X.509

X.509 is the international standard for digital certificates, used for HTTPS/TLS, S/MIME, and other internet security, now in version 3.

X.509 is the international standard, defined by ITU-T Recommendation X.509 and profiled for internet use in IETF RFC 5280, that specifies the structure of digital certificates used across the internet, including for HTTPS/TLS connections and S/MIME email encryption. It defines fields such as the subject's name, public key, issuer, validity period, serial number, and extensions, as well as the format of certificate revocation lists (CRLs), allowing systems everywhere to interpret certificates in the same way. Each certificate is digitally signed by its issuer, and its authenticity is validated by following a chain of certificates up to a trusted root within a Public Key Infrastructure (PKI). Version 3 (X.509v3) is the version in current use, as it added extension fields that support features like key usage restrictions and certificate revocation information.