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copied!<p>It's not that hard, if you're somewhat flexible in your requirements -- perhaps the scheme below would work for you.</p> <p>You could just produce K = [SN, H([X,SN,Y])] which is the concatenation of an incrementing serial number with a hash, where the hash is a secure hash function of the concatenation of the serial number between unique constants X and Y that are secret <a href="http://en.wikipedia.org/wiki/Rainbow_table#Defense_against_rainbow_tables" rel="noreferrer">"salt" you use to prevent the use of rainbow tables</a>. </p> <p>Use a well-known secure hash algorithm (e.g. SHA-1 or SHA-2; MD5 is probably also adequate, since the known weaknesses for MD5 are collision attacks, and <em>not</em> <a href="http://en.wikipedia.org/wiki/Preimage_attack" rel="noreferrer">preimage attacks</a>) and you should be all set, as least as far as the serial key part goes (you'll probably want to prevent two people from using the same key).</p> <p>The other thing you can do which is helpful is use K = [SN, T, H([X, SN, T, Y])] -- use both the serial number and a timestamp. This can be used to allow only a narrow use window for the serial key: it's valid within N seconds of the timestamp, so it would prevent reuse of the key outside that window.</p> <p>Then encode/decode K to a representation that can be used to easily allow users to enter the key (e.g. base64).</p> <p>It's best to have a simple and transparent overall algorithm -- obfuscation is not going to help you if someone reverse-engineers your scheme.</p>

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