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Keywords: smime, secure/multipurpose internet mail extensions
Network Working Group J. Park
Request for Comments: 4010 S. Lee
Category: Standards Track J. Kim
J. Lee
KISA
February 2005
Use of the SEED Encryption Algorithm
in Cryptographic Message Syntax (CMS)
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 specifies the conventions for using the SEED encryption
algorithm for encryption with the Cryptographic Message Syntax (CMS).
SEED is added to the set of optional symmetric encryption algorithms
in CMS by providing two classes of unique object identifiers (OIDs).
One OID class defines the content encryption algorithms and the other
defines the key encryption algorithms.
1. Introduction
This document specifies the conventions for using the SEED encryption
algorithm [SEED][TTASSEED] for encryption with the Cryptographic
Message Syntax (CMS)[CMS]. The relevant object identifiers (OIDs)
and processing steps are provided so that SEED may be used in the CMS
specification (RFC 3852, RFC 3370) for content and key encryption.
Park, et al. Standards Track [Page 1]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
1.1. SEED
SEED is a symmetric encryption algorithm developed by KISA (Korea
Information Security Agency) and a group of experts since 1998. The
input/output block size and key length of SEED is 128-bits. 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 takes less memory to implement than other algorithms and generates
keys without degrading the security of the algorithm. In particular,
it can be effectively adopted in a computing environment with a
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].
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 communications.
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 [RFC2119].
2. Object Identifiers for Content and Key Encryption
This section provides the OIDs and processing information necessary
for SEED to be used for content and key encryption in CMS. SEED is
added to the set of optional symmetric encryption algorithms in CMS
by providing two classes of unique object identifiers (OIDs). One
OID class defines the content encryption algorithms and the other
defines the key encryption algorithms. Thus, a CMS agent can apply
SEED either for content or key encryption by selecting the
corresponding object identifier, supplying the required parameter,
and starting the program code.
Park, et al. Standards Track [Page 2]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
2.1. OIDs for Content Encryption
SEED is added to the set of symmetric content encryption algorithms
defined in [CMSALG]. The SEED content-encryption algorithm in Cipher
Block Chaining (CBC) mode has the following object identifier:
id-seedCBC OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) korea(410) kisa(200004)
algorithm(1) seedCBC(4) }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain the value of Initialization Vector
(IV):
SeedCBCParameter ::= SeedIV -- Initialization Vector
SeedIV ::= OCTET STRING (SIZE(16))
The plain text is padded according to Section 6.3 of [CMS].
2.2. OIDs for Key Encryption
The key-wrap/unwrap procedures used to encrypt/decrypt a SEED
content-encryption key (CEK) with a SEED key-encryption key (KEK) are
specified in Section 3. Generation and distribution of key-
encryption keys are beyond the scope of this document.
The SEED key-encryption algorithm has the following object
identifier:
id-npki-app-cmsSeed-wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) korea(410) kisa(200004) npki-app(7)
smime(1) alg(1) cmsSEED-wrap(1) }
The parameter associated with this object identifier MUST be absent,
because the key wrapping procedure itself defines how and when to use
an IV.
3. Key Wrap Algorithm
SEED key wrapping and unwrapping is done in conformance with the AES
key wrap algorithm [RFC3394].
3.1. Notation and Definitions
The following notation is used in the description of the key wrapping
algorithms:
Park, et al. Standards Track [Page 3]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
SEED(K, W) Encrypt W using the SEED codebook with key K
SEED-1(K, W) Decrypt W using the SEED codebook with key K
MSB(j, W) Return the most significant j bits of W
LSB(j, W) Return the least significant j bits of W
B1 ^ B2 The bitwise exclusive or (XOR) of B1 and B2
B1 | B2 Concatenate B1 and B2
K The key-encryption key K
n The number of 64-bit key data blocks
s The number of steps in the wrapping process,
s = 6n
P[i] The ith plaintext key data block
C[i] The ith ciphertext data block
A The 64-bit integrity check register
R[i] An array of 64-bit registers where
i = 0, 1, 2, ..., n
A[t], R[i][t] The contents of registers A and R[i] after
encryption step t.
IV The 64-bit initial value used during the
wrapping process.
In the key wrap algorithm, the concatenation function will be used to
concatenate 64-bit quantities to form the 128-bit input to the SEED
codebook. The extraction functions will be used to split the 128-bit
output from the SEED codebook into two 64-bit quantities.
3.2. SEED Key Wrap
Key wrapping with SEED is identical to Section 2.2.1 of [RFC3394]
with "AES" replaced by "SEED".
The inputs to the key wrapping process are the KEK and the plaintext
to be wrapped. The plaintext consists of n 64-bit blocks containing
the key data being wrapped. The key wrapping process is described
below.
Inputs: Plaintext, n 64-bit values {P[1], P[2], ..., P[n]}, and
Key, K (the KEK).
