I'm working on a "license" system which will take IP address and expiration date, possibly some other data, and produce an "encrypted" license key string. The license key string will then sit in a file, various programs will access the file to see if this installation is appropriately licensed. This doesn't need to be bullet proof, just a bit of a bother for people who should be reminded when their license is up. And to keep people from spreading the licensed programs all over the place while still lying in bed. So, I think I need it to be a two-way thing, not too heavy on the calculations, output of ASCII. Any suggestions?