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That's not true
Op:
That's overkill, just use a hash function
For example, you could start with the number two
Multiply that by 32 and by itself 3 time, you are going to end with a number, give that as a license and reverse engineer it (Divide it by itself and by 32, 3 times) then check if the number is 2 and if so, the serial is decent
I'm going to try and make you an example later on, because this one is a bit naive, basically, make it as hard as possible for somebody to find the way to reverse engineer your license