RC5 is a set of encryption algorithms developed by Ronald Rivest in 1994, which are very powerful while simple to implement. This set of algorithms provide a wide range of mobility since it features 3 parameters: number of words per block, number of rounds, and length of key. For example, the cipher text of the RC5-32/12/9 contest is generated by an RC5 encryption algorithm using 32-word blocks, 12 rounds of encryptions, and an encryption key of length 9 bytes = 72 bits. As the key length increases, the number of possible keys increases exponentially fast, and therefore the difficulty of cracking the key also increases exponentially. For instance, to crack a cipher generated by a key length of 72 bits, there are 2^72 = 4,722,366,482,869,645,213,696 possible keys to attempt, which would take a very long time to run even if using thousands of computers.
On January 28, 1997, RSA Laboratories announced 13 cypher breaking challenges, 12 of which were encrypted by RC5-32/12/n where n = 5, 6, ..., 16, and the solver of each challenge is awarded between US$1,000 and US$10,000 according the difficulty of the challenge. The RC5-32/12/5 and RC5-32/12/6 challenges were solved in 3.5 hours and 13 days, respectively. The challenges with n = 7 and higher are much more difficult. It took nearly 9 months and 5 years, respectively, for the distributed computing organization distributed.net to conquer the challenges RC5-32/12/7 and RC5-32/12/8. In 2002 after distributed.net copleted RC5-32/12/8, it announced that it will continue to work on RC5-32/12/9.
In May 2007, over 10 years since the beginning of the RC5 challenges, RSA Laboratories announced the cancellations of the remaining challenges. Nevertheless, distributed.net dicided to sponsor the RC5-32/12/9 challenge on its own and offer the US$10,000 award according to its award distribution rule.
Since the encryption key of RC5-32/12/9 is 8 bits longer than that of RC5-32/12/8, the RC5-32/12/9 challenge is 2^8 = 256 times more complex than RC5-32/12/8. If the same resource dedicated for RC5-32/12/8 is used for RC5-32/12/9, it would take approximately 1,000 years to complete the challenge. However, due to the software/hardware improvement and the increase of the number of participants over the years, the computing power involved in this challenge has become several times higher than that when the challenge started. As of April 2010, the estimate time of completion has been reduced from approximately 1,000 years to approximately 200 years and is still decreasing steadily. To read more about and involve in the RC5-32/12/9 project, please visit its official website at http://www.distributed.net/rc5/