OpenID Connect Explained

OpenID Connect (OIDC) is an authentication protocol built on top of OAuth 2.0. While OAuth 2.0 allows applications to obtain authorization to access protected resources, OpenID Connect adds a standardized way to verify a user's identity. It enables applications to implement secure sign-in without handling user passwords directly.

Today, OpenID Connect is used by many identity providers including Google, Microsoft, Okta, Auth0 and numerous enterprise identity platforms. It powers Single Sign-On (SSO), social login and secure authentication across web, mobile and desktop applications.

What Is OpenID Connect?

OpenID Connect extends OAuth 2.0 by introducing identity information through standardized tokens and endpoints. After a user successfully authenticates, the identity provider issues an ID token that contains verified information about the authenticated user.

OAuth 2.0 vs OpenID Connect

OAuth 2.0 and OpenID Connect work together but solve different problems. OAuth focuses on authorization, while OpenID Connect provides authentication by confirming the user's identity.

Why OpenID Connect Is Important

Without OpenID Connect, applications using OAuth would know they had permission to access certain resources but would not have a standardized method for determining who the authenticated user actually is. OIDC solves this problem by defining identity tokens and standardized user information endpoints.

• Provides user authentication.
• Supports Single Sign-On (SSO).
• Works on top of OAuth 2.0.
• Uses standardized identity tokens.
• Reduces password handling by applications.

OIDC Participants

How OpenID Connect Works

The application redirects the user to the OpenID Provider for authentication. After successful login and user consent, the provider returns an authorization code. The application exchanges the code for tokens, including an ID token that proves the user's identity.

User
↓
OpenID Provider
↓
Authorization Code
↓
ID Token + Access Token
↓
Application

What Is an ID Token?

An ID token is a JSON Web Token (JWT) containing identity claims about the authenticated user. It is digitally signed by the identity provider so that the application can verify its authenticity before trusting its contents.

Common OIDC Use Cases

• Single Sign-On (SSO).
• Social login.
• Enterprise authentication.
• Cloud applications.
• Mobile apps.
• Web portals.

OIDC Tokens

OpenID Connect commonly uses three types of tokens. Each serves a different purpose during authentication and authorization, allowing applications to separate user identity from API access.

What Information Does an ID Token Contain?

An ID token is typically a signed JWT containing claims that identify the authenticated user and describe the authentication event. Applications validate these claims before establishing a user session.

{
  "iss": "https://identity.example.com",
  "sub": "248289761001",
  "aud": "my-client-app",
  "exp": 1766000000,
  "iat": 1765996400,
  "name": "Alice Smith",
  "email": "alice@example.com"
}

Standard OIDC Claims

UserInfo Endpoint

OpenID Connect defines a standardized UserInfo endpoint that allows client applications to retrieve additional profile information about the authenticated user using a valid access token. This avoids placing excessive user data inside the ID token itself.

OpenID Connect Scopes

OIDC introduces the 'openid' scope, which signals that authentication is requested. Additional scopes determine which user profile information the application may access.

Authentication Flow

Most modern applications use the Authorization Code Flow with PKCE. After the user authenticates, the application exchanges the authorization code for an ID token and an access token, validates the ID token and creates a local authenticated session.

OIDC vs Traditional Login

Where OpenID Connect Is Used

• Single Sign-On systems.
• Enterprise identity platforms.
• Cloud services.
• Social login providers.
• Mobile applications.
• Business web portals.
• Developer platforms.

Common Mistakes

OpenID Connect simplifies authentication, but incorrect implementations can reduce its security benefits. Many issues occur when applications misunderstand the purpose of ID tokens, skip validation steps or confuse authentication with authorization.

• Using OAuth 2.0 alone when user authentication is required.
• Skipping ID token signature validation.
• Using an ID token to authorize API requests.
• Requesting more user information than necessary.
• Ignoring token expiration.
• Sending tokens over unencrypted HTTP connections.

Best Practices

• Use the Authorization Code Flow with PKCE for modern applications.
• Always validate ID tokens before trusting their contents.
• Request only the scopes your application actually needs.
• Store tokens securely.
• Use HTTPS for all authentication traffic.
• Keep authentication sessions appropriately short and secure.

Frequently Asked Questions

Helpful API Tools

A JWT Inspector decodes and displays ID and access tokens, a JWT Claims Viewer helps examine token claims, a JWT Encoder is useful for creating sample JWTs during development and testing, an OpenAPI Viewer allows developers to inspect API specifications, and an HTTP Request Builder helps construct authenticated requests when testing OAuth and OpenID Connect integrations.

Conclusion

OpenID Connect extends OAuth 2.0 by providing a standardized and secure way to authenticate users. Through ID tokens, standardized scopes and well-defined authentication flows, OIDC enables Single Sign-On, social login and enterprise identity solutions without requiring applications to manage user passwords directly. Understanding how OpenID Connect works helps developers build secure authentication systems that are interoperable, scalable and easy for users to adopt.