OIDC Specification: Key Concepts Developers Actually Need

Modern applications rarely exist in isolation. They depend on identity systems to authenticate users, manage sessions, and protect APIs across devices, platforms, and environments. As applications scale, building authentication from scratch becomes both risky and expensive.

OpenID Connect (OIDC) has emerged as the industry standard for modern authentication. Built on OAuth 2.0, OIDC provides a secure, interoperable way for applications to verify user identities and obtain identity-related information. Yet, for many developers, the OIDC specification feels dense, abstract, and difficult to translate into real implementation decisions.

This article breaks down the OIDC specification into the key concepts developers actually need. Rather than restating the entire spec, it focuses on how OIDC works in practice: the roles involved, the tokens exchanged, the flows used, and the security mechanisms that keep authentication reliable and safe.

What Is OpenID Connect (OIDC)?

OpenID Connect is an identity layer built on top of OAuth 2.0. While OAuth 2.0 is designed for authorization (granting applications access to resources), OIDC adds a standardized way to authenticate users and convey identity information.

At its core, OIDC answers a simple question:
“Who is the user that just logged in?”

OIDC enables applications (known as clients) to verify the identity of a user based on authentication performed by an identity provider (IdP). It also provides structured identity data, such as a user ID, email address, or name, in a predictable, secure format.

This separation of responsibilities is critical. Applications no longer need to store passwords or manage credential security directly. Instead, they delegate authentication to a trusted provider that follows open standards, reducing security risk and improving interoperability across platforms.

Why Developers Should Care About the OIDC Specification

OIDC is not just another authentication option; it is the foundation of modern identity systems. Major identity providers, including Google, Microsoft, Apple, Okta, Auth0, and enterprise IAM platforms, rely on OIDC as their primary authentication protocol.

For developers, understanding OIDC means:

• Building authentication that works across web, mobile, and API-based system
• Avoiding vendor lock-in by relying on open standards
• Designing secure login flows that resist phishing, token leakage, and replay attacks
• Supporting advanced features like single sign-on (SSO), social login, and passwordless authentication

Without a clear grasp of OIDC’s concepts, implementations often become fragile, insecure, or tightly coupled to a single provider’s SDK. The specification exists to prevent that, but only if its core ideas are understood.

OIDC Roles and Actors

OIDC defines clear roles to separate responsibilities and reduce complexity. Every OIDC interaction involves three primary actors.

End User

The end user is the human who is attempting to authenticate. While this seems obvious, OIDC is designed to ensure that applications never directly handle the user’s credentials. The user authenticates only with the identity provider.

This separation improves security and user trust. Users can authenticate using passwords, MFA, passkeys, or biometrics without the application needing to understand how those credentials are managed.

Client (Relying Party)

The client is the application that wants to authenticate the user. This could be a web app, mobile app, single-page application (SPA), or backend service.

In OIDC terminology, the client is often referred to as the Relying Party (RP) because it relies on the identity provider’s authentication result. Clients are registered with the IdP and assigned identifiers and credentials that allow secure communication.

OpenID Provider (OP)

The OpenID Provider is the identity service responsible for authenticating the user and issuing tokens. It verifies credentials, enforces security policies, and generates cryptographically signed identity assertions.

The OP exposes standardized endpoints defined by the OIDC specification, enabling any compliant client to integrate without proprietary protocols.

OAuth 2.0 vs OIDC: Understanding the Difference

A common source of confusion is the relationship between OAuth 2.0 and OIDC.

OAuth 2.0 is an authorization framework. It answers the question:
“Is this application allowed to access a protected resource?”

OIDC builds on OAuth 2.0 to add authentication. It answers:
“Who is the user, and how did they authenticate?”

OIDC introduces:

• A new token type (the ID Token)
• Standard identity claims
• A user information endpoint
• Authentication-specific scopes and parameters

Without OIDC, OAuth alone cannot reliably identify users. With OIDC, identity becomes standardized and verifiable.

OIDC Tokens: The Core Building Blocks

Tokens are the heart of OIDC. They carry authentication results and authorization data between parties.

ID Token

The ID Token is the most important OIDC-specific concept. It is a JSON Web Token (JWT) that contains identity information about the authenticated user.

An ID Token includes:

• A subject identifier (sub) that uniquely identifies the user
  
  
• The issuer (iss) that generated the token
  
  
• The intended audience (aud) for the token
  
  
• Authentication time and expiration
  
  
• Optional identity claims like email or name

Clients must validate the ID Token’s signature and claims before trusting it. This cryptographic verification is what makes OIDC secure and tamper-resistant.

Access Token

Access tokens come from OAuth 2.0 and are used to access protected APIs. In OIDC flows, access tokens are often issued alongside ID Tokens.

Unlike ID Tokens, access tokens are not meant for the client to interpret. They are intended for resource servers, which validate them before granting access to APIs.

Refresh Token

Refresh tokens allow clients to obtain new access tokens without forcing the user to reauthenticate. They are long-lived credentials and must be stored securely.

