OAuth 2.0 Grant Types & Related Concepts Overview

 

Here’s an overview of the hierarchy of OAuth 2.0 grant types along with related concepts like Bearer Tokens, JWT, and OpenID Connect:

OAuth 2.0, Bearer Token, JWT, OpenID Connect Hierarchy Overview

1. OAuth 2.0

OAuth 2.0 is an authorization framework that allows third-party applications to obtain limited access to a resource, like user data, without exposing the user’s credentials. It works via various grant types for different scenarios.

1.1. Grant Types (Authorization Flows)

OAuth 2.0 defines multiple grant types, which are methods by which a client can obtain access tokens. These grant types are:

Ø  Authorization Code Grant: Used in server-side applications. The client receives an authorization code, which is exchanged for an access token.

Ø  Implicit Grant: Used in browser-based or mobile apps. The access token is returned directly to the client without the need for a code exchange (less secure, deprecated in favor of authorization code with PKCE).

Ø  Client Credentials Grant: Used when an application accesses its own resources, without a user being involved (server-to-server).

Ø  Resource Owner Password Credentials (ROPC) Grant: Used in trusted applications where the user provides their credentials directly (not recommended).

Ø  Refresh Token Grant: Used to obtain a new access token when the current one expires, without re-authenticating the user.

2. Bearer Tokens

After a client successfully completes a grant type flow, it receives an access token. These tokens are typically Bearer Tokens.

2.1 Bearer Tokens Overview

Ø  Bearer Tokens are tokens that provide access to protected resources. The term "bearer" means the party that has the token can use it to access resources (whoever "bears" the token can use it).

Ø  Tokens are passed in HTTP headers when accessing a protected resource, such as Authorization: Bearer <token>.

2.2 Types of Bearer Tokens

1. Opaque Tokens: Opaque tokens are arbitrary, non-transparent strings. They carry no information for the client and can only be validated by the authorization server.

The server maintains a session or a database that maps the token to the corresponding user and their permissions.

 Example: A random string like 3fa85f64-5717-4562-b3fc-2c963f66afa6

 

2. JWT (JSON Web Tokens): JWT is a self-contained token format that encodes claims (user information, token expiration time, etc.) directly into the token itself, in a structured way

 Structure:

Ø  Header: Metadata about the token, including the signing algorithm.

Ø  Payload: Contains claims (e.g., user details, token expiry).

Ø  Signature: Verifies the token’s integrity.

 Example:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9. eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ. SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

 

3. Macaroon Tokens: Macaroon tokens are a type of caveat-bearing token. They allow you to add constraints (caveats) to restrict the token’s use based on things like IP address, time, or action.

Structure: Similar to JWT, Macaroons are structured tokens, but they come with extra flexibility in applying fine-grained access controls.

Example: A base64-encoded string with attached caveats.

 

 

4. SAML Tokens (Security Assertion Markup Language): Though less common in modern OAuth flows, SAML tokens are often used in Single Sign-On (SSO) contexts. These tokens are based on XML and are exchanged as part of authentication or authorization.

Structure: SAML tokens are XML documents that include user information and assertions about the user’s identity.

Example: An XML structure that looks something like:

xml

<saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="abc123">     <saml:Issuer>https://auth.example.com</saml:Issuer>     <saml:Subject>         <saml:NameID>John Doe</saml:NameID>     </saml:Subject>     <saml:Conditions NotBefore="2024-01-01T12:00:00Z" NotOnOrAfter="2024-01-02T12:00:00Z"/> </saml:Assertion>

 

5. PASETO (Platform Agnostic Security Tokens): PASETO is a newer alternative to JWT, designed to be easier to use and less error-prone while addressing some of JWT’s known vulnerabilities. Unlike JWT, it removes algorithm flexibility to avoid certain attack vectors.

Structure: PASETO tokens have a simpler, more rigid structure:

Ø  Version (e.g., v2.local or v1.public).

Ø  Payload (encrypted or signed).

 Example: v2.local.AES256GCM_SECRET_PAYLOAD.

