02. Mod 26 - PicoCTF 2021 Writeup

Category: Cryptography | Author: Pandu | Points: 10

Hello All 👋,

This challenge is also quite an easy one; in fact, the hint says it all. Before moving forward, a quick note on cryptography.

Sometimes, it is not a good idea to leave things visible in plain sight. Imagine you want to send a message to your friend. But you don't trust the person who's taking the message to your friend. In a case like this, you and your friend can come to an agreement on a way to modify the information so that only you two can make sense of what it reads. A simple way would be to replace all the letter 'a's with the letter 's' and vice versa. The requirements are that the output can be reverted back to the original information and that this can only be done by someone who knows the 'key' to turn it back to the original information.

This is the main gist of what cryptography does: turn useful information into a controlled mess, which can only be reverted back by a key that turned it into this mess. Now anyone with the key is the only one who can read the information.

In our challenge, this is the same thing we are given. The text in red is the encrypted data. The first thing to do is to identify what the encryption method is. The hint tells us this: ROT13. A quick search on google tells us that ROT13 is a shift cypher where the alphabet is divided into two sections and the ith letter of each one is replaced by the other.

So, in order to decrypt the string, we do the same thing; replace each letter with its corresponding letter!

That's it! We have our flag!