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Encrypted communications could have an undetectable backdoor

Lucian Constantin | Oct. 12, 2016
Researchers warn about the use of standardized or hard-coded primes in existing cryptosystems

Furthermore, 1024-bit keys are still widely used online, despite the U.S. National Institute of Standards and Technology recommending a transition to larger key sizes since 2010. According to the SSL Pulse project, 22 percent of the internet's top 140,000 HTTPS-enabled websites use 1024-bit keys.

"Our results are yet another reminder that 1024-bit primes should be considered insecure for the security of cryptosystems based on the hardness of discrete logarithms," the researchers said. "The discrete logarithm computation for our backdoored prime was only feasible because of the 1024-bit size, and the most effective protection against any backdoor of this type has always been to use key sizes for which any computation is infeasible."

The researchers estimate that performing similar computations for 2048-bit keys, even with backdoored primes, would be 16 million times harder than for 1024-bit keys and will remain infeasible for many years to come. The immediate solution is to switch to 2048-bit keys, but in the future all standardized primes should be published together with their seeds, the researchers said.

Documents leaked in 2013 by former NSA contractor Edward Snowden suggested that the agency has the ability to decrypt a lot of VPN traffic. Last year, a group of researchers speculated that the reason for this was the widespread use in practice of a small number of fixed or standardized groups of primes.

"Performing precomputation for a single 1024-bit group would allow passive eavesdropping on 18% of popular HTTPS sites, and a second group would allow decryption of traffic to 66% of IPsec VPNs and 26% of SSH servers," the researchers said in their paper at that time. "A close reading of published NSA leaks shows that the agency’s attacks on VPNs are consistent with having achieved such a break."

 

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