However, Stevens didn't agree with Sotirov's estimate for the theoretical cost of using Amazon's cloud for the attack. That's because there's currently not enough information about the MD5 collision method used in the Flame attack.

Sotirov assumes the attack had a similar cost-per-attempt as the RapidSSL one, Stevens said. However, the Flame attackers might have used a method that was faster, or one that was slower.

He expects they used a slower method, but that's still being researched and the findings won't be released until later.

The Flame attackers might also have had free access to powerful computer hardware, which would have significantly reduced the time required to perform the attack.

"More powerful hardware reduces the wall clock time," Stevens said. "The collision attack is highly parallelizable and a big cluster can be used very efficiently."