Hashdesigner Internetwache CTF challenge crypto70

attack.py

#!/usr/bin/python2

from pwn import *
import random

def proof_of_work(prefix, rlen=15, hashfunc=hashlib.sha1):
    i = 0
    maxiter = 2 ** 28
    while i < maxiter:
        i += 1
        s = prefix
        for _ in range(rlen-len(s)):
            s += random.choice(string.lowercase+string.digits)
        h = hashfunc()
        h.update(s)
        digest = h.digest()
        binary = ''.join('{0:08b}'.format(ord(x)) for x in digest)
        if binary.endswith("0"*16):
            log.info("found proof of work after {} iterations"
                     .format(i))
            return s
    log.critical("couldn't find matching proof of work after {} iterations"
                 .format(i))
    return None

p = remote("188.166.133.53", 10009)


line = p.recvline()
prefix = line[line.find("It has ")+7:line.find(" as the prefix.")]

proof = proof_of_work(prefix)
p.recv(len("Enter work:"))
p.sendline(proof)
succ = p.recvline()
assert("Thank you" in succ)
p.sendline("DhvxGXEdJkRiv4QEDS")  # brute-forced offline
p.interactive()