WARNING: The demo app in this repository, while useful, does some very bad things that should not be copied for use in any production app. For further explanation, read the section on "Why is this app bad?" below.
This app demonstrate the use of the AppAuth for Android library to interact with four identity providers (IDPs): Facebook, GitHub, Google and Microsoft. While Google supports the current best-practices for OAuth2 from mobile devices (specifically, public clients and PKCE), the other providers do not:
All require the use of client secrets in order to acquire a refresh token. The use of the implicit flow to acquire short lived access tokens is generally unacceptable for mobile apps, as frequent usage of custom tabs to acquire new access tokens will disrupt the user experience.
All require that the redirect URI be an https URL. Prior to Android M, this would require the use of an "interstitial" web page to be used that could collect the response of the authorization request and forward it to the app via a custom scheme URI. With Android M and above this is less of an issue, as deep linking can be used to allow the app to directly handle the https redirect URI.
Github and Facebook allow re-authorization without any user interaction: the authorization endpoint will immediately redirect to the specified redirect URI if the user has already consented to the requested scopes. In order for an app to receive either a custom URI or deep link redirect, a user action must have triggered it, otherwise it is blocked as potentially malicious.
This app registers for deep linking on Android M and above, and uses an interstitial page (see here) to capture the response on earlier versions of Android. The experience is far from perfect, but it does work.
This app is a good example of what not to do when using AppAuth for Android:
For deep linking to work, the app's certificate must match that of the declared affiliation (see here). In order to make it possible for you to compile an app with this certificate, it is included in this repository.
The client IDs and secrets are embedded into this repository to make it easier for you to build the demo without having to configure four separate IDPs. Even if I had not done this, they could be extracted from the APK. Client secrets often come with a number of security assumptions that are invalid once they are no longer secret. It is trivial for any other app to now pretend to be mine.
DO NOT DO ANY OF THESE in your own apps. As things stand at the time of writing, one should not use AppAuth for Android to integrate directly with Facebook, GitHub or Microsoft. Once these providers support the recommendations of OAuth 2.0 for Native Apps then that will change.
For IDPs which rely on client secrets, all authorization should be performed with the support of your application's backend. In an ideal world, your application's backend itself would act as an OAuth2 or OpenID Connect authorization service: AppAuth would request authorization via your backend, which in turn would fan out to the IDP of the user's choice. The backend can then perform the exchange with the external IDP to secure a refresh token. When this succeeds, it can then create an authorization code to send to your app, which it exchanges for its own refresh token. All subsequent interaction with the external IDP would be mediated by your own backend.
This "ideal world" scenario is complex, and obviously requires that your app even has a backend / web app. It is our hope that as mobile applications become ever more important, identity providers will begin to support the recommendations that allow AppAuth to work as intended, with the client application fully in control of the authorization process.