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copied!<p>I've not got any experience with what people actually do to generate CD keys, but (assuming you're not wanting to go down the road of online activation) here are a few ways one could make a key:</p> <ul> <li><p>Require that the number be divisible by (say) 17. Trivial to guess, if you have access to many keys, but the majority of potential strings will be invalid. Similar would be requiring that the checksum of the key match a known value.</p></li> <li><p>Require that the first half of the key, when concatenated with a known value, hashes down to the second half of the key. Better, but the program still contains all the information needed to generate keys as well as to validate them.</p></li> <li><p>Generate keys by encrypting (with a private key) a known value + nonce. This can be verified by decrypting using the corresponding public key and verifying the known value. The program now has enough information to verify the key without being able to generate keys.</p></li> </ul> <p>These are still all open to attack: the program is still there and can be patched to bypass the check. Cleverer might be to encrypt part of the program using the known value from my third method, rather than storing the value in the program. That way you'd have to find a copy of the key before you could decrypt the program, but it's still vulnerable to being copied once decrypted and to having one person take their legit copy and use it to enable everyone else to access the software.</p>

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