cyberspacial musings
bits about the real and virtual worlds

16 Feb

SHA-1 Broken

Bruce Schneier reports on his blog that SHA-1 has been broken as described in a paper by Chinese researchers Xiaoyun Wang, Yiqun Lisa Yin, and Hongbo Yu. Federal Information Processing Standard 180 (FIPS-180) describes SHA-1 in the following way:

Explanation: This Standard specifies a Secure Hash Algorithm, SHA-1, for computing a condensed representation of a message or a data file. When a message of any length

The SHA-1 is called secure because it is computationally infeasible to find a message which corresponds to a given message digest, or to find two different messages which produce the same message digest. Any change to a message in transit will, with very high probability, result in a different message digest, and the signature will fail to verify. SHA-1 is a technical revision of SHA (FIPS 180). A circular left shift operation has been added to the specifications in section 7, line b, page 9 of FIPS 180 and its equivalent in section 8, line c, page 10 of FIPS 180. This revision improves the security provided by this standard. The SHA-1 is based on principles similar to those used by Professor Ronald L. Rivest of MIT when designing the MD4 message digest algorithm (“The MD4 Message Digest Algorithm,” Advances in Cryptology – CRYPTO ’90 Proceedings, Springer-Verlag, 1991, pp. 303-311), and is closely modelled after that algorithm.

The general conclusion of this paper is that collisions can be found after 2^69 hash operations, instead of the brute force 2^80. A collision is where two given messages are found to produce the same result. This effectively means that 2^11 fewer operations are required to produce a collision. Computationally, this means that if it took a week to compute 2^69 hash operations before a collision, it would have taken 2048 weeks to compute 2^80 hash operations before, which is about 39 years. That’s a pretty significant reduction in the amount of time necessary to break a hash. Now it still takes a long time to compute a hash and 2^69 of them is a huge amount, but as Moore’s law continues to give us faster processors, a 2^11 reduction in operations is very, very important. It effectively renders SHA-1 useless for the long-term, and maybe even for the short term.

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