Problem Writeup: Mod 26 - CTF 2

Category: Cryptography

Description

“Cryptography can be easy, do you know what ROT13 is? cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_uJdSftmh}”

Approach

This problem introduces us to the world of cryptography. We will explore the ancient Caesar cipher and learn how to use CyberChef, a very powerful tool we will be using a lot in our CTF problems.

Encryption

The problem description includes a string of seeming gibberish: cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_uJdSftmh}. Looking at the string more closely, we notice that it looks to be in the general format of a CTF flag; in fact, the first six letters could be an encrypted form of “picoCTF”.

Additionally, the problem description asks if we know what ROT13 is. A quick Google search tells us that ROT13 is a simple shift cipher, where every letter of the alphabet is shifted a certain number of places to the right to create an encoded message:

Input	ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
Output	NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm

Let’s begin, then, by assuming that the given string is the CTF flag encrypted by ROT13.

Cyberchef

To decrypt the string, we can write out the message one character at a time–or we could use a computer to speed up this task. There are plenty of ROT13 decrypters online, but we will use a website called cyberchef.com for this.

Copy the encrypted string and go to cyberchef.com. Search for ROT13 in the Search bar on the CyberChef website, and drag the ROT 13 box into the Recipe space. Then paste the encrypted string into the Input box, and immediately the decrypted string appears in the Output box.

You should bookmark the CyberChef website because we will return to it soon.