How to add a secret function for application's privacy

I've created a SMS application. However, anyone can open my application. What I want to do is to prevent unauthorized access to my application. How could I add a secret function for the privacy of my application.

Thank开发者_运维百科s.

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Create logon procedure. For instance during creation of your main Activity - check whether user logged in and if not forward to dialog/activity which will ask to log in