If your Android apps use a server-side back end to store and share data, you can take advantage of the auth APIs in Google Play services to let your back end know which app is calling and for which user.

The post below explains how an app can generate an ID token without having to ask for username and password, how it sends the token to the back end, and how the back end verifies it. The post includes example back-end code to get you started.
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