X-Standards

Background

The X standards are for “Data communication over the telephone network” (Eg. X-500, X-509)
These protocols are typically defined piecemeal throughout multiple specifications and ASN.1 modules. (See Custom Protocols)
There are other standards from ITU-T: ITU-T Recommendations (Eg. H265 is part of this)
Back in the day, X.500 (and X.400) got a lot of pushback in the IETF crowd because of the use of ASN.1, while all IETF protocols of the era were still plaintext.

X.500 is a commonly used name for a series of joint ISO/IEC and ITU-T standards specifying a distributed directory service.
First approved in 1988
It contains X.511, X.518, X.525, X.501, X.509, X.510

X.509v3
X.509 was designed to be the secure access method for updating X.500 before the WWW
X.500 offers a way to view which organization claims a specific root certificate
For browsers, trusted root certificates for supported certificate authorities were pre loaded into certificate storage areas on the personal computer or device.