What mechanism provides a reliable method for checking online certificate status?

The Online Certificate Status Protocol (OCSP) is the mechanism most suitable for reliably checking the status of an online certificate. OCSP is designed specifically to provide the revocation status of a digital certificate in real-time, making it an efficient solution for applications that require immediate verification.

When a client needs to check if a certificate is still valid, it can send a time-stamped request to an OCSP responder, which then provides a response indicating whether the certificate is valid, revoked, or unknown. This immediate feedback is crucial, especially in high-security environments where using a revoked certificate can lead to significant issues.

In contrast, other options like the Certificate Revocation List (CRL) provide a list of certificates that have been revoked but do not facilitate real-time checking without possibly needing to download and parse a potentially large list. LDAP (Lightweight Directory Access Protocol) is a protocol used for accessing directory services but is not specifically dedicated to checking certificate status. PKI (Public Key Infrastructure), while it encompasses the systems for managing digital certificates, does not by itself provide a mechanism for checking the revocation status of individual certificates. Thus, OCSP stands out as the most effective solution for this purpose.