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Opinion: Cryptanalysis of MD5 and SHA: Time for a new standard

Crypto researchers report weaknesses in common hash functions

By Bruce Schneier, Counterpane Internet Security Inc.
August 19, 2004 12:00 PM ET

Computerworld - At the Crypto 2004 conference in Santa Barbara, Calif., this week, researchers announced several weaknesses in common hash functions. These results, while mathematically significant, aren't cause for alarm. But even so, it's probably time for the cryptography community to get together and create a new hash standard.
One-way hash functions are a cryptographic construct used in many applications. They are used with public-key algorithms for both encryption and digital signatures. They are used in integrity checking. They are used in authentication. They have all sorts of applications in a great many different protocols. Much more than encryption algorithms, one-way hash functions are the workhorses of modern cryptography.
In 1990, Ron Rivest invented the hash function MD4. In 1992, he improved on MD4 and developed another hash function: MD5. In 1993, the National Security Agency published a hash function very similar to MD5, called the Secure Hash Algorithm (SHA). Then in 1995, citing a newly discovered weakness that it refused to elaborate on, the NSA made a change to SHA. The new algorithm was called SHA-1. Today, the most popular hash function is SHA-1, with MD5 still being used in older applications.
One-way hash functions are supposed to have two properties. One, they're one-way. This means that it's easy to take a message and compute the hash value, but it's impossible to take a hash value and re-create the original message. (By "impossible," I mean "can't be done in any reasonable amount of time.") Two, they're collision-free. This means that it's impossible to find two messages that hash to the same hash value. The cryptographic reasoning behind these two properties is subtle, and I invite curious readers to learn more in my book Applied Cryptography.
Breaking a hash function means showing that either -- or both -- of those properties aren't true. Cryptanalysis of the MD4 family of hash functions has proceeded in fits and starts over the past decade or so, with results against simplified versions of the algorithms and partial results against the whole algorithms.
This year, Eli Biham and Rafi Chen, and separately Antoine Joux, announced some pretty impressive cryptographic results against MD5 and SHA. Collisions have been demonstrated in SHA. And there are rumors, unconfirmed at this writing, of results against SHA-1.
The magnitude of these results depends on who you are. If you're a cryptographer, this is a huge deal. While not revolutionary, these results are substantial advances in the field. The techniques described by the researchers are likely to have other applications, and we'll be better able to design secure systems



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