crc_spoof.py

# 
# CRC-32 forcer (Python)
# Compatible with Python 2 and 3.
# 
# Copyright (c) 2017 Project Nayuki
# https://www.nayuki.io/page/forcing-a-files-crc-to-any-value
# 
# 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 (see COPYING.txt).
# If not, see <http://www.gnu.org/licenses/>.
# 

import os, sys, zlib

# reimplemented to work with buffers
def modify_buffer_crc32(buffer, offset, newcrc, printstatus=False):
	if newcrc & MASK != newcrc:
		return "Error: Invalid new CRC-32 value"

	length = len(buffer)

	if offset + 4 > length:
	   raise ValueError("Byte offset plus 4 exceeds buffer length")

	# Calculate original CRC-32 value
	crc = get_buffer_crc32(buffer)
	if printstatus:
		print("Buffer CRC-32: {:08X}".format(reverse32(crc)))
		
	# Compute the change to make
	delta = crc ^ newcrc
	delta = multiply_mod(reciprocal_mod(pow_mod(2, (length - offset) * 8)), delta)
	
	# Patch 4 bytes in the buffer
	bytes4 = bytearray(buffer[offset:offset+4])
	if len(bytes4) != 4:
		raise IOError("Cannot read 4 bytes at offset")
		
	if printstatus:
		print("Target bytes @ offset: {:s}".format(str(bytes4).encode('hex')))

	for i in range(4):
		bytes4[i] ^= (reverse32(delta) >> (i * 8)) & 0xFF
    
	if printstatus:
		print("Patched value: {:s}".format(str(bytes4).encode('hex')))

	result = str(buffer[0:offset] + bytes4 + buffer[offset+4:])
	
	if printstatus:
		print("Computed and wrote patch")
	
	# Recheck entire file
	if get_buffer_crc32(result) != newcrc:
		raise AssertionError("Failed to update CRC-32 to desired value")
	elif printstatus:
		print("New CRC-32 successfully verified")
	
	return result

# Public library function. path is str/unicode, offset is uint, newcrc is uint32, printstatus is bool.
# Returns None. May raise IOError, ValueError, AssertionError.
def modify_file_crc32(path, offset, newcrc, printstatus=False):
	with open(path, "r+b") as raf:
		raf.seek(0, os.SEEK_END)
		length = raf.tell()
		if offset + 4 > length:
			raise ValueError("Byte offset plus 4 exceeds file length")
		
		# Read entire file and calculate original CRC-32 value
		crc = get_file_crc32(raf)
		if printstatus:
			print("Original CRC-32: {:08X}".format(reverse32(crc)))
		
		# Compute the change to make
		delta = crc ^ newcrc
		delta = multiply_mod(reciprocal_mod(pow_mod(2, (length - offset) * 8)), delta)
		
		# Patch 4 bytes in the file
		raf.seek(offset)
		bytes4 = bytearray(raf.read(4))
		if len(bytes4) != 4:
			raise IOError("Cannot read 4 bytes at offset")
		for i in range(4):
			bytes4[i] ^= (reverse32(delta) >> (i * 8)) & 0xFF
		raf.seek(offset)
		raf.write(bytes4)
		if printstatus:
			print("Computed and wrote patch")
		
		# Recheck entire file
		if get_file_crc32(raf) != newcrc:
			raise AssertionError("Failed to update CRC-32 to desired value")
		elif printstatus:
			print("New CRC-32 successfully verified")


# ---- Utilities ----

POLYNOMIAL = 0x104C11DB7  # Generator polynomial. Do not modify, because there are many dependencies
MASK = (1 << 32) - 1

def get_buffer_crc32(buffer):
    crc = zlib.crc32(buffer)
    return reverse32(crc & MASK)

def get_file_crc32(raf):
	raf.seek(0)
	crc = 0
	while True:
		buffer = raf.read(128 * 1024)
		if len(buffer) == 0:
			return reverse32(crc & MASK)
		else:
			crc = zlib.crc32(buffer, crc)

def reverse32(x):
	y = 0
	for i in range(32):
		y = (y << 1) | (x & 1)
		x >>= 1
	return y


# ---- Polynomial arithmetic ----

# Returns polynomial x multiplied by polynomial y modulo the generator polynomial.
def multiply_mod(x, y):
	# Russian peasant multiplication algorithm
	z = 0
	while y != 0:
		z ^= x * (y & 1)
		y >>= 1
		x <<= 1
		if (x >> 32) & 1 != 0:
			x ^= POLYNOMIAL
	return z

# Returns polynomial x to the power of natural number y modulo the generator polynomial.
def pow_mod(x, y):
	# Exponentiation by squaring
	z = 1
	while y != 0:
		if y & 1 != 0:
			z = multiply_mod(z, x)
		x = multiply_mod(x, x)
		y >>= 1
	return z

# Computes polynomial x divided by polynomial y, returning the quotient and remainder.
def divide_and_remainder(x, y):
	if y == 0:
		raise ValueError("Division by zero")
	if x == 0:
		return (0, 0)
	
	ydeg = get_degree(y)
	z = 0
	for i in range(get_degree(x) - ydeg, -1, -1):
		if (x >> (i + ydeg)) & 1 != 0:
			x ^= y << i
			z |= 1 << i
	return (z, x)

# Returns the reciprocal of polynomial x with respect to the modulus polynomial m.
def reciprocal_mod(x):
	# Based on a simplification of the extended Euclidean algorithm
	y = x
	x = POLYNOMIAL
	a = 0
	b = 1
	while y != 0:
		q, r = divide_and_remainder(x, y)
		c = a ^ multiply_mod(q, b)
		x = y
		y = r
		a = b
		b = c
	if x == 1:
		return a
	else:
		raise ValueError("Reciprocal does not exist")

def get_degree(x):
	return x.bit_length() - 1