Pivot.md

Categorie: Cryptography

Author: r3s0lv3r

Solve: 16/20

Points: 70 pts (at first)| 55 pts (at end)

File : message.txt

Write-up by: Gojo

Description :

Wfno iuum yy fcalzfz odb qiefoi! Jwwqcmgiaqkahhn, wnu'xh ztswlupp wi! Lay wkqves lw IAN_npobrt_kzx_xemfdr_tlhirl_gwqwhdo. Cxyk vmtr ylxx hjxee lcj rk gjnb arbkq ykhanbd nkpyk! :).

Solution :

Fr Version :

For ENG version scroll down

Pour ce chall, nous pouvons voir dans la description que la partie IAN_ semble correspondre à la partie CTF de plus le nom du chall nous donne un indice, il s'agit d'un pivot, la place d'un caractère a été remplacée par un autre. Apres quelque recherche, nous voyons que c'est du Vigenère. Nous écrivons donc un script, vous faire le travail

def dechiffrer_vigenere(message_chiffre, cle):
    message_dechiffre = ""
    cle_repeated = (cle * (len(message_chiffre) // len(cle))) + cle[:len(message_chiffre) % len(cle)]
    for i in range(len(message_chiffre)):
        if message_chiffre[i].isalpha():
            decalage_message = ord(message_chiffre[i].upper()) - ord('A')
            decalage_cle = ord(cle_repeated[i].upper()) - ord('A')
            lettre_dechiffree = chr(((decalage_message - decalage_cle) % 26) + ord('A'))
            if message_chiffre[i].islower():
                lettre_dechiffree = lettre_dechiffree.lower()
            message_dechiffre += lettre_dechiffree
        else:
            message_dechiffre += message_chiffre[i]
    return message_dechiffre

# Exemple d'utilisation
message_chiffre = "Wfno iuum yy fcalzfz odb qiefoi! Jwwqcmgiaqkahhn, wnu'xh ztswlupp wi! Lay wkqves lw IAN_npobrt_kzx_xemfdr_tlhirl_gwqwhdo. Cxyk vmtr ylxx hjxee lcj rk gjnb arbkq ykhanbd nkpyk! :)."
cle = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
message_dechiffre = dechiffrer_vigenere(message_chiffre, cle)
print("Message déchiffré:\n\n", message_dechiffre)

Et nous obtenons ceci

Ξ ~/Downloads → python3 vigenere.py 
Message déchiffré:

 Well done on solving the riddle! Congratulations, you've unlocked it! The answer is CTF_decode_the_cipher_riddle_xmfkupz. Keep your wits about you as more brain teasers await! :).

---

Eng Version :

For this chall, we can see in the description that the IAN_ part seems to correspond to the CTF part plus the name of the chall gives us a clue, it is a pivot, the place of a character was replaced by another. After some research, we see that it is Vigenère. So we write a script, you do the work

def dechiffrer_vigenere(message_chiffre, cle):
    message_dechiffre = ""
    cle_repeated = (cle * (len(message_chiffre) // len(cle))) + cle[:len(message_chiffre) % len(cle)]
    for i in range(len(message_chiffre)):
        if message_chiffre[i].isalpha():
            decalage_message = ord(message_chiffre[i].upper()) - ord('A')
            decalage_cle = ord(cle_repeated[i].upper()) - ord('A')
            lettre_dechiffree = chr(((decalage_message - decalage_cle) % 26) + ord('A'))
            if message_chiffre[i].islower():
                lettre_dechiffree = lettre_dechiffree.lower()
            message_dechiffre += lettre_dechiffree
        else:
            message_dechiffre += message_chiffre[i]
    return message_dechiffre

# Exemple d'utilisation
message_chiffre = "Wfno iuum yy fcalzfz odb qiefoi! Jwwqcmgiaqkahhn, wnu'xh ztswlupp wi! Lay wkqves lw IAN_npobrt_kzx_xemfdr_tlhirl_gwqwhdo. Cxyk vmtr ylxx hjxee lcj rk gjnb arbkq ykhanbd nkpyk! :)."
cle = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
message_dechiffre = dechiffrer_vigenere(message_chiffre, cle)
print("Message déchiffré:\n\n", message_dechiffre)

And we get this

Ξ ~/Downloads → python3 vigenere.py 
Message déchiffré:

 Well done on solving the riddle! Congratulations, you've unlocked it! The answer is CTF_decode_the_cipher_riddle_xmfkupz. Keep your wits about you as more brain teasers await! :).