It would be an extremely fast rc5 cruncher, especially because of all the vector units. It should be possible to calculate 8 keys in parallel. I've been looking at N64 (Reality Signal Processor (RSP)) for a long time and they have a Vector Shift Left and a Vector Shift Right. The only issue is communication, I dont see an easy way to push blocks into the n64 and get results out. You could embed blocks into the program before you send it to the N64, but it would require too much user intervention.