Partial Key Verification (PKV) is a software licensing key technique that breaks a product key into multiple subkeys. With each version of your software, you check a different subset of the subkeys. The beauty of this approach is that a cracker cannot generate a complete keygen. They might be able to generate one for a given version, but it won't work on a different release (assuming you check a different subkey). Another nice feature of PVK is that the key contains an embedded serial number. This allows you to easily check the key against a list of stolen/posted/refunded keys. For more information about the PKV technique, see this blog post by Brandon Staggs.
My version of PKV differs slightly from the one discussed by Brandon Staggs.
Instead of using 8-bit subkeys, I used 32-bit subkeys (just check one key instead
of four). My version also Base-32 (5-bit) encodes the keys to shrink the key size
by 20%, and allows you to specify a different hash algorithm for each subkey.
This is a .NET Standard 1.0 library and distributed under the Simplified BSD license.
PM> Install-Package Partial.Key.Verification
To generate a key, create a PartialKeyGenerator class specifying the checksum and hash functions to use, along with the base values for each subkey. Then call the Generate function, passing it a serial number or a string (such as the customer's e-mail address) to generate a key. You can optionally tell the generator to add a separator between a certain number of characters in the key by setting the Spacing property.
var generator = new PartialKeyGenerator(new Adler16(), new Jenkins96(), new uint[] { 1, 2, 3, 4 }) { Spacing = 6 };
var key = generator.Generate("[email protected]");
This will generate the key: QDKZUO-JLLWPY-XWOULC-ONCQIN-5R5X35-ZS3KEQ. Adler16 is the checksum function and Jenkins96 is the hash function. You can have as many subkeys as you like, but each subkey adds 7 more characters to the key.
To validate the key, use the PartialKeyValidator static class. Again telling it the checksum and hash functions to use, along with which subkey to check and the base value for that subkey. For example, to check the first subkey of the key generated above:
var isValid = PartialKeyValidator.ValidateKey(new Adler16(), new Jenkins96(), key, 0, 1);
See the unit tests for more examples.