Outputs: Ciphertext, (n+1) 64-bit values {C[0], C[1], ..., C[n]}.
1) Initialize variables.
Set A[0] to an initial value (see Section 3.4)
For i = 1 to n
R[0][i] = P[i]
Park, et al. Standards Track [Page 4]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
2) Calculate intermediate values.
For t = 1 to s, where s = 6n
A[t] = MSB(64, SEED(K, A[t-1] | R[t-1][1])) ^ t
For i = 1 to n-1
R[t][i] = R[t-1][i+1]
R[t][n] = LSB(64, SEED(K, A[t-1] | R[t-1][1]))
3) Output the results.
Set C[0] = A[s]
For i = 1 to n
C[i] = R[s][i]
An alternative description of the key wrap algorithm involves
indexing rather than shifting. This approach allows one to calculate
the wrapped key in place, avoiding the rotation in the previous
description. This produces identical results and is more easily
implemented in software.
Inputs: Plaintext, n 64-bit values {P[1], P[2], ..., P[n]}, and
Key, K (the KEK).
Outputs: Ciphertext, (n+1) 64-bit values {C[0], C[1], ..., C[n]}.
1) Initialize variables.
Set A = IV, an initial value (see Section 3.4)
For i = 1 to n
R[i] = P[i]
2) Calculate intermediate values.
For j = 0 to 5
For i=1 to n
B = SEED(K, A | R[i])
A = MSB(64, B) ^ t where t = (n*j)+i
R[i] = LSB(64, B)
3) Output the results.
Set C[0] = A
For i = 1 to n
C[i] = R[i]
Park, et al. Standards Track [Page 5]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
3.3. SEED Key Unwrap
Key unwrapping with SEED is identical to Section 2.2.2 of [RFC3394],
with "AES" replaced by "SEED".
The inputs to the unwrap process are the KEK and (n+1) 64-bit blocks
of ciphertext consisting of previously wrapped key. It returns n
blocks of plaintext consisting of the n 64-bit blocks of the
decrypted key data.
Inputs: Ciphertext, (n+1) 64-bit values {C[0], C[1], ..., C[n]},
and Key, K (the KEK).
Outputs: Plaintext, n 64-bit values {P[1], P[2], ..., P[n]}.
1) Initialize variables.
Set A[s] = C[0] where s = 6n
For i = 1 to n
R[s][i] = C[i]
2) Calculate the intermediate values.
For t = s to 1
A[t-1] = MSB(64, SEED-1(K, ((A[t] ^ t) | R[t][n]))
R[t-1][1] = LSB(64, SEED-1(K, ((A[t]^t) | R[t][n]))
For i = 2 to n
R[t-1][i] = R[t][i-1]
3) Output the results.
If A[0] is an appropriate initial value (see Section 3.4),
Then
For i = 1 to n
P[i] = R[0][i]
Else
Return an error
The unwrap algorithm can also be specified as an index based
operation, allowing the calculations to be carried out in place.
Again, this produces the same results as the register shifting
approach.
Inputs: Ciphertext, (n+1) 64-bit values {C[0], C[1], ..., C[n]},
and Key, K (the KEK).
Outputs: Plaintext, n 64-bit values {P[0], P[1], ..., P[n]}.
Park, et al. Standards Track [Page 6]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
1) Initialize variables.
Set A = C[0]
For i = 1 to n
R[i] = C[i]
2) Compute intermediate values.
For j = 5 to 0
For i = n to 1
B = SEED-1(K, (A ^ t) | R[i]) where t = n*j+i
A = MSB(64, B)
R[i] = LSB(64, B)
3) Output results.
If A is an appropriate initial value (see Section 3.4),
Then
For i = 1 to n
P[i] = R[i]
Else
Return an error
3.4. Key Data Integrity -- the Initial Value
The initial value (IV) refers to the value assigned to A[0] in the
first step of the wrapping process. This value is used to obtain an
integrity check on the key data. In the final step of the unwrapping
process, the recovered value of A[0] is compared to the expected
value of A[0]. If there is a match, the key is accepted as valid,
and the unwrapping algorithm returns it. If there is not a match,
then the key is rejected, and the unwrapping algorithm returns an
error.
The exact properties achieved by this integrity check depend on the
definition of the initial value. Different applications may call for
somewhat different properties; for example, whether there is a need
to determine the integrity of key data throughout its lifecycle or
just when it is unwrapped. This specification defines a default
initial value that supports the integrity of the key data during the
period it is wrapped (in Section 3.4.1). Provision is also made to
support alternative initial values (in Section 3.4.2).
Park, et al. Standards Track [Page 7]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
3.4.1. Default Initial Value
The default initial value (IV) is defined to be the hexadecimal
constant:
A[0] = IV = A6A6A6A6A6A6A6A6
The use of a constant as the IV supports a strong integrity check on
the key data during the period that it is wrapped. If unwrapping
produces A[0] = A6A6A6A6A6A6A6A6, then the chance that the key data
is corrupt is 2^-64. If unwrapping produces A[0] = any other value,
then the unwrap must return an error and not return any key data.
