Authenticating with OAuth2 for an app *and* a website

I'm developing a website that is primarily accessed via an app, and I want to use OAuth2 for user registration and authentication. Since it is an Android app I will start using Google's OAuth2 stuff, since it provides a decent UI on Android.

Google states that "You can choose to use Google's authentication system as a way to outsource user authentication for your application. This can remove the need to create, maintain, and secure a username and password store." which is what I want to do. However when I go through all their examples and whatnot, I can only find stuff about having a website or an app authenticate a user against Google's services.

And indeed, when I go to register my app ("client") with Google's OAuth2 there are options for website clients and "installed" clients (i.e. a mobile app) but not both. I can create two separate clients but I read the OAuth2 draft and I think there will be a problem, which I will now explain.

Here's how I did envisage it working:

1. User asks MyApp to access his private data.
2. App uses Android's AccountManager class to request an access token for Google's APIs.
3. Android says to user "The app 'MyApp' wants access to your Basic Information on Google. Is this ok?"
4. User says yes.
5. AccountManager connects to Google's OAuth2 server using the credentials stored on the phone, and asks for an access token.
6. Access token (which follows the green lines) is returned.
7. AccountManager returns the access token to MyApp.
8. MyApp sends a request to MySite for the user's private data, including the access token.
9. MySite needs to verify the user, using the access token. It validates the token as described here, with Google - "Google, is this token valid?".
10. Now, what I want to happen is that Google says "Yes, whoever gave it to you is indeed that user.", but what I think will actually happen (based on the OAuth2 draft and Google's documentation) is that it will say "No way! That token is only valid for MyApp, and you're MySite. GTFO!".

So how should I do this? And PLEASE don't say "Use OpenID" or "Don't use OAuth2" or other similarly unhelpful answers. Oh and I would really like to keep using the nice AccountManager UI rather than crappy popup WebViews

Edit

Provisional answer (I will report back if it works!) from Nikolay is that it should actually work, and Google's servers won't care where the access token came from. Seems a bit insecure to me, but I will see if it works!

Update

I implemented this pattern with Facebook instead of Google and it totally works. The OAuth2 server doesn't care where the access token comes from. At least Facebook's doesn't, so I assume Google's doesn't either.

In light of that it is a very very bad idea to store access tokens! But we also don't want to have to hit Facebook/Google's servers to check authentication for every request since it will slow everything down. Probably the best thing is to add an additional authentication cookie for your site that you hand out when their access token is validated, but a simpler way is just to treat the access token like a password and store a hash of it. You don't need to salt it either since access tokens are really really long. So the steps above become something like:

9. MySite needs to verify the user, using the access token. First it checks its cache of hashed valid access tokens. If the hash of the token is found there it knows the user is authenticated. Otherwise it checks with Google as described here, with Google - "Google, is this token valid?".

10. If Google says the access token is invalid, we tell the user to GTFO. Otherwise Google says "Yes that is a valid user" and we then check our registered user database. If that Google username (or Facebook id if using Facebook) is not found we can create a new user. Then we cache the hashed value of the access token.

I just posted an answer to a similar StackOverflow question.

Google calls this Hybrid Apps and explains how an "Android app obtains offline access for Web back-end".

The gist of it is that you'll have to pass a massaged scope string into GoogleAuthUtil.getToken in order to get it to return an Authorization Code (not an OAuth2 Token). That Authorization Code can be passed from your mobile app to your server and be exchanged for an OAuth2 Token and Refresh Token, according to this schematic.

The scope parameter needs to look something like this:

oauth2:server:client_id:<your_server_client_it>:api_scope:<scope_url_1> <scope_url_2> ...

You can use the access token retrieved by the mobile application anywhere else. Drive SDK has a nice and simple intro that goes through the flow on https://developers.google.com/drive/quickstart-android

You probably need OpenID Connect, which uses OAuth tokens for authentication. As for AccountManager, the current OAuth support is a bit hacky, the new Google Play Services, set to be released 'soon' should hopefully make this better. See here for a demo.

At least with Google, the access token eventually expires. This is why the android AccountManager has the invalidateAuthToken method--the cached access token has expired, and you need to tell the AccountManager to stop giving you the old one and instead get a new one. This makes it somewhat safer to cache the token, as the token itself doesn't give you eternal access as that user. Instead, when valid, it merely says "at some point in the recent past, this token was acquired by a trusted source."

Here are a couple of things I've found helpful when working with tokens. The first is Google's tokeninfo endpoint. The token itself is just base64-encoded JSON. This means it isn't encrypted, so you need to be sure to be using HTTPS for communication. However, it also means that you can examine the token and have a better idea of what's going on.

https://www.googleapis.com/oauth2/v1/tokeninfo?id_token=

If your token was "abcdef", you would navigate to:

https://www.googleapis.com/oauth2/v1/tokeninfo?id_token=abcdef

and Google would unpack the token for you. It is a simple JSON object that includes an "expires_in" field telling you the number of seconds for which the token is still valid. At 6:03 in the video below you can see the unpacked token:

https://developers.google.com/events/io/sessions/383266187

That video includes a thorough overview of OAuth2 and is well worth watching in its entirety if you're going to be dealing with OAuth and tokens. The speaker also discusses other forms of Oauth2 tokens, that are not access tokens, that do not expire.

Another useful resource is the OAuth Playground. This lets you do basic things like request scopes, make up requests, and get back tokens. This link seems to work sporadically, and on Chrome I had to install the Oauth Playground app:

https://developers.google.com/oauthplayground/

And here is a tutorial by Tim Bray, the speaker in the video, explaining how to use access tokens to communicate to a server from an Android app. This was useful to me because I began to understand how the different things in the Google API Console work together:

http://android-developers.blogspot.in/2013/01/verifying-back-end-calls-from-android.html

With regards to the actual answer to your question, I would say you never need to cache the access token on the server. As explained in the Verifying Back End Calls from Android link above, verifying a token is almost always a fast static call, meaning there's no reason to cache the tokens:

>The libraries can cache the Google certs and only refresh them when required, so the verification is (almost always) a fast static call.

Finally, you can indeed use the AccountManager to get access tokens. However, Google now instead encourages the use of the GoogleAuthUtil class in the Play Services library instead:

https://stackoverflow.com/questions/14365219/in-a-nutshell-whats-the-difference-from-using-oauth2-request-getauthtoken-and-g

Here note the comment by Tim Bray, the same guy yet again from the above links, saying that they are putting their efforts into the GoogleAuthUtil route. Note, however, that this means you would be limited to Google authentication. I believe that the AccountManager could be used to get, for example, a Facebook token instead--not the case with GoogleAuthUtil.

it describes exactly what you want: https://developers.google.com/identity/protocols/CrossClientAuth

When we had a need to do something similar on a non-google OAuth Server, we kept the tokens in a DB on the website. The app would then use web services to request the token when needed to request data.

The user could do the OAuth registration on either the web or app. They shared the same application token, so they could share the same access token. After the registration we would store the access and refresh tokens in the DB for use from whichever app needed it.