Which protocol does NOT involve checking a certificate status in PKI?

The reason that the selected answer is correct stems from the nature of the Certificate Signing Request (CSR) protocol in Public Key Infrastructure (PKI). A CSR is generated when an entity wishes to obtain a digital certificate. It includes information such as the public key and the identity of the requester but does not inherently involve checking or verifying the status of a certificate.

In contrast, the Online Certificate Status Protocol (OCSP) and Certificate Revocation List (CRL) are protocols specifically designed to check the status of certificates. OCSP provides real-time status information about certificates, while CRL is a list of certificates that have been revoked by the certificate authority (CA). A Registration Authority (RA) plays a role in the verification and processing of requests for certificates but is also not directly involved in checking the status of a certificate itself.

Thus, the CSR is purely a request mechanism without any functionality related to certificate status checking, distinguishing it from the other protocols.