.1. SEED.....................................................3 1.2. Terminology..............................................3 2. Cryptographic Transforms......................................3 2.1. Counter..................................................3 2.1.1. Message Authentication/Integrity: HMAC-SHA1.........4 2.2. Counter with CBC-MAC (CCM)...............................4 2.2.1. Nonce Format........................................5 2.3. Galois/Counter Mode (GCM)................................5 3. Key Derivation: SEED-CTR PRF..................................5 4. Default and mandatory-to-implement Transforms.................6 5. Security Considerations.......................................6 6. References....................................................7 6.1. Normative References.....................................7 6.2. Informative References...................................7 APPENDIX A: Test Vectors.........................................8 A.1. SEED-CTR Test Vectors....................................8 A.2. SEED-CCM Test Vectors....................................9 A.3. SEED-GCM Test Vectors...................................10 Author's Addresses..............................................11 Yoon, et al. Expires November 18, 2009 [Page 2] Internet-Draft SEED-SRTP November 5, 2008 1. Introduction This document describes the use of the SEED [RFC4009] block cipher algorithm in the Secure Real-time Transport Protocol (SRTP) [RFC3711] for providing confidentiality for the Real-time Transport Protocol (RTP) [RFC3550] traffic and for the control traffic for RTP, the Real-time Transport Control Protocol (RTCP) [RFC3550] 1.1. SEED SEED is a Korean National Industrial Association standard [TTASSEED] and is widely used in South Korea for electronic commerce and financial services that are operated on wired and wireless communications. SEED is a 128-bit symmetric key block cipher that has been developed by KISA (Korea Information Security Agency) and a group of experts since 1998. The input/output block size of SEED is 128-bit and the key length is also 128-bit. SEED has a 16-round Feistel structure. 1.2. Terminology 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 [RFC-2119]. 2. Cryptographic Transforms All symmetric block cipher algorithms share common characteristics and valuables, including mode, key size, weak keys, and block size. The following sections contain description of the relevant characteristics of SEED. SEED does not have any restrictions for modes of operation that are used with this block cipher. We define three modes of running SEED, (1) SEED in Counter Mode, (2) SEED in Counter with CBC-MAC (CCM) Mode and (3) SEED in Galois/Counter Mode (GCM) Mode. 2.1. Counter Section 4.1.1 of [RFC3711] defines AES counter mode encryption, which it refers to as AES-CM. SEED counter mode is defined in a similar manner, and is denoted as SEED-CTR. 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 SEED-CTR uses SEED as the underlying encryption primitive. Implementations of this specification MUST use SEED-CTR in conjunction with an authentication function. Yoon, et al. Expires November 18, 2009 [Page 3] Internet-Draft SEED-SRTP November 5, 2008 2.1.1. Message Authentication/Integrity: HMAC-SHA1 HMAC-SHA1 [RFC2104], as defined in section 4.2.1 of [RFC3711], SHALL be the default message authentication code. The default session authentication key-length SHALL be 160 bits, the default authentication tag length SHALL be 80 bits, and the SRTP_PREFIX_LENGTH SHALL be zero for HMAC-SHA1. For SRTP, smaller values are NOT RECOMMENDED, but MAY be used after careful consideration of the issues in section 7.5 and 9.5 of [RFC3711]. 2.2. Counter with CBC-MAC (CCM) CCM is a generic authenticate-and-encrypt block cipher mode [RFC3610]. In this specification, CCM is used with the SEED block cipher is denoted as SEED-CCM. SEED-CCM first authenticates and then encrypts a message to construct an SRTP packet. This does not comply with the SRTP packet processing defined in section 3.3 of [RFC3711]. SEED-CCM has two parameters: M M indicates the size of the authentication tag. In SRTP, a full 80-bit authentication-tag SHOULD be used and implementation of this specification MUST support M values of 10 octets. L L indicates the size of the length field in octets. The number of octets in the nonce SHOULD be 12, i.e., L is 3. SEED-CCM has four inputs: Key A single key is used to calculate the authentication tag