 

Types of Bearer Tokens

2.3 Types of Bearer Tokens Difference

Token Type	Description	Advantages	Disadvantages
Opaque Tokens	Random strings with no inherent meaning. Require server-side lookup.	Simple, secure, cannot be introspected by the client.	Server must store tokens, performance overhead.
JWT	Structured tokens with encoded claims.	Self-contained, widely supported, easy to validate.	If leaked, data in payload is exposed, not easily revoked.
Macaroon Tokens	Flexible tokens with constraints (caveats).	Fine-grained access control, highly customizable.	More complex to manage and verify.
SAML Tokens	XML-based tokens used in SSO environments.	Rich metadata, used in enterprise SSO.	Heavy and verbose, less common in token-based APIs.
PASETO	Modern, secure alternative to JWT with fewer vulnerabilities.	Safer and simpler than JWT.	Limited library and tool support compared to JWT.

 

 

Bearer tokens can take different forms, but one of the most commonly used forms in modern systems is JWT.

3. JWT (JSON Web Tokens)

3.1 Overview

JWT is a compact, self-contained token format commonly used in OAuth 2.0 and OpenID Connect flows.

Ø  Structure: A JWT consists of three parts:

1. Header: Metadata about the token, typically specifying the signing algorithm.
2. Payload: Contains the claims, such as the user's ID and token expiration time.
3. Signature: Used to verify the token’s authenticity.

JWT tokens are signed (and optionally encrypted), which makes them secure and tamper-resistant.

 

3.2 Types of JWT:

JSON Web Tokens (JWT) are a popular way of securely transmitting information between parties as a JSON object. The structure and functionality of JWTs can vary depending on their use case, with different types and features available. Here are the different types of JWTs based on their content, purpose, and the algorithms used for their creation:

1. Based on Purpose and Usage:

1.1 Access Tokens

Ø  Purpose: Used to access protected resources, such as APIs. When a client wants to access a resource, it sends the JWT in the Authorization header.

Ø  Structure: Contains claims such as sub (subject), iss (issuer), exp (expiration time), and aud (audience).

Ø  Example Use Case: OAuth 2.0 tokens for API access.

Ø  Common Claim: exp (expiration time), ensuring the token has a limited lifetime.

1.2 ID Tokens

Ø  Purpose: Used in OpenID Connect (OIDC) to authenticate users. It provides information about the user (identity) and their authentication status.

Ø  Structure: Includes claims like sub (subject), name, email, auth_time (authentication time), and nonce.

Ø  Example Use Case: Authentication in OIDC flows to confirm user identity.

Ø  Common Claim: sub (subject) - the unique identifier for the user.

1.3 Refresh Tokens

Ø  Purpose: Used to obtain new access tokens when the original access token expires, without requiring the user to re-authenticate.

Ø  Structure: Typically longer-lived tokens. They may or may not be JWTs (depends on implementation).

Ø  Example Use Case: OAuth 2.0 for long-lived sessions.

Ø  Common Claim: exp (expiration) - though it often has a longer validity than access tokens.

 

2. Based on Token Signature and Integrity:

2.1 Signed JWT (JWS - JSON Web Signature)

Ø  Description: These JWTs are signed using a cryptographic algorithm to ensure the data has not been tampered with. The signature confirms the token's authenticity.

Ø  Structure:

ü  Header: Contains the signing algorithm (e.g., HS256, RS256).

ü  Payload: The claims.

ü  Signature: Created by applying the signing algorithm to the header and payload using a secret or private key.

Ø  Example Use Case: Authentication and authorization tokens, where integrity is critical.

Ø  Common Algorithms:

ü  HS256 (HMAC with SHA-256): Symmetric key signing.

ü  RS256 (RSA with SHA-256): Asymmetric key signing (private/public key pair).

2.2 Encrypted JWT (JWE - JSON Web Encryption)

Ø  Description: These JWTs are encrypted to provide confidentiality. The payload is encrypted, making it unreadable to anyone without the decryption key.

Ø  Structure:

ü  Header: Contains the encryption algorithm and key management algorithm.

ü  Encrypted Key: Symmetric key used to encrypt the JWT.

ü  Ciphertext: Encrypted payload.

ü  Authentication Tag: Ensures message integrity.

Ø  Example Use Case: Use cases where sensitive information (e.g., personally identifiable information) is included in the JWT.

Ø  Common Algorithms:

ü  RSA-OAEP: For key management (asymmetric encryption).

ü  A256GCM: For content encryption (symmetric encryption).

2.3 Signed and Encrypted JWT

Ø  Description: A combination of JWS and JWE where the JWT is first signed and then encrypted. This ensures both integrity (via the signature) and confidentiality (via encryption).

