RSA Secret-Key Challenge
The RSA Secret-Key Challenge is a series of cryptographic contests that were started by RSA Laboratories on January 28, 1997 with the intent of helping to demonstrate the relative security of different encryption algorithms. The challenge consists of one DES contest and twelve contests based around the block cipher RC5.
The distributed.net group has actively participated in this challenge by making use of distributed computing to perform a brute force attack. They have successfully used that technique to provide the solutions of the DES, RC5-32/12/7 ("RC5-56"), and RC5-32/12/8 ("RC5-64") contests.
There are still eight remaining contests that have not yet been solved, although distributed.net is working on "RC5-72" and is over 0.3% through (as of March 2006).