I want to develop an Android Application that allows users to sign in with their Google Accounts (i think it's always an email address) instead of forcing the user to create a new one. I have 3 questions regarding this:
When the user starts the application for the first time i'll have to use the AccountManager, right? If the user has only one com.google account i'll use that one, if not, i'll ask him which one he wants to use. My question is, can i be sure that every com.google account in the AccountManager has been properly authenticated and the application can be sure that the user is who he says he is so that i don't have to ask him to choose the account every time the application launches?
The application will have a server that will store on a database what restricted content the user has unlocked, that's why it needs the Google Account, to match the foreign key of the user with the foreign key of the content to know what that user has unlocked. How do you suggest i save the Google Account in the database as the user table's primary key? I could store the email of the Google Account, but isn't there any privacy problems with that? What else can i use? I suppose there is no Facebook ID-like integer value for Google Accounts.
Do i need to use OpenID or oAuth 2.0 for any of these operations i've mentioned? I'm asking this because when the application doesn't have internet connection i want the user to still be able to access the restricted content he unlocked and previously downloaded into the phone. If i use OpenID that requires internet connection right? So the user shouldn't be able to enter the application and that's not what i want.
Thanks
Using the account manager to check which accounts are available on the phone and ask the user which account he want to use sounds like a good idea. I think it's a good idea to use OAuth 2.0 and grab the OAuth 2.0 access token for userinfo in your client application (https://www.googleapis.com/auth/userinfo.profile is probably the scope you want to use) and send this to your server when the client communicates with the server. Then using the userinfo Google API your server can use the access token to make sure the user is who he claims to be. With this api you can get hold of the user id which you can use instead of the user's email.
Getting the auth token will require internet access, but since it's only needed when you communicate with your own server you can grab the token at that time.
I'm not 100% sure if the accounts given by the AccountManager can be trusted, but it will provide you with the google accounts available on the phone and I don't know of any way of adding an account to the phone without having access to the account. If this was possible it would be a really big security issue as well so I think you can trust that the accounts given by the AccountManager are authentic.
When you get the OAuth 2.0 token on the client I suggest you use the Google Play's GoogleAuthUtil instead of the account manager if it's possible. For more details on this see: In a nutshell what's the difference from using OAuth2 request getAuthToken and getToken