OAuth 2.0 and OIDC 1 of 2

OAuth 2.0 and OpenID Connect (OIDC 1.0) are different but highly related protocols and they are often confused. When we talk about IAM (identity and access management), we should first distinguish between Authentication (AuthN) and Authorization (AuthZ):

  • AutheNtication (AuthN, aka Identity Management) is about validating user’s identity by verifying that the user trying to connect is actually who it claims itself to be;
  • AuthoriZation (AuthZ, aka Access Management) refers to granting or denying access to specific resources based on the requesting user’s identity. It is usually performed after a user is identified through authentication. The most common approach is Role-Based Access Control (RBAC).

In a nutshell, OAuth 2.0 deals with authorization. OIDC is a layer later developed on top of OAuth 2.0, to deal with authentication. This post is greatly influenced by a presentation (1 hour) delivered by Nate Barbettini from Okta, with the slides available here. There is also an abridged and illustrated video (16 min) by Okta available here.

OAuth 2.0

OAuth was originally developed by Twitter and Google in 2006 as an open standard for API authorization. OAuth 2.0 is published in 2012. It allows user to delegate authorization. The original scenario is a user signed up to a new application and allows it to automatically import her Gmail contact. The technical problem to solve is: how can a user (Resource Owner) let an app (Client) to access his contact list stored in Google server (Resource Server)? The proposal is that it redirects user to Google Account page (Authorization Server) for user to log in. Then Google Account issues token to the application (Client) with user’s approval. Note that the user did NOT log in to the application itself with her Google account. From the application’s standpoint, the user had been authenticated already, and was simply importing contact after logging in. The roles involved in OAuth 2.0 are:

  • Resource: the contact list of the user
  • Resource owner: the user
  • Client: the application
  • Resource Server: contact.google.com
  • Authorization Server: accounts.google.com

The diagram below illustrates the interactions:

Authorization Code Flow
OAuth 2.0 Authorization Code Flow

This page has further details for each step. Note that at step 3 to 5 may seem unnecessary because Auth Server could have send Access Token Grant to Application via User-Agent at step 3, which could have eliminate the need for step 4 and 5. In fact this design is to avoid sending critical information (Access Token Grant) to User-Agent (browser) which is considered in secure. In other words, it avoids front channel (User-agent to auth server) and prefers back channel (Client to Auth server) for security. This is the difference between Authorization code flow and the implicit flow.

With OAuth 2.0, there are a number of flows:

  • Authorization code (front channel and back channel)
  • Implicit (front channel only, token returned to user agent directly)
  • Resource owner password credentials (back channel only)
  • Client credentials (back channel only)

In Authorization Code Flow, the Application (client) needs a one-time registration with the Auth Server and is given a client ID and client secret, which are sent to Auth Server at step 4 along with Access Token Request, to prove the identity of the client application.

Note that OAuth 2.0 is an inherently insecure protocol since it does not support signature, encryption, channel binding or client verification. The protocol relies entirely on the underlying transport layer security (TLS) to provide confidentiality and integrity.

Also note that throughout the process (Authorization Code Flow as an example), the Client application eventually is granted access to user’s data. However, it does not know anything about the user itself. Neither the authorization code grant, nor the access token grant is obligated to present information about the user itself. Therefore, OAuth 2.0 is designed strictly for permission purpose without the intent to address identity issue. In the flow, the auth server does the authentication (for the purpose of granting access to resources, but none of the authentication. A user logs in to client application as Bob, when he requests to imports contact, he is redirected to account.google.com and there he could put in the credential of Alice and therefore load Alice’s Google contacts into Bob’s App account!

Pseudo-authentication with OAuth 2.0

In many real life OAuth 2.0 implementations, at step 3, the Auth server chooses to include a field about the user’s identity. This makes user’s identity visible to the client, and the client is therefore able to confirm user’s identity in its own code. This also allows client application to use OAuth 2.0 as an authentication method, which is referred to as pseudo-authentication. The access token acts as a kind of “valet key” that the application can include with its request to the auth server, as a proof that it has user’s permission to access the resources (or APIs).

Because the identity provider (auth server) typically (but not always) authenticates the user as part of the process of granting an OAuth access token, it’s tempting to view a successful OAuth access token request as an authentication method itself. However, because OAuth was not designed with this use case in mind, making this assumption can lead to major security flaws.

Nate’s presentation outlined some scenarios where OAuth 2.0 is applied up to 2012, and which ones are misuses:

  • Simple login – pseudo authentication with OAuth 2.0
  • Single sign-on across sites – pseudo authentication with OAuth 2.0
  • Mobile app login – pseudo authentication with OAuth 2.0
  • Delegated authorization – the only intended use case for OAuth 2.0

To address the authentication issue properly, and in a standard approach, we need OpenID Connect.

OpenID Connect (OIDC)

OpenID Connect is an open standard for authentication, promoted by the non-profit OpenID Foundation. It allows user to be authenticated using a third-party service called identity providers. User may choose to use their preferred OpenID Connect providers to log in to websites that accept the OpenID Connect authentication scheme. For example, a user uses her Facebook to login to an online application.

OpenID Connect is an extension to OAuth 2.0 with a just few additions:

  • In addition to access token, an ID token is returned by the authorization server;
  • Userinfo end point is provided in case Id token is not sufficient and more user information is needed;
  • “openid” is passed as a parameter in the Scope during the initial call to the authorization server;

Therefore OpenID Connect is considered an identity layer on top of OAuth 2.0. Many application supports OpenID Connect such as Apache Nifi. OIDC is comparable with SAML in the sense that both provide SSO feature (federated identity). Here is a comparison table:

OpenID ConnectSAML
Main PurposeSSO for consumer/mobile applicationsSSO for enterprise applications
LoadRelatively light weightHeavy weight due to the size of XML messages
Use caseSatisfies both authentication and authorization use cases, often combined with OAuth 2.0Generally not used for API security
TransportHTTP GET and HTTP POSTHTTP Redirect (GET) binding, SAML SOAP binding, HTTP POST binding, et

Here are more details about their differences. In general SAML is more common in the enterprise world for SSO and it has been around for a while. When developing new applications for enterprise it is advised to consider OIDC first.

OIDC also has authorization code flow, with the additional fields on top of its counterpart in OAuth 2.0. The authorization server returns both access and ID tokens, wrapped in a data structure named JWT (JSON Web Token). The JWT includes a signature field, allowing the client application to verify it with authorization server’s public key. Nate’s presentation proposes the following flows for each application type:

  • Web application with server backend: authorization code flow
  • Native mobile app: authorization code flow with PKCE
  • Java Script app (SPA) with API backend: implicit flow
  • Microservices and APIs: client credential flow

In addition, this page from Okta developer has a good summary of how to select flow type (grant) based on each use case.


OpenID Connect is an authentication protocol for the purpose of validating user’s identity. OAuth 2.0 is an authorization protocol. You should use OAuth 2.0 for granting access to your API, or access to user data in other systems. If you need to log user in, or make your accounts available in other systems, you need OIDC.

The next post about OAuth and OIDC was posted in 2023.