The Last Dance

The code is using ChaCha20 but is using the same key and nonce. So I’ll perform the following:

• Get the encoded data from out.txt
• Calculate the XOR between both texts

from binascii import unhexlify
 
# Known original message (M1)
known_message = (b"Our counter agencies have intercepted your messages and a lot "
                 b"of your agent's identities have been exposed. In a matter of "
                 b"days all of them will be captured")
 
# Load data from "out.txt"
with open("out.txt", "r") as f:
    iv = unhexlify(f.readline().strip())  # Convert IV from hex to bytes
    ciphertext1 = unhexlify(f.readline().strip())  # Encrypted message
    ciphertext2 = unhexlify(f.readline().strip())  # Encrypted FLAG
 
# Compute the keystream XOR between both ciphertexts
keystream_xor = bytes(a ^ b for a, b in zip(ciphertext1, known_message))
 
# Decrypt the FLAG
flag = bytes(a ^ b for a, b in zip(ciphertext2, keystream_xor))
 
print("FLAG:", flag.decode(errors="ignore"))  # Decode while ignoring errors

python3 decryptor.py
 
#HTB{und3r57AnD1n9_57R3aM_C1PH3R5_15_51mPl3_a5_7Ha7}