What is OCSP?
Also known as: Online Certificate Status Protocol
OCSP (Online Certificate Status Protocol) lets a client check a certificate's revocation status in real time by querying a responder, avoiding bulky CRLs.
OCSP is a protocol defined in RFC 6960 that allows a client (typically a web browser or server) to determine whether a specific X.509 digital certificate has been revoked by its issuing Certificate Authority. Instead of downloading a large Certificate Revocation List (CRL) periodically, a client sends a lightweight request containing the certificate's serial number to an OCSP responder. The responder returns a signed response indicating "good," "revoked," or "unknown." This real-time, on-demand check reduces bandwidth and latency compared to CRL distribution.
Technically, OCSP uses HTTP as its transport (often on port 80 or 443) with a request/response structure encoded in ASN.1 DER, signed by the CA or a delegated responder. The response includes a timestamp (thisUpdate and nextUpdate) and may include extensions for nonces or delegated trust. While OCSP eliminates the need to maintain a local CRL cache, it introduces a new trust dependency: the OCSP responder must be reachable and trusted. If the responder is unavailable, the client must decide whether to "soft-fail" (allow the connection) or "hard-fail" (block it), a choice with security trade-offs. Many modern implementations combine OCSP with stapling (see TLS Certificate Status Request extension, RFC 6066) where the server fetches and attaches the OCSP response during the TLS handshake, reducing client latency and responder load.
Key facts
- Replaced by RFC 6960, obsoleting the original RFC 2560.
- Returns one of three statuses: good, revoked, or unknown.
- Uses HTTP transport; default port is 80, but often runs over 443 with HTTPS.
- Signed response prevents tampering; includes a timestamp for freshness.
- OCSP stapling moves the check to the TLS handshake (RFC 6066).
How it works in practice
Related terms
References
More in TLS & PKI
ACME Protocol
ACME (Automated Certificate Management Environment) is a protocol that automates the issuance, renewal, and revocation of TLS certificates, defined in RFC 8555.
Certificate Authority
A Certificate Authority (CA) is a trusted entity that issues digital certificates after verifying that the requester controls the domain or identity named in the certificate.
Certificate Chain
A certificate chain is an ordered list of certificates, starting with the server certificate and ending with a root CA, that a client validates to establish trust in the server's identity.
Certificate Pinning
A security technique where an application trusts only a specific, pre-selected certificate or public key for a given server, bypassing the standard chain of trust.
Cipher Suite
A cipher suite is a named set of cryptographic algorithms negotiated during a TLS handshake, specifying key exchange, authentication, encryption, and integrity protection for secure communications.
Domain Validation
Domain Validation (DV) is the lowest level of certificate validation used in TLS/SSL, where the certificate authority verifies only that the applicant controls the domain name, typically via an HTTP or DNS challenge.
ECH
Encrypted Client Hello (ECH) is a TLS extension that encrypts the Client Hello message, including the Server Name Indication (SNI), to prevent on-path observers from learning the target hostname during the handshake.
Extended Validation
Extended Validation (EV) is the highest level of TLS certificate assurance, requiring the certificate authority to perform rigorous, human-verified checks on the legal identity and operational existence of the requesting organization before issuance.
HSTS
HTTP Strict Transport Security (HSTS) is a web security policy mechanism that forces browsers to interact with a website only over HTTPS, preventing downgrade attacks and cookie hijacking.
Intermediate Certificate
An intermediate certificate is a subordinate CA certificate signed by a root CA, used to sign end-entity certificates and enable path validation while the root remains offline.