Which protocol enables a server to communicate the status of a requested certificate efficiently?

The Online Certificate Status Protocol (OCSP) is the appropriate choice as it allows a server to efficiently communicate the status of a requested certificate. OCSP is specifically designed to provide real-time verification of a certificate's validity without the need for the client to download the entire Certificate Revocation List (CRL).

Using OCSP, a client can send a request to an OCSP responder, which quickly provides a response indicating whether the requested certificate is valid, revoked, or unknown. This real-time exchange is particularly advantageous in scenarios where immediate validation is needed, such as when a secure connection is established.

In contrast, a Certificate Revocation List (CRL) presents an entire list of revoked certificates; while it is useful, it is less efficient since it requires downloading potentially large lists, especially in environments with many certificates. The Registration Authority (RA) focuses on validating the identity of entities requesting certificates but does not directly manage or communicate certificate status. A Certificate Signing Request (CSR) is a message sent from the applicant to the certificate authority (CA) to apply for a digital certificate, but it does not pertain to the status of an existing certificate.

Thus, OCSP stands out as the protocol specifically designed for the efficient communication of certificate