ZoTrus Application Security Cloud Service is an innovative cloud service that uses digital signature and encryption to ensure the security of application software. It realizes the always verification for the digital signature of application software to ensure that the source of application software is trust, thereby ensuring the security of software operation. At the same time, for IoT devices that need to download updated software, a trusted https encrypted channel must be used to download the update package, and the digital signature and timestamp signature of the update package must be verified. Only software updates with trusted digital signature and timestamp signature are installed. It can effectively ensure the security of OTA upgrade of IoT devices and prevent attacks and extortion events in software upgrades.