Use of the SEED Encryption Algorithm in Cryptographic Message Syntax (CMS)
draft-ietf-smime-cms-seed-02

S/MIME Working Group                                Jongwook Park (KISA)
Internet Draft                                        Sungjae Lee (KISA)
Document: draft-ietf-smime-cms-seed-02.txt            Jeeyeon Kim (KISA)
Expires: Feburary 2005                                  Jaeil Lee (KISA)
Target category : Standard Track                             August 2004

                  Use of the SEED Encryption Algorithm
                 in Cryptographic Message Syntax (CMS)

                   <draft-ietf-smime-cms-seed-02.txt>

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

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).

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   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 3369, RFC 3370) for content and key encryption.

1.1 SEED

   SEED is a symmetric encryption algorithm that had 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
   generated from the key scheduling.

   SEED is easily implemented in various software and hardware because
   it takes less memory to implement that than other algorithms and
   generates keys without degrading the security of the algorithm. In
   particular, it can be effectively adopted to a computing environment
   with a restricted resources such as a mobile devices, smart cards and
   so on.

   SEED is robust against known attacks including DC (Differential
   cryptanalysis), LC (Linear cryptanalysis) and related key attacks,
   etc. SEED has gone through wide public scrutinizing procedures.
   Especially, it has been evaluated and also considered
   cryptographically secure by credible organizations such as ISO/IEC
   JTC 1/SC 27 and Japan CRYTEC (Cryptography Reasearch and Evaluation
   Comittees) [ISOSEED][CRYPTEC].

   SEED is a national industrial association standard [TTASSEED] and is
   widely used in South Korea for electronic commerce and financial
   services operated on wired & 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].

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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.

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

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   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 Defintions

   The following notation is used in the description of the key wrapping
   algorithms:

         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[t][i] 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

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              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]

   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.

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      Set C[0] = A
      For i = 1 to n
        C[i] = R[i]

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).

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     Outputs: Plaintext, n 64-bit values {P[0], P[1], ..., P[n]}.

   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 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 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).

3.4.1 Default Initial Value

   The default initial value (IV) is defined to be the hexadecimal
   constant:

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     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 or the desired duration for more than just the period
   that it is wrapped. Also, if the key data is not just an 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
   [RFC3394], NIST will define alternative initial values in future key
   management publications as needed. In order to accommodate a set of
   alternatives that may evolve over time, key wrap implementations that
   are not application-specific will require some flexibility in the way
   that 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 2633 [RFC2633], 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

   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

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   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, legal
   restrictions, and so on. If local policy requires the use of SEED for
   symmetric encryption, then the 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, and the other mechanisms are 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 Reference

   [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]       R. Housley, "Cryptographic Message Syntax", RFC 3369,
               August 2002.

   [CMSALG]    R. Housley, "Cryptographic Message Syntax (CMS)
               Algorithms", RFC 3370, August 2002.

   [RFC2633]   Ramsdell, B., Editor.  S/MIME Version 3 Message
               Specification.  RFC 2633.  June 1999.

   [RFC3394]   J. Schaad and R. Housley, "Advanced Encryption Standard
                 (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   [RFC2119]   S. Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

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6.2 Informative Reference

   [SEED]      Jongwook Park, Sungjae Lee, Jeeyeon Kim, Jaeil Lee,
               "The SEED Encryption Algorithm", draft-park-seed-01.txt

   [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

7. Authors' Address

    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

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Appendix. 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)

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    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 Capabilty parameter

    SeedSMimeCapability ::= NULL

  END

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