OCSPstapling

OCSP stapling is a technique used to improve the efficiency and privacy of certificate revocation checks in Transport Layer Security (TLS). In this approach, the web server periodically queries the certificate authority’s OCSP responder to obtain the revocation status of its TLS certificate and then sends that OCSP response to clients during the TLS handshake. This avoids clients having to contact the CA directly for revocation information, while also reducing latency and CA load.

How it works: The server caches an OCSP response for its certificate and updates it at regular intervals before expiry. When a client connects, the server supplies the stapled OCSP response as part of the TLS negotiation. The client can then verify the signature and status contained in the response to determine whether the certificate is still valid. If no stapled response is available or if the stapled response is invalid, the client may fall back to its own OCSP checks or consider the certificate status unknown.

Benefits and limitations: OCSP stapling improves privacy (clients do not reveal which sites they visit to the CA), reduces round-trips and latency, and decreases CA infrastructure load. It also helps during network outages when the client cannot reach OCSP responders. Limitations include dependence on the server’s ability to fetch and refresh the stapled response, the potential for stale information if the stapled data expires, and the fact that stapling does not guarantee revocation status beyond the stapled response’s validity period. Some clients may ignore stapled data if it is absent or invalid.

References: This technique is defined by standards and widely implemented in TLS stacks and servers.