OpenID Connect (OIDC) is an identity layer built on top of the OAuth 2.0 protocol, allowing clients to verify the identity of end-users and obtain basic profile information. OIDC is widely used for single sign-on (SSO) and user authentication scenarios. Here’s an overview of the main OpenID Connect flows:
1. Authorization Code Flow
The Authorization Code Flow is designed for server-side applications and provides the highest level of security.
Use Case: Secure server-side applications where the server can keep a client secret.
Summary:
- The client directs the user to the authorization server.
- The user authenticates and consents.
- The authorization server redirects back to the client with an authorization code.
- The client exchanges the authorization code for an ID token and access token.
2. Implicit Flow
The Implicit Flow is designed for client-side applications where the client secret cannot be kept confidential.
Use Case: Single-page applications or mobile apps.
Summary:
- The client directs the user to the authorization server.
- The user authenticates and consents.
- The authorization server redirects back to the client with an ID token and optionally an access token.
3. Hybrid Flow
The Hybrid Flow allows the client to obtain tokens directly from the authorization server as well as through an authorization code.
Use Case: Applications that need to obtain tokens both at the front-end and back-end.
Summary:
- The client directs the user to the authorization server.
- The user authenticates and consents.
- The authorization server redirects back to the client with an ID token and an authorization code.
- The client can use the authorization code to obtain additional tokens (ID token, access token, and refresh token).
4. Client Credentials Flow
The Client Credentials Flow is used for server-to-server interactions where the client acts on its own behalf, not on behalf of a user.
Use Case: Machine-to-machine communication.
Summary:
- The client authenticates with the authorization server using its client credentials (client ID and client secret).
- The authorization server issues an access token.
5. Resource Owner Password Credentials Flow
The Resource Owner Password Credentials Flow allows the client to obtain tokens by directly using the resource owner’s credentials.
Use Case: Trusted applications where the user’s credentials are securely handled by the client.
Summary:
- The client collects the user’s credentials.
- The client sends the credentials to the authorization server.
- The authorization server issues an access token and ID token.
Key Components
- Client: The application requesting authentication and authorization.
- Resource Owner: The user who authorizes the client to access their information.
- Authorization Server: The server issuing tokens (access tokens, ID tokens) after successfully authenticating the user and obtaining authorization.
- Resource Server: The server hosting the protected resources.
- ID Token: A token containing information about the authenticated user.
- Access Token: A token that the client uses to access the resource owner’s protected resources.
- Refresh Token: A token used to obtain new access tokens without re-authenticating the user.
Configuring OIDC in an Application
- Register the Application:
- Register your application with the OIDC provider (e.g., Google, Okta, Auth0) to obtain client credentials (client ID and client secret).
- Define Scopes:
- Specify the scopes required by your application, such as
openid,profile,email, etc.
- Configure Redirect URI:
- Specify the callback URL where the OIDC provider should redirect the user after authentication.
- Implement the Flow:
- Implement the chosen OIDC flow in your application by directing the user to the OIDC provider for authentication and handling the tokens returned by the provider.
Using Tokens
- ID Token: Verifies the user’s identity and contains claims about the user (e.g., name, email).
- Access Token: Grants the client access to protected resources on behalf of the user.
- Refresh Token: Allows the client to obtain new access tokens without requiring the user to re-authenticate.
Conclusion
OpenID Connect provides a robust and flexible framework for user authentication and SSO by building on OAuth 2.0. Understanding the different OIDC flows and their appropriate use cases helps in choosing the right authentication strategy for your application.