[egypturnash]

Sell keys. Why are you trying to make this complicated? Don't encrypt the user's data and hold it for ransom, that's just insane.

You could also just release it as shareware. Especially if you're writing this because you take part in this kind of event and have gotten a lot out of the general weightlifting community; now you're giving something back. Have a splash screen asking people to send you some money (maybe with a suggested donation) or a note with their thanks.