X.509 is a PKI standard used for secure communications in the Internet. It is used as part of many standard cryptographic protocols such as TLS/SSL, ISAKMP/IKE and IKEv2 to provide strong authentication of cryptographic entities.
Internet X.509 Public Key Infrastructure: Certificate and Certificate Revocation List (CRL) Profile
Internet X.509 Public Key Infrastructure: Certificate and Certificate Revocation List (CRL) Profile
Internet X.509 Public Key Infrastructure: Logotypes in X.509 Certificates