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I did some research and experiments on integrating LINE login with Firebase Auth using Flutter. I have some questions:

  1. Looks like in Firebase Auth, there are AuthProvider, and in Flutter source code, I also found an OAuthProvider. There you can create Credential to include your idToken and access token. But I don't know how to specify the providerId in that Credential. I guess it is not possible, because Firebase hasn't integrate LINE login. The client side api: logInWithCredential can ONLY work with Firebase supported login methods, and you have to enable them in your Firebase console. Am I right?

  2. So it looks like now I have to setup my own server to exchange LINE access token to Firebase custom token. In my server, I first verify access token and grab the LINE user profile, then I create a custom token, but there I have to decide a UID, which I have to use some pattern like LINE:${LINE_UID}. This looks like some hacking, is there a better way?

  3. Admin API to create custom token only accept UID or optionally a user claim, I have no way to set its display name or some other basic info. So if I directly send the token with UID like LINE:${LINE_UID} to a client, then the client logInWithToken, it will create a user without display name if it doesn't exist. The only workaround I can image is, in the server-side, generate the LINE:${LINE_UID} and look up it in Admin API, if it doesn't exist, then I create a user with a proper display name. This looks again not so good, because the document said if you do logInWithToken, it will create one if it is not there, and we cannot use that because I want to set it's display name when it is created. Any better solution?

  4. I want to link a user with multiple auth provider. I saw in the Firebase JWT, they are well included, that is cool. But those linked elements are user profile get via credential. So can we link a user to a LINE login? Which is not built-in Firebase Auth Provider? Is auth provider linking only valid for Firebase built-in provider?

Regards, Xiang.

Frank van Puffelen
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Xiang Zhang
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  • See https://firebase.googleblog.com/2016/11/authenticate-your-firebase-users-with-line-login.html, which describes the general flow, and https://github.com/firebase/functions-samples/tree/master/line-auth, which shows how to use Cloud Functions as the server component for this. If you get stuck implementing those for Flutter, post the [**minimal**, complete/standalone code that reproduces this issue](http://stackoverflow.com/help/mcve). – Frank van Puffelen Feb 28 '20 at 14:49

3 Answers3

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You're asking way too much in a single post, which makes it hard to help. I'll try to address below, but please follow the advice on How do I ask a good question going forward.

The general approach for adding LINE as an authentication provider is shown in this blog post: Authenticate your Firebase users with LINE Login. There is also a example, which shows how to use Cloud Functions as the server component for this. If you get stuck implementing those for Flutter, post the minimal, complete/standalone code that reproduces this issue.

  1. The client side api: logInWithCredential can ONLY work with Firebase supported login methods, and you have to enable them in your Firebase console. Am I right?

Nope, you're wrong. You can also provide your own sign-in provider for Firebase, and use the tokens you mint there with the Firebase Authentication SDK.

  1. This looks like some hacking, is there a better way?

The UID can be whatever you want, but you'll have to ensure it is unique. A common way to ensure global uniqueness is to embed some identifier for the provider in the UID. If you prefer another way to ensure global uniqueness, you're free to use that too of course.

  1. I have no way to set its display name or some other basic info [when creating the account]

This is indeed a common problem with some providers, as you for example can't set the display name for the email+password provider until after the account has been created. With a custom provider you can put whatever information you want in the initial JWT already though.

  1. can we link a user to a LINE login? Which is not built-in Firebase Auth Provider?

See Linking custom auth provider with Firebase

Frank van Puffelen
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  • Thanks for reply. For the 1st question, I mean, there are two client login methods: `logInWithCredential` and `logInWithCustomToken`. So if I mint a Firebase custom token, which means I MUST use `logInWithCustomToken`, right? So my question is, is the `logInWithCredential` method only works with built-in provider? – Xiang Zhang Feb 28 '20 at 15:38
  • about question 4, I read that post, not a perfect workaround, because it won't be embedded to firebase user Id token. But thanks, that gives me another idea which I've never thought of :) – Xiang Zhang Feb 28 '20 at 15:46
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Late to the party . but This article helps you which I publish on Medium. I'm using A Flutter plugin LINE's native SDKs in Flutter apps with Dart. The plugin helps you integrate LINE Login features in your app. You can redirect users to LINE or a web page where they log in with their LINE credentials. package

Also, I make a git hub repo for the same.

Ashutosh singh
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Did you check out the firebase_auth plugin? It has lots of helpfull features.

https://pub.dev/packages/firebase_auth

Manuel Mühlemann
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  • Sure I'm using it. But LINE is not supported, and I did some experiments but had these questions which I cannot find answers by Googling. – Xiang Zhang Feb 28 '20 at 11:26
  • SignInWithCustomToken: https://pub.dev/documentation/firebase_auth/latest/firebase_auth/FirebaseAuth/signInWithCustomToken.html – Manuel Mühlemann Feb 28 '20 at 11:35
  • Yes that is the method I have to use. I have to create token in server side using Admin API, and in the document it says if the UID in the custom token does not exist, it will create that user account, that is what I want. But one of the problem is, I cannot include some basic information such as display name in the custom token. So I have to create user in my server side and add some basic info from my LINE profile I got with the access token. Which means, this auto-create-account feature is not very useful. – Xiang Zhang Feb 28 '20 at 11:42
  • Also, there is https://pub.dev/documentation/firebase_auth/latest/firebase_auth/FirebaseAuth/signInWithCredential.html , which you just wrap your idToken and accessToken into a credential object, and I guess Firebase will do the OAuth stuff for you so you don't need to create a server for that. And in source code of Flutter Firebase Auth, I also found a `OAuthProvider` which can create such credential, that puzzles me, maybe they can handle all OAuth? Line login support OAuth2 so if that is possible, then I don't even need to create my own server. – Xiang Zhang Feb 28 '20 at 11:50
  • @XiangZhang did you ever figure out how to get the OAuthProvider's `credential` method working? Based on the documentation's comments you should be able to use it with anything, but it doesn't seem to accept either ID token or Access token from LINE. – Tristan Bennett Dec 01 '21 at 23:51