消息解密

$decrypt = $app->encryptor->decryptData($ciphertext, $iv, $session_key);

详见https://smartprogram.baidu.com/docs/develop/api/open_log/#%E7%94%A8%E6%88%B7%E6%95%B0%E6%8D%AE%E7%9A%84%E7%AD%BE%E5%90%8D%E9%AA%8C%E8%AF%81%E5%92%8C%E5%8A%A0%E8%A7%A3%E5%AF%86/