J.H. Yoon
S.L. Lee
J.I. Lee
KISA
October 2005
The SEED Cipher Algorithm and Its Use with IPsec
Status of This Memo
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This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
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Copyright © The Internet Society (2005).
Abstract
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This document describes the use of the SEED block cipher algorithm in the Cipher Block Chaining Mode, with an explicit IV, as a confidentiality mechanism within the context of the IPsec Encapsulating Security Payload (ESP).
1. Introduction
1.1. SEED
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SEED is a 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 the 16-round Feistel structure. A 128-bit input is divided into two 64-bit blocks, and the right 64- bit block is an input to the round function with a 64-bit subkey that is generated from the key scheduling.
SEED is easily implemented in various software and hardware, and it can be effectively adopted to a computing environment with restricted resources, such as mobile devices and smart cards.
SEED is robust against known attacks including DC (Differential cryptanalysis), LC (Linear cryptanalysis), and related key attacks. SEED has gone through wide public scrutinizing procedures. It has been evaluated and is considered cryptographically secure by credible organizations such as ISO/IEC JTC 1/SC 27 and Japan CRYPTREC (Cryptography Research and Evaluation Committees)[ISOSEED][CRYPTREC].
The remainder of this document specifies the use of SEED within the context of IPsec ESP. For further information on how the various pieces of ESP fit together to provide security services, please refer to [ARCH], [ESP], and [ROAD].
1.2. Terminology
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The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase, as shown) are to be interpreted as described in RFC 2119 [KEYWORDS].
2. The SEED Cipher Algorithm
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All symmetric block cipher algorithms share common characteristics and variables, including mode, key size, weak keys, block size, and rounds. The following sections contain descriptions of the relevant characteristics of SEED.
The algorithm specification and object identifiers are described in [ISOSEED] [SEED]. The SEED homepage, http://www.kisa.or.kr/seed/seed_eng.html, contains a wealth of information about SEED, including a detailed specification, evaluation report, test vectors, and so on.
2.1. Mode
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NIST has defined 5 modes of operation for the Advanced Encryption Standard (AES) [AES] and other FIPS-approved ciphers [MODES]: CBC (Cipher Block Chaining), ECB (Electronic Codebook), CFB (Cipher FeedBack), OFB (Output FeedBack), and CTR (Counter). The CBC mode is well-defined and well-understood for symmetric ciphers, and is currently required for all other ESP ciphers. This document specifies the use of the SEED cipher in the CBC mode within ESP. This mode requires an Initialization Vector (IV) that is the same size as the block size. Use of a randomly generated IV prevents generation of identical ciphertext from packets that have identical data that spans the first block of the cipher algorithm's block size
The IV is XOR'd with the first plaintext block before it is encrypted. Then for successive blocks, the previous ciphertext block is XOR'd with the current plaintext before it is encrypted.
More information on the CBC mode can be obtained in [MODES] [CRYPTO-S]. For use of the CBC mode in ESP with 64-bit ciphers, please see [CBC].
2.2. Key Size and Numbers of Rounds
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SEED supports 128-bit key and has the 16-round Feistel structure.
2.3. Weak Keys
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At the time this document was written, there were no known weak keys for SEED.
2.4. Block Size and Padding
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SEED uses a block size of 16 octets (128 bits).
Padding is required by SEED to maintain a 16-octet (128-bit) blocksize. Padding MUST be added, as specified in [ESP], such that the data to be encrypted (which includes the ESP Pad Length and Next Header fields) has a length that is a multiple of 16 octets.
Because of the algorithm specific padding requirement, no additional padding is required to ensure that the ciphertext terminates on a 4- octet boundary (i.e., maintaining a 16-octet blocksize guarantees that the ESP Pad Length and Next Header fields will be right aligned within a 4-octet word). Additional padding MAY be included, as specified in [ESP], as long as the 16-octet blocksize is maintained.
