Network Working Group H.J. Lee

Network Working Group H.J. Lee
Request for Comments: 4196 J.H. Yoon
Category: Standards Track S.L. Lee
J.I. Lee
KISA
October 2005

The SEED Cipher Algorithm and Its Use with IPsec

Status of This Memo

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

Copyright (C) The Internet Society (2005).

Abstract

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

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.

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RFC 4196 The Use of SEED with IPsec October 2005

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

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

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

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.

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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

SEED supports 128-bit key and has the 16-round Feistel structure.

2.3. Weak Keys

At the time this document was written, there were no known weak keys
for SEED.

2.4. Block Size and Padding

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

Performance figures of SEED are available at
http://www.kisa.or.kr/seed/seed_eng.html

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RFC 4196 The Use of SEED with IPsec October 2005

3. ESP Payload

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

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

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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

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RFC 4196 The Use of SEED with IPsec October 2005

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 24e5bd80

Original packet:
IP header (20 bytes) : 45000030 08fe0000 4001fa16 c0a87b03 c0a87b64
Data (28 bytes) :
0800b5e8 a80a0500 a69c083d 0b660e00 77777777 77777777 77777777

Augment 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 01020201

Post-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

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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

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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

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RFC 4196 The Use of SEED with IPsec October 2005

5. Interaction with IKE

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

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

For Phase 2 negotiations, IANA has assigned an ESP Transform
Identifier of (21) for ESP_SEED_CBC.

5.3. Key Length Attribute

Since the SEED supports 128-bit key lengths, the Key Length attribute
is set with 128 bits.

5.4. Hash Algorithm Considerations

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

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

IANA has assigned ESP Transform Identifier (21) to ESP_SEED_CBC.

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RFC 4196 The Use of SEED with IPsec October 2005

8. Acknowledgments

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

[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

[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

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RFC 4196 The Use of SEED with IPsec October 2005

[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

Hyangjin Lee
Korea Information Security Agency
Phone: +82-2-405-5446
Fax : +82-2-405-5319
EMail : [email protected]

Jaeho Yoon
Korea Information Security Agency
Phone: +82-2-405-5434
Fax : +82-2-405-5219
EMail : [email protected]

Seoklae Lee
Korea Information Security Agency
Phone: +82-2-405-5230
Fax : +82-2-405-5219
EMail : [email protected]

Jaeil Lee
Korea Information Security Agency
Phone: +82-2-405-5200
Fax : +82-2-405-5219
EMail: [email protected]

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RFC 4196 The Use of SEED with IPsec October 2005

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