Tags: dcode crypto
Rating:
## 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](https://en.wikipedia.org/wiki/Caesar_cipher).
## 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.
```py
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}
```
