Internet Draft Carlisle Adams (Entrust Technologies)
S/MIME Working Group
Expires in 6 months March 2000
Use of the CAST-128 Encryption Algorithm in CMS
<draft-ietf-smime-cast-128-01.txt>
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Copyright Notice
Copyright (C)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].
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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
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}
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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
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.
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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) MAY be used to indicate supported key length.
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).
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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] 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
E-Mail: cadams@entrust.com
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