2.5. Performance
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Performance figures of SEED are available at http://www.kisa.or.kr/seed/seed_eng.html
3. ESP Payload
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The ESP Payload is made up of the Initialization Vector(IV) of 16 octets followed by the encrypted payload. Thus, the payload field, as defined in [ESP], is broken down according to the following diagram:
+---------------+---------------+---------------+---------------+ | | + Initialization Vector (16 octets) + | | +---------------+---------------+---------------+---------------+ | | ~ Encrypted Payload (variable length, a multiple of 16 octets) ~ | | +---------------------------------------------------------------+The IV field MUST be the same size as the block size of the cipher algorithm being used. The IV MUST be chosen at random and MUST be unpredictable.
Including the IV in each datagram ensures that decryption of each received datagram can be performed, even when some datagrams are dropped or re-ordered in transit.
To avoid CBC encryption of very similar plaintext blocks in different packets, implementations MUST NOT use a counter or other low-hamming distance source for IVs.
4. Test Vectors
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The first 2 test cases test SEED-CBC encryption. Each test case includes key, the plaintext, and the resulting ciphertext. All data are hexadecimal numbers (not prefixed by "0x").
The last 4 test cases illustrate sample ESP packets using SEED-CBC for encryption. All data are hexadecimal numbers (not prefixed by "0x").
Case #1 : Encrypting 32 bytes (2 blocks) using SEED-CBC with 128-bit key Key : ed2401ad 22fa2559 91bafdb0 1fefd697 IV : 93eb149f 92c9905b ae5cd34d a06c3c8e PlainText : b40d7003 d9b6904b 35622750 c91a2457 5bb9a632 364aa26e 3ac0cf3a 9c9d0dcb CipherText : f072c5b1 a0588c10 5af8301a dcd91dd0 67f68221 55304bf3 aad75ceb 44341c25 Case #2 : Encrypting 64 bytes (4 blocks) using SEED-CBC with 128-bit key Key : 88e34f8f 081779f1 e9f39437 0ad40589 IV : 268d66a7 35a81a81 6fbad9fa 36162501 PlainText : d76d0d18 327ec562 b15e6bc3 65ac0c0f 8d41e0bb 938568ae ebfd92ed 1affa096 394d20fc 5277ddfc 4de8b0fc e1eb2b93 d4ae40ef 4768c613 b50b8942 f7d4b9b3 CipherText : a293eae9 d9aebfac 37ba714b d774e427 e8b706d7 e7d9a097 228639e0 b62b3b34 ced11609 cef2abaa ec2edf97 9308f379 c31527a8 267783e5 cba35389 82b48d06 Case #3 : Sample transport-mode ESP packet (ping 192.168.123.100) Key : 90d382b4 10eeba7a d938c46c ec1a82bf SPI : 4321 Source address : 192.168.123.3 Destination address : 192.168.123.100 Sequence number : 1 IV : e96e8c08 ab465763 fd098d45 dd3ff893 Original packet : IP header (20 bytes) : 45000054 08f20000 4001f9fe c0a87b03 c0a87b64 Data (64 bytes) : 08000ebd a70a0000 8e9c083d b95b0700 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 Augment data with : Padding : 01020304 05060708 090a0b0c 0d0e Pad length : 0e Next header : 01 (ICMP) Pre-encryption Data with padding, pad length and next header(80 bytes): 08000ebd a70a0000 8e9c083d b95b0700 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 01020304 05060708 090a0b0c 0d0e0e01 Post-encryption packet with SPI, Sequence number, IV : IP Header : 45000054 08f20000 4001f9fe c0a87b03 c0a87b64 SPI/Seq # : 00004321 00000001 IV : e96e8c08 ab465763 fd098d45 dd3ff893 Encrypted Data (80 bytes) : e7ebaa03 cf45ef09 021b3011 b40d3769 be96ebae cd4222f6 b6f84ce5 b2d5cdd1 60eb6b0e 5a47d16a 501a4d10 7b2d7cc8 ab86ba03 9a000972 66374fa8 f87ee0fb ef3805db faa144a2 334a34db 0b0f81ca Case #4 : Sample transport-mode ESP packet (ping -p 77 -s 20 192.168.123.100) Key : 90d382b4 10eeba7a d938c46c ec1a82bf SPI : 4321 Source address : 192.168.123.3 Destination address : 192.168.123.100 Sequence number : 8 IV : 69d08df7 d203329d b093fc49 24e5bd80Original packet:
