PKI Standards Work
The dominant PKI-related standards can be found at the following organisations and working groups.
PKIX - the public key working group of the IETF
IETF S/MIME Mail Security (see also the Internet Mail Consortium S/MIME site)
IETF Transport Layer Security (TLS)
IEEE Standards for Public Key Cryptography
ANSI X9.79 - Financial Industry PKI Standard
RSA PKCS standards series, most of which have moved into the public domain
NIST Federal PKI Technical Working Group (now inactive)
The chair of the IETF's PKIX Working Group once named these as the most important of their RFCs to do with public key security.
RFC3820 X.509 PKI Proxy Certificate Profile
RFC2560 Online Certificate Status Protocol - OCSP
RFC2527 X.509 Certificate Policy and Certification Practices Framework. Superseded by RFC 3647.
RFC3647 - X.509 Certificate Policy and Certification Practices Framework. Supersedes RFC 2527.
RFC2511 - Certificate Request Message Format
RFC2797 - Certificate Management Messages over CMS
RFC3039 - PKI Qualified Certificates Profile
RFC3161 - Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)
RFC3281 - An Internet Attribute Certificate Profile for Authorization.