"This document defines a framework within which security services may be applied to MIME body parts. MIME@ an acronym for ""Multipurpose Internet Mail Extensions""@ defines the format of the contents of Internet mail messages and provides for multi-part textual and nontextual message bodies. The new content types are subtypes of multipart: signed and encrypted. Each will contain two body parts: one for the protected data and one for the control information necessary to remove the protection. The type and contents of the control information body parts are determined by the value of the protocol parameter of the enclosing multipart/signed or multipart/encrypted content type@ which is required to be present."
RFC 1847-1995 history
1995RFC 1847-1995 Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted