xor: how come so slow?
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:
> 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_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'
More information about the Python-list