What's the difference between OpenID and OAuth?

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OpenID and OAuth are both authentication and authorization protocols, but they serve different purposes and have distinct use cases:

OpenID:

  1. Purpose:

    • OpenID is primarily an authentication protocol used for verifying the identity of a user.
    • It allows users to log in to multiple websites and applications using a single set of credentials (single sign-on or SSO).

  2. Example:

    • Imagine you want to log in to an online forum using your Google account. Instead of creating a new username and password for the forum, you click a "Sign in with Google" button. This uses OpenID to authenticate you via your Google account.

  3. Key Terminology:

    • OpenID Provider (OP): The service that authenticates the user (e.g., Google, Facebook).
    • Relying Party (RP): The website or application that allows users to log in using their OpenID credentials.
    • OpenID Connect (OIDC): An extension of OpenID that adds identity and profile information. OIDC is often used for SSO and user profile data.

OAuth:

  1. Purpose:

    • OAuth is primarily an authorization protocol used for granting access to resources or data without exposing user credentials.
    • It enables third-party applications to access resources on behalf of a user (e.g., accessing your Google Calendar from a third-party app).

  2. Example:

    • Suppose you want to allow a fitness app to access your Google Fit data. Instead of sharing your Google username and password, you use OAuth to grant the fitness app limited access to your Google Fit data.

  3. Key Terminology:

    • Resource Owner: The user who owns the data or resources being accessed.
    • Client: The third-party application requesting access to the resources.
    • Resource Server: The server hosting the protected resources (e.g., Google Calendar server).
    • Authorization Server: The server responsible for issuing access tokens after the user's authorization.

In summary:

  • OpenID focuses on user authentication and single sign-on, allowing users to prove their identity across multiple websites and applications.
  • OAuth focuses on authorization and delegated access, allowing third-party applications to access a user's resources or data without exposing the user's credentials.

Both protocols are often used together in scenarios where authentication (OpenID) and authorization (OAuth) are needed, such as in federated identity and single sign-on solutions.

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