X.509

X.509 is a standard defined by ITU-T (International Telecommunication Union - Telecommunication Sector) that specifies the format of electronic certificates and certificate revocation lists (CRLs) used in PKI (Public Key Infrastructure). The first version was published in 1988. The widely adopted version "X.509v3," released in 1996, forms the basis for electronic certificates used in technologies like S/MIME for encrypting and digitally signing emails, and SSL/TLS for secure communication over the internet.

The electronic certificates defined by X.509 include the following information:

• X.509 version
• Certificate serial number
• Signature algorithm
• Issuer of the certificate
• Validity period of the certificate
• Subject (the entity being certified)
• Subject’s public key and its algorithm
• X.509v3 extensions
• Signature algorithm
• Certification Authority’s (CA) digital signature