What is OpenID Connect (OIDC)?
OpenID Connect or OIDC is an identity protocol that utilizes the authorization and authentication mechanisms of OAuth 2.0. The OIDC final specification was published on February 26, 2014, and is now widely adopted by many identity providers on the Internet.
OIDC was developed by the OpenID Foundation, which includes companies like Google and Microsoft. While OAuth 2.0 is an authorization protocol, OIDC is an identity authentication protocol and may be used to verify the identity of a user to a client service, also called Relying Party. In addition, users’ claims like, for example, name, email address, etc., may also be shared on request.
A wide variety of clients may use OpenID Connect (OIDC) to identify users, from single-page applications (SPA) to native and mobile apps. It may also be used for Single Sign-On (SSO) across applications. OIDC uses JSON Web Tokens (JWT), HTTP flows and avoids sharing user credentials with services.
OpenID Connect has consent built-in. This is important as OIDC is often used in consumer-facing services (e.g., a Relying Party), where the sharing of personal data requires the user’s explicit consent.
These features, along with the simplicity of implementation, make OpenID Connect a useful protocol when a user’s identity is required and a powerful alternative to the more complex SAML 2.0. It is also especially suitable for mobile apps.
How Does OpenID Connect Fit with OAuth2?
OIDC utilizes OAuth 2.0 as an underlying protocol. The principal extensions are a special scope value (“openid”), the use of an extra token (the ID Token, which encapsulates the identity claims in JSON format), and the emphasis on authentication rather than authorization. Also, in OIDC, the term “flow” is used in place of OAuth2 “grant”
Principles and Definitions in OpenID Connect
Being built on top of OAuth 2.0, OpenID Connect uses tokens to provide a simple identity layer integrated with the underlying authorization framework. This integration implies the use of the following types of token:
ID Token: Specific to OIDC, the primary use of this token in JWT format is to provide information about the authentication operation’s outcome. Upon request, it may provide the identity data describing a user profile. The data about the authentication result and the user profile information are called claims. The user profile claims may be any data that is pertinent to the Relying Party for identification purposes, such as a persistent ID, email address, name, etc.
Access Token: Defined in OAuth2, this (optional) short lifetime token provides access to specific user resources as defined in the scope values in the request to the authorization server.
Refresh Token: Coming from OAuth2 specs, this token is usually long-lived and may be used to obtain new access tokens.
ID Tokens should be digitally signed to prevent tampering. They may also be encrypted to provide additional privacy, although, in many cases, transport layer security (HTTPS) is sufficient. For SPAs and mobile apps, ID Token encryption is not useful, as the decryption key can be discovered easily.
The choice of OpenID Connect flow depends on the type of application and its security requirements. There are three common flows:
Implicit Flow: In this flow, commonly used by SPAs, tokens are returned directly to the RP in a redirect URI.
Hybrid Flow: Combining Implicit and Authorization Code flows, here, the ID Token is returned directly to the RP, but the access token is not. Instead, an authorization code is returned that is exchanged for an access token.
What Can an Identity Provider Use to Authenticate Users Using OIDC?
The OpenID Provider determines the authentication methods available to authenticate users when they sign in to their IdP account and possibly consent to release their identity data to the RP. OIDC specs say nothing about the mechanics of user authentication itself. The IdP can offer single or multiple factors e.g.
Single-use code delivered out of band, e.g., by SMS or email
Code generated by App (OATH, TOTP or HOTP)
Biometric via App
Federated (e.g., Facebook, GoogleID)
The RP can also have a say on IdP authentication. For example, multifactor authentication may be requested. However, the way an IdP authenticates users is out of the scope of OIDC.