The RapidSSL attackers had a one-second window to obtain a legitimate certificate with a serial number they predicted in advance and whose signature could be copied over to their rogue certificate. It took several attempts to time this right, and after each failed attempt they had to generate a new rogue certificate, which took several hours.

The Flame attackers had only a one-millisecond window to obtain the right certificate signed by Microsoft, said Sotirov, co-founder and chief scientist at security firm Trail of Bits, in at the SummerCon conference on Saturday. That means they probably needed a far greater number of attempts to succeed.

The RapidSSL attack would have cost around US$20,000 if it had been performed on Amazon's EC2 cloud. The Flame attack would have cost between 10 and 100 times more, Sotirov said.

"[Sotirov's] analysis on the time window seems to be correct and is excellent research," said Marc Stevens, a scientific staff member in the cryptology group at the Dutch national research center for mathematics and computer science (CWI), via email.

"This would significantly increase the overall cost and I agree with that assessment," said Stevens, another member of the team that performed the RapidSSL attack in 2008.