Claire wishes to verify if a certificate has been revoked or not. Which protocol is utilized for validating certificates?

The Online Certificate Status Protocol (OCSP) is used to verify the status of a certificate. RTCP, CRBL, and PKCRL are not real and were created for this question. OCSP is a protocol that is used to verify the status of a digital certificate.

When a client attempts to access a resource that requires a digital certificate, the server can use OCSP to verify the status of the certificate. When a certificate is issued, the issuing authority (such as a Certificate Authority or CA) records the certificate's serial number, public key, and expiration date in a database. If the certificate is revoked for any reason (such as if the private key is compromised), the issuing authority marks the certificate as revoked in the database.

When a client attempts to access a resource that requires a certificate, the server sends a request to the issuing authority's OCSP responder, providing the certificate's serial number. The OCSP responder checks its database to see if the certificate has been revoked. If the certificate has been revoked, the responder sends a response indicating that the certificate is invalid. If the certificate has not been revoked, the responder sends a response indicating that the certificate is valid.

OCSP provides a more efficient method of checking certificate status than Certificate Revocation Lists (CRLs), which are lists of revoked certificates that must be downloaded and checked by clients. With OCSP, the client only needs to send a request to the OCSP responder to obtain the status of a certificate, rather than downloading and checking an entire CRL.