To quickly check the validity of an SSL certificate, which method provides the fastest results?

The fastest method to check the validity of an SSL certificate is through the Online Certificate Status Protocol (OCSP). OCSP allows clients to query the status of a specific SSL certificate in real-time, providing an immediate response regarding its validity. This method is more efficient than other techniques, as it does not require downloading a complete list of revoked certificates.

In contrast, a Certificate Revocation List (CRL) requires downloading a potentially large file that contains information on all revoked certificates, which can lead to delays in checking legitimacy. The Registration Authority (RA) is responsible for verifying the identity of entities requesting certificates and does not play a role in validating them once issued. A Certificate Signing Request (CSR) is a request to create a certificate and is not used for checking its validity. Thus, OCSP is the preferred method for quick verification of certificate status in SSL communications.