S/MIME Version 3 Message Specification
1999
This memo defines how to create a MIME body part that has been cryptographically enhanced according to CMS [CMS], which is derived from PKCS #7 .This memo also defines the application/pkcs7mime MIME type that can be used to transport those body parts.This memo also discusses how to use the multipart/signed MIME type defined in [MIME-SECURE] to transport S/MIME signed messages.This memo also defines the application/pkcs7-signature MIME type, which is also used to transport S/MIME signed messages.In order to create S/MIME messages, an S/MIME agent has to follow specifications in this memo, as well as the specifications listed in the Cryptographic Message Syntax [CMS].Throughout this memo, there are requirements and recommendations made for how receiving agents handle incoming messages.There are separate requirements and recommendations for how sending agents create outgoing messages.In general, the best strategy is to "be liberal in what you receive and conservative in what you send".Most of the requirements are placed on the handling of incoming messages while the recommendations are mostly on the creation of outgoing messages.The separation for requirements on receiving agents and sending agents also derives from the likelihood that there will be S/MIME systems that involve software other than traditional Internet mail clients.S/MIME can be used with any system that transports MIME data.An automated process that sends an encrypted message might not be able to receive an encrypted message at all, for example.Thus, the requirements and recommendations for the two types of agents are listed separately when appropriate.1.