S/MIME Working Group Jongwook Park (KISA) Internet Draft Sungjae Lee (KISA) Document: draft-ietf-smime-cms-seed-01.txt Jeeyeon Kim (KISA) Expires: October 2004 Jaeil Lee (KISA) Target category : Standard Track April 2004 Use of the SEED Encryption Algorithm in CMS Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments or suggestions for improvement may be made on the "ietf- smime" mailing list, or directly to the author. Abstract This document specifies the conventions for using the SEED encryption algorithm for encryption with the Cryptographic Message Syntax (CMS). 1. Introduction This document specifies the conventions for using the SEED encryption algorithm [SEED][TTASSEED] for encryption with the Cryptographic Park, et. al. Expires - October 2004 [Page 1] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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 is designed to increase the efficiency of memory storage and the simplicity in generating 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 trustworhty 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 PKI. 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 Park, et. al. Expires - October 2004 [Page 2] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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 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 [AES-WRAP][RFC3394]. Park, et. al. Expires - October 2004 [Page 3] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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 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) Park, et. al. Expires - October 2004 [Page 4] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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. Set C[0] = A For i = 1 to n C[i] = R[i] Park, et. al. Expires - October 2004 [Page 5] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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]}. 1) Initialize variables. Park, et. al. Expires - October 2004 [Page 6] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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: A[0] = IV = A6A6A6A6A6A6A6A6 Park, et. al. Expires - October 2004 [Page 7] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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 Park, et. al. Expires - October 2004 [Page 8] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 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 users require SEED for symmetric encryption, it MUST be supported by the S/MIME clients on both the sending and receiving side, and it MUST be set in the user preferences. 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 [AES-WRAP][RFC3394] can be applied to this document. No security problem has been found on SEED [CRYPTREC]. 6. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Park, et. al. Expires - October 2004 [Page 9] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 7. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 8. References 8.1 Normative Reference [CMS] R. Housley, "Cryptographic Message Syntax", RFC 3369, August 2002. [CMSALG] R. Housley, "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370, August 2002. [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. Park, et. al. Expires - October 2004 [Page 10] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 [AES-WRAP] National Institute of Standards and Technology. AES Key Wrap Specification. 17 November 2001. http://csrc.nist.gov/encryption/kms/key-wrap.pdf 8.2 Informative Reference [SEED] Jongwook Park, Sungjae Lee, Jeeyeon Kim, Jaeil Lee, "The SEED Encryption Algorithm", draft-park-seed-00.txt [SEED-WEB] KISA, "SEED Algorithm Specification", http://www.kisa.or.kr/seed/seed_eng.html" [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 [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 9. Authors' Address Jongwook Park Korea Information Security Agency 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 Park, et. al. Expires - October 2004 [Page 11] INTERNET-DRAFT Use of the SEED Algorithm in CMS April 2004 Phone: +82-2-405-5300 FAX : +82-2-405-5499 Email: jilee@kisa.or.kr 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(25) } DEFINITIONS IMPLICIT TAGS ::= BEGIN id-seedCBC OBJECT IDENTIFIER ::= { iso(1) member-body(2) korea(410) kisa(200004) algorithm(1) seedCBC(4) } -- Initialization Vector 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 Park, et. al. Expires - October 2004 [Page 12]