How can I create a random keycode generation ?

I am trying to figure out a way to make a keycode to prevent game being opened/ran without one. Like how you need one to run in unity pro mode. I have a site who is going to host my downloadable game and need the key code to generate a different code for each download. I am having trouble figureing out how to make this happen. So once it is bought it can't be given to someone else to run without them purchasing a keycode as well. I am not sure if I explained this in a good way, hope I did