Username or EmailPassword
I would say probably into your licence key, or in a system string.
I agree that encryption would give a single weak point, but I would only be cracked by the motivated people. whereas right now disassembling is just a click away (or a few command away anyway). giving one extra step might discourage most of the hacker wannabe.
Even if I am sugesting a solution, I would say giving people a mean to see your code, might give them more confidence in it as they would be more sure that your application is doing what it is suppose to do.
What would be your solution anyway ?