21. PEM & S/MIME
PEM (Privacy Enhanced Mail) was developed by the Internet community in the late '80s and early '90s as a means of adding encryption, source authentication, and integrity protection to ordinary text messages—the dominant form of email at the time. It is documented in four pieces. RFC 1421 describes the message formats. RFC 1422 describes the CA hierarchy. RFC 1423 describes a base set of cryptographic algorithms that can be used with PEM. RFC 1424 describes mail message formats for requesting certificates and for requesting or posting certificate revocation lists.
At about the same time, email was evolving to be more than just text. MIME (Multipurpose Internet Mail Extensions, RFC 2045) specified a standard ...