bozhu/aes-gcm-python

A Python implementation of the authenticated encryption mode Galois/Counter Mode (GCM).

aes-gcm-python's Issues

Add AES-256 support

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...

Faster AES_GCM.__pre_table

I noticed that changing the implementation of AES_GCM.__pre_table to the code below fills the cache much faster than assigning all values in a nested loop directly.

For example I see user time of my script going from 0.430ms to 0.056ms. Using CPython 2.7.10 here.

class GCMulCacheProxy:

    __fields__ = ('gcmc', 'i')

    def __init__(self, gcmc, i):
        self.gcmc = gcmc
        self.i = i

    def __getitem__(self, j):  # i=0..15, j=0..255
        val = self.gcmc.cache[self.i][j]
        if val is None:
            val = gf_2_128_mul(self.gcmc.auth_key, j << (8 * self.i))
            self.gcmc.cache[self.i][j] = val
        return val

class GCMulCache:

    __fields__ = ('auth_key', 'cache')

    def __init__(self, auth_key, time_invariant=True):
        self.auth_key = auth_key
        self.cache = [[None]*256]*16
        if time_invariant:
            for i in range(16):
                for j in range(256):
                    self[i][j]

    def __getitem__(self, i):
        return GCMulCacheProxy(self, i)

and

class AES_GCM:
        [...]
    def change_key(self, master_key):
        [...]
        self.__pre_table = GCMulCache(self.__auth_key)