using CBC-MAC and to perform payload encryption using counter mode. SEED only supports key size of 128 bits. Nonce The size of the nonce depends on the value selected for the parameter L. L equals 3 and hence the nonce size equals 12 octets. Payload The payload of the RTP packet. Yoon, et al. Expires November 18, 2009 [Page 4] Internet-Draft SEED-SRTP November 5, 2008 Additional Authentication Data (AAD) The header of the RTP packet. SEED-CCM generates one output: The final result c consists of the encrypted RTP payload followed by the authentication tag. 2.2.1. Nonce Format The nonce passed to the CCM-SEED encryption algorithm has the following layout: 0 0 0 0 0 0 0 0 0 0 1 1 0 1 2 3 4 5 6 7 8 9 0 1 +--+--+--+--+--+--+--+--+--+--+--+--+ |00|00| SSRC | ROC | SEQ |---+ +--+--+--+--+--+--+--+--+--+--+--+--+ | | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Salt | |->(+) +--+--+--+--+--+--+--+--+--+--+--+--+ | | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Initialization Vector |<--+ +--+--+--+--+--+--+--+--+--+--+--+--+ Figure 1: Initialization Vector formation. The salt field is 24bits and the initialization vector is 64bits. SSRC, ROC SHALL be taken from the RTP header; ROC is from the cryptographic context. 2.3. Galois/Counter Mode (GCM) GCM is a block cipher mode of operation providing both confidentiality and data origin authentication [GCM].GCM used with the SEED block cipher is denoted as SEED-GCM. SEED-GCM has four inputs: a key, a payload, a nonce and an additional authenticated data (AAD) all described in section 2.1.2. 3. Key Derivation: SEED-CTR PRF Section 4.3.3 of [RFC3711] defines AES-128 counter mode key derivation function, which it refers to as "AES-CM PRF". The SEED-CTR PRF is defined in a similar manner. Yoon, et al. Expires November 18, 2009 [Page 5] Internet-Draft SEED-SRTP November 5, 2008 The currently defined PRF, keyed by 128 bit master key, has input block size m = 128 and can produce n-bit outputs for n up to 2^23. SEED-PRF_n(k_master, x) SHALL be SEED in Counter Mode as described in section 2.1, applied to key k_master, and IV equal to (x*2^16), and with the output keystream truncated to the n first (left-most) bits. 4. Default and mandatory-to-implement Transforms The default transforms also are mandatory-to-implement transforms in SRTP. Of course, "mandatory-to-implement" does not imply "mandatory- to-use". Table 1 summarizes the pre-defined transforms. The default values below are valid for the pre-defined transforms. man.-to-impl. optional default encryption SEED-CTR SEED-CCM,SEED-GCM SEED-CTR message integrity HMAC-SHA1 SEED-CCM,SEED-GCM HMAC-SHA1 key derivation (PRF) SEED-CTR - SEED-CTR Table 1: Mandatory-to-implement, optional and default transforms in SRTP and SRTCP. 5. Security Considerations No security problem has been found on SEED. SEED is secure against all known attacks including Differential cryptanalysis, linear cryptanalysis, and related key attacks. The best known attack is only an exhaustive search for the key. For further security considerations, the reader is encouraged to read [SEED-EVAL]. See [RFC3610] and [GCM] for security considerations regarding the CCM and GCM Modes of Operation, respectively. In the context of SRTP, the procedures in [RFC3711] ensure the critical property of non-reuse of counter values. Yoon, et al. Expires November 18, 2009 [Page 6] Internet-Draft SEED-SRTP November 5, 2008 6. References 6.1. Normative References [GCM] Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", U.S. National Institute of Standards and Technology http://csrc.nist.gov/Publications/nistpubs/800-38D/SP- 800-38D.pdf [RFC2104] Krawczyk, H.,Bellare, M. and R. Canetti, "HMAC: keyed- Hashing for Message Authentication", RFC 2104, February 1997. [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", RFC3550, July 2003 [RFC3711] M. Baugher, D. McGrew, M. Naslund, E.Carrara, K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. 