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In cryptography, **MD5CRK** was a distributed effort (similar to distributed.net) launched by Jean-Luc Cooke and his company, CertainKey Cryptosystems, to demonstrate that the MD5 message digest algorithm is insecure by finding a collision — two messages that produce the same MD5 hash. The project went live on March 1, 2004. The project ended on August 24, 2004 after researchers independently demonstrated a technique for generating collisions in MD5 using analytical methods by Xiaoyun Wang, Feng, Xuejia Lai, and Yu.^{[1]} CertainKey awarded a 10,000 Canadian Dollar prize to Wang, Feng, Lai and Yu for their discovery.^{[2]}

A technique called Floyd's cycle-finding algorithm was used to try to find a collision for MD5. The algorithm can be described by analogy with a random walk. Using the principle that any function with a finite number of possible outputs placed in a feedback loop will cycle, one can use a relatively small amount of memory to store outputs with particular structures and use them as "markers" to better detect when a marker has been "passed" before. These markers are called *distinguished points*, the point where two inputs produce the same output is called a *collision point*. MD5CRK considered any point whose first 32 bits were zeroes to be a distinguished point.

The expected time to find a collision is not equal to where is the number of bits in the digest output. It is in fact , where is the number of function outputs collected.

For this project, the probability of success after MD5 computations can be approximated by: .

The expected number of computations required to produce a collision in the 128-bit MD5 message digest function is thus:

To give some perspective to this, using Virginia Tech's System X with a maximum performance of 12.25 Teraflops, it would take approximately seconds or about 3 weeks. Or for commodity processors at 2 gigaflops it would take 6,000 machines approximately the same amount of time.

**^**Xiaoyun Wang; Dengguo Feng; Xuejia Lai; Hongbo Yu (17 August 2004). "Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD".**^**"Popular, Yet Obsolete, Banking Algorithm Broken".*CertainKey News*(Press release). 17 February 2005. Archived from the original on 13 May 2011.

- Paul C. van Oorschot; Michael J. Wiener.
*Parallel Collision Search with Application to Hash Functions and Discrete Logarithms*(PDF). ACM Conference on Computer and Communications Security 1994. pp. 210&ndash, 218.