Decrypt a block of cipher text using the AES block cipher.

def decrypt(self, ciphertext):
        'Decrypt a block of cipher text using the AES block cipher.'

        if len(ciphertext) != 16:
            raise ValueError('wrong block length')

        rounds = len(self._Kd) - 1
        (s1, s2, s3) = [3, 2, 1]
        a = [0, 0, 0, 0]

        # Convert ciphertext to (ints ^ key)
        t = [(_compact_word(ciphertext[4 * i:4 * i + 4]) ^ self._Kd[0][i]) for i in xrange(0, 4)]

        # Apply round transforms
        for r in xrange(1, rounds):
            for i in xrange(0, 4):
                a[i] = (self.T5[(t[ i          ] >> 24) & 0xFF] ^
                        self.T6[(t[(i + s1) % 4] >> 16) & 0xFF] ^
                        self.T7[(t[(i + s2) % 4] >>  8) & 0xFF] ^
                        self.T8[ t[(i + s3) % 4]        & 0xFF] ^
                        self._Kd[r][i])
            t = copy.copy(a)

        # The last round is special
        result = [ ]
        for i in xrange(0, 4):
            tt = self._Kd[rounds][i]
            result.append((self.Si[(t[ i           ] >> 24) & 0xFF] ^ (tt >> 24)) & 0xFF)
            result.append((self.Si[(t[(i + s1) % 4] >> 16) & 0xFF] ^ (tt >> 16)) & 0xFF)
            result.append((self.Si[(t[(i + s2) % 4] >>  8) & 0xFF] ^ (tt >>  8)) & 0xFF)
            result.append((self.Si[ t[(i + s3) % 4]        & 0xFF] ^  tt       ) & 0xFF)

        return result