Like a driver’s license, a certificate specifies a period of time during which it is valid. Attempts to use a certificate for authentication before or after its validity period fails. Therefore, mechanisms for managing certificate renewal are essential for any certificate management strategy. For example, an administrator may wish to be notified automatically when a certificate is about to expire, so that an appropriate renewal process can be completed in plenty of time without causing the certificate’s subject any inconvenience. The renewal process may involve reusing the same public-private key pair or issuing a new one.
A driver’s license can be suspended even if it has not expired—for example, as punishment for a serious driving offense. Similarly, it’s sometimes necessary to revoke a certificate before it has expired—for example, if an employee leaves a company or moves to a new job within the company.
Certificate revocation can be handled in several different ways. For some organizations, it may be sufficient to set up servers so that the authentication process includes checking the directory for the presence of the certificate being presented. When an administrator revokes a certificate, the certificate can be automatically removed from the directory, and subsequent authentication attempts with that certificate fails even though the certificate remains valid in every other respect. Another approach involves publishing a certificate revocation list (CRL)—that is, a list of revoked certificates—to the directory at regular intervals and checking the list as part of the authentication process. For some organizations, it may be preferable to check directly with the issuing CA each time a certificate is presented for authentication. This procedure is sometimes called real-time status checking.