On 2/1/2018 at 4:43 AM, Octane_Test said:

User ID Requirements:

 1. Independent of the device's platform
 2. Offline implementation (no communication with any servers)
 3. Without sign-up process

On 2/1/2018 at 4:43 AM, Octane_Test said:

EDIT:

There are lot of responses to use UUID/GUID. Generally, this approach looks fine but I am looking for a solution which can generate same User ID even if the user reinstall the game.

I think with your edit you are completely out of luck with these requirements.  Cannot use any device based ID (MAC address, UUID/GUID, etc), no communication to servers to check for existing/duplicate IDs (which eliminates #3 as an option anyway) and now you want it to generate the same User ID even after a reinstall so anything date/time based is also out.

On 2/1/2018 at 4:43 AM, Octane_Test said:

There are lot of responses to use UUID/GUID. Generally, this approach looks fine but I am looking for a solution which can generate same User ID even if the user reinstall the game.

If you want to deal gracefully with cases such as the user migrating to a new phone, then you are absolutely going to need to integrate with platform-specific APIs. Both Apple and Google seem to recommend generating a GUID on first install, and then storing it via their platforms' respective secure storage APIs, so that it gets backed up via the cloud and transferred to new devices.