What is a Certificate Revocation List (CRL)?

A Certificate Revocation List (CRL) is fundamentally a list of digital certificates that have been revoked before their scheduled expiration date. This means that these certificates are no longer trusted by the certificate authority (CA) due to various reasons, such as compromise, loss of the private key, or a change in the status of the entity to which the certificate was issued.

The CRL is published by the CA and used by applications and systems to check the validity of a certificate before establishing secure communications. If a certificate is on the CRL, it indicates that it should not be trusted, regardless of its original validity period. This is crucial in maintaining the integrity and trustworthiness of communications in a security context, ensuring that only valid and trustworthy certificates are being utilized.

Other options like lists of expired certificates, pending approval certificates, or certificates with missing information do not serve the same critical purpose of identifying currently untrustworthy certificates, which is the primary function of a CRL.