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Bad certificates happen

Even if CAs tightened the vetting process for the credentials they issued and locked down their networks against hack attacks, bad certificates will occasionally get issued. In theory, there is a mechanism for checking the validity of certificates in real-time to protect against this. Using a process known as OCSP, or Online Certificate Status Protocol, browsers are supposed to check a database maintained by the CA that issued the credential to make sure it hasn't been revoked.

But researchers have demonstrated that OCSP, and an earlier database known as certificate revocation lists, are easily defeated. Since those performing man-in-the-middle attacks by definition have the ability to monitor, block and alter data being sent to and from an intended victim, it's trivial for them to replace a CA message warning that the credential is no longer valid with a response that says the server is temporarily down. SSLStrip, the SSL-hacking tool Marlinspike released in early 2009 performs this OCSP-workaround automatically.

With the exception of Opera, every major browser that receives the faux error message will proceed with the transaction. The reason for the so-called soft fail: OCSP server outages are far too common. If browsers invalidated a certificate each time they were unable to get an up-or-down response from a database, there's the potential for millions of transactions to be rejected. E-commerce companies would be sure to howl, as would their customers.

Google researcher Adam Langley recently documented the myriad other failures of certificate revocation.

“I really wish that browser people put a requirement to every single certification authority to say: 'Guys, you have to have this infrastructure that can respond within these parameters,” said Comodo CEO Melih Abdulhayoglu. “The industry is not able to achieve that because we have to have the lowest common denominator in the equation. The industry has to have a requirement to get OCSP fully deployed.”

Abdulhayoglu is also critical of the entire certificate market for selling credentials for as little as $8 apiece. The low cost means CAs can only turn a profit by doing as little vetting as possible and relying on automated mechanisms that are more susceptible to attacks than those that require the intervention of humans. As a result, he said, more than 50 percent of certificates fall into a category known as DV, or domain-validated, which certify only that the applicant had some measure of control over the domain name at issue.

Organization-validated and extended-validated certificates, by contrast, require much higher levels of vetting that can only be carried out by an issuer's employees. They therefore come at a much higher cost, about $80 for the former and $300 for the latter. Only about 3 percent of certificates use extended validation, he said.

“We as a company don't believe in DV certificates, although we do provide them because of the commercial pressure,” he said. “Because the likes of VeriSign and GoDaddy push $10 certificates to people, it causes commercial pressure to us.”

Abdulhayoglu has proposed that the industry provide DV certificates for free, with the caveat that browsers no longer display a padlock icon when connected to sites that use them.

Of course, the Comodo CEO is the subject of plenty of criticism himself. After all, last month's counterfeited certificates for Gmail, Skype, Hotmail, Yahoo and Mozilla used his company's root certificate, which Comodo alone is responsible for securing. The forgeries came about after someone breached the servers of one of Comodo's certificate resellers, which had access to the CA's signing mechanism.

Abdulhayoglu has responded by revoking privileges for all of Comodo's so-called registration authorities. He declined to specify how many RAs his company used, except to say there were fewer than 100 of them.

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