Certificate Authorities form group to push for better certificate-revocation checking

The newly formed Certificate Authority Security Council will raise awareness about OCSP stapling

Several Certificate Authorities (CAs) have formed an advocacy group called the Certificate Authority Security Council (CASC), which will focus on promoting new security standards, policies and best practices for SSL (Secure Sockets Layer) deployment on the Web.

CASC was launched on Thursday and its member list includes representatives from Symantec, Trend Micro, Comodo, DigiCert, Entrust, GlobalSign and Go Daddy. CAs are organizations that issue digital certificates.

The group's first efforts will focus on educating Web server administrators, software vendors, browser developers and end users about the importance of certificate-revocation checking and the benefits of adopting and implementing a mechanism called OCSP stapling -- OCSP stands for Online Certificate Status Protocol.

Ensuring that certificate-revocation information reaches users in a timely manner is very important, especially in light of several security breaches that affected Certificate Authorities during the past few years and resulted in rogue certificates for high-profile domains being issued fraudulently.

CAs use two mechanisms to announce certificate revocations: certificate-revocation lists (CRLs) that are updated periodically and OCSP responders -- servers that respond on-the-fly to certificate-revocation checks initiated by clients.

When encountering a digital certificate, modern browsers first verify that it is signed by a trusted CA and then send an OCSP request to that CA's OCSP responder in order to check if the certificate is still valid -- whether it has, for example, expired or has been revoked for some reason.

However, if their OCSP and CRL requests fail because of network errors, most browsers will, by default, accept the certificate and proceed to establish the connection. This is a soft-fail approach that favors usability over security.

For example, an attacker who has the ability to launch a man-in-the-middle attack using a rogue digital certificate can intercept the OCSP requests of the victim's browsers and block them, forcing the browser to accept the certificate even if the issuing CA revoked it in the meantime.

OCSP also raises privacy concerns because by contacting the CA to check the validity of a certificate, the browser indirectly tells the CA that the user visited a certain website.

OCSP stapling resolves both the security and privacy issues of current OCSP implementations, because the client no longer contacts the CA's OCSP responder directly. Instead, the HTTPS server queries the OCSP responder periodically and caches a signed and time-stamped response which is then included -- stapled -- in the SSL handshake with clients.

OCSP stapling also improves performance because it decreases the time required to establish a HTTPS connection by eliminating the additional request sent by the client to a third party.

"The CASC plans to educate affected parties in the coming months through blog posts, conference presentations, and other resources that will help people learn about OCSP stapling and make it easier for web server administrators to understand how to enable OCSP stapling on their servers," CASC said Thursday on its website. "We also plan to encourage wider adoption of this critical technology among software vendors and browsers."

According to CASC, OCSP stapling is supported by Microsoft's IIS (Internet Information Services), the newest versions of Apache and nginx Web servers, as well as the major browsers.

Ivan Ristic, the director of engineering at security firm Qualys, who runs the SSL Labs and SSL Pulse projects, believes that encouraging OCSP stapling adoption is a very good idea. However, it will take some time until the technology is widely adopted, because there are still some issues, he said.

For example, OCSP stapling is supported in Apache 2.4.x, but this version of the server is not widely used, Ristic said. "It's not installed by default in any of the major [Linux] distributions, as far as I know," he said.

If you want to support OCSP stapling today, you have to go out of your way, download Apache 2.4.x from the Apache Software Foundation website, compile it from source and so on, Ristic said. "So yes, it will take some time until this capability [OCSP stapling] is distributed widely so that people can use it."

"We'll have to work on raising awareness and from that perspective I think that what CASC is doing is great, because the community as a whole needs to learn how to do it," Ristic said.

SSL Labs, a project that monitors the SSL implementation on the world's top 180,000 HTTPS-enabled websites doesn't collect information about OCSP stapling. However, according to statistics released by Opera developer Yngve Nysaeter Pettersen, who runs the TLS Prober project, roughly 7 percent of HTTPS servers supported OCSP stapling as of October 2012.

Support on the client side is hard to estimate because there are so many products and old software out there, Ristic said. "I have a project that performs passive client SSL monitoring and I use it on ssllabs.com. I've just run some statistics on the collected information and exactly 30 percent of browser clients supported OCSP stapling. This number is very dependent on the type of visitor/browser one has on a Web site, but can be taken as a rough measurement," he said.

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Lucian Constantin

IDG News Service
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