X.509 Certificate

X.509 is the ITU-T standard that defines the syntax of public-key certificates used across the internet, including TLS, S/MIME, code signing and IPsec. A certificate contains the subject's distinguished name, public key, serial number, validity period, issuer, signature algorithm, the issuer's signature and a set of extensions such as Subject Alternative Name, Key Usage, Extended Key Usage, Basic Constraints, and CRL/AIA distribution points. Relying parties build a path from the certificate up to a trusted root, verifying each signature, validity period, name constraints and revocation status. Version 3 is the format used today, encoded in DER and typically distributed in PEM form.