4aa85aa621
[MS-CAB] is pretty unclearly worded in this area. Turns out that if the sequence of bytes being checksummed is not a multiple of 4 bytes, you are supposed to _first_ reverse the order of the trailing (n % 4) bytes, and _then_ append zero bytes to pad to a multiple of 4, before you do the main checksum operation of XORing together all the 32-bit words of the input.
111 lines
3.7 KiB
Python
Executable file
111 lines
3.7 KiB
Python
Executable file
#!/usr/bin/env python
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import sys
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import os
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import time
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import zlib
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import struct
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from collections import namedtuple
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CFHEADER_s = struct.Struct("<4sLLLLLBBHHHHH")
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CFHEADER = namedtuple("CFHEADER", "sig res0 size res1 firstfile res2 "
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"verminor vermajor folders files flags setid icabinet")
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CFHEADER_sig = "MSCF"
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CFFOLDER_s = struct.Struct("<LHH")
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CFFOLDER = namedtuple("CFFOLDER", "firstdata ndata compresstype")
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CFFILE_s = struct.Struct("<LLHHHH")
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CFFILE = namedtuple("CFFILE", "size offset ifolder date time attrs")
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CFDATA_s = struct.Struct("<LHH")
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CFDATA = namedtuple("CFDATA", "checksum compressedlen uncompressedlen")
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def mszip(data):
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compressor = zlib.compressobj(9, zlib.DEFLATED, -zlib.MAX_WBITS,
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zlib.DEF_MEM_LEVEL, zlib.Z_DEFAULT_STRATEGY)
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compressed = compressor.compress(data)
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compressed += compressor.flush()
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return "CK" + compressed # add MSZIP header
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def packdate(y,m,d):
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return ((y - 1980) << 9) | (m << 5) | d
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def packtime(h,m,s):
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return ((h << 11) | (m << 5) | (s >> 1))
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def checksum(data):
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data_full_words = data[:len(data) & ~3]
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data_last_word = data[len(data_full_words):]
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data_last_word = "".join(reversed(data_last_word))
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data_last_word += "\0" * (3 & -len(data)) # pad to multiple of 4 bytes
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data = data_full_words + data_last_word
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toret = 0
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for offset in xrange(0, len(data), 4):
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toret ^= struct.unpack_from("<L", data, offset)[0]
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return toret
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def build_cab(files):
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uncompressed_data = ""
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fileheaders = ""
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for name, data, mtime in files:
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mtime_u = time.gmtime(mtime)
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fileheader = CFFILE(
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size=len(data), offset=len(uncompressed_data), ifolder=0, attrs=0,
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date=packdate(mtime_u.tm_year, mtime_u.tm_mon, mtime_u.tm_mday),
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time=packtime(mtime_u.tm_hour, mtime_u.tm_min, mtime_u.tm_sec))
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uncompressed_data += data
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fileheaders += CFFILE_s.pack(*fileheader) + name + "\0"
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compressed_data = ""
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offset = 0
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n_data_blocks = 0
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while offset < len(uncompressed_data):
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uncompressed_block = uncompressed_data[offset:offset+0x8000]
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compressed_block = mszip(uncompressed_block)
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blockheader = CFDATA(
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checksum=0,
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compressedlen=len(compressed_block),
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uncompressedlen=len(uncompressed_block))
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header_after_checksum = CFDATA_s.pack(*blockheader)[4:]
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blockheader = blockheader._replace(
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checksum=checksum(header_after_checksum + compressed_block))
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compressed_data += CFDATA_s.pack(*blockheader) + compressed_block
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offset += len(uncompressed_block)
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n_data_blocks += 1
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totalsize = (CFHEADER_s.size +
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CFFOLDER_s.size +
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len(fileheaders) +
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len(compressed_data))
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header = CFHEADER(
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sig=CFHEADER_sig, res0=0, res1=0, res2=0,
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vermajor=1, verminor=3, folders=1, files=len(files),
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flags=0, setid=0, icabinet=0, size=totalsize,
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firstfile=CFHEADER_s.size + CFFOLDER_s.size)
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folder = CFFOLDER(
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ndata=n_data_blocks, compresstype=1,
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firstdata = (CFHEADER_s.size + CFFOLDER_s.size + len(fileheaders)))
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return (CFHEADER_s.pack(*header) +
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CFFOLDER_s.pack(*folder) +
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fileheaders +
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compressed_data)
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def main():
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args = sys.argv[1:]
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outfile = args.pop(0)
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files = []
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while len(args) > 0:
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cabname = args.pop(0)
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filename = args.pop(0)
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with open(filename, "rb") as f:
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filedata = f.read()
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files.append((cabname, filedata, os.stat(filename).st_mtime))
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cabdata = build_cab(files)
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with open(outfile, "wb") as f:
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f.write(cabdata)
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if __name__ == '__main__':
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main()
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