GCHQ Challenge - PNG Inspector

# Dr Gareth Owen, University of Greenwich
# PNG Image parser and inspector for stenographic content

import sys
import zlib
import struct
import collections
#import file

idat = ''
width = 0
height = 0
Header = collections.namedtuple('Header', 'width height bpp coltype comptype filtype intype')
hdr = ''

def readChunk(f):
  global idat, hdr
  (len,) = struct.unpack('>L', f.read(4))
  type = f.read(4)

  print "Len %d Type %s" % (len, type)

  data = f.read(len)

  if type == "IHDR":

    hdr = Header._make(struct.unpack('>LLBBBBB', data))
    print hdr
  elif type == "iTXt":
    print data
  elif type == "tIME":
    Tm = collections.namedtuple('Tm', 'year month day hour minute second')
    print Tm._make(struct.unpack('>HBBBBB', data))
  elif type == "IDAT":
    idat += data

  crc = f.read(4)

  return type

inf = open('cyber.png', 'r')
sig = inf.read(8)

while readChunk(inf) != 'IEND':
  pass

unpacked = zlib.decompress(idat)
print "Decom len %d " % (len(unpacked))

# has extra byte at beginning of each scan line indicating encoding type
o = open('cyber.raw', 'w')
o.write(unpacked)
o.close()

# check Least significant bits for stenographic content
#for y in range(0, hdr.height):
#  for x in range(1, hdr.width):
#    print ord(unpacked[y*hdr.width + x]) & 0x01,