IP header (20 bytes) : 45000030 08fe0000 4001fa16 c0a87b03 c0a87b64 Data (28 bytes) :
0800b5e8 a80a0500 a69c083d 0b660e00 77777777 77777777 77777777Augment data with : Padding : 0102 Pad length : 02 Next header : 01 (ICMP)
Pre-encryption Data with padding, pad length and
next header(32 bytes):
0800b5e8 a80a0500 a69c083d 0b660e00
77777777 77777777 77777777 01020201Post-encryption packet with SPI, Sequence number, IV : IP header : 4500004c 08fe0000 4032f9c9 c0a87b03 c0a87b64 SPI/Seq # : 00004321 00000008 IV : 69d08df7 d203329d b093fc49 24e5bd80 Encrypted Data (32 bytes) : b9ad6e19 e9a6a2fa 02569160 2c0af541 db0b0807 e1f660c7 3ae2700b 5bb5efd1 Case #5 : Sample tunnel-mode ESP packet (ping 192.168.123.200) Key : 01234567 89abcdef 01234567 89abcdef SPI : 8765 Source address : 192.168.123.3 Destination address : 192.168.123.200 Sequence number : 2 IV : f4e76524 4f6407ad f13dc138 0f673f37 Original packet : IP header (20 bytes) : 45000054 09040000 4001f988 c0a87b03 c0a87bc8 Data (64 bytes) : 08009f76 a90a0100 b49c083d 02a20400 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 Augment data with : Padding : 01020304 05060708 090a Pad length : 0a Next header : 04 (IP-in-IP) Pre-encryption Data with original IP header, padding, pad length and next header (96 bytes) : 45000054 09040000 4001f988 c0a87b03 c0a87bc8 08009f76 a90a0100 b49c083d 02a20400 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 01020304 05060708 090a0a04 Post-encryption packet with SPI, Sequence number, IV : IP header : 4500008c 09050000 4032f91e c0a87b03 c0a87bc8 SPI/Seq # : 00008765 00000002 IV : f4e76524 4f6407ad f13dc138 0f673f37 Encrypted Data (96 bytes): 2638aa7b 05e71b54 9348082b 67b47b26 c565aed4 737f0bcb 439c0f00 73e7913c 3c8a3e4f 5f7a5062 003b78ed 7ca54a08 c7ce047d 5bec14e4 8cba1005 32a12097 8d7f5503 204ef661 729b4ea1 ae6a9178 59a5caac 46e810bd 7875bd13 d6f57b3d Case #6 : Sample tunnel-mode ESP packet (ping -p ff -s 40 192.168.123.200) Key : 01234567 89abcdef 01234567 89abcdef SPI : 8765 Source address : 192.168.123.3 Destination address : 192.168.123.200 Sequence number : 5 IV : 85d47224 b5f3dd5d 2101d4ea 8dffab22 Original packet : IP header (20 bytes) : 45000044 090c0000 4001f990 c0a87b03 c0a87bc8 Data (48 bytes) : 0800d63c aa0a0200 c69c083d a3de0300 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff Augment data with : Padding : 01020304 05060708 090a Pad length : 0a Next header : 04 (IP-in-IP) Pre-encryption Data with original IP header, padding, pad length and next header (80 bytes): 45000044 090c0000 4001f990 c0a87b03 c0a87bc8 0800d63c aa0a0200 c69c083d a3de0300 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff 01020304 05060708 090a0a04 Post-encryption packet with SPI, Sequence number, IV : IP header : 4500007c 090d0000 4032f926 c0a87b03 c0a87bc8 SPI/Seq # : 00008765 00000005 IV : 85d47224 b5f3dd5d 2101d4ea 8dffab22 Encrypted Data (80 bytes) : 311168e0 bc36ac4e 59802bd5 192c5734 8f3d29c8 90bab276 e9db4702 91f79ac7 79571929 c170f902 ffb2f08b d448f782 31671414 ff29b7e0 168e1c87 09ba2b67 a56e0fbc 4ff6a936 d859ed57 6c16ef1b
5. Interaction with IKE
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This section describes the use of IKE [IKE] to establish IPsec ESP security associations (SAs) that employ SEED in CBC mode.
