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interencdec

Challenge information

Level: Easy
Tags: picoCTF 2024, Cryptography, base64, browser_webshell_solvable, caesar
Author: NGIRIMANA SCHADRACK
 
Description:
Can you get the real meaning from this file.

Download the file here.

Hints:
1. Engaging in various decoding processes is of utmost importance

Challenge link: https://play.picoctf.org/practice/challenge/418

Solution

Base64-decoding

The given enc_file file contains the following:

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2024/Cryptography/interencdec]
└─$ cat enc_flag               
YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclh6ZzJhMnd6TW1zeWZRPT0nCg==

The padding characters (=) at the end reveals that this is likely base64-encoded data.

Let's decode it with base64:

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2024/Cryptography/interencdec]
└─$ cat enc_flag | base64 -d
b'd3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrXzg2a2wzMmsyfQ=='

Still base64-endoded but in python byte-format.

Another round of decoding:

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2024/Cryptography/interencdec]
└─$ echo "d3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrXzg2a2wzMmsyfQ==" | base64 -d
wpjvJAM{jhlzhy_k3jy9wa3k_86kl32k2}

Now this looks like a rotation cipher like Caesar or ROT13. The caesar cipher rotates 3 positions whereas ROT13 rotates 13 positions.

Get the flag - caesar tool solution

We can try to bruteforce the cipher with the caesar tool from the bsdgames package.
The tool uses English letter frequency statistics to crack the cipher. Install it with sudo apt install bsdgames if needed.

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2024/Cryptography/interencdec]
└─$ echo "d3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrXzg2a2wzMmsyfQ==" | base64 -d | caesar 
picoCTF{<REDACTED>}

Success, we get the flag.

Get the flag - python script solution

A more manual approach is to brute-force the cipher with a python script

#!/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

# Encrypted data after base64-decoding (twice)
enc_data = 'wpjvJAM{jhlzhy_k3jy9wa3k_86kl32k2}'

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

This also gives us the flag

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2024/Cryptography/interencdec]
└─$ ./solve.py                                                        
ROT-19: picoCTF{<REDACTED>}

and tells us that the rotation used was 19 characters.

For additional information, please see the references below.

References