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IETF RFC 5669
The SEED Cipher Algorithm and Its Use with the Secure Real-Time Transport Protocol (SRTP)
Last modified on Tuesday, August 31st, 2010
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Internet Engineering Task Force (IETF) S. Yoon
Request for Comments: 5669 J. Kim
Category: Standards Track H. Park
ISSN: 2070-1721 H. Jeong
Y. Won
Korea Internet & Security Agency
August 2010
The SEED Cipher Algorithm and Its Use
with the Secure Real-Time Transport Protocol (SRTP)
 Abstract
This document describes the use of the SEED block cipher algorithm in
the Secure Real-time Transport Protocol (SRTP) for providing
confidentiality for Real-time Transport Protocol (RTP) traffic and
for the control traffic for RTP, the Real-time Transport Control
Protocol (RTCP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5669.
Copyright Notice
Copyright (c) 2010 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Yoon, et al. Standards Track PAGE 1 
RFC 5669 SEED-SRTP August 2010
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
1.1. SEED .......................................................3
1.2. Terminology ................................................3
1.3. Definitions ................................................3
2. Cryptographic Transforms ........................................4
2.1. Counter ....................................................4
2.1.1. Message Authentication/Integrity: HMAC-SHA1 .........4
2.2. Counter with CBC-MAC (CCM) .................................4
2.3. Galois/Counter Mode (GCM) ..................................6
3. Nonce Format for CCM and GCM ....................................6
3.1. Nonce for SRTP .............................................6
3.2. Nonce for SRTCP ............................................6
4. Key Derivation: SEED-CTR PRF ....................................7
5. Mandatory-to-Implement Transforms ...............................7
6. Security Considerations .........................................7
7. IANA Considerations .............................................8
8. Acknowledgements ................................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
Appendix A. Test Vectors ..........................................10
A.1. SEED-CTR Test Vectors .....................................10
A.2. SEED-CCM Test Vectors .....................................11
A.3. SEED-GCM Test Vectors .....................................12
Yoon, et al. Standards Track PAGE 2
RFC 5669 SEED-SRTP August 2010
1. Introduction
This document describes the use of the SEED [RFC 4269] block cipher
algorithm in the Secure Real-time Transport Protocol (SRTP) [RFC 3711]
for providing confidentiality for Real-time Transport Protocol (RTP)
[RFC 3550] traffic and for the control traffic for RTP, the Real-time
Transport Control Protocol (RTCP) [RFC 3550].
1.1. SEED
SEED is a symmetric encryption algorithm that was developed by the
Korea Information Security Agency (KISA) and a group of experts,
beginning in 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 generated from the key scheduling.
SEED is easily implemented in various software and hardware because
it is designed to increase the efficiency of memory storage and the
simplicity of generating keys without degrading the security of the
algorithm. In particular, it can be effectively adopted in a
computing environment that has restricted resources such as mobile
devices, smart cards, and so on.
SEED is a national industrial association standard [TTASSEED] and is
widely used in South Korea for electronic commerce and financial
services operated on wired and wireless PKI.
The algorithm specification and object identifiers are described in
[RFC 4269]. The SEED homepage, http://seed.kisa.or.kr/eng/main.jsp,
contains a wealth of information about SEED, including detailed
specification, evaluation report, test vectors, and so on.
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].
1.3. Definitions
|| concatenation
XOR exclusive or
Yoon, et al. Standards Track PAGE 3
RFC 5669 SEED-SRTP August 2010
2. Cryptographic Transforms
All symmetric block cipher algorithms share common characteristics,
including mode, key size, weak keys, and block size. The following
sections contain descriptions 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 mode with CBC-MAC
(CCM), and (3) SEED in Galois/Counter Mode (GCM).
2.1. Counter
Section 4.1.1 of [RFC 3711] defines AES counter mode encryption, which
that document 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. When SEED-CTR is used, it MUST be used only in
conjunction with an authentication function.
2.1.1. Message Authentication/Integrity: HMAC-SHA1
HMAC-SHA1 [RFC 2104], as defined in Section 4.2.1 of [RFC 3711], SHALL
be the default message-authentication code to be used with SEED-CTR.
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 discussed in Sections 7.5 and 9.5 of
[RFC 3711].
