Introduction

In this task, we can see a simple Caesar cipher with a lot of random characters on both sides of the flag. In the LoooongCaesarCipher.txt file, there is a 100 0000 characters. As Caesar’s cipher only contains a-z characters, we can make a simple script to brute force the shift. You can learn about the Caesar cipher here.

Solving

Thanks to the fact that we know that every flag starts with utflag{, we can easily bruteforce the shift. I used multithreading to speed up the process as python is not the fastest language :P.

import concurrent.futures

line = open("./LoooongCaesarCipher.txt").readline()


def caesar_cipher_crack(k: int) -> str:
    ans = ""
    for char in line:
        if char.isalpha():
            shift = -k
            if char.islower():
                ans += chr(((ord(char) - ord("a") + shift) % 26) + ord("a"))
            else:
                ans += chr(((ord(char) - ord("A") + shift) % 26) + ord("A"))
        else:
            ans += char

    return ans


with concurrent.futures.ThreadPoolExecutor() as executor:
    for i in range(len(line)):
        if "utflag{" in (result := executor.submit(caesar_cipher_crack, i).result()):
            print(f"utflag{{{result.split("utflag{")[1].split("}")[0]}}}")
            break

After running the script, we get the flag:

utflag{rip_dcode}