RFC 1847: Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted
Defines MIME framework for signed and encrypted message parts, foundation for S/MIME and PGP/MIME.
Digital signatures
RFC 5751: Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Defines S/MIME 3.2 for signing and encrypting email using X.509 certificates.
