Posted June 14, 2016
Lantea: so there has to be a way for the game to tell it is getting a valid key, they could use the same method on GOG to determine that you are entering a valid game key for a specific game.
I think I already gave an example with the Luhn algorithm that is used for credit cards. The check that happens is to see if the serial you enter returns true when used in a function. There are multiple such keys, but the publisher only uses a subset of those. A very basic example (that I doubt has been used) would be to check if the number entered is odd or even, and only allow even numbers. The publisher creates serial in the range of 1000000-1500000, but the check would return true for the number 222222222 as well. That is how most of the keygens worked back in the days, though the scene groups usually tagged their keys by having the group's name in the generated ones. So no, a key that passes the installer's check is not necessarily a valid publisher's key.