function add($userName, $password, $salt = null)
    {
        if ($salt !== null) {
            $key = \Thruway\Common\Utils::getDerivedKey($password, $salt);
        } else {
            $key = $password;
        }

        $this->users[$userName] = ["authid" => $userName, "key" => $key, "salt" => $salt];
    }