Multiple Auth Providers with AppEngine, Webapp2, and Cloud Endpoints Proto Datastore

Question

I'm working on a webapp that will allow users to authenticate using simpleauth. For now I will be supporting Google and Facebook. Other than logging in and out (using webapp2), the webapp will consist of Cloud Endpoint APIs. The clients will be web, Android, and iOS.

My questions is, using Endpoints Proto Datastore, can I have user_required=True and call endpoints.get_current_user() to get my user from an @Model.method if the user's auth provider is Facebook (or any of the other supported OAuth2 providers? If it is not possible, does that mean I should not have user_required=True, and should instead get a permanent user id from the provider given the OAuth2 token and keep it in the datastore, generate my own auth token for that user, and then pass that token to each request?

EDIT: Instead of passing the auth token around, would it make sense to have an authenticated user request an "API token" that they can pass to the API methods? Would this token have to be included in the POST or GET body, or can it be put in a header/cookie (I saw some questions elsewhere on SO regarding headers and cookies with Cloud Endpoints, but it's been some time since then). This is all assuming that non-Google auth won't work.

Answer 1

This answer is not going to directly answer your question but should give you a good idea how you can implement authentication in a safe way. I've implemented something similar recently and spent quite some time figuring out which is the best way to do authentication with Google AppEngine environment.

Google supports OpenId Connect protocol. Facebook's implementation should be very similar according to Getting Started with OAuth 2.0 book. I will focus more on Google's implementation, as I am more familiar with it but the concepts should be very similar when using other OAuth providers.

OpenId Connect will give you an id_token, after successfully authenticating the user. It will also give you an access token. Access token is something that should be kept a secret. Never send it over the wire. Id token on the other hand is very handy. It contains some information about your user, and it's encrypted so it doesn't expose any information about the user "just like that". You'd have to crack the id_token to find out anything about user. In the unlikely event that it gets cracked it doesn't expose anything critical about the user. What you can do you can store it as a cookie and then use it in all subsequent requests to verify it the user by checking if there's an access token that matches the id_token. The only drawback is that id_token is quite long - it takes around 650bytes. That would mean that every http request carries that payload. If sending that much information is too much for your use case you can send only first few characters, maybe 12 or so and then match just the first part. The id_token has can also be useful when analysing your data. It will show up when analysing http requests but will not reveal any information about the user and you can still differentiate requests that came from different users.

Also on a side note, don't try using AppEngine's users service as it doesn't work very well with any kind of custom authentication.

Hope this gives you an idea and puts you on the right track.