encrypt_block : (mode : Mode) -> Vect (get_n_k mode * 4) Bits8 -> Vect 16 Bits8 -> Vect 16 Bits8
decrypt_block : (mode : Mode) -> Vect (get_n_k mode * 4) Bits8 -> Vect 16 Bits8 -> Vect 16 Bits8