I currently do not have a license time-out requirement. This could also be accomplished by embedding a date into the encrypted registration key string and then calculating the offset to determine the timeout period. Renewal would have to depend on another key combination. I would have a third key in the table (renewal key). After they have renewed with me, I would send the corresponding renewal key for their external key. This would trigger an update to the entire key sequence. Get a new internal and external key randomly (make sure not same as current) and then store this as the new registration in the registry with a new date. This could then be repeated for any length of time without comprimising the keys and would not permit the reuse of a renewal key.

The only problem would be how to handle reinstallations since the date is set at the renewal time. This would require more thought...