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#!/usr/bin/python
"""Calculate and manipulate CRC32.
http://en.wikipedia.org/wiki/Cyclic_redundancy_check
-- StalkR
"""
import struct
import sys
# Polynoms in reversed notation
POLYNOMS = {
'CRC-32-IEEE': 0xedb88320, # 802.3
'CRC-32C': 0x82F63B78, # Castagnoli
'CRC-32K': 0xEB31D82E, # Koopman
'CRC-32Q': 0xD5828281,
}
class Error(Exception):
pass
class CRC32(object):
"""A class to calculate and manipulate CRC32.
Use one instance per type of polynom you want to use.
Use calc() to calculate a crc32.
Use forge() to forge crc32 by adding 4 bytes anywhere.
"""
def __init__(self, type="CRC-32-IEEE"):
if type not in POLYNOMS:
raise Error("Unknown polynom. %s" % type)
self.polynom = POLYNOMS[type]
self.table, self.reverse = [0]*256, [0]*256
self._build_tables()
def _build_tables(self):
for i in range(256):
fwd = i
rev = i << 24
for j in range(8, 0, -1):
# build normal table
if (fwd & 1) == 1:
fwd = (fwd >> 1) ^ self.polynom
else:
fwd >>= 1
self.table[i] = fwd & 0xffffffff
# build reverse table =)
if rev & 0x80000000 == 0x80000000:
rev = ((rev ^ self.polynom) << 1) | 1
else:
rev <<= 1
rev &= 0xffffffff
self.reverse[i] = rev
def calc(self, s):
"""Calculate crc32 of a string.
Same crc32 as in (binascii.crc32)&0xffffffff.
"""
crc = 0xffffffff
for c in s:
crc = (crc >> 8) ^ self.table[(crc ^ ord(c)) & 0xff]
return crc^0xffffffff
def forge(self, wanted_crc, s, pos=None):
"""Forge crc32 of a string by adding 4 bytes at position pos."""
if pos is None:
pos = len(s)
# forward calculation of CRC up to pos, sets current forward CRC state
fwd_crc = 0xffffffff
for c in s[:pos]:
fwd_crc = (fwd_crc >> 8) ^ self.table[(fwd_crc ^ ord(c)) & 0xff]
# backward calculation of CRC up to pos, sets wanted backward CRC state
bkd_crc = wanted_crc^0xffffffff
for c in s[pos:][::-1]:
bkd_crc = ((bkd_crc << 8)&0xffffffff) ^ self.reverse[bkd_crc >> 24] ^ ord(c)
# deduce the 4 bytes we need to insert
for c in struct.pack('<L',fwd_crc)[::-1]:
bkd_crc = ((bkd_crc << 8)&0xffffffff) ^ self.reverse[bkd_crc >> 24] ^ ord(c)
res = s[:pos] + struct.pack('<L', bkd_crc) + s[pos:]
assert(crc32(res) == wanted_crc)
return res
if __name__=='__main__':
if len(sys.argv) > 1:
arg = sys.argv[1]
else:
arg = "test"
# CRC32 with default polynom
crc = CRC32().calc(arg)
print "CRC32(%s) = 0x%08x" % (arg, crc)
# check with library
from binascii import crc32
assert(crc == crc32(arg)&0xffffffff)
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