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Mod 26

Challenge information

Points: 10
Tags: picoCTF 2021, Cryptography
Author: PANDU

Description:
Cryptography can be easy, do you know what ROT13 is? 

cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_GYpXOHqX}

Hints:
1. This can be solved online if you don't want to do it by hand!

Challenge link: https://play.picoctf.org/practice/challenge/144

Solution

There are several ways to solve this challenge and here are some of them.

CyberChef solution

As the hint suggested you can use an online site such as CyberChef and use the 'ROT13' recipe.

Enter 'rot13' in the Operations search bar, then drag and drop it to the Recipe.
Copy the scrambled flag to the Input pane and press BAKE.

Use a rot13 commandline tool in Linux

There are at least two sets of packages that contains prepacked rot13 tools:

Install them with either sudo apt install hxtools or sudo apt install bsdgames.

The tool from hxtools installs as /usr/bin/rot13 and is a script that invokes the tr command more or less as described below.

The tool from bsdgames installs as /usr/games/rot13 and calls the caesar tool (which is also included in the package) but with a rotation of 13.

After one of these tools have been installed you can run

┌──(kali㉿kali)-[~]
└─$ echo "cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_GYpXOHqX}" | rot13
picoCTF{next_time_<REDACTED>}

Use the tr tool in Linux

Alternatively, you can use the tr tool to "manually" do the decoding

┌──(kali㉿kali)-[~]
└─$ echo "cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_GYpXOHqX}" | tr 'A-Za-z' 'N-ZA-Mn-za-m'
picoCTF{next_time_<REDACTED>}

For additional information, please see the references below.

References