Le Chiffre Indéchiffrable
🔍 Details
| Attribute | Details |
|---|---|
| Challenge Name | Le Chiffre Indéchiffrable |
| Category | Cryptography |
| Difficulty | 🟢 Easy |
| Flag | F4H{JGAH4vdvX1**********} |
📝 Description
A centuries-old encryption method once thought to be unbreakable hides a secret message.
Encript the message using a key: indecipherable
📥 Download ciphertext.txt
🧩 Hints
- Vigenère cipher is what you are looking for.
💡 Solution
The name Le Chiffre Indéchiffrable ("The Indecipherable Cipher") refers to the Vigenère cipher — a method that uses a repeating keyword to apply multiple Caesar shifts to a message.
To solve this challenge:
- Use the provided key:
indecipherable - Decrypt the contents of
ciphertext.txtusing a Vigenère decryption method. - You can use an online tool like CyberChef (opens in a new tab) or any Vigenère decoder.
📚 Insights
The Vigenère cipher is a polyalphabetic substitution cipher that uses a repeating keyword to shift letters in the plaintext by varying amounts. Unlike the Caesar cipher (which uses a fixed shift), Vigenère’s strength comes from the way it obscures letter frequency by cycling through different Caesar shifts.
Each letter of the key determines how much to shift the corresponding letter of the plaintext. For example, if the key is KEY and the message is HELLO, the encryption applies the shifts for K, E, and Y repeatedly over the message.
Though once considered unbreakable, the Vigenère cipher can now be cracked using known-plaintext attacks, frequency analysis, or simply by knowing the key — as in this challenge.