OCSP

OCSP, short for Online Certificate Status Protocol, is a protocol used to determine the revocation status of X.509 digital certificates in real time. It allows a client to query an OCSP responder about a particular certificate and receive a signed response that indicates whether the certificate is still valid, has been revoked, or if the status is unknown. OCSP responses include validity intervals and may carry a revocation reason. The protocol is defined in RFC 6960.

Operation: The client constructs a certificate identifier (certID) including the issuer's name hash, issuer's public key hash, and the certificate's serial number, hashed with a chosen algorithm. The request is sent to the URL specified in the certificate's Authority Information Access extension. The responder returns a BasicOCSPResponse that contains the status and a chain of signer certificates. The response is time-stamped with thisUpdate and nextUpdate, and can include a nonce to prevent replay.

Transport and deployment: OCSP commonly uses HTTP as the transport. It supports GET requests or POST with DER-encoded data. To reduce latency and improve privacy, many servers deploy OCSP stapling, where the web server obtains and periodically caches the OCSP response and presents it during TLS handshakes. There is also a Must-Staple X.509 extension that enforces the presence of a stapled OCSP response.

Limitations and policy: OCSP can reveal which certificates a client is using to the responder, raising privacy concerns; the availability of responders is critical for timely revocation information. Browsers may implement soft-fail or hard-fail policies when an OCSP response cannot be obtained. As an alternative to OCSP, Certificate Revocation Lists (CRLs) provide revocation data as a signed list.