I'm a wee bit late to the show, but for future reference for anyone googling this issue, If you take the combined key you get from the Diffie-Hellman function and run it through the md5 function it comes out as a 128 bit binary. it is really odd that the key generator makes keys that are too long for the only real encryption program that came with it. There's also a set of libraries and programs available on open programs, called lua-lockbox, that have a whole heap of different encoding and encrypting files and functions, though i haven't tried them myself, so i'm unsure if they still work