Internet Draft S. Teiwes, draft-ietf-smime-idea-02.txt P. Hartmann, February 22, 1999 D. Kuenzi, Expires in six months iT_Security Ltd. Incorporation of the IDEA Encryption Algorithm in S/MIME 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. To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 1. Introduction This memo specifies how to incorporate IDEA (International Data Encryption Algorithm) [IDEA] into S/MIME [SMIME2, SMIME3] as an additional algorithm for symmetric encryption. Today, IDEA is widely applied in electronic business applications. But it is not considered in the specification [SMIME3]. Encryption algorithms are part of an organizations' security policy. Typically, organizations like to have their own preferences in this respect. Therefore, it is beneficial to have the choice between different well-known encryption algorithms. Especially for those organization who make already use of IDEA on a wide scale it is of high interest that IDEA is also available in S/MIME. It is the intention of this memo to provide the OIDs and algorithms required to include IDEA in S/MIME for symmetric content and key encryption. The general functional capabilities and preferences of S/MIME are specified by the registered list of S/MIME object identifiers (OIDs). This list of OIDs is maintained by the Internet Mail Consortium at <http://www.imc.org/ietf-smime/oids.html>. The set of S/MIME functions provided by a client is expressed by the S/MIME capabilities attribute. This attribute contains a list of OIDs of supported cryptographic functions. This draft is being discussed on the "ietf-smime" mailing list. To subscribe, send a message to: ietf-smime-request@imc.org with the single word subscribe in the body of the message. There is a Web site for the mailing list at <http://www.imc.org/ietf-smime/> In this document, the terms MUST, MUST NOT, SHOULD, and SHOULD NOT are used in capital letters. This conforms to the definitions in [MUSTSHOULD]. 2. Object Identifier for Content and Key Encryption The Cryptographic Message Syntax [CMS], derived from PKCS#7 [PKCS7], is the framework for the implementation of cryptographic functions in S/MIME. It specifies data formats and encryption processes without naming the cryptographic algorithms. Each algorithm which is used for encryption purposes must be specified by a unique algorithm identifier. For example, in the special case of content encryption the ContentEncryptionAlgorithmIdentifier specifies the algorithm to be applied. However, according to [CMS] any symmetric encryption algorithm that a CMS implementation includes as a content-encryption algorithm must also be included as a key-encryption algorithm. IDEA is added to the set of optional symmetric encryption algorithms in S/MIME by providing two unique object identifiers (OIDs). One OID defines content encryption and the other one key encryption. Thus an S/MIME agent can apply IDEA either for content or key encryption by selecting the corresponding object identifier, supplying the required parameter, and starting the program code. For content encryption the use of IDEA in cipher block chaining (CBC) mode is recommended. The key length is fixed to 128 bits. The IDEA content-encryption algorithm in CBC mode has the object identifier IDEA-CBC OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) usdod(6) oid(1) private(4) enterprises(1) ascom(188) systec(7) security(1) algorithms(1) 2 } The identifier's parameters field contains the initial vector IV as an optional parameter. IDEA-CBCPar ::= SEQUENCE { IV OCTET STRING OPTIONAL -- exactly 8 octets } If IV is specified as above, it MUST be used as initial vector. In this case, the ciphertext MUST NOT include the initial vector. If IV is not specified, the first 64 bits of the ciphertext MUST be considered as the initial vector. However, this alternative of not including the IV SHOULD NOT be applied in S/MIME. The key-wrap/unwrap algorithms used to encrypt/decrypt an IDEA content-encryption key with an IDEA key-encryption key are specified in the following section. Generation and distribution of IDEA key-encryption keys are beyond the scope of this document. The IDEA key-encryption algorithm has the object identifier id-alg-CMSIDEAwrap OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) usdod(6) oid(1) private(4) enterprises(1) ascom(188) systec(7) security(1) algorithms(1) 6 } The identifier's parameters field MUST be NULL. 3. Key-Wrapping and Unwrapping In the following subsections IDEA key-wrap and key-unwrap algorithms are specified in conformance with [CMS], section 12.3. 3.1 IDEA Key Wrap The IDEA key-wrap algorithm encrypts an IDEA content-encryption key with an IDEA key-encryption key. The IDEA key-wrap algorithm is defined by: 1. Let the content-encryption key (16 octets) be called CEK 2. Compute an 8 octet key checksum value on CEK as described in [CMS], section 12.6.1, call the result ICV. 3. Let CEKICV := CEK || ICV. 4. Generate 8 octets at random, call the result IV. 5. Encrypt CEKICV using IDEA in CBC mode and the key-encryption key. Use the random value generated in the previous step as the initialization vector (IV). Call the ciphertext TEMP1. 6. Let TEMP2 = IV || TEMP1. 7. Reverse the order of the octets in TEMP2. That is, the most significant (first) octet is swapped with the least significant (last) octet, and so on. Call the result TEMP3. 8. Encrypt TEMP3 using IDEA in CBC mode and the key-encryption key. Use an initialization vector (IV) of 0x4adda22c79e82105. The ciphertext is 32 octets long. 3.2 IDEA Key Unwrap The IDEA key-unwrap algorithm decrypts an IDEA content-encryption key using an IDEA key-encryption key. The IDEA key-unwrap algorithm is defined by: 1. If the wrapped content-encryption key is not 32 octets, then error. 