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.

A technique called Pollard's rho algorithm (a cycle detection 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.