The challenge generates a mustache based on argv[1] input. Challenge authors supplied us with the target mustache, which we had to match.

user@KARCH ~/ctf/papa % ./papa_bear AAAAA
   ______   _______  ______   _______      ______   _______  _______  ______
  (_____ \ (_______)(_____ \ (_______)    (____  \ (_______)(_______)(_____ \
   _____) ) _______  _____) ) _______      ____)  ) _____    _______  _____) )
  |  ____/ |  ___  ||  ____/ |  ___  |    |  __  ( |  ___)  |  ___  ||  __  /
  | |      | |   | || |      | |   | |    | |__)  )| |_____ | |   | || |  \ \
  |_|      |_|   |_||_|      |_|   |_|    |______/ |_______)|_|   |_||_|   |_|

            dWMM=-        dWWMWWMMWWMWb  dWMMWWMWWMMWb        -=WMWb
          dWMMP       dWWMWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMb        qMMb
          MMMMb   dMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMb    dMMM
          qMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMP
            QMMMMMMMMMMMMMMMMMMMMMMMMMP  QMMMMMMMMMMMMMMMMMMMMMMMMMMP
              QMMMMMMMMMMMMMMMMMMMP          QMMMMMMMMMMMMMMMMMMMP
                     QMMMMMMP                         QMMMMMMP

Like in babybear, the input transformation into the mustache was linear. So this time I tried to solve it without reversing at all, just by bruteforce.

Patching

I tried to run the binary with python subprocess.getoutput aaaand it did not work. (output was always empty) A look with strace on the binary revealed that the output is written to stdin, and not stdout. As there was only one position where the output is written, I just patched the file descriptor from 0 to 1. (r2 -w papa_bear, seek to the write, and patch the rdi assignment to wa mov rdi, 1) Now everything works as expected.

Bruteforcing

We now can just guess byte by byte until we match the desired mustache. Sometimes manual intervention is necessary to prevent a path explosion (it seems like the @ symbol can cause recursion).

import subprocess
import string

soll = """
dWWW=- dWWMWWWWWMWMb dMMWWWWWWWWWb -=MMMb
dWMWP dWWWMWWWMMWMMMWWWWWMMMMMMWMMMWWWMMMb qMWb
WMWWb dMWWMMMMMMWWWWMMWWWMWWWWWWMMWWWWMWMWMMMWWWWb dMMM
qMMWMWMMMWMMWWWMWMMMMMMMMWMMMMWWWMMWWMWMWMMWWMWWWWMWWMMWMMWP
QWWWWWWWMMWWWWWWWMMWWWWMMWP QWWWMWMMMMWWWWWMMWWMWWWWWWMP
QWMWWWMMWWMWMWWWWMWWP QWWMWWMMMWMWMWWWWMMMP
QMWWMMMP QMMMMMMP
"""


def normalize(data):
    return ''.join(filter(lambda c: c if c in ['W', 'M'] else '', data)).replace('W', '1').replace('M', '0')


soll = normalize(soll)

states = ['HackTM{F4th3r bEaR s@y$: Smb']
while states:
    newstates = []
    for state in states:
        for c in string.digits + string.ascii_letters + string.whitespace + r"""!"#%&'()*+,-./:;<=>?@[\]^_`{|}~""":
            ist = subprocess.getoutput("./papa_bear '{}'".format((state + c).replace("'", "'\\''")))
            ist = ''.join(ist.splitlines()[7:])
            ist = normalize(ist)
            ist = ist[:ist.rfind('1') + 1]
            if soll.startswith(ist):
                newstates.append(state + c)
    states = newstates
    print(states)