BASED (picoCTF2019)

16 Nov 2019

Description

Solution

In this task we need to convert data from different bases (binary, octal, hex) to proper word. Any time we connect to the server we get diffrent words. The easiest way to to this task is write a simple Python script which get data from console, convert it, and send “decrypted” word back to server.

#!/usr/bin/env python3

from pwn import *
import socket

# Connection to server
try:
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(("2019shell1.picoctf.com", 25180))
except Exception:
    print("Connection failed")

text = s.makefile('rb')

def Task_1(linijka,tekst):
    tablica = linijka.split(' ')  # split this line to single words
    x = tablica.index('the')  # finding index of last word before our "binary text"
    y = tablica.index('as')  # finding index of first word after our "binary text"
    tablica = tablica[x + 1:y]  # cut only binary elements from list
    dlugosc_tablicy = len(tablica)
    i = 0
    wynik = ''

    # converting list elements from string "binary" elements to letters and save it in string
    while i != dlugosc_tablicy:
        wynik += chr(int(tablica[i], base=2))
        i += 1
    wynik += "\n"

    # finding input place
    tekst = text.readline()
    s.send(wynik.encode("utf-8"))

def Task_2(linijka,tekst):
    tablica = linijka.split(' ')
    x = tablica.index('the')
    y = tablica.index('as')
    tablica = tablica[x + 2:y]
    dlugosc_tablicy = len(tablica)
    i = 0
    wynik = ''

    while i != dlugosc_tablicy:
        wynik += chr(int(tablica[i], base=8))
        i += 1
    wynik += "\n"
    tekst = text.readline()
    s.send(wynik.encode("utf-8"))

def Task_3(linijka,teskt):
    tablica = linijka.split(' ')
    x = tablica.index('the')
    y = tablica.index('as')
    tablica = tablica[x + 1:y]
    wartosc = tablica[0]
    tablica = []
    j = 0

    # converting data from string to list of letters in hex type
    for j in range(0, len(wartosc), 2):
        tablica.append(wartosc[j] + wartosc[j+1])
    i = 0
    dlugosc_tablicy = len(tablica)
    wynik = ''
    while i != dlugosc_tablicy:
        wynik += chr(int(tablica[i], base=16))
        i += 1
    wynik += "\n"
    tekst = text.readline()
    s.send(wynik.encode("utf-8"))
# receiving text from console
while True:
    tekst = text.readline()
    linijka = repr(tekst)
    print(linijka)

    # finding line with first task
    if "Please give the" in linijka:
        Task_1(linijka,tekst)

    # finding line with second task
    if "Please give me the  " in linijka:
        Task_2(linijka,tekst)

    # finding line with third task (to find difference between second and third task we take 6 and 7 to searching string
    # all data starts on 6 or 7
    if "Please give me the 6" in linijka:
        Task_3(linijka,tekst)
    elif "Please give me the 7" in linijka:
        Task_3(linijka,tekst)

    if "WRONG!" in linijka:
        break

    if "picoCTF" in linijka:
        break

After runnig script we get the flag:

Flag: picoCTF{learning_about_converting_values_d57e7f86}