2. Decrypt the wrapped content-encryption key using IDEA in CBC mode with the key-encryption key. Use an initialization vector (IV) of 0x4adda22c79e82105. Call the output TEMP3. 3. Reverse the order of the octets in TEMP3. That is, the most significant (first) octet is swapped with the least significant (last) octet, and so on. Call the result TEMP2. 4. Decompose the TEMP2 into IV and TEMP1. IV is the most significant (first) 8 octets, and TEMP1 is the remaining (last) 24 octets. 5. Decrypt TEMP1 using IDEA in CBC mode with the key-encryption key. Use the IV value from the previous step as the initialization vector. Call the plaintext CEKICV. 6. Decompose the CEKICV into CEK and ICV. CEK is the most significant (first) 16 octets, and ICV is the least significant (last) 8 octets. 7. Compute an 8 octet key checksum value on CEK as described in [CMS], section 12.6.1. If the computed key checksum value does not match the decrypted key checksum value, ICV, then error. 8. Use CEK as the content-encryption key. 4. SMIMECapabilities Attribute An S/MIME client can announce the set of cryptographic functions it supports by using the S/MIME capabilities attribute as specified in [SMIME3]. This attribute provides a partial list of OIDs of cryptographic functions and must be signed by the client. These OIDs should be logically separated in functional categories and MUST be ordered with respect to their preference. If an S/MIME client is required to support symmetric encryption and key wrapping based on IDEA, the capabilities attribute MUST contain the above specified OIDs in the category of symmetric algorithms and key encipherment algorithms. IDEA does not require additional OID parameters since it has a fixed key length of 128 bits. The SMIMECapability SEQUENCE representing the IDEA symmetric encrytion algorithm MUST include the IDEA-CBC OID in the capabilityID field and the parameters field MUST be absent. The SMIMECapability SEQUENCE for IDEA encryption SHOULD be included in the symmetric encryption algorithms portion of the SMIMECapabilities list. The SMIMECapability SEQUENCE representing IDEA MUST be DER-encoded as follows: 300F 060B 2B06 0104 0181 3C07 0101 0205 00. The SMIMECapability SEQUENCE representing the IDEA key wrapping algorithm MUST include the id-alg-CMSIDEAwrap OID in the capabilityID field and the parameters field of KeyWrapAlgorithm MUST be absent. The SMIMECapability SEQUENCE for IDEA key wrapping SHOULD be included in the key encipherment algorithms portion of the SMIMECapabilities list. The SMIMECapability SEQUENCE representing IDEA key wrapping MUST be DER-encoded as follows: 300F 060B 2B06 0104 0181 3C07 0101 0605 00. 5. Activation of IDEA in S/MIME 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, etc. If users require IDEA 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. A. References [IDEA] X. Lai, "On the design and security of block ciphers", ETH Series in Information Processing, J.L. Massey (editor), vol. 1, Hartung-Gorre Verlag Konstanz, Technische Hochschule (Zurich), 1992. A. J. Menezes, P.C. v. Oorschot, S.A. Vanstone, "Handbook of Applied Cryptography," CRC Press New York, 1997, p. 265. B. Schneier, "Applied Cryptography," 2nd ed., John Wiley & Sons Inc. New York, 1996, pp. 319-325. [SMIME2] "S/MIME Version 2 Message Specification", RFC 2311, and "S/MIME Version 2 Certificate Handling", RFC 2312. [SMIME3] "S/MIME Version 3 Certificate Handling", RFC 2632, and "S/MIME Version 3 Message Specification", RFC 2633. [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119. [CMS] "Cryptographic Message Syntax", RFC 2630. [PKCS7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315. B. Comments on IDEA Security and Standards The IDEA algorithm was developed in a joint project involving the Swiss Federal Institute of Technology in Zurich (Dr. X. Lai and Prof. J.L. Massey) and Ascom Ltd. The aim of the project was to develop an encryption algorithm which would replace the DES algorithm. IDEA uses 128-bit secret keys and encrypts one 64-bit block at a time. Experts in cryptography consider IDEA to be a highly secure symmetric cipher [IDEA]. It was particularly strengthened to protect against differential cryptoanalysis attacks. For the full 8-round IDEA there is no attack known which is better than exhaustive search on the total 128-bit key space. IDEA permits the implementation of standard Electronic Data Interchange applications. It has been entered in the ISO/IEC register for encryption algorithms and incorporated in the "SECURITY GUIDE LINES" code list by the UNI/EDIFACT "SECURITY JOINT WORKING GROUP". More information on IDEA can be found at <http://www.it-sec.com/idea_alg.html>. C. Intellectual Property Notice IDEA (TM) is protected by international copyright law and in addition it has been patented in the United States, Japan, and in most of the European countries. The patent is held by Ascom Ltd. Non-commercial use of IDEA is free. Commercial licenses can be easily obtained via online order or by contacting idea@it-sec.com. Detailed licensing information can be found at <http://www.it-sec.com/idea_lic.html>. D. Acknowledgements We would like to thank Jim Schaad and Francois Zeller for their contributions to this document. E. Authors' Address iT_Security Ltd. Badenerstrasse 530 CH-8048 Zurich, Switzerland Phone: +41 1 236 9900 Fax : +41 1 236 9990 Email: {stephan.teiwes,peter.hartmann,diego.kuenzi}@it-sec.com F. Full Copyright Statement Copyright (C) The Internet Society (date). All Rights Reserved. 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