Internet Draft                     Carlisle Adams (Entrust Technologies)
S/MIME Working Group
Expires in 6 months                                       September 1999

           Use of the CAST-128 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.

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Copyright Notice

  Copyright (C)The Internet Society (1999). All Rights Reserved.


  This document specifies how to incorporate CAST-128 [RFC2144] into
  S/MIME as an additional algorithm for symmetric encryption.  The
  relevant OIDs and processing steps are provided so that CAST-128
  may be included in the S/MIME Cryptographic Message Syntax (CMS)
  specification [RFC2630] for symmetric content and key encryption.

  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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1. Motivation

  S/MIME (Secure/Multipurpose Internet Mail Extensions) [SMIME2,
  SMIME3] is a set of specifications for the secure transport of MIME
  objects.  In the current (S/MIME v3) specifications the mandatory-
  to-implement symmetric algorithm for content encryption and key
  encryption is triple-DES (3DES).  While this is perfectly acceptable
  in many cases because the security of 3DES is generally considered to
  be high, for some environments 3DES may be seen to be too slow.  In
  part to help alleviate such performance concerns, S/MIME has allowed
  any number of (optional) additional algorithms to be used for
  symmetric content and key encryption.

  The CAST-128 encryption algorithm [RFC2144, Adams] is a well-studied
  symmetric cipher that has a number of appealing features, including
  relatively high performance and a variable key size (from 40 bits
  to 128 bits).  It is available royalty-free and license-free for
  commercial and non-commercial uses worldwide [IPR], and therefore
  is widely used in a number of applications around the Internet.  It
  thus seems to be a suitable optional encryption algorithm for S/MIME.

  This document describes how to use CAST-128 within the S/MIME CMS

2. Specification

  This section provides the OIDs and processing information necessary
  for CAST-128 to be used for content and key encryption in CMS.

2.1 OIDs for Content and Key Encryption

  CAST-128 is added to the set of optional symmetric encryption
  algorithms in S/MIME by providing two unique object identifiers
  (OIDs).  One OID defines the content encryption algorithm and the
  other defines the key encryption algorithm.  Thus an S/MIME agent can
  apply CAST-128 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 CAST-128 in cipher block chaining
  (CBC) mode is RECOMMENDED.  The key length is variable (from 40 to 128
  bits in 1-octet increments).

  The CAST-128 content-encryption algorithm in CBC mode has the
  following object identifier:

    cast5CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2)
        us(840) nt(113533) nsn(7) algorithms(66) 10}

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  The parameter associated with this object identifier contains the
  initial vector IV and the key length:

    cast5CBCParameters ::= SEQUENCE {
        iv         OCTET STRING DEFAULT 0,
        -- Initialization vector
        keyLength  INTEGER
        -- Key length, in bits

  Comments regarding the use of the IV may be found in [RFC2144].

  The key-wrap/unwrap procedures used to encrypt/decrypt a CAST-128
  content-encryption key with a CAST-128 key-encryption key are
  specified in the Section 2.2.  Generation and distribution of
  key-encryption keys are beyond the scope of this document.

  The CAST-128 key-encryption algorithm has the following object

    cast5CMSkeywrap OBJECT IDENTIFIER ::= { iso(1)
        member-body(2) us(840) nt(113533) nsn(7)
        algorithms(66) 15}

  The parameter associated with this object identifier contains only
  the key length (because the key wrapping procedure itself defines
  how and when to use an IV):

    cast5CMSkeywrapParameter ::= INTEGER
      -- key length, in bits

2.2 Key Wrapping and Unwrapping

  CAST-128 key wrapping and unwrapping is done in conformance with CMS

2.2.1 CAST-128 Key Wrap

  Key wrapping with CAST-128 is identical to [RFC2630], Sections 12.6.1
  and 12.6.4, with "RC2" replaced by "CAST-128" in the introduction to

2.2.2 CAST-128 Key Unwrap

  Key unwrapping with CAST-128 is identical to [RFC2630], Sections
  12.6.1 and 12.6.5, with "RC2" replaced by "CAST-128" in the
  introduction to 12.6.5.

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3. Discussion

  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 CAST-128, the capabilities attribute
  MUST contain the above specified OIDs in the category of symmetric
  algorithms.  The parameters associated with the OIDs given above may
  be used to indicate supported key length.  Alternatively, the
  following parameter from
  may be used in the S/MIME capabilities attribute instead:

    SMIMECapabilitiesParametersForCast5CBC ::= INTEGER
    --        CAST Key Length (number of bits)
    --        (key size:  40, 64, 80 or 128)

  Note that this parameter restricts the range of possible CAST-128 key
  sizes to four, but this should be acceptable for many environments.

  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 CAST-128 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

4. Security Considerations

  This document specifies the use of the CAST-128 symmetric cipher for
  encrypting the content of an S/MIME message and for encrypting the
  symmetric key used to encrypt the content of an S/MIME message.
  Although CAST-128 allows keys of variable length to be used, it must
  be recognized that smaller key sizes (e.g., 40, 56, or 64 bits) may be
  unacceptably weak for some environments.  The use of larger key sizes
  (e.g., 128 bits) is always RECOMMENDED (when relevant import, export,
  or other laws permit).  It is also RECOMMENDED that the size of the
  content-encryption key and the size of the key-encryption key be equal
  (since the security of the content will be at most the smaller of
  these two values).

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  [Adams]   C. Adams, "Constructing Symmetric Ciphers using the CAST
            Design Procedure", Designs, Codes, and Cryptography, vol.12,
            no.3, November 1997, pp.71-104.

  [IPR]     See the "IETF Page of Intellectual Property Rights Notices",

  [RFC2144] C. Adams, "The CAST-128 Encryption Algorithm", Internet
            Request for Comments RFC 2144, May 1997.

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

  [RFC2630] R. Housley, "Cryptographic Message Syntax", Internet
            Request for Comments RFC 2630, June 1999.

  [SMIME2]  S. Dusse, P. Hoffman, B. Ramsdell, L. Lundblade, L. Repka,
            "S/MIME Version 2 Message Specification", Internet Request
            for Comments RFC 2311, March 1998.
            S. Dusse, P. Hoffman, B. Ramsdell, J. Weinstein, "S/MIME
            Version 2 Certificate Handling", Internet Request for
            Comments RFC 2312, March 1998.

  [SMIME3]  B. Ramsdell, "S/MIME Version 3 Certificate Handling",
            Internet Request for Comments RFC 2632, June 1999.
            B. Ramsdell, "S/MIME Version 3 Message Specification",
            Internet Request for Comments RFC 2633, June 1999.

Author's Address

  Carlisle Adams
  Entrust Technologies
  750 Heron Road, Suite E08,
  Ottawa, Ontario, Canada K1V 1A7

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