Add AES-256 support

[cptwunderlich]

What would be necessary to add support for AES-256?
Am I correctly assuming, that it's mostly about creating an equivalent function for "gf_2_128_mul"?
I don't really understand the maths behind it well enough to touch it...

[zhuzhuor]

The block size (plaintext/ciphertext bit-length) of AES-256 is still 128. The 256 denotes only its key size.
So the code shouldn't need significant modification except replacing AES-128 by AES-256. I am not 100% sure though. I may check it thoroughly once I have time later.

[meuter]

I know this thread is a bit old, but zhuzhuor is right. The gf function and the block size remain the same whatever the key length. I patched the implementation to support AES192/256. The only change is in aes_gcm.py, in the function change_key():

`
def change_key(self, master_key):
if (len(master_key)*8 not in (128, 192, 256)):
raise InvalidInputException("valid AES key must be 128, 192 or 256 bit long")

    self.__master_key = master_key
    self.__aes_ecb = AES.new(self.__master_key, AES.MODE_ECB)
    self.__auth_key = bytes_to_long(self.__aes_ecb.encrypt(b'\x00' * 16))

    # precompute the table for multiplication in finite field
    table = []  # for 8-bit
    for i in range(16):
        row = []
        for j in range(256):
            row.append(gf_2_128_mul(self.__auth_key, j << (8 * i)))
        table.append(tuple(row))
    self.__pre_table = tuple(table)

    self.prev_init_value = None  # reset`

Note that the master key is now passed as a byte array instead of an int, which I personally find easier. Using this I was able to successfully run some test vectors from the NIST CAVP test suite (http://csrc.nist.gov/groups/STM/cavp/block-cipher-modes.html#gcm-gmac).

I can do a pull request and patch it if someone is still interested.