xor: how come so slow?

Seun Osewa seun.osewa at gmail.com
Wed Oct 15 13:43:17 CEST 2008


My answer is: never do things like this with python.
You will find this module useful: www.pycrypto.org

On Oct 15, 12:19 pm, Michele <mich... at nectarine.it> wrote:
> Hi,
> I'm trying to encode a byte data. Let's not focus on the process of
> encoding; in fact, I want to emphasize that the method
> create_random_block takes 0.5s to be executed (even Java it's faster) on
> a Dual-Core 3.0Ghz machine:
>
> took 46.746999979s, avg: 0.46746999979s
>
> Thus I suppose that the xor operation between bytes raise the execution
> time to 0.5; why I suppose that?
> Because in Python there's no support for bytes and even for xoring
> bytes, so I used a workaround:
> I cycle on the two strings to be xor-red
>     for every char in the strings
>         convert one char on integer and then xor them; (ord)
>         insert one char in the result, transforming the previous integer
> in char (chr)
>
> I suppose that ord() and char() are the main problems of my
> implementation, but I don't know either way to xor two bytes of data
> (bytes are represented as strings).
> For more information, see the code attached.
>
> How should I decrease the execution time?
>
> Thank you
>
> from __future__ import division
> import random
> import time
> import sha
> import os
>
> class Encoder(object):
>     def create_random_block(self, data, seed, blocksize):
>         number_of_blocks = int(len(data)/blocksize)
>         random.seed(seed)
>         random_block = ['0'] * blocksize
>         for index in range(number_of_blocks):
>             if int(random.getrandbits(1)) == 1:
>                 block = data[blocksize*index:blocksize*index+blocksize]
>                 for bit in range(len(block)):
>                     random_block[bit] =
> chr(ord(random_block[bit])^ord(block[bit])) # workaround per fare xor
> bit a bit di str; xor e' solo supportato per int -> ord
>         return ''.join(random_block)
>
> x = Encoder()
> piece = os.urandom(1024*1024)
> blocksize = 16384
> t1 = time.time()
> for l in range(100):
>     seed = random.getrandbits(32)
>     block = x.create_random_block(piece, seed, blocksize)
> t2 = time.time()
> print 'took ' + str(t2-t1) + 's, avg: ' + str((t2-t1)/100.0) + 's'




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