Changes:

  1) Add new exported function: encrypt_aes_128_random_mode
  2) Add docs to hamming_distance().
  3) Clean up the code a little in hamming_distance().
  4) Add code to decrypt_aes_128_ecb() to verify that the ctext length is an
     even multiple of the key length.
  5) New function: encrypt_aes_128_ecb. This just transparently calls
     decrypt_aes_128_ecb.
  6) New internal function: generate_random_aes_key
Forgot to export encrypt_aes_128_ecb; fixed now.
Set 2, Challenge 11.