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What is an X.509 Digital Certificate?

X.509 is a standard format for public key certificates, digital documents that securely associate cryptographic key pairs with identities such as websites, individuals, or organizations.

An X.509 certificate contains information about the identity to which a certificate is issued and the identity that issued it. Standard information in an X.509 certificate includes:

  • Version – which X.509 version applies to the certificate (which indicates what data the certificate must include)
  • Serial number – the identity creating the certificate must assign it a serial number that distinguishes it from other certificates
  • Algorithm information – the algorithm used by the issuer to sign the certificate
  • Issuer distinguished name – the name of the entity issuing the certificate (usually a certificate authority)
  • Validity period of the certificate – start/end date and time
  • Subject distinguished name – the name of the identity the certificate is issued to
  • Subject public key information – the public key associated with the identity
  • Extensions (optional)