OAuth Authorization vs Authentication

OAuth is a specification for authorization

OAuth 2.0 is a specification for authorization, but NOT for authentication. RFC 6749, 3.1. Authorization Endpoint explicitly says as follows:

The authorization endpoint is used to interact with the resource owner and obtain an authorization grant. The authorization server MUST first verify the identity of the resource owner. The way in which the authorization server authenticates the resource owner (e.g., username and password login, session cookies) is beyond the scope of this specification.

OAuth authentication?

Authentication deals information about "who one is". Authorization deals information about "who grants what permissions to whom". Authorization flow contains authentication as its first step. It is the reason people are often confused.

There are many libraries and services that use OAuth 2.0 for authentication. It is often called "social login" and It makes people more confused. If you see "OAuth authentication" (not "OAuth authorization"), it is a solution using OAuth for authentication.

OpenID Connect

OpenID 1.0 and OpenID 2.0 are old specifications for authentication. Those who made the specifications expected people to use OpenID for authentication. However, some people began to use OAuth 2.0 for authentication (not for authorization) and OAuth authentication has prevailed rapidly.

From a viewpoint of OpenID guys, authentication based on OAuth was not secure enough, but they had to admit that people preferred OAuth authentication. As a result, OpenID guys decided to define a new specification, OpenID Connect, on top of OAuth 2.0.

Yes, this has made people much more confused.

One-sentence definitions of OAuth 2.0 and OpenID Connect

OAuth 2.0 is a framework where a user of a service can allow a third-party application to access his/her data hosted in the service without revealing his/her credentials (ID & password) to the application.

OpenID Connect is a framework on top of OAuth 2.0 where a third-party application can obtain a user's identity information which is managed by a service.

^{(Sorry, these definitions are excerpts from the overview page of my company)}

Definitions from a viewpoint of implementors

Authentication is a process to determine the subject (= unique identifier) of an end-user. There are many ways to determine the subject. ID & password, fingerprints, iris recognition, etc.

Authorization is a process to associate the subject with the requested permissions and the client application that requested the permissions. An access token represents the association.

See Also

1. Full-Scratch Implementor of OAuth and OpenID Connect Talks About Findings
2. Diagrams And Movies Of All The OAuth 2.0 Flows
3. Diagrams of All The OpenID Connect Flows
4. The Simplest Guide To OAuth 2.0