Posted by Tim Bray
Most Android apps have some sort of server-side back end, to persist and share data. Even the most basic game needs to remember its players’ high scores. When you’re building your back end, one problem you have to solve is how the back-end code knows what app it’s talking to and who the person using it is.
You probably have HTTP endpoints for communicating with your client apps, but how can the server-side code be sure who’s sending messages to it? After all, anyone can send HTTP POST requests from anywhere; could they impersonate your users if they could guess their identities?
It’s really user-unfriendly to ask people to type in usernames and passwords on mobile devices. In particular, if someone has installed your app and given it permission to use the Internet and know your identity, they shouldn’t be pestered any more.
It turns out that Google Play services, now available on every compatible device running Android release 2.2 or higher, offers a good solution to this problem, based on the use of Google Accounts.
Doing this is a multi-step process, which I’ll outline in full, but here’s the short version: You use the
GoogleAuthUtil class, available through Google Play services, to retrieve a string called an “ID Token”. You send the token to your back end and your back end can use it to quickly and cheaply verify which app sent it and who was using the app.
This capability is built into Google facilities such as App Engine’s new Cloud Endpoints feature, which bakes app/back-end identity into a simple programming model.
Now let’s get to the details.
You’re going to have to use the Google API Console quite a bit in this process. You’ll need to make a new project for this purpose; while you can give it a nice human-readable name and graphical branding, it turns out that those resources aren’t used in this particular scenario.
You can also authorize this project to access a large number of different Google APIs; but once again, you don’t need to in this scenario.
You should give serious thought to the people you autho...