need "license string" ideas

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?