source: lib/bletchley/buffertools.py @ 7

'''
A collection of tools to manipulate buffers of encrypted content

Copyright (C) 2011-2012 Virtual Security Research, LLC
Author: Timothy D. Morgan, Jason A. Donenfeld

 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Lesser General Public License, version 3,
 as published by the Free Software Foundation.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
'''

import sys
import zlib

# Computes the block-wise differences between two strings
# Blobs must be the same length and their length must be a multiple of block_size
#
# Returns a list of block numbers that are different from one another.  
# Examples:
# blockWiseDiff(8, '1234567812345678', '12345678XXXXXXXX')
# => [1]
#
# blockWiseDiff(1, '12345', '12345')
# => []
#

def blockWiseDiff(block_size, blob1, blob2):
    if len(blob1) != len(blob2):
        sys.stderr.write("ERROR: Ciphertexts not the same length.\n")
        return None

    if (len(blob1) % block_size) != 0:
        sys.stderr.write("ERROR: Ciphertexts do not have an even multiple of blocks.\n")
        return None

    blocks1 = [blob1[o:o+block_size] for o in range(0,len(blob1),block_size)]
    blocks2 = [blob2[o:o+block_size] for o in range(0,len(blob2),block_size)]

    ret_val = []
    for b in range(0,len(blocks1)):
        if blocks1[b] != blocks2[b]:
            ret_val.append(b)

    return ret_val



def blockWiseColorMap(block_size, blobs):
    '''
    Accepts a sequence of blobs and compares all individual blocks 
    (of block_size) within those blobs.  Returns a dictionary where the
    keys are blocks from the original blobs whose values were repeated
    at least once. (Unique blocks in the original blobs will not be
    represented in the return value.) The values of the returned
    dictionary are a 32bit integer hash value of the blocks they are
    associated with.
    '''
    block_counts = {}
    for blob in blobs:
        for block_off in range(0,len(blob),block_size):
            block = blob[block_off:block_off+block_size]
            count = block_counts.get(block, None)
            if count == 0:
                block_counts[block] = 1 # more than one exists
            elif count == None:
                block_counts[block] = 0  # one exists

    colors = {}
    for block,count in block_counts.iteritems():
        if count == 1:
            # mask needed for portability
            colors[block] = zlib.crc32(block)%0xFFFFFFFF 

    return colors



# XORs two buffers (bytes/bytearrays) and returns result
#
# If buffers not the same length, returned buffer length 
# will be that of the shorter buffer.
#
def xorBuffers(buff1, buff2):
    max_len = min(len(buff1), len(buff2))

    ret_val = bytearray(buff1[0:max_len])
    other = bytearray(buff2[0:max_len])
    for i in range(0,len(ret_val)):
        ret_val[i] ^= other[i]

    return ret_val


def splitBuffer(buf, block_size):
        '''
        Splits a buffer into evenly sized blocks.
        '''
        return [buf[i:i + block_size] for i in xrange(0, len(buf), block_size)]

def iterBuffer(buf, block_size):
        '''
        Iterates through a buffer in evenly sized blocks.
        '''
        return (buf[i:i + block_size] for i in xrange(0, len(buf), block_size))

def pkcs7PadBuffer(buf, block_size):
        '''
        Pads the end of a buffer using PKCS#5/PKCS#7 padding.
        '''
        padding = block_size - (len(buf) % block_size)
        return buf + (chr(padding) * padding)