I have a racing game. In this game, the user will be allowed to purchase additional cars. How do I keep track of what cars the user purchased and make sure he or she did not steal any cars? I came up with the idea of storing data on all cars in an XML file that is bundled with the app, and making the app parse the file. The problem is that I have no idea how to check to see if the file was modified. I can already see ways that people could steal car data from others.