OAuth 2.0 vs. OpenID Connect

OpenID Connect is a profile built on top OAuth 2.0. OAuth talks about access delegation while OpenID Connect talks about authentication. In other words, OpenID Connect builds an identity layer on top of OAuth 2.0.

Authentication is the act of confirming the truth of an attribute of a datum or entity. If I say, I am Peter - I need to prove that. I can prove that with something I know, something I have or with something I am. Once proven who I claim I am, then the system can trust me. Sometimes systems do not just want to identify end users just by the name. By name could help to identify uniquely - but how about other attributes. Before you get through the border control - you need to identify your self - by name - by picture - and also by fingerprints and eye retina. Those are validated in real-time against the data from the VISA office which issued the VISA for you. That check will make sure its the same person who claimed to have the VISA enters in to the country.

That is proving your identity. Proving your identity is authentication. Authorization is about what you can do. Your capabilities.

You could prove your identity at the border control by name - by picture - and also by fingerprints and eye retina - but it's your VISA that decides what you can do. To enter into the country you need to have a valid VISA that has not expired. A valid VISA is not a part of your identity - but a part of what you can do. Also what you can do inside the country depends on the VISA type. What you do with a B1 or B2 differs from what you can do with an L1 or L2. That is authorization.

OAuth 2.0 is about authorization. Not about authentication.

With OAuth 2.0, the client does not know about the end user (only exception is resource owner credentials grant type). It simply gets an access token to access a resource on behalf of the user. With OpenID Connect, the client will get an ID Token along with the access token. ID Token is a representation of the end user’s identity. What does it mean by securing an API with OpenID Connect ? Or is it totally meaningless ? OpenID Connect is at the Application level or at the Client level - not at the API level or at the Resource Server level. OpenID Connect helps, client or the application to find out who the end user is, but for the API that is meaningless. Only thing API expects is the access token. If the resource owner, or the API wants to find who the end user is - it has to query the Authorization Server. The OAuth Token Introspection specification currently does not support sending back the end user identity in the introspection response, but, it would be quite useful to have an user ID Token in the response (as in OpenID Connect) and was proposed to the OAuth IETF working group.