Thomas acknowledged that the developers' fix corrected the problem his DecryptoCat tool exploits. "For Cryptocat version 2.0.42 this will take 1,000 computer-years to generate [cracked keys], 500 computer-years on average to use, and 40 petabytes to store," he said. "So the only ones capable of doing this are large companies and governments."
Is Cryptocat now safe to use? In fact, Thomas said his bug report wasn't meant to be exhaustive, and warned against relying on the application for encrypted communications. "I'm sure there are plenty of bugs and other bad crypto in other parts because I only looked at random generation and found a bug, at public key algorithm and found a bug, and quickly looked where random is used and found something scary, and random (BigInt.randBigInt) used in two-party messaging and found a bug," he said.
The exploitable Cryptocat vulnerabilities are notable, given that interest in using encrypted communications tools has been growing in the wake of leaks by former National Security Agency (NSA) contractor Edward Snowden, who revealed the existence of numerous NSA data and metadata-interception programs.
But ensuring that communications can't be intercepted depends in large part on the applications not including exploitable vulnerabilities or bugs. For example, Bruce Schneier, chief security technology officer of BT, Monday detailed four techniques for protecting communications against snooping: use vulnerability-free applications, choose secure passwords, manage those passwords securely and know the threat you're facing.
Although Schneier's advice pertained to securing email, it also applies to chat -- and it's notable that the first tip is to use applications that don't include vulnerabilities. His reasoning is simple: No matter how great the cryptography used by an application, if someone wants to intercept messages sent using that application, they're going to first see if "breaking the engineering" might suffice. In the case of Cryptocat, prior to the patch, that would have been the case.
Then again, according to leaked NSA docs, the agency is legally allowed to retain encrypted communications indefinitely, meaning that simply by using an application such as Cryptocat, people might already be putting themselves at greater risk of having their communications intercepted and studied. Thus, anyone who used a vulnerable version of Cryptocat might have had their encrypted communications stored. With Thomas' bug information in hand, decrypting those messages would now be a trivial task.