In all honesty, Train's probably right in that there have been, and still are "bad" blocks comming in. While it's not suprising that we haven't found the key yet(65% out of 100% is a lot, but isn't the whole keyspace), it's definately possible that the "right" key was lost to a client bug, or a cracked block, ect. At the very least, the RC5 project is experimental; for something that was one of the first few projects(behind only the original RC5-56, and ECCp I think), it's not something that is going to be perfect in every way. The question when the project is over will be what went wrong, and how to fix it for the next big project.