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rotation.md

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rotation

Challenge information

Points: 100
Tags: picoCTF 2023, Cryptography
Author: LOIC SHEMA
 
Description:
You will find the flag after decrypting this file

Download the encrypted flag here.

Hints:
1. Sometimes rotation is right

Challenge link: https://play.picoctf.org/practice/challenge/373

CyberChef solution

Open the file in CyberChef and use the 'ROT13' recipe.
The default rotation is 13 steps. Change the amount until you find the flag. The correct amount is 18.

Python solution

Even though it takes a bit longer time it's more fun to write a small python script called solve.py to bruteforce the challenge.

#!/usr/bin/python

import string

alphabet = string.ascii_lowercase
alpha_len = len(alphabet)

def shift(cipher_text, key):
    result = ''
    for c in cipher_text:
        if c.islower():
            result += alphabet[(alphabet.index(c) + key) % alpha_len]
        elif c.isupper():
            result += alphabet[(alphabet.index(c.lower()) + key) % alpha_len].upper()
        else:
            result += c
    return result

# Read the encoded flag
with open("encrypted.txt", 'r') as fh:
    enc_flag = fh.read().strip()

for i in range(1, alpha_len+1):
    plain = shift(enc_flag, i)
    if ('picoCTF' in plain):
        print("ROT-%02d: %s" % (i, plain))

Then make the script executable and run it

┌──(kali㉿kali)-[/picoCTF/picoCTF_2023/Cryptography/rotation]
└─$ chmod +x solve.py

┌──(kali㉿kali)-[/picoCTF/picoCTF_2023/Cryptography/rotation]
└─$ ./solve.py
ROT-18: picoCTF{<REDACTED>}

For additional information, please see the references below.

References