Definition
The online Certificate Status Protocol (OCSP)is a network protocol that obtains X.509 digital certificate revocation status. It is part of the Internet Public Key Infrastructure (PKI) that protects web communications.
OCSP enables clients (like web browsers) to confirm with the Certificate Authority (CA) whether a digital certificate is valid or has been revoked. This technique is more efficient and prompter in verifying certificate status than older methods like Certificate Revocation Lists (CRLs), improving digital transactions and communications security.
How Does OCSP Work?
When checking for the validity of a certificate, the client sends an OCSP request to an OCSP responder (a server operated by the issuing CA). Then, the OCSP responder verifies the request’s validity with a trustworthy CA, which responds with current, revoked, or unknown. Many popular browsers, including Microsoft Edge, Internet Explorer, Apple Safari, and Mozilla Firefox, support OCSP.
History of Online Certificate Status Protocol
- Certificate revocation lists (CRLs): These were lists of revoked certificates that CAs published and were used to check the digital certificates’ revocation status. However, as the internet expanded, they became obsolete because of their size and infrequent updates.
- Emergence of OCSP: The Internet Engineering Task Force (IETF) introduced OCSP in 1999 to improve CRLs by facilitating real-time, on-demand checks of the certificate’s revocation status.
- OCSP in SSL/TLS: OCSP enables clients (such as web browsers) to check the validity of a website’s SSL/TLS certificate.
- OCSP stapling: OCSP stapling addressed the protocol’s privacy concerns and improved efficiency.
- Continued evolution: OCSP continues to improve with updates that address various issues, such as security vulnerabilities, response time, and scalability.