2.2. Counter with CBC-MAC (CCM)
CCM [RFC 3610] is a generic authenticate-and-encrypt block cipher
mode. In this specification, CCM used with the SEED block cipher is
denoted as SEED-CCM.
Section 3.3 of [RFC 3711] defines procedures to construct or to
authenticate and decrypt SRTP packets. For SEED-CCM, however, the
sender performs Step 7 before Step 5 and the receiver performs the
second half of Step 5 (verification) after Step 6. This means that
authentication is performed on the plaintext rather than the
ciphertext. This applies equally to SRTCP.
Yoon, et al. Standards Track PAGE 4
RFC 5669 SEED-SRTP August 2010
All SRTP packets MUST be authenticated and encrypted. Unlike SRTP,
Secure Real-time Transport Control Protocol (SRTCP) packet encryption
is optional (but authentication is mandatory). A sender can select
which packets to encrypt and indicates this choice with a 1-bit
encryption flag (located in the leftmost bit of the 32-bit word that
contains the SRTCP index).
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 MUST 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 a key size of 128 bits.
Nonce
The size of the nonce depends on the value selected for the
parameter L. It is 15-L octets. L equals 3, and hence the
nonce size equals 12 octets.
Plaintext
In the case of SRTP, the payload of the RTP packet, the RTP
padding, and the RTP pad count field (if the latter two fields
are present) are treated as plaintext.
In the case of SRTCP, when the encryption flag is set to 1,
the Encrypted Portion described in Fig.2 of [RFC 3711] is
treated as plaintext. When the encryption flag is set to 0,
the plaintext is zero-length.
Additional Authentication Data (AAD)
In the case of SRTP, the header of the RTP packet, including
the contributing source (CSRC) identifier (if present) and the
RTP header extension (if present), is considered AAD.
Yoon, et al. Standards Track PAGE 5
RFC 5669 SEED-SRTP August 2010
In the case of SRTCP, when the encryption flag is set to 0,
the Authentication Portion described in Fig.2 of [RFC 3711] is
treated as AAD. When the encryption flag is set to 1, the
first 8 octets, the encryption flag, and the SRTCP index are
treated as AAD.
SEED-CCM accepts these four inputs and returns a ciphertext field.
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 plaintext, a nonce, and the
additional authenticated data (AAD), all described in Section 2.2.
The bit length of the tag, denoted t, is 12, and an authentication
tag with a length of 12 octets (96 bits) is used.
3. Nonce Format for CCM and GCM
3.1. Nonce for SRTP
The nonce for SRTP SHALL be formed in the following way:
Nonce = (16 bits of zeroes || SSRC || ROC || SEQ) XOR Salt
The 4-octet SSRC and the 2-octet SEQ SHALL be taken from the RTP
header. The 4-octet ROC is from the cryptographic context. The
12-octet Salt SHALL be produced by the SRTP key derivation function.
3.2. Nonce for SRTCP
The nonce for SRTCP SHALL be formed in the following way:
Nonce = (16 bits of zeroes || SSRC || 16 bits of zeroes ||
SRTCP index) XOR Salt
The 4-octet SSRC SHALL be taken from the RTCP header and the 31-bit
SRTCP index (packed zero-filled and right-justified into a 4-octet
field) is from each packet. The 12-octet Salt SHALL be produced by
the SRTP key derivation function.
Yoon, et al. Standards Track PAGE 6
RFC 5669 SEED-SRTP August 2010
4. Key Derivation: SEED-CTR PRF
Section 4.3.3 of [RFC 3711] defines the 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, but with each invocation of
AES replaced with an invocation of SEED.
The currently defined PRF, keyed by the 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; it SHALL be applied to key k_master, have IV equal to
(x*2^16), and have the output keystream truncated to the first n
(leftmost) bits.
5. Mandatory-to-Implement Transforms
"Mandatory-to-implement" means conformance to this specification, and
that Table 1 in this document does not supercede a similar table in
Section 5 of [RFC 3711]. An RTP implementation that supports SEED
MUST implement the modes listed in Table 1 of this document.
mandatory-to-implement optional
encryption SEED-CTR SEED-CCM,SEED-GCM
message integrity HMAC-SHA1 SEED-CCM,SEED-GCM
key derivation (PRF) SEED-CTR -
Table 1: Mandatory-to-implement and optional transforms in SRTP and
SRTCP
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. For further security
considerations, the reader is encouraged to read [SEED-EVAL].
