rc5 has your cpu check decoding keys for an encryption algorithm. a cpu can decode lots of keys in 1 second, but theres 2^64 keys (64 bit encryption). rsa labs came up with the encryption, and had a contest for someone to break it, to prove how hard it is to break. distributed.net was set up to handle this, and after 3 years hasn't broken it yet. every ket will have been checked in the 14 months or so though, so the contest will end by then. of course, it could end tomorrow if they find the right key.
(i figure 14 months because currently the keyrate is double the average keyrate over the last 36 months, so without the keyrate increasing, it would take half the time. since the keyrate is increasing, i did a little bit of guesstimating that it will shave 4 months off. of course, the way dennifloss is going, the keyspace could be exhausted in a year or less)
