A certificate revocation list (CRL) is a component of the International Telecommunication Union's (ITU) X.509 security standard. According to the X.509 standard, a certificate authority (CA) can use a CRL to either place a hold on, or explicitly revoke, any digital security certificate that it has issued and which has not expired. The CRL is then distributed and used by various computer programs to confirm the validity of the security certificates used to identity a source.
The generation of a security certificate by a CA falls under what's called a public key infrastructure (PKI). Through a PKI, any user can be identified by the public key of their security key pair, the user's private key being the other half of the pair. A user then contacts a CA and, using his public key as identification, requests a security certificate. After some measure of vetting the user's actual identity, the CA can then issue a certificate that is bound to the user's public key. By this method, the CA acts as a trusted third party, guaranteeing the identity of the user who's been issued a certificate.
A digital security certificate is typically given a one- or two-year lifespan. After the certificate expires, the user needs to renew his existing certificate by re-validating his identity or by requesting a new certificate outright. The expiration date of a certificate is included in the certificate itself, so computer software knows when to no longer honor an expired certificate. There are times, however, when a certificate may need to be revoked before its expiration date. For those instances, a CA must maintain a certificate revocation list that lists any certificates that have not expired but cannot be trusted for some reason.
A certificate revocation list contains a number of possible reasons for revoking a certificate. The most common being that the private key for the owner of the certificate is no longer safe, at which point the certificate remains on the listing until its expiration date. In this case, the user must generate a new key pair and request an entirely new certificate.
There are, of course, other reasons a certificate may appear in the CRL. A certificate can be listed if it has been superseded by another or there is some change to the information contained in the certificate about its owner, or if the CA itself has been compromised, whereupon the CA itself will appear on what's called an authority revocation list (ARL). Another reason a certificate may appear on a CRL is because the certificate is being placed on hold for some reason. In the case of a certificate listed as being held, it can then be reinstated in the next CRL distributed by the CA. The many, frequent changes to the statuses of digital security certificates mean a certificate revocation list usually has a life expectancy of about 24 hours, though sometimes less.