Oauth 2.0 Openid Connect And Identity Server CSharp DotNetCore OIDC

Feb 16th, 2018 - written by Kimserey with .

When it comes to authentication and authorization, the most used standard is OAuth 2.0 with OpenID Connect (OIDC). Few weeks ago I discussed Resource owner password and Implicit flows focusing mainly on implementations with Identity Server. There is a lot of confusion revolving around OAuth 2.0 and OIDC, what they are, how they differ and even what Identity Server is and what is it used for. Today I will give more insights on what is OAuth 2.0 and OIDC are and how Identity Server relates to them.

  1. What is OAuth 2.0
  2. What is OpenID Connect
  3. What is Identity Server 4

1. What is OAuth 2.0

OAuth 2.0 is an authorization protocol enabling applications to have a limited access to protected resources. The authorization is handled in the Identity provider (Idp) who is in charge of delivering an access token to the client apppication after having authenticated the resource owner (usually the user).

Why do we need it?

Let’s take an example.

I have an application where you are a user. When you log in, I want to get your profile picture from Facebook to display it on my application. The straight forward approach would be for my application to ask your Facebook username and password and to log into your Facebook. But this will cause multiple problems:

  1. you would be giving your credentials to a my application,
  2. you wouldn’t know what I will be doing with it,
  3. I will have full access to your account, there is no way for you to restrict the access to only the parts you would want to share,
  4. it would be hard to revoke access, the only way would be to change your password.

This is where OAuth 2.0 comes into picture. Facebook implements OAuth 2.0 which allows my application to request from you a piece of your information on Facebook. To do that, you are redirected to the official Facebook login on Facebook site, hence your credentials would only be handled by Facebook whom you trust, and then Facebook would issue me an access token allowing me to access only the resources you have granted my applicaction to access.

The access token delivery providing limited access to a protected resource (like Facebook) is what OAuth 2.0 was designed for.

OAuth 2.0 makes it possible via different flows; Authorization code, Implicit, Client credentials and Resource owner password flows:

  • Authorization code is used for clients who can keep a secret between themselves and the idp like hosted frontend.
  • Implicit flow is used for pure frontend clients like SPA who can’t keep a secret.
  • Client credentials is used when the client needs to authenticate as itself, for example when your own application needs access to your own protected resource.
  • Resource owner password is used when the client is trusted. Be very cautious with this flow as the user credentials will need to be given to the client for the client to pass it to the idp.

OAuth 2.0 protocol also defines how clients are registered and how exchanges occur on which HTTP endpoints.

But what it does not define is how to identify the user requesting the access token. A common practice is to return a second token signed by the identity provider, deserializable by the recipient containing the identity of the user. But because this was not defined by the protocol, every company providing external login capabilities had to create their own way.

Then came OIDC.

2. What is OpenID Connect

Initially every company was writing their own way of providing the identity of the user, either by including an identifier within the access token returned or by providing an extra token as part of the OAuth 2.0 flow. From there OpenID Connect was created as a simple authentication protocol layer on top of OAuth 2.0 with the goal of providing a unified way of authenticating users.

OIDC standardized the delivery of the id_token within the existing flows of OAuth 2.0.

OIDC standardizes the way to identify the user by providing an id_token together with the OAuth access_token within the current flows available. For example in an implicit flow it will be provided at the authorization endpoint together with the access token while for an authorization code flow, it will be provided by the token endpoint.

The important part to understand is that the access_token provided by OAuth 2.0 is for the resource to be accessed while the id_token is for the client application to identify the user authenticated.

Now that we know what is OAuth 2.0 and OIDC, we need an implemenation.

3. What is Identity Server 4

Now that we know what is OAuth 2.0 and OpenID Connect, we need a framework which allows us to implement the protocols.

Identity Server 4 is a framework implementing OAuth 2.0 and OIDC in the .NET ecosystem and most importantly in ASP .NET Core.

In previous blog post I have covered two flow implementations:

But all the flows are actually supported and there are examples backing up the flows on the github of identity server https://github.com/identityserver. The project is totally open sourced and very well maintained and is OpenID certified.

An important point to note is that while it is responsible for the implementation of the protocols, Identity server does not manage users. The responsability to store users, manage their information together with credentials is left to the developers. This allows us to choose which database we want to use and how we want to store our user. There are already package created to support common implementation of membership systems like ASP .NET Core Identity.

With Identity Server 4, we will get OAuth 2.0 and OIDC, coupled with ASP .NET Core Identity for storing and managing users, we will have the full picture for authentication, authorization and management of users.


Today we saw what was OAuth 2.0 and OIDC. We also deciphered some of the concepts around authorization versus authentication and lastly we saw Identity Server 4, a .NET implementation of it. Hope this post was helpful! See you next time!

Designed, built and maintained by Kimserey Lam.