(n.) A widely used standard for defining
digital certificates. X.509 (Version 1)
was first issued in 1988 as a part of the ITU X.500
Directory Services standard. When X.509 was revised in 1993, two more fields
were added resulting in the Version 2 format. These two additional fields
support directory access control. X.509 Version 3 defines the format for
certificate extensions used to store additional information regarding the
certificate holder and to define certificate usage. Collectively, the term X.509
refers to the latest published version, unless the version number is stated.
X.509 is published as ITU recommendation ITU-T X.509 (formerly CCITT X.509) and
ISO/IEC/ITU 9594-8 which defines a standard certificate
format for public key certificates and certification validation. With minor
differences in dates and titles, these publications provide identical text in
the defining of public-key and attribute certificates.