3.4.2. Alternative Initial Values
When the key wrap is used as part of a larger key management protocol
or system, the desired scope for data integrity may be more than just
the key data, and the desired duration may be more than just the
period that it is wrapped. Also, if the key data is not just a SEED
key, it may not always be a multiple of 64 bits. Alternative
definitions of the initial value can be used to address such
problems. According to RFC 3394 [RFC3394], NIST will define
alternative initial values in future key management publications as
they are needed. To accommodate a set of alternatives that may
evolve over time, non-application-specific key wrap implementations
will require some flexibility in the way the initial value is set and
tested.
4. SMIMECapabilities Attribute
An S/MIME client SHOULD announce the set of cryptographic functions
it supports by using the S/MIME capabilities attribute. This
attribute provides a partial list of OIDs of cryptographic functions
and MUST be signed by the client. The functions' OIDs SHOULD be
logically separated in functional categories and MUST be ordered with
respect to their preference.
RFC 3851 [RFC3851], Section 2.5.2 defines the SMIMECapabilities
signed attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs)
to be used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support.
If an S/MIME client is required to support symmetric encryption with
SEED, the capabilities attribute MUST contain the SEED OID specified
above in the category of symmetric algorithms. The parameter
associated with this OID MUST be SeedSMimeCapability.
SeedSMimeCapabilty ::= NULL
Park, et al. Standards Track [Page 8]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
The SMIMECapability SEQUENCE representing SEED MUST be DER-encoded as
the following hexadecimal strings:
30 0C 06 08 2A 83 1A 8C 9A 44 01 04 05 00
When a sending agent creates an encrypted message, it has to decide
which type of encryption algorithm to use. In general, the decision
process involves information obtained from the capabilities lists
included in messages received from the recipient, as well as other
information, such as private agreements, user preferences and legal
restrictions. If local policy requires the use of SEED for symmetric
encryption, then both the sending and receiving S/MIME clients must
support it, and SEED must be configured as the preferred symmetric
algorithm.
5. Security Considerations
This document specifies the use of SEED for encrypting the content of
a CMS message and for encrypting the symmetric key used to encrypt
the content of a CMS message, with the other mechanisms being the
same as the existing ones. Therefore, the security considerations
described in the CMS specifications [CMS][CMSALG] and the AES key
wrap algorithm [RFC3394] can be applied to this document. No
security problem has been found on SEED [CRYPTREC].
6. References
6.1. Normative References
[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
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
Park, et al. Standards Track [Page 9]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[SEED] Park, J., Lee, S., Kim, J., and J. Lee, "The SEED
Encryption Algorithm", RFC 4009, February 2005.
[ISOSEED] ISO/IEC, ISO/IEC JTC1/SC 27 N 256r1, "National Body
contributions on NP 18033 Encryption algorithms in
response to document SC 27 N 2563", October, 2000
[CRYPTREC] Information-technology Promotion Agency (IPA), Japan,
CRYPTREC. "SEED Evaluation Report", February, 2002
http://www.kisa.or.kr
Park, et al. Standards Track [Page 10]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
Appendix A. ASN.1 Module
SeedEncryptionAlgorithmInCMS
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs9(9) smime(16) modules(0) id-mod-cms-seed(24) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
id-seedCBC OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) korea(410) kisa(200004)
algorithm(1) seedCBC(4) }
-- Initialization Vector (IV)
SeedCBCParameter ::= SeedIV
SeedIV ::= OCTET STRING (SIZE(16))
-- SEED Key Wrap Algorithm identifiers - Parameter is absent.
id-npki-app-cmsSeed-wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) korea(410) kisa(200004) npki-app(7)
smime(1) alg(1) cmsSEED-wrap(1) }
-- SEED S/MIME Capability parameter
SeedSMimeCapability ::= NULL
END
Park, et al. Standards Track [Page 11]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
Authors' Addresses
Jongwook Park
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, 138-803
REPUBLIC OF KOREA
Phone: +82-2-405-5432
FAX : +82-2-405-5499
EMail: khopri@kisa.or.kr
Sungjae Lee
Korea Information Security Agency
Phone: +82-2-405-5243
FAX : +82-2-405-5499
EMail: sjlee@kisa.or.kr
Jeeyeon Kim
Korea Information Security Agency
Phone: +82-2-405-5238
FAX : +82-2-405-5499
EMail: jykim@kisa.or.kr
Jaeil Lee
Korea Information Security Agency
Phone: +82-2-405-5300
FAX : +82-2-405-5499
EMail: jilee@kisa.or.kr
Park, et al. Standards Track [Page 12]
RFC 4010 The SEED Encryption Algorithm in CMS February 2005
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Acknowledgement
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Park, et al. Standards Track [Page 13]