A certificate is a small binary file that contains identifying information about the issuer of the certificate, the holder of the certificate, and the certificate's public key. The contents and format of the certificate conform to the international standard X.509v3. Most information in a certificate is typically considered public knowledge and can be distributed freely. The exception is the private key that complements the public key in the certificate. If the private key is compromised, you need to revoke the certificate and reissue a new one with a new public and private key pair. For security reasons, certificates have expiration dates, so you must occasionally renew certificates.