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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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
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