Information, Computer and Network Security Terms Glossary and Dictionary

X.509 is an ITU-T standard for public key infrastructure (PKI). X.509 specifies, amongst other things, standard formats for public key certificates and a certification path validation algorithm. X.509 was initially issued in 1988 and was begun in association with the X.500 standard and assumed a strict hierarchical system of certificate authorities (CAs) for issuing the certificates. The X.500 system has never been fully implemented, and the IETF's public-key infrastructure working group has adapted the standard to the more flexible organization of the Internet. In fact, the term X.509 certificate usually refers to the IETF's PKI Certificate and CRL Profile of the X.509 v3 certificate standard, as specified in RFC 3280, commonly referred to as PKIX.

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X.509