title: Google Sign-In backend server authentication
body: <p>I'm writing an Android app for voice chatting and decided to use Google Sign-In for a simple user authentication with my backend server. However, I don't understand how the app should authenticate with my backend. When a user signs-in using his Google account and I receive the ID token, I can send the ID token to the server, then the server verifies it. And what's then? How to authenticate following requests, for example when the user sends/receives a voice message and the app needs to upload/download the message to/from the server? Server needs to know which user is making the request, but the ID token is inappropriate because it expires soon and its integrity verification is a complex and relatively long process.</p>