drmless - PlaidCTF 2013

Posted by br0ns on 06 May 2013

Challenge description:

 You should check out our cool stories, bros.

Link: drmless.tgz

Inside the archive are five files:

  • .drmlicense
  • cool_story.enc
  • cooler_story.enc
  • drmless
  • readme.txt

In readme.txt we find this text:

 Here's a cool story from PPP!
We wrote an even cooler story, but you need to pay
us if you want to unlock it. TEN THOUSAND DOLLAR.

The file drmless is a 32bit ELF. Running it we see that it behaves exactly as less except that it is also able to open cool_story.enc. However we can't open cooler_story.enc.

Looking in the file .drmlicense we find the 16 bytes:

00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff

Flipping one bit in .drmlicense and opening cool_story.enc again we see that every 16th byte has the same bit flipped. So we guess that the 16 bytes in .drmless are xor'ed into the DRM'ed file ECB style. However the xor difference between the DRM'ed file and the plaintext is not .drmlicense, so something else is going on. Our initial guess is that a pseudorandom stream is xor'ed into the DRM'ed file along with the license.

So we try putting all zeroes in .drmlicense in order to extract the pseudorandom stream. However drmless now says that the file contains binary data and when prompted to show it anyway shows the DRM'ed file, not the "plaintext".

The binary is not stripped so running readelf -s drmless gives us a lot of symbols. Specifically we are interested in these (readelf -s drmless|grep drm):

1024: 0804eb69   197 FUNC    GLOBAL DEFAULT   14 drmprotected
1192: 0804eb34    53 FUNC    GLOBAL DEFAULT   14 undrm

Going out on a limb we patch drmprotected to always return true:

0804eb69 <drmprotected>:
  804eb69:       31 c0                   xor    eax,eax
  804eb6b:       40                      inc    eax
  804eb6c:       c3                      ret

Running the program again we see the "plaintext". Bam. We see that changing the license file doesn't change the pseudorandom stream.

So now we do the same trick with cooler_story.enc to find it's stream (which is not the same as the one for cool_story.enc by the way). Now trying each byte value for each of the 16 bytes in .drmlicense we compile a list of candidates that will produce printable characters everywhere in the plaintext (script below).

We find two candidates for each of the sixteen bytes, and after some quick guesswork (chaning "!" to " ", etc.) we arrive at the answer:

TWELFTH NIGHT; OR, WHAT YOU WILL

by PPP
"freeShakespeare_downWithDRM"

doit.py:

from pwn import *
import os, string

log.waitfor('Generating "plaintext"')
os.system('./drmless cooler_story.enc > cooler_story')
log.succeeded()

ct = read('cooler_story.enc')
pt = read('cooler_story')
lic = read('.drmlicense')

stream = xor(ct, pt, cut='min')
stream = xor(stream, lic, cut='max')

ct = xor(ct, stream, cut='min')

groups = zip(*group(16, ct))

log.waitfor('Finding valid license bytes')
for i in range(len(groups)):
    log.status_append('\n%2d:' % i)
    for x in range(256):
        xs = xor(groups[i], x)
        if all(x in string.printable for x in xs):
            log.status_append(' ' + enhex(x))
log.succeeded()