pdf.py

#!/usr/bin/env python

# script to craft MD5 collisions of 2 PDFs via mutool and UniColl
# Ange Albertini 2018

# uses mutool from https://mupdf.com/index.html

import os
import sys
import hashlib

def EnclosedString(d, starts, ends):
  off = d.find(starts) + len(starts)
  return d[off:d.find(ends, off)]

def getCount(d):
  s = EnclosedString(d, "/Count ", "/")
  count = int(s)
  return count

def procreate(l): # :p
  return " 0 R ".join(l) + " 0 R"


if len(sys.argv) == 1:
  print("PDF MD5 collider")
  print("Usage: pdf.py <file1.pdf> <file2.pdf>")
  sys.exit()

os.system('mutool merge -o first.pdf %s' % sys.argv[1])
os.system('mutool merge -o second.pdf %s' % sys.argv[2])
os.system('mutool merge -o merged.pdf dummy.pdf %s %s' % (sys.argv[1], sys.argv[2]))

with open("first.pdf", "rb") as f:
  d1 = f.read()

with open("second.pdf", "rb") as f:
  d2 = f.read()

with open("merged.pdf", "rb") as f:
  dm = f.read()


COUNT1 = getCount(d1)
COUNT2 = getCount(d2)


kids = EnclosedString(dm, "/Kids[", "]")

# we skip the first dummy, and the last " 0 R" string
pages = kids[:-4].split(" 0 R ")[1:]

template = """%%PDF-1.4

1 0 obj
<<
  /Type /Catalog

  %% for alignments (comments will be removed by merging or cleaning)
  /MD5_is__ /REALLY_dead_now__
  /Pages 2 0 R
  %% to make sure we don't get rid of the other pages when garbage collecting
  /Fakes 3 0 R
  %% placeholder for UniColl collision blocks
  /0123456789ABCDEF0123456789ABCDEF012
  /0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0
>>
endobj

2 0 obj
<</Type/Pages/Count %(COUNT2)i/Kids[%(KIDS2)s]>>
endobj 

3 0 obj
<</Type/Pages/Count %(COUNT1)i/Kids[%(KIDS1)s]>>
endobj

4 0 obj %% overwritten - was a fake page to fool merging
<< >>
endobj

"""

KIDS1 = procreate(pages[:getCount(d1)])

KIDS2 = procreate(pages[getCount(d1):])


with open("hacked.pdf", "wb") as f:
  f.write(template % locals())
  # adjust parents for the first set of pages
  f.write(dm[dm.find("5 0 obj"):].replace("/Parent 2 0 R", "/Parent 3 0 R", COUNT1))

# let's adjust offsets - -g to get rid of object 4 by garbage collecting
# (yes, errors will appear because we modified objects without adjusting XREF)
print
print "KEEP CALM and IGNORE THE NEXT ERRORS"
os.system('mutool clean -gggg hacked.pdf cleaned.pdf')

with open("cleaned.pdf", "rb") as f:
  cleaned = f.read()

# some mutool versions do different stuff :(
cleaned = cleaned.replace(
  " 65536 f \n0000000016 00000 n \n",
  " 65536 f \n0000000018 00000 n \n",
  1)

with open("pdf1.bin", "rb") as f:
  prefix1 = f.read()

with open("pdf2.bin", "rb") as f:
  prefix2 = f.read()

file1 = prefix1 + "\n" + cleaned[192:]
file2 = prefix2 + "\n" + cleaned[192:]

with open("collision1.pdf", "wb") as f:
  f.write(file1)

with open("collision2.pdf", "wb") as f:
  f.write(file2)

os.remove('first.pdf')
os.remove('second.pdf')
os.remove('merged.pdf')
os.remove('hacked.pdf')
os.remove('cleaned.pdf')

md5 = hashlib.md5(file1).hexdigest()

assert md5 == hashlib.md5(file2).hexdigest()

# to prove the files should be 100% valid
print
os.system('mutool info -X collision1.pdf')
print
print
os.system('mutool info -X collision2.pdf')

print
print "MD5: %s" % md5
print "Success!"