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OCSP stapling, formally known as the TLS Certificate Status Request extension, is an alternative approach to the Online Certificate Status Protocol (OCSP) for checking the revocation status of X.509 digital certificates. It allows the presenter of a certificate to bear the resource cost involved in providing OCSP responses, instead of the issuing Certificate Authority (CA).
OCSP has several advantages over older Certificate Revocation List (CRL)-based certificate revocation-checking approaches; however, it can introduce a significant penalty for certificate authorities who are now required to provide responses to every client of a given certificate in real time. When the certificate is issued to a legitimate high traffic web site, for instance, this can result in enormous volumes of OCSP request traffic, all of which serves to indicate that the certificate is valid and can be trusted.
OCSP checking also creates a privacy impairment, since it requires the client to contact a third party to confirm certificate validity. A way to verify validity without disclosing browsing behavior would be desirable for some groups of users.
OCSP stapling resolves both problems in a fashion reminiscent of the Kerberos Ticket. In a stapling scenario, the certificate holder queries the OCSP server themselves at regular intervals, obtaining a signed time-stamped OCSP response. When the site's visitors attempt to connect to the site, this response is included ("stapled") with the TLS/SSL Handshake via the Certificate Status Request extension response (note: the TLS client must explicitly include a Certificate Status Request extension in its ClientHello TLS/SSL handshake message). It may appear that allowing the site operator to control verification responses would allow a fraudulent site to issue a false verification for a revoked certificate, however, the stapled response is signed by the certificate authority, not the certificate holder (site operator), so stapled responses can't be forged (without the CA's signing key). Also, an invalid stapled response (or no stapled response) will just cause the client to ask the OCSP server directly. The only increased risk with this approach is a time delay in propagating the notification of revocation for a period of time up to the length of the query interval.
As a result, clients continue to have verifiable assurance from the certificate authority that the certificate is presently valid (or was quite recently), but no longer need to individually contact the OCSP server. This means that the brunt of the resource burden is now placed back on the certificate holder. It also means that the client software no longer needs to disclose users' browsing habits to any third party.
Overall performance is also improved: When the client fetches the OCSP response directly from the CA, it usually involves the lookup of the domain name of the CA's OCSP server in the DNS as well as establishing a connection to the OCSP server. When OCSP stapling is used, the certificate status information is delivered to the client through the established channel, which improves performance.
The TLS Certificate Status Request extension (colloquially known as OCSP stapling) is specified in RFC 6066, Section 8. A draft proposal for an X509v3 extension field specifying that a compliant server presenting a certificate carrying the extension MUST return a valid OCSP token in its response if the status_request extension is specified in the TLS client hello expired in April 2013. TLS developer Adam Langley discussed the extension in an April 2014 article following the repair of the Heartbleed OpenSSL bug.
OCSP stapling is designed to reduce the cost of an OCSP validation---both for the client and the OCSP responder---especially for large sites serving many simultaneous users. However, OCSP stapling supports only one OCSP response at a time, which is insufficient for sites which use several different certificates for a single page.
- P. Hallam-Baker, X.509v3 Extension: OCSP Stapling Required
- A. Langley, No, don't enable revocation checking, April 19, 2014.
- Apache HTTP Server mod_ssl documentation - SSLUseStapling directive
- nginx-announce mailing list - nginx-1.3.7
- Release Log - Litespeed Tech. Retrieved 2014-02-07.
- OCSP Stapling in Firefox, retrieved 2013-07-30
- Mozilla NSS Bug 360420, Comment by Adam Langley
- Mozilla NSS Bug 611836 - Implement multiple OCSP stapling extension