What's a good approach for developing a simple serial number generator/verifier?

Question

I\'m working on an app I\'d like to sell some day -- sooner rather than later! I\'d like to develop a reasonably simple serial number scheme to protect it.

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Answer 1

I would not use any strong cryptography, since you have to decrypt it in program anyways, making keygens or at least cracks easy to do.

I would do the following - take a, say, 25 digit number. Now add some rules, such as: - number must be divisible by 31 - it must start and end with the last letter ...

Always generate keys using these rules. Use 20 rules or more (more the better). When deploying the app, use smaller number of rules, e.g. 10 to check if key is valid. These rules will then be disassemled and used to create keygen.

On every update enable one of the rules you didn't use before. If rules are selected correctly, you will disable most of keys generated by keygens.