The OIDC specification builds on OAuth best practices to limit refresh token exposure, particularly in browser-based or mobile environments.

Claims and Scopes in OIDC

OIDC uses claims to represent identity attributes and scopes to request access to those claims.

Claims

Claims are key-value pairs included in tokens or returned by the UserInfo endpoint. Common claims include:

• sub (subject identifier)
• email
• name
• preferred_username

Claims allow applications to personalize experiences while maintaining a consistent identity model.

Scopes

Scopes control what information the client can request. The openid scope is mandatory for OIDC and signals that the request is an authentication request.

Additional scopes such as profile, email, or custom scopes define which claims are returned. This scoped approach minimizes data exposure and supports privacy-by-design.

OIDC Endpoints Developers Must Know

OIDC defines a set of standardized endpoints that clients interact with.

Authorization Endpoint

This is where authentication begins. The client redirects the user to the authorization endpoint to authenticate and consent to requested scopes.

The authorization request includes parameters such as:

• client_id
• redirect_uri
• scope
• response_type

Correct construction of this request is critical for security.

Token Endpoint

After successful authentication, the client exchanges authorization data for tokens at the token endpoint. This step occurs server-to-server, protecting sensitive credentials.

UserInfo Endpoint

The UserInfo endpoint allows clients to retrieve additional identity claims using an access token. This is optional but useful when identity data changes over time.

JWKS Endpoint

The JSON Web Key Set (JWKS) endpoint exposes public keys used to verify token signatures. Clients use this endpoint to validate ID Tokens securely.

OIDC Authentication Flows Explained

OIDC defines multiple flows to support different application types.

Authorization Code Flow

This is the most secure and widely recommended flow. It is used by server-side web applications and modern SPAs with PKCE.

The flow separates user authentication from token exchange, reducing token leakage risk.

Authorization Code Flow with PKCE

PKCE (Proof Key for Code Exchange) protects public clients that cannot securely store secrets. It prevents authorization code interception attacks and is mandatory for SPAs and mobile apps.

Implicit Flow (Deprecated)

The implicit flow was designed for browser-based apps but is now discouraged due to security risks. Modern implementations should avoid it entirely.

Token Validation and Security Best Practices

Implementing OIDC correctly requires strict token validation.

Clients must verify:

• Token signature using trusted keys
• Issuer and audience values
• Expiration and nonce values

Skipping these checks undermines the security guarantees of OIDC and exposes applications to replay and impersonation attacks.

Common OIDC Implementation Mistakes

Many OpenID Connect implementation problems stem from incorrect usage rather than limitations of the specification itself. OIDC provides strong security guarantees, but only when its components are applied correctly. 

The following mistakes are frequently seen in real-world implementations and can significantly weaken authentication security.

Using Access Tokens for User Identification

Access tokens are designed to grant applications permission to access protected APIs, not to identify users. Treating them as proof of authentication can result in inaccurate user identification and security gaps. Developers should always rely on the ID Token, which is specifically issued to confirm a user’s identity.

Failing to Properly Validate ID Tokens

Receiving an ID Token alone is insufficient. Applications must validate the token’s signature, issuer, audience, expiration time, and nonce to ensure it is legitimate. Skipping these checks leaves systems vulnerable to replay attacks and token misuse.

Requesting Excessive Scopes

Over-requesting scopes can expose unnecessary user data and violate privacy best practices. Applying the principle of least privilege helps minimize data exposure and supports regulatory compliance.

Insecure Token Storage in Client Applications

Storing tokens in insecure browser storage increases exposure to cross-site scripting attacks. Using secure storage mechanisms and short-lived tokens reduces the risk of token compromise and strengthens overall security.

How OIDC Enables Modern Authentication Use Cases

OIDC is the backbone of:

• Single sign-on (SSO)
• Social login
• Passwordless authentication
• Enterprise federation
• Zero-trust architectures

Its flexibility allows organizations to evolve authentication strategies without rewriting application logic.

Bottom Line

The OIDC specification is not just a technical document; it is a blueprint for modern identity systems. While the full spec is extensive, developers do not need to memorize it to build secure authentication.

By understanding key concepts such as roles, tokens, flows, claims, and endpoints, developers can implement OIDC confidently and correctly. This knowledge reduces security risk, improves interoperability, and future-proofs authentication systems.

Platforms like Authgear simplify OIDC implementation by handling protocol complexity, security best practices, and compliance requirements out of the box. This allows development teams to focus on building products, not reinventing identity infrastructure.

Explore Authgear today to simplify OIDC implementation and future-proof your authentication stack.

FAQs

When should developers use OIDC?

OIDC should be used whenever applications need secure, scalable user authentication, especially for SSO, social login, MFA, or multi-platform identity integration.

What is the difference between an ID Token and an Access Token?

An ID Token confirms a user’s identity and is meant for the client. An Access Token authorizes API access and is meant for resource servers, not user identification.

Is PKCE required for OIDC?

PKCE is required for public clients like SPAs and mobile apps. It is strongly recommended for all clients to prevent authorization code interception attacks.

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