CTFs | 404CTF_2024 | Steganographie | L'absence

L’absence

Here we are given the following text:

bonsoir, désolé pour le déranGement. je n'ai pas pu Y aller hier pour l'épreuve de barres asyMétriques. désolé si je N'ai pas été à lA hauteur de voS attenTes, je feraIs mieux aux épreuves publiQUes de dEmain. 


bises.
franciS vigenere.

ps :
Kl qsfwm, r'qc hm s'ynfefmmh wej rc peahxik xi eg lmgigg i uni voqevmmem fuv vkq srnk jcy psmryurnl yiyli hkppee ehv fuck ! Syuf ahkmi orw rmztuw kmsbijifq, w'aa xvvcr ha jq eelkwkpij. Rc hbiub : 404KJZ{RwBmxrzHtaBywVxybramqAlj}

As we can see, there are a lot of uppercase letters and the person that sent the letter is franciS vigenere. This helps us guess that the PS section is encrypted using the Vigenere cipher.

The basic idea of the Vigenere cipher is that we are going to do a shift of letter that is based on the key. But we don’t have any key here??? Don’t we ?

I split the clear text into a file called “message” and ran the following code to get the key (concatenation of all the uppercase letters):

f = open("message")
key = ""

for letter in f.read():
	if letter.isupper():
		key += letter
print("[+] KEY : ", key)
f.close()

We get:

Now we can use online tool to decode the message:

So as we can see, the flag is 404CTF{NeVolezPasLesDrapeauxSvp}.