'Factory method for creating the cryptographically bound
safe cookie data for the session and the user.
Raises SafeCookieError if session_id is None.'

@classmethod
def create(cls, session_id, user_id):

    cls._validate_cookie_params(session_id, user_id)
    safe_cookie_data = SafeCookieData(cls.CURRENT_VERSION, session_id, key_salt=get_random_string(), signature=None)
    safe_cookie_data.sign(user_id)
    return safe_cookie_data
