alright, i'm making a game i intend to sell (i don't know if it will or not, but that doesn't matter). Now, the easiest way by far to have people buy my game would be through paypal or credit card over the internet. So what i want to do is allow people to download my entire game, but unless they have a key, only play the beginning part of it (a demo if you will). Now of course i can't simply have one key, or once one person buys the game, he or she could give everyone in the world the key (in thoery). So i need my game to check when it is started if it already has a key. If it does, all is well, the user paided for the game and has full access. If it doesn't have a key, it checks if it has a code (that the user sends to me, so that i can give them the correct key). If has a code, it tells the user the code so that hopefully he or she will buy the game, and allows the user to play the beingning of the game only. If it doesn't yet have a code, it will create a code and tell the user the code so he or she will hopefully buy the game. now my question is, i need the game to create a code that if i enter into another program i will write, will give me a key. which i can then give to the user so that he or she has access to the full game. So for every code x, i need a key y. what mathimatial fomula will allow me to do this? i've seen it done in other games, so i know it can be done. thank you for your time and any help you can give me. EDIT: i'm going to try to explain this in more detail of what is going on. my game creates the code 123. now my game knows (but the user doesn't) that the correct key for this is 456. now when i enter the code 123 into my decoder (if you will) program, it produces the key 456. but i need millions of different codes and keys. so i need a mathimatial fomula to produce them. hope that helps.

Experienced crackers will have no trouble bypassing this, but if it's a small game it probably won't draw a ton of crackers. If you're going to do it, I wouldn't worry too much about it being a great algorithm, since if one was going to crack it they would simply bypass the check of the code. Therefore as long as your algorithm produces semi-unique numbers you should be fine.

Something as simple as (gamecode << a) - (gamecode % b) + c for some constants a,b,c should generate reasonable values.
[large# for gamecode, b and c, small# for a]

PS. I'd also recommend not packaging all resources, for example if it's a puzzle game don't ship levels 10-100 or in some other kind of game don't ship all the graphics needed for the full game.

it is an adventure game (think old zeldaish).

why would experenced crackers have no problems bypassing this though?