This spell is an implementation of the ArcFour, an encryption algorithm
which is believed to be RC4, and is used in programs like ssh and (atleast
its RC4 cousin) Netscape.

The algorithm works by starting with a 256-byte array containing the
values 0 to 255, which is seeded by the encryption-key. Each input-byte is
encrypted by using this array.

The algorithm works two ways:
plain data [arcfour] key => encrypted data
encrypted data [arcfour] key => plain data

For more information on how the algorithm works, please read a book :)
