Which protocol can be used to check if a certificate has been revoked?

The Online Certificate Status Protocol (OCSP) is specifically designed to enable clients to check the revocation status of digital certificates in real time. It provides a way for users to verify whether a particular certificate is still valid or if it has been revoked by the Certificate Authority (CA). When a system or application needs to validate a certificate, it can query the OCSP server with details about the certificate in question. The server then responds with up-to-date information regarding the certificate status, thereby ensuring that only trusted certificates are in use.

When considering other protocols, SMTP (used for sending emails), HTTP (used for transferring hypertext), and FTP (used for transferring files), none of these are designed to specifically handle certificate status checks. Their purposes do not include interacting with certificate revocation lists or validating the path of trust for certificates, which is the primary role of OCSP. This is why OCSP is the correct choice for checking the revocation status of a certificate.