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Chapter: Cryptography and Network Security

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S/MIME Functionality and Messages

1. Functions 2. Cryptographic Algorithms

S/MIME Functionality


In terms of general functionality, S/MIME is very similar to PGP. Both offer the ability to sign and/or encrypt messages. In this subsection, we briefly summarize S/MIME capability. We then look in more detail at this capability by examining message formats and message preparation.


1. Functions


S/MIME provides the following functions:


·        Enveloped data: This consists of encrypted content of any type and encrypted-content encryption keys for one or more recipients.  


·        Signed data: A digital signature is formed by taking the message digest of the content to be signed and then encrypting that with the private key of the signer. The content plus signature are then encoded using base64 encoding. A signed data message can only be


·        viewed by a recipient with S/MIME capability.  


·        Clear-signed data: As with signed data, a digital signature of the content is formed. However, in this case, only the digital signature is encoded using base64. As a result, recipients without S/MIME capability can view the message content, although they cannot verify the signature.  


·        Signed and enveloped data: Signed-only and encrypted-only entities may be nested, so that encrypted data may be signed and signed data or clear-signed data may be encrypted.  


2. Cryptographic Algorithms


·        hash functions: SHA-1 & MD5


·        digital signatures: DSS & RSA


·        session key encryption: ElGamal & RSA


·        message encryption: Triple-DES, RC2/40 and others


·        have a procedure to decide which algorithms to use.


S/MIME uses the following terminology, taken from RFC 2119 to specify the requirement level:


·        Must: The definition is an absolute requirement of the specification. An implementation must include this feature or function to be in conformance with the specification.  


·        Should: There may exist valid reasons in particular circumstances to ignore this feature or function, but it is recommended that an implementation include the feature or function.

S/MIME Messages


S/MIME makes use of a number of new MIME content types. All of the new application types use the designation PKCS. This refers to a set of public-key cryptography specifications issued by RSA Laboratories and made available for the S/MIME effort.


We examine each of these in turn after first looking at the general procedures for S/MIME message preparation.

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