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S/MIME uses the marriage of the MIME and RSA PKCS #7 data types, using RSA PKCS #10 for Certification Requests. Data is always a MIME entity (a body part, attachment or the whole message with all of its sub-parts), which is handed to the PKCS processes, thereby producing a PKCS object. This object is then wrapped up as a MIME message and sent.

S/MIME products use RSA public key methods for key exchanges and digital signatures. Bulk message encryption is conducted using private key methods--either DES, Triple-DES or RSA RC2. In addition, some v endors may use proprietary methods (Entrust's CAST, for example) at the expense of interoperability.

Because of some ambiguity in the S/MIME specification, significant differences between some S/MIME implementations exist. For example, S/MIME does not require the use of Certificate Revocation Lists, nor does it adequately address how IMAP messages are to be handled. Some IMAP features--like separately downloadable headers--may not function properly with S/MIME content, as the individual components of the message may not be signed or encrypted. Systems must also support the multipart or signed format if you want non-S/MIME clients to view messages that are signed but not encrypted. However, this may cause message loss when crossing non-MIME environments.

S/MIME alone does not serve as a complete business-to-business commerce solution. It supports sour ce nonrepudiation (the sender can't deny he or she sent a message), but it does not support nonrepudiation of receipt or delivery (the receiver can deny message receipt). Issues such as these have an affect on the use of S/MIME for electronic commerce and are being addressed by the EDIINT working group of the IETF (see "Safe and Secure Electronic Commerce," www.NetworkComputing.com/719/ 719cn4.html, and "Signed, Sealed & Delivered: CommerceNet Test Results," September 15, page 88).

The major vendors of proprietary messaging products--Microsoft, Novell and Lotus--have pledged support for S/MIME. These same vendors are also incorporating X.509 certificate services into their messaging products, Web servers and operating systems. Currently, S/MIME products are available from a range of vendors, including Netscape, ConnectSoft, Entrust Technologies, Innosoft International, OpenSoft and Worldtalk. With this much market momentum, S/MIME will provide the best road to multivendor messaging security.

David Willis can be reached at dwillis@nwc.com.