How to validate an authentication token against firebase?

Question

I don't mean custom authentication with firebase. What I need is slightly different from that custom authentication that generates tokens in application server and allows access in firebase. Actually, I'm trying to authenticate in firebase with e-mail and password , for instance, and with that authentication be able to access restful services in some application server. Is this possible ? I think that in some way an token could be sent to application server after firebase authentication and that server would validate the auth token against firebase.

Client --------authenticates ------->> Firebase
Client <<--------auth token ---------- Firebase
Client --------- sends ------------->> Application server (NodeJS)
App Server ------- validates (auth token) ---->> Firebase

Thanks in advance.

Answer 1

Client --------authenticates ------->> Firebase

import firebase from 'firebase/app';
import 'firebase/auth';
import 'firebase/firestore';

const googleAuthProvider = new firebase.auth.GoogleAuthProvider();
const auth = firebase.auth();

const authenticates = await auth.signInWithPopup(googleAuthProvider).then(user => user).catch(err => err)

Client <<--------authtoken ---------- Firebase

you will get data from authenticates response

authtoken = authenticates.credential.idToken
email = authenticates.user.email
...

Client --------- sends ------------->> Application server (NodeJS)

const sends = await axios({
    method: 'post',
    url: `${API_BASE_URL}/request`,
    headers: {
        'Authorization': `Bearer ${authtoken}`,
    },
    data: {
        from: next_cursor,
        size: next_cursor + 100,
    }
});

App Server ------- validates (auth token) ---->> Firebase

We will have app_oauth2_client_id when we initialize firebase authentication

import { OAuth2Client } from 'google-auth-library';

const oauth2Client = new OAuth2Client(process.env.app_oauth2_client_id);

function verifyOauth2Token(token) {
  const ticket = await oauth2Client.verifyIdToken({
    idToken: token,
    audience: [process.env.app_oauth2_client_id]
  });
  return ticket.getPayload();
}

const tokenInfo = await verifyOauth2Token(token);

for tokenInfor

{
  iss: 'accounts.google.com',
  azp: '671303332471-5n8014rorllmd09n8mmadesc2qidpda5.apps.googleusercontent.com',
  aud: '671303332471-5n8014rorllmd09n8mmadesc2qidpda5.apps.googleusercontent.com',
  sub: '100037911230177975416',
  email: '[email protected]',
  email_verified: true,
  at_hash: '3rxsMOftrr9NZWlBkYznuQ',
  iat: 1635842823,
  exp: 1635846423
}