Firebase Auth across multiple projects with different Providers - firebase

Question

I am looking for the Firebase to be authenticated seamlessly across multiple projects with different auth providers.
Example.
Suppose I have a website with 10 pages, with different data coming from 2 firebase projects ( project-1, project-2)
project-1 contains all the Users accounts
When I sign up/Login into firebase project (project-1). I am using Google/Phone/Password/Facebook. this creates users account if not exists.
Now I can see the data coming from the firebase (project-1).
Now few of my pages website want to access data from the project-2.
How to maintain the authentication state across the 2 projects so that, project-2 will get authenticated seamlessly with out prompting signup again. Probably with the same auth token which already created for project-1
I read this blog which was created by Ian Barber Working with multiple Firebase projects in an Android app. Which is only discussed about the google and android case only.
Thanks.

Answer 1

You can use a combination of firebase-admin on a server and a custom firebase token as described here.
Essentially, you can get a user's JWT token for your first app using the methods described in this answer. Then you could make a request to your own server that would validate that your JWT token originated from your first project and could then issue you a second JWT token you can use to authenticate with the second project.

Answer 2

I think you can do the following:
Configure an OIDC provider on project-2 that points out to the firebase auth from project-1 .
The issuer should be https://securetoken.google.com/<project-1-id>
The client id its the project-1-id
After you signIn on project-1 you can use the idToken from that app, to login with an OAuthProvider to the second app (project-2):
const provider = new OAuthProvider('oidc.project-1');
const credential = provider.credential({
idToken: 'idTokenFromProject1',
});
signInWithCredential(auth, credential)
idTokenFromProject1 can be fetched through getIdToken() method on the project-1 app

Answer 3

This is now easily possible with OIDC configuration (reference answer by Alenjadro Barone above )
Configure an OIDC provider on project-2 that points out to the firebase auth from project-1 .
The issuer should be https://securetoken.google.com/<project-1-id>
Then You can follow Code below . Its starter but definitely works : -
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/foundation.dart';
// Create secondary firebase app Instance
FirebaseApp secondaryApp = Firebase.app('secondary');
class Auth {
// default Firebase Auth app instance: PROJECT 1
final firebaseAuth = FirebaseAuth.instance;
// Secondary Firebase App Instance : PROJECT 2
FirebaseAuth secondaryFirebaseAuth =
FirebaseAuth.instanceFor(app: secondaryApp);
signin() async {
// Sign in method of your choice from PTOJECT 1
await firebaseAuth.signInWithEmailAndPassword(
email: "abc#abc.com",//registered user email
password: "password",//registered user password
);
// Declare OAuth Provider for PROJECT 2
// name of OIDC provider as set in Project
final provider = OAuthProvider('oidc.provider');
try {
// get IdToken for signedin user
final userIdToken = await firebaseAuth.currentUser!.getIdToken();
// Get access Token for User using IdTokenResult
final idTokenResult = await firebaseAuth.currentUser!.getIdTokenResult();
final userAccessToken = idTokenResult.token;
// Create OAuthCredentials with idToken & AccessToken
final credential = provider.credential(
idToken: userIdToken,
accessToken: userAccessToken,
);
// Sign in to Secondary app or PROJECT 2
await secondaryFirebaseAuth.signInWithCredential(credential);
} catch (e) {
if (kDebugMode) {
print("This is error $e");
}
}
}
}