Nuclex.Support/Documents/ShuffleTableGenerator.md

Shuffle Table Generator

The LicenseKey class converts a GUID (or any 128 bits of data) into a license key similar to the ones used by Microsoft products (5 groups of 5 alphanumeric characters).

For additional obfuscation, bits can be mapped in a random order. That will offer some protection when you have, for example, a running serial number. With shuffling, each increment will result in likely several alphanumeric characters changing in seemingly unpredictable ways (base-2 to base-36 conversion pretty much guarantees that any small change affects many following alphanumeric "digits").

/// <summary>Generates a new random shuffle table</summary>
/// <param name="iterationCount">
///   Number of iterations in which to randomize the shuffle table
/// </param>
/// <returns>The new random shuffle table</returns>
public static byte[] generateShuffleTable(int iterationCount) {
  byte[] shuffleTable = new byte[128];
  for(int index = 0; index < 128; ++index)
    shuffleTable[index] = index;

  Random rng = new Random();

  for(int iteration = 0; iteration < iterationCount; ++iteration) {
    int firstIndex = rng.Next() % 128;
    int secondIndex = rng.Next() % 128;

    byte temp = shuffleTable[firstIndex];
    shuffleTable[firstIndex] = shuffleTable[secondIndex];
    shuffleTable[secondIndex] = temp;
  }

  return shuffleTable;      
}