picoCTF Cryptography Guide

interencdec

Name: interencdec
Description: Can you get the real meaning from this file. Download the file here.
Author: NGIRIMANA Schadrack
Tags: Easy, Cryptography, picoCTF 2024, base64, browser_webshell_solvable, caesar
Challenge from: picoCTF 2024
Files: enc_flag
Hints:
1. Engaging in various decoding processes is of utmost importance

Theory

According to the description, to get the flag we'll have to decode something from the file, and it'll be Base64 and Caesar Cypher because of the tags. So yeah nothing else to get out of that, so let's go to look at that file.

Solution

Download the file and see what it has:

shukularuni-picoctf@webshell:~$ wget https://artifacts.picoctf.net/c_titan/111/enc_flag
--2025-04-28 14:41:20--  https://artifacts.picoctf.net/c_titan/111/enc_flag
Resolving artifacts.picoctf.net (artifacts.picoctf.net)... 3.160.22.92, 3.160.22.128, 3.160.22.43, ...
Connecting to artifacts.picoctf.net (artifacts.picoctf.net)|3.160.22.92|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 73 [application/octet-stream]
Saving to: 'enc_flag'

enc_flag                                                            100%[=================================================================================================================================================================>]      73  --.-KB/s    in 0s      

2025-04-28 14:41:20 (19.1 MB/s) - 'enc_flag' saved [73/73]

shukularuni-picoctf@webshell:~$ cat enc_flag
YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclh6ZzVNR3N5TXpjNWZRPT0nCg==

Yup, that's Base64, let's use the online decoder I always use:

BASE64 DECODE (Base64Decode.com)

INPUT: YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclh6ZzVNR3N5TXpjNWZRPT0nCg==

OUTPUT: b'd3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrXzg5MGsyMzc5fQ=='

Again? But now it's wrapped around byte stuff, Idk just remove that and decode it again:

Caesar Cypher (online tool i think)

INPUT: wpjvJAM{jhlzhy_k3jy9wa3k_890k2379}

SHIFT: 7

OUTPUT:
>7 (<19) - picoCTF{caesar_d3cr9pt3d_890d2379}
<7 (>19) - dwqcQHT{qosgof_r3qf9dh3r_890r2379}

There we go! That's the flag.

I rated this level as "good"! :3