How to sync firebase users across several locations? (extension + website) - firebase

Question

I'm working on chrome extension (provides main functionality) and the complementary website (mostly profile and billing related functionality) both backed with firebase backend.
I'm wondering if it's possible to implement the below scenario:
user logs in with the extension using firebase authentication (with firebaseUI lib)
I store a token that I can use to reauthenticate that user (is there such a token?)
when user opens the website, I login that user automatically with the token.
While both the extension and the website has their login/signup forms I'm wondering if it's possible to login user in the extension and to somehow automatically login that same user on the website so they don't have to enter their credentials twice?
So far I was hoping that I could use something like below:
firebase.auth().currentUser.getIdToken(true).then(function(idToken) {
console.log("idToken = ", idToken)
})
And then to use that idToken like this, since if I understand correctly, it's an AWT:
firebase.auth().signInWithCustomToken(idToken).catch(function(error) {
// Handle Errors here.
var errorCode = error.code;
var errorMessage = error.message;
console.log("signInWithCustomToken: error = ", error)
})
But it gives the following error:
code: "auth/invalid-custom-token"
message: "The custom token format is incorrect. Please check the documentation."
I can parse the token on https://jwt.io/ which shows all the user information but in the end it says "invalid signature"
So I guess this token can be only used to check authentication (like admin.auth().verifyIdToken(idToken)) but not to login user. Am I right?
I know I can create a custom token, but is there any straightforward way to workaround that and to login user from one place only using firebase funstionality? (of course without storing username/password)

Answer 1

You can't sign in with a Firebase ID token. What you can do is the following:
Keep the user session in the chrome extension and run all authenticated requests from there. Use postMessage (with origin verification) to talk with extension from app anytime a request is to be sent. With this you don't have to worry about session synchronization and no Firebase tokens are stored or passed to the web app or every web app that can access the extension.
Add a postMessage API to get an ID token from the extension after verifying the origin of the request. You can then send the request from the web app with the ID token. (less secure than 1 but easier to implement and session is stored in one place).
Create an HTTP endpoint that takes an ID token and returns a custom token. This would verifyIdToken and then create a corresponding custom token for that user using createCustomToken provided by Admin SDK. You then postMessage that from chrome extension to the web page after verifying origin and signInWithCustomToken with that custom token in that web app. This is the least secure as you are providing an endpoint to exchange a short lived ID token with an indefinite session. You will also deal with synchronization issues (user signs out from chrome extension, you have to sign out from websites, etc).