Bad idea, really bad idea! Sorry if this comes out rude, but if you don't choose your keys carefully you're messing up security. I'm unable to give you a mathematical proove of this (see e.g. Bruce Schneier "Applied Cryptography" for a scientific text), but you have to get a key that isn't vulnerable to prime factorization (or something of the like), in other words: a prime number.

So, if you are to bet security on your keys, make sure they work. I'd suggest you take a look at GPG (GNU Privacy Guard) which contains Blowfish encryption. Maybe you can use that program to generate keys?

Andreas

Update:
Oups, I'm sorry, it seeems like I really jumped on this too fast (something triggered the alarm bells in the head, and off they go). /me makes a mental note not to post after having two beers. ;-))
mdillon and lhoward are right about prime number factorization and guessable keys/key space usage.
Once again, sorry to jump on you, Mushy!