Yes, pseudo-random permutation. The basic algorithm goes like this. To generate the codes generate_codes(range valid_range) for(i in valid_range)

Yes, pseudo-random permutation. The basic algorithm goes like this. To generate the codes generate_codes(range valid_range) for(i in valid_range) encoded = RSA.Encode(key, i) code = base32.Encode(encoded) print code and then, to verify a code verify(string code, range valid_range) encoded = base32.Decode(code) value = RSA.Decode(key, encoded) return valid_range.contains(value) where of course RSA is an implementation of http://en.wikipedia.org/wiki/RSA_(cryptosystem) http://en.wikipedia.org/wiki/RSA_(cryptosystem) and base32 refers to http://en.wikipedia.org/wiki/Base32 http://en.wikipedia.org/wiki/Base32. On top of that, we needed codes to be of a specified maximum length and with a specified alphabet. Thus, instead of base32 we used a different base with different letters, but the same algorithm. To fix the length of the codes, we needed the output of RSA encode to be less than a specific value. In RSA this can be controlled by choosing the modulus part of the key. There are more subtleties to this, but the gist is there.