The Online Certificate Status Protocol (OCSP) is a protocol used to determine whether a certificate has been revoked by the issuing CA. When a certificate is received by SSL/TLS, the client checks the certificate revocation lists (CRLs) of the publisher to determine whether a certificate is still valid. These lists are not suitable for a quick check, however, because they can be large and need to be downloaded from the issuer. Therefore, systems using OCSP obtain the status of a specific certificate from the publisher that will use an OCSP Responder, to provide the up-to-date status of the certificate being checked.