Decrypt FortiGate configuration secrets CVE-2019-6693

fortigate_decrypt.py

#!/usr/bin/env python3
from Cryptodome.Cipher import AES
import base64
import sys

key = b'Mary had a littl'

data = base64.b64decode(sys.argv[1])

iv = data[0:4] + b'\x00' * 12
ct = data[4:]

cipher = AES.new(key, iv=iv, mode=AES.MODE_CBC)
pt = cipher.decrypt(ct)
print(pt)

Can you help me decrypt a key I am not familiar with python?

what?