It should be said that all software must be signed before it can be used. There is no signature, which means that the software has only been developed and has not passed the formalities of putting it on the shelves. The function of signature is simply to tell the system that the program has been approved by the signer, who is responsible for its security, and the system issues the corresponding behavior of the program according to the level of signature.

Signatures are usually divided into three categories:

1. System-level signature: The program is completely recognized by the system and can realize all functions, including actions involving sensitive permissions.

2. Author-level signature: Some permissions will be restricted, and behaviors that do not affect system security will be released.

3. Development signature: It is usually only used for testing and can only be run on the specified equipment.

The higher the level, the more complicated and strict the application process of the signed document is, and the stricter the audit of the program is, thus ensuring the security and controllability of the software in the process.