Migrating from Google Sign-In for Android to Firebase Authentication

Currently, we plan to use Google Sign-In for Android, as our server authentication method.

This is what we plan to do.

Client side (Google Sign-In for Android)

GoogleSignInAccount account = completedTask.getResult(ApiException.class);
// This idToken will sent to backend server.
String idToken = account.getIdToken();

Server side (Google Sign-In for Android)

// Based on received idToken from client, backend server will call https://www.googleapis.com/oauth2/v3/tokeninfo?id_token=... 
// to identify who is this user.

{
 // These six fields are included in all Google ID Tokens.
 "iss": "https://accounts.google.com",
 "sub": "110169484474386276334",
 "azp": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "aud": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "iat": "1433978353",
 "exp": "1433981953",

 // These seven fields are only included when the user has granted the "profile" and
 // "email" OAuth scopes to the application.
 "email": "testuser@gmail.com",
 "email_verified": "true",
 "name" : "Test User",
 "picture": "https://lh4.googleusercontent.com/-kYgzyAWpZzJ/ABCDEFGHI/AAAJKLMNOP/tIXL9Ir44LE/s99-c/photo.jpg",
 "given_name": "Test",
 "family_name": "User",
 "locale": "en"
}

---

In the future, we might want to migrate to provide more login option. This is my future migration plan, to migrate from Google Sign-In for Android to Firebase Authentication.

Client side (Firebase Authentication)

FirebaseUser mUser = FirebaseAuth.getInstance().getCurrentUser();
mUser.getIdToken(true)
    .addOnCompleteListener(new OnCompleteListener<GetTokenResult>() {
        public void onComplete(@NonNull Task<GetTokenResult> task) {
            if (task.isSuccessful()) {
                // This idToken will sent to backend server.
                String idToken = task.getResult().getToken();

            } else {
                // Handle error -> task.getException();
            }
        }
    });

Server side (Google Sign-In for Android)

# idToken comes from the client app (shown above)
decoded_token = auth.verify_id_token(idToken)
uid = decoded_token['uid']

---

My questions are

1. For Google Sign-In for Android, We plan to store "sub": "110169484474386276334", as an unique identifier to represent a user. Is that a correct field to be used? Is it unique per user? My testing so far is, at client side, we might get different idToken for a same user (At different day). Different idToken from same user, will still yield same sub at server side.

2. One day, we might migrate to Firebase Authentication to support more login methods. Is it still backward compatible with Google Sign-In for Android. Is Firebase Authentication able to return same "sub" as what is previously returned by Google Sign-In for Android? As you can see in the code example, Firebase Authentication is returning uid.

How can I compare new Firebase Authentication's uid, with previous stored Google Sign-In's sub?

from Migrating from Google Sign-In for Android to Firebase Authentication