How to create a parse _User account from a Android Google token?

Note: This answer does not apply to the Open Source Parse Server, as it uses revocable sessions only. Check out parse-server/issues/1392 for further update

Update (Jan-2016):

You need to turn off Revocable Session in order to call getSessionToken on Parse.User. Go to App Settings >> Users >> Turn off Require revocable sessions. This is not new in 2016, but at the time of giving answer, the author did not know of this change.

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I will break into 2 cases for easier to follow: New User and Returning User.

1. New User

The flow is as below:

1. User authorizes and a token is acquired
2. We create a new user with a random password

You can create a ParseUser using following code inside the newChooseAccountIntent() method that return email.

ParseUser user = new ParseUser();
user.setUsername(mEmail);
user.setPassword(randomPassword);
user.setEmail(mEmail);
user.signUpInBackground(new SignUpCallback() {
  public void done(ParseException e) {
    if (e == null) {
      // Hooray! Let them use the app now.
    } else {
      // Sign up didn't succeed. Look at the ParseException
      // to figure out what went wrong
    }
  }
});

2. Returning User

This is the where most of people stuck, as I researched over the Internet. The flow is as below:

1. User authorizes and the app gets a token
2. We pass this token to Cloud Code to validate. We need to check if this token is signed by Google and if it is meant for us (android-developers (2013)).
3. After you can verify that the token is valid, you can query for the user in Cloud Code using Parse.Cloud.useMasterKey() method and return the session key by using getSessionToken() method on the query result.
4. Use the session key to save login state on disk by calling becomeInBackground method

To validate the token, you can send Parse.Cloud.httprequest to this endpoint: https://www.googleapis.com/oauth2/v3/tokeninfo?access_token=. This is instructed in Google Identity Documentation. You will receive data as below:

{
 "iss": "https://accounts.google.com",
 "sub": "110169484474386276334",
 "azp": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "email": "[email protected]",
 "at_hash": "X_B3Z3Fi4udZ2mf75RWo3w",
 "email_verified": "true",
 "aud": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "iat": "1433978353",
 "exp": "1433981953"
}

Things need to compare are "aud", "azp" and "email" which are translated as audience, authorized party and email.

To query for the current user on Cloud Code:

var query = new Parse.Query(Parse.User);
query.equalTo("email",mEmail);
query.first({
  success: function(user) {
    // Use user..getSessionToken() to get a session token
  },
  error: function(user, error) {
    //
  },
  useMasterKey: true
});

Note: Make sure you have following scope so that the email will show up when you check on Cloud Code: https://www.googleapis.com/auth/plus.profile.emails.read