6.2. Informative References [ISOSEED] ISO/IEC JTC 1/SC 27, "National Body contributions on NP 18033 "Encryption Algorithms" in Response to SC 27 N2563 (ATT.3 Korea Contribution)", ISO/IEC JTC 1/SC 27 N2656r1 (n2656_3.zip), October, 2000. [SEED-EVAL] KISA, "Self Evaluation Report", http://www.kisa.or.kr/kisa/seed/data/Document_pdf/SEED_Ev aluation_Report_by_CRYPTREC.pdf [TTASSEED] Telecommunications Technology Association (TTA), South Korea, "128-bit Symmetric Block Cipher (SEED)", TTAS.KO- 12.0004, September, 1998 (In Korean) http://www.tta.or.kr/English/new/main/index.htm [RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with CBC-MAC (CCM), RFC 3610, September 2003. [RFC4009] Park, J., Lee, S., Kim, J., and J. Lee, "The SEED Encryption Algorithm", RFC 4009, February 2005. Yoon, et al. Expires November 18, 2009 [Page 7] Internet-Draft SEED-SRTP November 5, 2008 APPENDIX A: Test Vectors All values are in hexadecimal. A.1. SEED-CTR Test Vectors Session Key: 0c5ffd37a11edc42c325287fc0604f2e Rollover Counter: 00000000 Sequence Number: 315e SSRC: 20e8f5eb Session Salt: cd3a7c42c671e0067a2a2639b43a Initial Vector: cd3a7c42e69915ed7a2a263985640000 RTP Payload: f57af5fd4ae19562976ec57a5a7ad55a 5af5c5e5c5fdf5c55ad57a4a7272d572 62e9729566ed66e97ac54a4a5a7ad5e1 5ae5fdd5fd5ac5d56ae56ad5c572d54a e54ac55a956afd6aed5a4ac562957a95 16991691d572fd14e97ae962ed7a9f4a 955af572e162f57a956666e17ae1f54a 95f566d54a66e16e4afd6a9f7ae1c5c5 5ae5d56afde916c5e94a6ec56695e14a fde1148416e94ad57ac5146ed59d1cc5 Encrypted RTP Payload: df5a89291e7e383e9beff765e691a737 70d5b9319162589956544855ce99a71f 48c90e413272cbb576447855e691a78c 70c58101a9c56889666458ca7999a727 cf6ab98ec1f55036e1db78dade7e08f8 3cb96a4581ed5048e5fbdb7d5191ed27 bf7a89a6b5fd582699e754fec60a8727 bfd51a011ef94c32467c5880c60ab7a8 70c5a9bea976bb99e5cb5cdada7e9327 d7c168504276e7897644267169766ea8 Authentication Tag: 28b7a194b1e3df3c573d Yoon, et al. Expires November 18, 2009 [Page 8] Internet-Draft SEED-SRTP November 5, 2008 A.2. SEED-CCM Test Vectors Key: 974bee725d44fc3992267b284c3c6750 Rollover Counter: 00000000 Sequence Number: 315e SSRC: 20e8f5eb Nonce: b446cb282d9c7cf8d620670d Payload: f57af5fd4ae19562976ec57a5a7ad55a 5af5c5e5c5fdf5c55ad57a4a7272d572 62e9729566ed66e97ac54a4a5a7ad5e1 5ae5fdd5fd5ac5d56ae56ad5c572d54a e54ac55a956afd6aed5a4ac562957a95 16991691d572fd14e97ae962ed7a9f4a 955af572e162f57a956666e17ae1f54a 95f566d54a66e16e4afd6a9f7ae1c5c5 5ae5d56afde916c5e94a6ec56695e14a fde1148416e94ad57ac5146ed59d1cc5 AAD: 8008315ebf2e6fe020e8f5eb Encrypted RTP Payload: 39b63931862d59ae5ba209b696b61996 96390929093139099619b686bebe19be ae25be59aa21aa25b609868696b6192d 9629311931960919a629a61909be1986 2986099659a631a621968609ae59b659 da55da5d19be31d825b625ae21b65386 599639be2dae39b659aaaa2db62d3986 5939aa1986aa2da28631a653b62d0909 962919a63125da092586a209aa592d86 312dd848da258619b609d8a21951d009 Authentication Tag: 1ea5f4dabf178ebf8cec Yoon, et al. Expires November 18, 2009 [Page 9] Internet-Draft SEED-SRTP November 5, 2008 A.3. SEED-GCM Test Vectors Key: e91e5e75da65554a48181f3846349562 Rollover Counter: 00000000 Sequence Number: 315e SSRC: 20e8f5eb Initial Vector: a679885189eb9982113f7152 Payload: f57af5fd4ae19562976ec57a5a7ad55a 5af5c5e5c5fdf5c55ad57a4a7272d572 62e9729566ed66e97ac54a4a5a7ad5e1 5ae5fdd5fd5ac5d56ae56ad5c572d54a e54ac55a956afd6aed5a4ac562957a95 16991691d572fd14e97ae962ed7a9f4a 955af572e162f57a956666e17ae1f54a 95f566d54a66e16e4afd6a9f7ae1c5c5 5ae5d56afde916c5e94a6ec56695e14a fde1148416e94ad57ac5146ed59d1cc5 AAD: 8008315ebf2e6fe020e8f5eb Encrypted RTP Payload: 05863f37b87ac8ac5977b2bb66c22d70 bc5e4240c7d175adc56273aaafe63a3d 3b28292f2484056e14cc3b1bdd59bffa 8e8f3ad87d92657a1f673a9a8b69e1d7 feeab9c0b0892892cad0e544473970b8 ed8c4ea0482be1a2275089198f336a31 343b6be5ac00065a7d8ed54b92d65d07 a14a243948b327d391fe4405bd3bbbe3 3dcb6d42cc8d9e71d4e181f7ef146438 e2793f7a7cecd803eabe1ce4f2dd62ac Authentication Tag: d35f789b1ee2ff180c1d Yoon, et al. Expires November 18, 2009 [Page 10] Internet-Draft SEED-SRTP November 5, 2008 Author's Addresses Seokung Yoon Korea Information Security Agency 78, Karak-dong, Songpa-Gu, Seoul, KOREA Email: seokung@kisa.or.kr Joongman Kim Korea Information Security Agency 78, Karak-dong, Songpa-Gu, Seoul, KOREA Email: seopo@kisa.or.kr Haeryong Park Korea Information Security Agency 78, Karak-dong, Songpa-Gu, Seoul, KOREA Email: hrpark@kisa.or.kr Hyuncheol Jeong Korea Information Security Agency 78, Karak-dong, Songpa-Gu, Seoul, KOREA Email: hcjung@kisa.or.kr Yoojae Won Korea Information Security Agency 78, Karak-dong, Songpa-Gu, Seoul, KOREA Email: yjwon@kisa.or.kr Yoon, et al. Expires November 18, 2009 [Page 11] Internet-Draft SEED-SRTP November 5, 2008 Copyright Statement Copyright (C) The IETF Trust (2008). 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. 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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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Yoon, et al. Expires November 18, 2009 [Page 12]