ctf-writeup/2023/KnightCTF 2023/Encode Mania

Encode Mania

Encoding random stuffs is so cool! I just want to encode it over and over and over again ...

About the Challenge

We were given a python script to encrypt the flag and the encrypted file. And if we open the script, the script will be look like this

import base64
from random import randint

flag = "kctf{************}"

def encrypt(s, option):
    if option == 0:
        ret = base64.b64encode(s)
    elif option == 1:
        ret = base64.b32encode(s)
    elif option == 2:
        ret = base64.b16encode(s)
    else:
        ret = base64.b85encode(s)

    return ret


encrypted_flag = flag.encode('utf-8')

for _ in range(12):
    option = randint(0, 3)
    encrypted_flag = encrypt(encrypted_flag, option)

with open("encode_mania.txt", 'w') as f:
    f.write(encrypted_flag.decode())

The program will encode the flag 12 times with different random encoding (base64, base32, base16, and base85) as you can see in this line (You can get the script here)

for _ in range(12):
    option = randint(0, 3)
    encrypted_flag = encrypt(encrypted_flag, option)

How to Solve?

To solve this i created a python script to bruteforce each possibility 12 times with all encoding (You can get the script here)

import base64
import re

encoded_flag = "GUZDGMRUIQ3T......"
def decrypt(s, option):
    if option == 0:
        ret = base64.b64decode(s)
    elif option == 1:
        ret = base64.b32decode(s)
    elif option == 2:
        ret = base64.b16decode(s)
    else:
        ret = base64.b85decode(s)
    return ret

for _ in range(12):
    for option in range(4):
        try:
            dec = decrypt(encoded_flag, option)
            if re.findall(r"^\w+", dec.decode()):
                print(dec.decode())
            encoded_flag = dec.decode()
        except:
            pass

And the flag will be printed in the terminal

KCTF{dfs_0r_b4u7e_f04ce}