A purely functional, and somewhat naive, implementation and proof of
correctness of the AES (also known as "Rijndael") algorithm from Rijmen  
and Daemen. The AES homepage is

    http://csrc.nist.gov/encryption/aes/

Unlike most implementations, no arrays or vectors are used in our
code. The main datastructure operated on is the `state', which is a 4x4
block of bytes (8 bit words). We model it by a 16-tuple of bytes. 
