IceCTF A Strong Feeling Solution

solve_a_strong_feeling.py

#!/usr/bin/env python2

"""
Angr would probably be the nice way to solve this. Oh well.
Brute force worked for me.

Trying different inputs, it seems the length doesn't matter.
The output changes depending on how many characters at the beginning match the key.
Brute force possible keys watching for output changes (to know when we got the right letter)


duck@computer:~/Downloads$ python solve_a_strong_feeling.py 
Leapfrog tcp buffer linux Leslie Lamport int L0phtCrack snarf headers bang.
I
Suitably small values machine code bypass infinite loop sudo lib.
Ic
Snarf foad mainframe firewall ifdef all your base are belong to us.
Ice
Segfault daemon pwned ack highjack eaten by a grue then linux stack hello world bang gcc do int hexadecimal.
IceC

"""

from subprocess import Popen, PIPE
import string

def try_flag(s):
    process = Popen(["./a_strong_feeling"], stdout=PIPE, stdin=PIPE)
    (output, err) = process.communicate(s + "\n")
    # exit_code = process.wait()
    return output

flag = ""

while True:
    # Start by guessing "." and assume that we are wrong.
    last = try_flag(flag + ".")
    for s in string.printable:
        out = try_flag(flag + s)
        if out != last:
            flag += s
            print out
            print flag
            break