Thursday, August 19, 2004

MD5 is Broken!!!

A new discovery on cryptography!!. There are rumors that said that SHA-1, had already been half-compromised. These rumors are circulating at the Crypto conference, which is being held this week in Santa Barbara, when somebody is about to announce a partial break of the SHA-1 cryptographic hashfunction. If true, this will have a big impact, especially on e-commerce where SHA-1 is one of the major encryption method that is widely used. Another major impact was on the popular cryptographic protocols, including the ones used to secure email and secure web connections.

SHA-1 is the most popular cryptographic hashfunction (CHF). A CHF is a mathematical operation which, roughly speaking, takes a pile of data and computes a fixed size "digest" of that data. To be cryptographically sound, a CHF should have two main properties. (1) Given a digest, it must be essentially impossible to figure out what data generated that digest. (2) It must be essentially impossible to find find a "collision", that is, to find two different data values that have the same digest.

Antoine Joux has already announced that he had found collision in SHA-0. One of the Chinese authors (Wang, Feng, Lai, and Yu) reported a family of collisions in MD5 (fixing the previous bug in their analysis), and also reported that their method can efficiently (2^40 hash steps) find a collision in SHA-0. This speaker received a standing ovation, from at least part of the audience, at the end of her talk. Eli Biham announced new results in cryptanalyzing SHA-1, including a collision in a reduced-round version of SHA-1. The full SHA-1 algorithm does 80 rounds of scrambling. At present, Biham and Chen can break versions of SHA-1 that use up to about 40 rounds, and they seem confident that their attacks can be extended to more rounds. This is a significant advance, but it's well short of the dramatic full break that was rumored.

Where does this leave us? MD5 is fatally wounded; its use will be phased out. SHA-1 is still alive but the vultures are circling. A gradual transition away from SHA-1 will now start. The first stage will be a debate about alternatives, leading to a consensus among practicing cryptographers about what the substitute will be.