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Internet Draft                                         S. Teiwes,
draft-ietf-smime-idea-01.txt                           P. Hartmann,
August 26, 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),
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1. Introduction

   S/MIME (Secure/Multipurpose Internet Mail Extensions) [SMIME2,
   SMIME3] is a specification for secure sending and receiving of MIME
   [MIME] data. Based on the Internet MIME standard, S/MIME provides
   cryptographic services for authentication, message integrity and
   non-repudiation of origin by using digital signatures. It also
   supports privacy and data security by using encryption. The S/MIME
   framework can be flexibly extended to meet future requirements.
   However, its extentability can also be used to introduce specific
   add-on functionality which might be desired in messaging applications.

   This memo specifies how to incorporate IDEA (International Data
   Encryption Algorithm) into S/MIME as an additional algorithm for
   symmetric encryption. Today, IDEA is widely applied in e-business
   applications. However, it is not part of the S/MIME specification given
   in [SMIME3]. Often, encryption algorithms are part of a security policy,
   and organizations usually 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 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/>


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.

   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 memo.

   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. Consequence on S/MIME Capabilities 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. If an S/MIME client is required to
   support symmetric encryption with IDEA, the capabilities attribute
   must contain the above specified OIDs in the category of symmetric
   algorithms. IDEA does not require additional OID parameters as it
   uses a fixed key length of 128 bits.


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.

   [MIME] The primary definition of MIME.
   "MIME Part 1: Format of Internet Message Bodies", RFC 2045.
   "MIME Part 2: Media Types", RFC 2046.
   "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC 2047.
   "MIME Part 4: Registration Procedures", RFC 2048.
   "MIME Part 5: Conformance Criteria and Examples", RFC 2049.

   [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 and source code can be found at
   <http://www.ascom.com/infosec/idea.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@ascom.ch. Detailed licensing information can be found
   at <http://www.ascom.com/infosec/idea.html>.


D. 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}@itsec.ch


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