Протокол OCSP: валидация сертификатов и проблемы безопасности.

Communications protocol
Some web browsers (e. g., Firefox) use OCSP to validate HTTPS certificates, while others have disabled it. Most OCSP revocation statuses on the Internet disappear soon after certificate expiration.

Since an OCSP response contains less data than a typical certificate revocation list (CRL), it puts less burden on network and client resources. Since an OCSP response has less data to parse, the client side libraries that handle it can be less complex than those that handle CRLs. OCSP discloses to the responder that a particular network host used a particular certificate at a particular time. OCSP does not mandate encryption, so other parties may intercept this information. where a signed, 'good' response is captured by a malicious intermediary and replayed to the client at a later date after the subject certificate may have been revoked. OCSP allows a nonce to be included in the request that may be included in the corresponding response. Because of high load, most OCSP responders do not use the nonce extension to create a different response for each request, instead using presigned responses with a validity period of multiple days. Thus, the replay attack is a major threat to validation systems. OCSP can support more than one level of CA. OCSP requests may be chained between peer responders to query the issuing CA appropriate for the subject certificate, with responders validating each other's responses against the root CA using their own OCSP requests. An OCSP responder may be queried for revocation information by delegated path validation (DPV) servers. OCSP does not, by itself, perform any DPV of supplied certificates. The key that signs a response need not be the same key that signed the certificate. The certificate's issuer may delegate another authority to be the OCSP responder. In this case, the responder's certificate (the one that is used to sign the response) must be issued by the issuer of the certificate in question, and must include a certain extension that marks it as an OCSP signing authority (more precisely, an extended key usage extension with the OID {iso(1) identified organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) keyPurpose(3) ocspSigning(9)})

OCSP checking creates a privacy concern for some users, since it requires the client to contact a third party (albeit a party trusted by the client software vendor) to confirm certificate validity. OCSP stapling is a way to verify validity without disclosing browsing behavior to the CA. The MustStaple TLS extension in a certificate can require that the certificate be verified by a stapled OCSP response, mitigating this problem.) supports OCSP checking. All versions of Mozilla Firefox support OCSP checking. Firefox 3 enables OCSP checking by default. Safari on macOS supports OCSP checking. It is enabled by default as of Mac OS X 10.7 (Lion). Prior to that, it has to be manually activated in Keychain preferences. Versions of Opera from 8.0 to the current version support OCSP checking. However, Google Chrome is an outlier. Google disabled OCSP checks by default in 2012, citing latency and privacy issues and instead uses their own update mechanism to send revoked certificates to the browser.

Several open source and proprietary OCSP implementations exist, including fully featured servers and libraries for building custom applications. OCSP client support is built into many operating systems, web browsers, and other network software due to the popularity of HTTPS and the World Wide Web.