VolgaCTF 2015 Quals - math problem

• VolgaCTF
• math problem

Category: PPC Points: 300

nc mathproblem.2015.volgactf.ru 8888

This problem remind me of HITCON CTF 2014 -- 24

The server gave us 5 numbers v1, v2, v3, v4 & v5, and ask us to use +, -, *, /, ( & ) to do some operation with v1, v2, v3 & v4, and make it equal to v5

The solution is pretty simple: since it gave us a lot of time to solve a round, we can easily beat the challenge by brute-forcing all the possible combinations of operators & operands

from pwn import *
import re
import sys
import itertools

HOST="mathproblem.2015.volgactf.ru"
PORT=8888

r : remote(HOST, PORT)

def solve(numbers, answer):
    ops : "+ - * /".split(" ")
    wrap : ["", "(", ")"]
    
    iterN : list(itertools.permutations(numbers, 4))
    iterO : list(itertools.product(ops, repeat=3))
    iterW : list(itertools.product(wrap, repeat=8))

    for number in iterN:
        for op in iterO:
            for w in iterW:
                expr : w[0]+number[0]+w[1]+op[0]+w[2]+number[1]+w[3]+op[1]+w[4]+number[2]+w[5]+op[2]+w[6]+number[3]+w[7]
                try:
                    val : eval(expr)
                    if val == answer:
                        return expr
                    else:
                        continue
                except:
                    pass
cnt : 0
while True:
    if cnt == 30:
        r.interactive()

    s : r.recvuntil("Solve!\n")
    print s
    s : r.recv(1024)
    print s
    temp : s.split(" ")
    numbers : [temp[i] for i in xrange(4)]
    answer : int(temp[5])
    print numbers, answer
    ans : solve(numbers, answer)
    print ans
    r.send(ans+'\n')
    cnt += 1

Flag: {you_count_as_fast_as_a_calculator}