How do I authenticate/validate an Android app on an App Engine Server with OAuth 2?

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http://stackoverflow.com – I have a simple application that lets a user draw pictures. There are Android, IOS, and web-based versions. I also let users store their pictures on our App-engine servers and I want them to be able to collaborate with other users. I want to use Google accounts for authentication and the basis of some permission scheme. I do not know how to validate/authenticate a user’s Google account on android (or IOS). I am hoping somebody can help or point me in the right direction. Here is what I understand so far: On the Web-based client, I just use Google-web toolkits UserService. However for my a (HowTos)