Browsers Cut Off an Indian Certificate Authority Over Trivially Guessable Keys

Major web browser makers have moved to stop trusting security certificates tied to eMudhra, a certificate authority based in India, after researchers found that the secret keys behind several of its intermediate certificates were so weak they could be guessed in under a second.

Certificate authorities vouch for the identity of websites. When a browser shows that a connection is secure, it is relying on a chain of certificates that traces back to an authority it trusts. If an attacker obtains the private key behind one of those certificates, the attacker can issue convincing fake certificates for websites they do not control and potentially intercept traffic that users believe is protected.

Mozilla received a report on Saturday, Oct. 17, that the private keys for eight eMudhra intermediate certificates had been compromised, The Register reported. Dennis Jackson of Mozilla said the keys could allow attackers to issue fraudulent HTTPS certificates. Ars Technica reported that the compromised keys were only 3 to 8 bits long.

The security researcher Scott Helme wrote on Tuesday that one of the keys was simply the number 4, and that he recovered others, including 56 and 115, in less than a second. A properly generated key of the type eMudhra used should be a random number roughly 384 bits long, drawn from a range so large that guessing it is practically impossible. Other researchers reported finding keys with similarly small values.

Mozilla, Google’s Chrome and Apple have taken steps so that their browsers no longer trust the affected certificates, though users need to install updates for the change to take effect, The Register reported. eMudhra has revoked the certificates, and several open-source software projects have issued updates that reject them. The company, which says it has issued more than 60 million certificates, had not answered The Register’s questions about how the keys were generated as of Tuesday.

The browsers’ quick response should limit the risk for most people browsing the web on up-to-date software. The larger concern is everything else. Certificates signed with the weak keys still exist, and some services and applications that rely on eMudhra certificates may not check revocation lists the way browsers do. Security experts urged organizations to review their systems for any dependence on the company’s certificates and replace them.

The Register noted that eMudhra’s key-generation practices are now a central concern. How keys that small were created, and whether any were misused before the problem was reported, remained unanswered on Tuesday.

For ordinary users, the advice is simple: keep browsers and operating systems updated so the changes take effect.