See [RFC 3610] and [GCM] for security considerations regarding the CCM
and GCM Modes of Operation, respectively. In the context of SRTP,
the procedures in [RFC 3711] ensure the critical property of non-reuse
of counter values.
Yoon, et al. Standards Track PAGE 7
RFC 5669 SEED-SRTP August 2010
7. IANA Considerations
[RFC 4568] defines SRTP "crypto suites". In order to allow the
Session Description Protocol (SDP) to signal the use of the
algorithms defined in this document, IANA has registered the
following crypto suites into the subregistry for SRTP crypto suites
under the Media Stream Transports of the SDP Security Descriptions:
SEED_CTR_128_HMAC_SHA1_80
SEED_128_CCM_80
SEED_128_GCM_96
8. Acknowledgements
The authors would like to thank David McGrew, Eric Rescorla, Alexey
Melnikov, Alfred Hoenes, Colin Perkins, Young-Chan Shin, the AVT WG
(in particular, the chairmen Roni Even and Tom Taylor), and the Real-
time Applications and Infrastructure Area Directors for their reviews
and support.
9. References
9.1. 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
[RFC 2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC 3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
Yoon, et al. Standards Track PAGE 8
RFC 5669 SEED-SRTP August 2010
[RFC 4269] Lee, H., Lee, S., Yoon, J., Cheon, D., and J. Lee, "The
SEED Encryption Algorithm", RFC 4269, December 2005.
[RFC 4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for
Media Streams", RFC 4568, July 2006.
[TTASSEED] Telecommunications Technology Association (TTA), South
Korea, "128-bit Symmetric Block Cipher (SEED)",
TTAS.KO-12.0004/R1, December 2005, (In Korean)
http://www.tta.or.kr/English/index.jsp.
9.2. Informative References
[SEED-EVAL] KISA, "Self Evaluation Report",
http://seed.kisa.or.kr/eng/main.jsp
Yoon, et al. Standards Track PAGE 9
RFC 5669 SEED-SRTP August 2010
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
Authentication Key: f93563311b354748c978913795530631
Session Salt: cd3a7c42c671e0067a2a2639b43a
Initialization Vector: cd3a7c42e69915ed7a2a263985640000
RTP Payload: f57af5fd4ae19562976ec57a5a7ad55a
5af5c5e5c5fdf5c55ad57a4a7272d572
62e9729566ed66e97ac54a4a5a7ad5e1
5ae5fdd5fd5ac5d56ae56ad5c572d54a
e54ac55a956afd6aed5a4ac562957a95
16991691d572fd14e97ae962ed7a9f4a
955af572e162f57a956666e17ae1f54a
95f566d54a66e16e4afd6a9f7ae1c5c5
5ae5d56afde916c5e94a6ec56695e14a
fde1148416e94ad57ac5146ed59d1cc5
Encrypted RTP Payload: df5a89291e7e383e9beff765e691a737
49c9e33139ad3001cd8da73ad07f69a2
805a70358b5c7c8c60ed359f95cf5e08
f713c53ff7b808250d79a19ccb8d1073
4e3cb72ed1f0a4e85b002b248049ab07
63dbe571bec52cf9153fdf2019e421ef
779cd6f4bd1c8211da8c272e2fce4393
4b9eabb87362510f254149f992599036
f5e43102327db1ac5e78adc4f66546ed
7abfb5a4db320fb7b9c52a61bc554e44
Authentication Tag: a5cdaa4d9edc53763855
Yoon, et al. Standards Track PAGE 10
RFC 5669 SEED-SRTP August 2010
A.2. SEED-CCM Test Vectors
Key: 974bee725d44fc3992267b284c3c6750
