Modes of Operation Starter


The previous set of challenges showed how AES performs a keyed permutation on a block of data. In practice, we need to encrypt messages much longer than a single block. A mode of operation describes how to use a cipher like AES on longer messages.

All modes have serious weaknesses when used incorrectly. The challenges in this category take you to a different section of the website where you can interact with APIs and exploit those weaknesses. Get yourself acquainted with the interface and use it to take your next flag!


This page offers a convenient way for you to interact with the challenge functions. You can also use GET requests to send and receive data directly from the listed routes/endpoints if you wish. For more information see the FAQ.

Your aim is to recover the FLAG value. Once you have have it, submit it on the CryptoHack Symmetric Ciphers page.


from Crypto.Cipher import AES

KEY = ?
FLAG = ?

def decrypt(ciphertext):
    ciphertext = bytes.fromhex(ciphertext)

    cipher =, AES.MODE_ECB)
        decrypted = cipher.decrypt(ciphertext)
    except ValueError as e:
        return {"error": str(e)}

    return {"plaintext": decrypted.hex()}

def encrypt_flag():
    cipher =, AES.MODE_ECB)
    encrypted = cipher.encrypt(FLAG.encode())

    return {"ciphertext": encrypted.hex()}


Hex Input Only

XOR tool

Use this form to XOR two hex strings together.

Hex Encoder/Decoder

This is a convenient encoder designed for ASCII <-> Hex translations. It won't work for decoding hex to byte streams and will just show [unprintable] in that case.