Use of the CAST-128 Encryption Algorithm in CMS

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2000). All Rights Reserved.

Abstract

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

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 specification.

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 CMS by providing two unique object identifiers (OIDs). One OID defines the content encryption algorithm and the other defines the key encryption algorithm. Thus a CMS 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}

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 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 identifier:

     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 [RFC2630].

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 12.6.4. Only 128-bit CAST-128 keys may be used as key-encryption keys, and they MUST be used with the cast5CMSkeywrapParameter set to 128. It is 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).

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.

3. Using CAST-128 in S/MIME Clients

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 cast5CBC OID specified above in the category of symmetric algorithms. The parameter associated with this OID (see above) MUST be used to indicate supported key length. For example, when the supported key length is 128 bits, the

SMIMECapability SEQUENCE representing CAST-128 MUST be DER-encoded as the following hexadecimal string:

301106092A864886F67D07420A300402020080.

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 preferences.

4. Security Considerations

This document specifies the use of the CAST-128 symmetric cipher for encrypting the content of a CMS message and for encrypting the symmetric key used to encrypt the content of a CMS 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).

References

   [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", http://www.ietf.cnri.reston.va.us/ipr.html
   
   [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
             May 1997.
   
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
             1999.
   
   [SMIME2]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and L.
             Repka, "S/MIME Version 2 Message Specification", RFC 2311,
             March 1998.
   
             Dusse, S., Hoffman, P., Ramsdell, B. and J. Weinstein,
             "S/MIME Version 2 Certificate Handling", RFC 2312, March
             1998.
   
   [SMIME3]  Ramsdell, B., "S/MIME Version 3 Certificate Handling", RFC
             2632, June 1999.
   
             Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
             2633, June 1999.

Author's Address

Carlisle Adams
Entrust Technologies
1000 Innovation Drive,
Kanata, Ontario, Canada K2K 3E7

EMail:

          cadams@entrust.com

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