X.509

X.509 is a standard that defines the certificate formats and fields for public keys

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X.500

is a DIRECTORY SERVICE implemented by ITU (International Telecommunications Union)

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X509 V3 is the most widely used digital certificate standard

the X.500 standard was implemented by the International Telecommunications Union (ITU), an international standards group. For directory services in the late 1980’s. the X.500 directory structure was the basis for later models of directory structure. Such as LDAP. The major problem in the industry in implementing a full-blown X.500 structure revolved around the complexity of the implementation. Novell was one of the first manufacturers to implement X.500 in its NetWare NDS product.