Introduction

The challenge was part of the NorthSec 2018 edition. This write-up has also been shared on the ctf repository, which seems to be slowly replaced by ctftime.org

• Category: forensic
• Points: 3
• Description: Can you decrypt the message on that picture?

Write-up

For this challenge, you had to download the font displayed in the image and decrypt the message. Here is a table for the translation

After a lot of pain, and three eye surgeries… We resolve with some cryptic message that is spaced a bit like a normal language.

tivvgrmt hglk gsv urihh szou lu
gsv uozt rh gsv mfnyvi gdl
nfmwivw zmw hvevm ulooldvw yb
gsv gdvougs ovggvi lu gsv
zokszyvg. hglk mvcg rh gsv
mfnyvi 3 zmw gsvm gsv hrcgs
ovggvi lu gsv zlkszyvg ulooldvw
yb gsv mrmvgvvmgs ovggvi hglk
zoo oldvixzhv

You can see some pattern in the language, maybe some kind of permutation like in rot13. But it yielded gibberish for any kind of permutation.

You can decode it using an atbash decipher to get the message that lets you create the flag:

greeting stop the firss half of
the flag is the number two
mundred and seven followed by
the twelfth letter of the
alphabet. stop next is the
number 3 and then the sixth
letter of the aophabet followed
by the nineteenth letter stop
all lowercase

I think they obviously left some errors in the text, this font being very difficult to read. That didn’t make me doubt and after following the instructions, it gave the flag 207l3fs.

The Atbash Cipher simply reverses the plaintext alphabet to create the ciphertext alphabet. It was a substitution cypher used apparently to encrypt hebrew alphabet.