OCSP stapling

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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).

Motivation

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.

Solution

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 introduces an opportunity for fraud. Since the response is signed by the certificate authority, not the certificate holder, though, and since lack of a valid stapled response will just cause the client to ask the OCSP server directly, there is actually no increased risk with this approach.

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 channel already established causing no additional delay.

Specification

The TLS Certificate Status Request extension (colloquially known as OCSP stapling) is specified in RFC 6066, Section 8.

Deployment

OCSP stapling has not seen broad deployment to date, however this is changing. The OpenSSL project included support in their 0.9.8g release with the assistance of a grant from the Mozilla Foundation.

Apache HTTP Server supports OCSP stapling since version 2.3.3,[1] the nginx web server since version 1.3.7,[2] and LiteSpeed Web Server since version 4.2.4.[3]

On the browser side, OCSP stapling was implemented in Firefox Nightly builds on July 29, 2013.[4]

Limitations

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.[5][6]

References

  1. ↑ Apache HTTP Server mod_ssl documentation - SSLUseStapling directive
  2. ↑ nginx-announce mailing list - nginx-1.3.7
  3. ↑ Release Log - Litespeed Tech. Retrieved 2014-02-07.
  4. ↑ OCSP Stapling in Firefox, retrieved 2013-07-30
  5. ↑ Mozilla NSS Bug 360420, Comment by Adam Langley
  6. ↑ Mozilla NSS Bug 611836 - Implement multiple OCSP stapling extension
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