To make license keys unforgeable, you have to go public key. Typically some sort of digital signature. This won't make your application uncrackable, but will ma

To make license keys unforgeable, you have to go public key. Typically some sort of digital signature. This won't make your application uncrackable, but will make it impossible to fake legit licenses. A good reference on how to get these things right is Schneier et al's 'Cryptography Engineering'. The standard advice line here, however, is "get professional help".