http://www.rfc-editor.org/rfc/rfc4262.txt
RFC 4262: X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities, S. Santesson2005-12-01T23:00:00-00:00This document defines a certificate extension for inclusion of Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities in X.509 public key certificates, as defined by RFC 3280. This certificate extension provides an optional method to indicate the cryptographic capabilities of an entity as a complement to the S/MIME Capabilities signed attribute in S/MIME messages according to RFC 3851. [STANDARDS-TRACK]