5.1. Phase 1 Identifier
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For Phase 1 negotiations, the object identifier of SEED-CBC is defined in [SEED].
algorithm OBJECT IDENTIFIER ::= { iso(1) member-body(2) korea(410) kisa(200004) algorithm(1) } id-seedCBC OBJECT IDENTIFIER ::= { algorithm seedCBC(4) }
5.2. Phase 2 Identifier
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For Phase 2 negotiations, IANA has assigned an ESP Transform Identifier of (21) for ESP_SEED_CBC.
5.3. Key Length Attribute
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Since the SEED supports 128-bit key lengths, the Key Length attribute is set with 128 bits.
5.4. Hash Algorithm Considerations
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HMAC-SHA-1 [HMAC-SHA] and HMAC-MD5 [HMAC-MD5] are currently considered of sufficient strength to serve both as IKE generators of 128-bit SEED keys and as ESP authenticators for SEED encryption using 128-bit keys.
6. Security Considerations
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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 (by [CRYPTREC]). For further security considerations, the reader is encouraged to read [CRYPTREC], [ISOSEED], and [SEED-EVAL].
7. IANA Considerations
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IANA has assigned ESP Transform Identifier (21) to ESP_SEED_CBC.
8. Acknowledgments
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The authors want to thank Ph.D Haesuk Kim of Future Systems Inc. and Brian Kim of OULLIM Information Technology Inc. for providing expert advice on Test Vector examples.
9. References
9.1. Normative References
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[CBC] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher Algorithms", RFC 2451, November 1998. [ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [SEED] Park, J., Lee, S., Kim, J., and J. Lee, "The SEED Encryption Algorithm", RFC 4009, February 2005. [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
9.2. Informative Reference
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[AES] NIST, FIPS PUB 197, "Advanced Encryption Standard(AES), November 2001. http://csrc.nist.gov/publications/fips/fips197/fips-197. {ps,pdf} [ARCH] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [CRYPTO-S] Schneier, B., "Applied Cryptography Second Edition", John Wiley & Sons, New York, NY, 1995, ISBN 0-471-12845-7. [CRYPTREC] Information-technology Promotion Agency (IPA), Japan, CRYPTREC. "SEED Evaluation Report", February, 2002 http://www.kisa.or.kr/seed/seed_eng.html [HMAC-MD5] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within ESP and AH", RFC 2403, November 1998. [HMAC-SHA] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP and AH", RFC 2404, November 1998. [ISOSEED] ISO/IEC JTC 1/SC 27 N3979, "IT Security techniques - Encryption Algorithms - Part3 : Block ciphers", June 2004. [MODES] Symmetric Key Block Cipher Modes of Operation, http://www.nist.gov/modes/. [ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security Document Roadmap", RFC 2411, November 1998.[SEED-EVAL] KISA, "Self Evaluation Report",
http://www.kisa.or.kr/seed/data/Document_pdf/ SEED_Self_Evaluation.pdf"
Authors' Address
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Hyangjin Lee Korea Information Security Agency Phone: +82-2-405-5446 Fax : +82-2-405-5319 EMail : jiinii@kisa.or.kr Jaeho Yoon Korea Information Security Agency Phone: +82-2-405-5434 Fax : +82-2-405-5219 EMail : jhyoon@kisa.or.kr Seoklae Lee Korea Information Security Agency Phone: +82-2-405-5230 Fax : +82-2-405-5219 EMail : sllee@kisa.or.kr Jaeil Lee Korea Information Security Agency Phone: +82-2-405-5200 Fax : +82-2-405-5219 EMail: jilee@kisa.or.kr
Full Copyright Statement
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Copyright © The Internet Society (2005).
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Acknowledgement
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Funding for the RFC Editor function is currently provided by the Internet Society.