public function get_crypto_key($key)
    {
        // Bomb out if the requested key does not exist
        if (!array_key_exists($key, $this->prop) || empty($this->prop[$key])) {
            rcube::raise_error(array(
                'code' => 500, 'type' => 'php',
                'file' => __FILE__, 'line' => __LINE__,
                'message' => "Request for unconfigured crypto key \"$key\""
            ), true, true);
        }

        return $this->prop[$key];
    }