What OpenID Connect authorization flow to authenticate mobile app users?

Question

I am building a cross-platform mobile app that interacts with a RESTful API, and I want to use OpenID Connect to authenticate my users. I will be building my own OpenID Connect provider server.

OpenID.net claims that:

OpenID Connect allows for clients of all types, including browser-based JavaScript and native mobile apps, to launch sign-in flows and receive verifiable assertions about the identity of signed-in users.

However, I can't find any documentation explaining how to actually authenticate for a mobile app client.

This StackExchange answer makes it clear that OpenID Connect does not support the "resource owner password-based grant" flow or the "client credentials" flow.

That just leaves the "authorization code" flow (normally used by server-side apps) and the "implicit grant" flow (normally used by client-side apps). Both of these seem to rely on redirecting the user to the provider's authorisation endpoint, and having the provider redirect back to the client URL. I don't see how this can apply to a mobile app.

Can anyone explain to me (or even better, point me at a tutorial or some example code) which explains how to do this?

Update

To clarify: OpenID Connect relies on the client redirecting the user to the Authorization Endpoint, and then the provider redirecting the user back to the client. In the case where the client isn't a web app, how can this work?

Answer 1

Mobile apps, at least on iOS and Android, can register custom URL schemes so that a redirect from a browser can send the user back to your app along with some query parameters.

So, you can use these flows in a native mobile app, but it involves sending the user to a web browser (either an external browser app or a web view built into your application) in order for them to authenticate with the OP.

A complete article presenting how to implement the "Authorization Code Grant" flow securely on a native mobile app is available here : Building an OpenID Connect flow for mobile. It is based on latest IETF OAuth 2.0 Security Best Current Practice.

Please also note that the use of the "Implicit Grant" flow is now highly discouraged.

Answer 2

I think that the Hybrid flow from the OpenID Connect spec is probably the one which you want to use. OpenID Connect Core Spec.

This does rely upon having a configured return URI, but as James says you would use a custom URI scheme to enable the mobile OS to redirect after login to your own app. Your app would then have an access code which it can use to obtain access tokens as needed (assuming that you are using Oauth2 to protect your back-end API services which the mobile app uses).

There is a vulnerability which would allow a malicious app to hijack your URI scheme and grab the tokens, There is a draft spec to overcome that Proof Key for Code Exchange by OAuth Public Clients which is worth considering implementing.