Computes the authenticator field (RFC 2865 - Section 3)

def compute_authenticator(self, packed_request_auth, shared_secret):
        """
        Computes the authenticator field (RFC 2865 - Section 3)
        """

        data = prepare_packed_data(self, packed_request_auth)
        radius_mac = hashlib.md5(data + shared_secret)
        return radius_mac.digest()