Brute-force script to decode the Artificial Heart Level 4 easter-egg image

joco_brute_decode.py

from base64 import b64decode
import struct
from zlib import crc32

b64alphabet = []
b64alphabet.extend([chr(x) for x in range(ord('A'),ord('Z')+1)])
b64alphabet.extend([chr(x) for x in range(ord('a'),ord('z')+1)])
b64alphabet.extend([chr(x) for x in range(ord('0'),ord('9')+1)])
b64alphabet.extend(['+', '/'])

with open('joco.b64', 'rb') as f:
    src = f.read()

subst1 = ''

# http://libpng.org/pub/png/spec/iso/index-object.html#5Chunk-layout
def check_chunk(name, startfrom):
    start = picstr.find(name, startfrom)
    if start < 0:
        return (True, -1)
    size = struct.unpack("!I", picstr[start-4:start])[0]
    end = start + 4 + size
    newcrc = crc32(picstr[start:end])
    oldcrc = struct.unpack("!i", picstr[end:(end+4)])[0]
    return (newcrc == oldcrc, end)

for subst in (''.join([x,y]) for x in b64alphabet for y in b64alphabet):
    picstr = b64decode(src.replace('code', subst).replace('python', 'hon'))
    next = 0
    # The string "code" appears by itself in the first four IDAT chunks,
    # so we check those only on this first run to reduce the search space
    for i in range(0,4):
        (valid, next) = check_chunk('IDAT', next)
        if not valid:
            break
    if valid:
        subst1 = subst
        break

if len(subst1) != 2:
    print "'code' couldn't be solved!"
    exit()
    
print "'code' was '%s'"%(subst1,)

for subst in (''.join([x,y,z]) for x in b64alphabet for y in b64alphabet for z in b64alphabet):
    picstr = b64decode(src.replace('code', subst1).replace('python', subst))
    next = 0
    # This time through, check all the IDAT chunks
    while next >= 0:
        (valid, next) = check_chunk('IDAT', next)
        if not valid:
            break
    if valid:
        # And also check the IEND chunk (just to be sure)
        (valid, next) = check_chunk('IEND', 0)
        if valid:
            subst2 = subst
            with open('joco_correct.png', 'wb') as out:
                out.write(picstr)
            break

print "'code' was '%s', 'python' was '%s'"%(subst1, subst2)

Wow, very nicely done. I'm sure glad you're not my nemesis. Unless, of course, you are.