SHA-1 compromised further

Crypto researchers point the way to feasible attack

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Crypto researchers have discovered a new, much faster, attack against the widely-used SHA-1 hashing algorithm. Xiaoyun Wang, one of the team of Chinese cryptographers that demonstrated earlier attacks against SHA-0 and SHA-1, along with Andrew Yao and Frances Yao, have discovered a way to produce a collision in SHA-1 over just 263 hash operations compared to 269 hash operations previously. A brute force attack should take 280 operations.

One-way hashing is used in many applications such as creating checksums used to validate files, creating digital certificates, authentication schemes and in VPN security hardware. Collisions occur when two different inputs produce the same output hash. In theory this might be used to forge digital certificates but it shouldn't be possible to find collisions except by blind chance. Wang and her team have discovered an algorithm for finding collisions much faster than brute force. The researchers released a paper (PDF) on their finding at the Crypto 2005 conference in Santa Barbara, California earlier this week.

"The SHA-1 collision search is squarely in the realm of feasibility," writes noted cryptographer Bruce Schneier in a posting to his web log. "Some research group will try to implement it. Writing working software will both uncover hidden problems with the attack, and illuminate hidden improvements. And while a paper describing an attack against SHA-1 is damaging, software that produces actual collisions is even more so."

The US National Institute of Standards and Technology (NIST) recently advised the US government to phase out SHA-1 in favor of SHA-256 and SHA-512. NIST is holding a workshop on the subject in late October. ®

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