Rollover Counter: 00000000
Sequence Number: 315e
SSRC: 20e8f5eb
Nonce: 000020e8f5eb00000000315e
Payload: f57af5fd4ae19562976ec57a5a7ad55a
5af5c5e5c5fdf5c55ad57a4a7272d572
62e9729566ed66e97ac54a4a5a7ad5e1
5ae5fdd5fd5ac5d56ae56ad5c572d54a
e54ac55a956afd6aed5a4ac562957a95
16991691d572fd14e97ae962ed7a9f4a
955af572e162f57a956666e17ae1f54a
95f566d54a66e16e4afd6a9f7ae1c5c5
5ae5d56afde916c5e94a6ec56695e14a
fde1148416e94ad57ac5146ed59d1cc5
AAD: 8008315ebf2e6fe020e8f5eb
Encrypted RTP Payload: 486843a881df215a8574650ddabf5dbb
2650f06f51252bccaeb4012899d6d71e
30c64dad5ead5d8ba65ffe9d79aaf30d
c9e6334490c07e7533d704114a9006ec
b3b3bff59ecf585485bc0bd286ed434c
fd684d19a1ad514ca5f37b71d93288c0
7cf4d5e9b83db8becc8c692a7279b6a9
ac62ba970fc54f46dcc926d434c0b5ad
8678fbf0e7a03037924dae342ef64fa6
5b8eaea260fecb477a57e3919c5dab82
Authentication Tag: b0a8274cf6a8bb6cc466
Yoon, et al. Standards Track PAGE 11
RFC 5669 SEED-SRTP August 2010
A.3. SEED-GCM Test Vectors
Key: e91e5e75da65554a48181f3846349562
Rollover Counter: 00000000
Sequence Number: 315e
SSRC: 20e8f5eb
Nonce: 000020e8f5eb00000000315e
Payload: f57af5fd4ae19562976ec57a5a7ad55a
5af5c5e5c5fdf5c55ad57a4a7272d572
62e9729566ed66e97ac54a4a5a7ad5e1
5ae5fdd5fd5ac5d56ae56ad5c572d54a
e54ac55a956afd6aed5a4ac562957a95
16991691d572fd14e97ae962ed7a9f4a
955af572e162f57a956666e17ae1f54a
95f566d54a66e16e4afd6a9f7ae1c5c5
5ae5d56afde916c5e94a6ec56695e14a
fde1148416e94ad57ac5146ed59d1cc5
AAD: 8008315ebf2e6fe020e8f5eb
Encrypted RTP Payload: 8a5363682c6b1bbf13c0b09cf747a551
2543cb2f129b8bd0e92dfadf735cda8f
88c4bbf90288f5e58d20c4f1bb0d5844
6ea009103ee57ba99cdeabaaa18d4a9a
05ddb46e7e5290a5a2284fe50b1f6fe9
ad3f1348c354181e85b24f1a552a1193
cf0e13eed5ab95ae854fb4f5b0edb2d3
ee5eb238c8f4bfb136b2eb6cd7876042
0680ce1879100014f140a15e07e70133
ed9cbb6d57b75d574acb0087eefbac99
Authentication Tag: 36cd9ae602be3ee2cd8d5d9d
Yoon, et al. Standards Track PAGE 12
RFC 5669 SEED-SRTP August 2010
Authors' Addresses
Seokung Yoon
Korea Internet & Security Agency
IT Venture Tower, Jungdaero 135
Songpa-gu, Seoul, Korea 138-950
EMail: seokung@kisa.or.kr
Joongman Kim
Korea Internet & Security Agency
IT Venture Tower, Jungdaero 135
Songpa-gu, Seoul, Korea 138-950
EMail: seopo@kisa.or.kr
Haeryong Park
Korea Internet & Security Agency
IT Venture Tower, Jungdaero 135
Songpa-gu, Seoul, Korea 138-950
EMail: hrpark@kisa.or.kr
Hyuncheol Jeong
Korea Internet & Security Agency
IT Venture Tower, Jungdaero 135
Songpa-gu, Seoul, Korea 138-950
EMail: hcjung@kisa.or.kr
Yoojae Won
Korea Internet & Security Agency
IT Venture Tower, Jungdaero 135
Songpa-gu, Seoul, Korea 138-950
EMail: yjwon@kisa.or.kr
Yoon, et al. Standards Track PAGE 13
The SEED Cipher Algorithm and Its Use with the Secure Real-Time Transport Protocol (SRTP)
RFC TOTAL SIZE: 24918 bytes
PUBLICATION DATE: Tuesday, August 31st, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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