#!/usr/bin/env python
# -*- coding: utf-8 -*-

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# 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/>

"""Flip a single bit of a file and check the CRC of the file
until a match is found."""

import sys
import optparse
import zlib

def main(options, args):
	file_name = args[0]
	crc32 = int(args[1], 16)
	
	# read complete file into memory
	with open(file_name, 'rb') as volume:
		data = volume.read()
	start = 0
	end = len(data)
	print("File size: %d" % end)
	print("CRC32: %0.X" % crc32)
	
	# naive way and flipping it all
	for cur_byte in range(start, end + 1):
		if cur_byte % 10 == 0:
			print(cur_byte)
			
		# calculate crc32
		first_crc = zlib.crc32(data[start:cur_byte])
		# 8 bit flips for each byte
		cur_byte_data = ord(data[cur_byte])
		for i in range(8):
			flip = chr(cur_byte_data ^ (0x80 >> i))
			test_crc = zlib.crc32(flip + data[cur_byte+1:end], first_crc)
			
			if test_crc == crc32:
				print("Found in %d!" % cur_byte)
				print("Bit %d" % i)
				
				# write out good file
				with open(file_name + ".bin", 'wb') as result:
					result.write(data[start:cur_byte])
					result.write(flip)
					result.write(data[cur_byte+1:end])
				break
		else:
			continue  # executed if the loop ended normally (no break)
		break  # executed if 'continue' was skipped (break)			
		
if __name__ == '__main__':
	parser = optparse.OptionParser(
		usage="Usage: %prog file_name CRC32\n"
		"This tool will flip each bit and stops when a CRC match is found.\n",
		version="%prog 0.1 (2014-10-07)") # --help, --version
	
	# no arguments given
	if len(sys.argv) < 2:
		print(parser.format_help())
	else:
		(options, args) = parser.parse_args()
		main(options, args)