Python Zlib … Son I Am Disapoint

Wyatt • Aug 19, 2013 • Rants

Every now and then, I find myself digging through some arbitrarily compressed binary and in IDA, when you have to keep doing it over and over again, you should write a script or a loader to handle that (as any good programmer would). So I started wiring up a loader in python and thought that I’d use the zlib library to decompress things … boy was I wrong. Not only did zlib fail to actually work correctly (because it can’t actually handle ZIP files, more on that in a moment), but the error messages were basically the same low-level messages you got out of zlib’s internal functions. Really? This is the best we can do right now? What I tried:

[wyatt@lazarus:~/Downloads]$ zip derp.zip Untitled\ drawing.png
[wyatt@lazarus:~/Downloads]$ cat Whatsnew.txt derp.zip > file.out
[wyatt@lazarus:~/Downloads]$ python
Python 2.7.3 (default, Apr 10 2013, 06:20:15)
[GCC 4.6.3] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.

import struct
import zlib
f = open(‘file.out’,‘rb’)
c = f.read()
f.close()
offset = c.find(‘PK’)
uncmp_size = struct.unpack("<l",c[offset+22:offset+22+4])
z = zlib.decompressobj()
out = z.decompress(c[offset:],int(uncmp_size[]))
Traceback (most recent call last):
File "", line 1, in
zlib.error: Error –3 while decompressing: incorrect header check

This of course fails because zlib doesn’t actually work right with zip files (you’ll find a vauge note to such things in the ) and of course … I should have really known that ZIP isn’t actually zlib. Instead of trying to be clever, I decided to give up and be lazy. What actually worked:

import subprocess
subprocess.call([‘7z’,‘e’,‘file.out’])

Processing archive: file.out

Extracting Untitled drawing.png

Everything is Ok

Size: 24513
Compressed: 22290

So yes … apparently this is the best we can do with the zlib library.