Ø  Structure: The token first follows the structure of a signed JWT (JWS) and then is wrapped in the structure of an encrypted JWT (JWE).

Ø  Example Use Case: High-security environments where both the integrity and confidentiality of the JWT are critical.

3. Based on Token Content:

3.1 Standard JWT

Ø  Description: Contains standard claims as defined by the JWT specification, like iss (issuer), sub (subject), aud (audience), exp (expiration time), and iat (issued at).

Ø  Example Use Case: Most common in OAuth 2.0 and OIDC for access tokens and ID tokens.

Ø  Common Claims:

ü  iss: The issuer of the token.

ü  exp: The expiration time of the token.

ü  sub: The subject or user ID.

3.2 Custom JWT

Ø  Description: Contains additional custom claims specific to the application's needs beyond the standard JWT claims.

Ø  Example Use Case: Applications that need to include application-specific data (e.g., user roles, permissions) in the JWT.

Ø  Common Custom Claims:

ü  roles: The roles assigned to the user.

ü  permissions: Specific permissions granted to the user.

4. Based on Expiration Strategy:

4.1 Short-Lived JWT

Ø  Description: JWTs that have a very short exp (expiration) time to minimize the window of attack if a token is compromised.

Ø  Example Use Case: Security-sensitive applications where tokens are refreshed frequently to reduce risk.

Ø  Common Strategy: Combine with refresh tokens for a balance between security and usability.

4.2 Long-Lived JWT

Ø  Description: JWTs that have a longer exp time, reducing the need for frequent token refreshes but increasing risk if a token is compromised.

Ø  Example Use Case: Less critical applications or systems where frequent re-authentication is impractical.

Ø  Common Strategy: Typically used with additional security measures, like token revocation lists.

Types of JWT

Summary of JWT Types:

JWT Type	Description	Use Cases
Access Token	Used to access protected resources.	API access in OAuth 2.0
ID Token	Used in OpenID Connect to authenticate users.	User authentication in OIDC
Refresh Token	Used to obtain new access tokens without re-authenticating.	Maintaining long-lived sessions in OAuth 2.0
Signed JWT (JWS)	Ensures data integrity and authenticity with a signature.	Authorization and authentication tokens
Encrypted JWT (JWE)	Provides confidentiality by encrypting the payload.	Handling sensitive information in a secure way
Signed and Encrypted JWT	Combines signature and encryption for both integrity and confidentiality.	High-security environments requiring both protection and validation
Standard JWT	Uses standard claims like iss, sub, aud, exp, iat.	General use in OAuth 2.0 and OpenID Connect
Custom JWT	Includes custom claims specific to application needs.	Applications needing to convey additional user data (e.g., roles, permissions)
Short-Lived JWT	JWTs with a very short expiration time for higher security.	Security-sensitive applications
Long-Lived JWT	JWTs with longer expiration times to reduce the need for frequent re-authentication.	Less critical applications or where frequent re-authentication is impractical

 

4. OpenID Connect (OIDC)

OpenID Connect is an identity layer built on top of OAuth 2.0 that enables authentication (OAuth 2.0 primarily handles authorization).

Ø  Difference from OAuth 2.0: OAuth 2.0 provides access to resources, while OpenID Connect provides authentication (verifying the user's identity).

Ø  ID Tokens: OIDC uses ID tokens (which are JWTs) to convey information about the authenticated user, including claims like their identity, email, etc.

5. Grant Types Hierarchy Overview Diagram

 

Grant Types Hierarchy Overview Diagram

 

This structure shows how OAuth 2.0 manages authorization via different grant types, uses bearer tokens for accessing protected resources, and how JWT and OpenID Connect extend these concepts for security and identity management.

 

Each OAuth 2.0 grant type serves a specific use case

Each OAuth 2.0 grant type serves a specific use case and addresses different security requirements, resulting in variations in how the authorization token is issued and handled. Below, I’ll describe the purpose of each grant type and provide a problem statement that illustrates why each grant type is necessary.

1) Authorization Code Grant

Use Case:

This grant type is typically used for server-side web applications (confidential clients) where the client application needs to securely access protected resources on behalf of the user.

Problem Statement:

A web application (App A) needs to access a resource (R) on App B on behalf of a user. The user must authenticate with App B, but the app should never directly see the user’s credentials to ensure security. How can App A obtain access to App B without directly exposing the user’s credentials?

Solution:

The Authorization Code Grant addresses this problem by allowing App A to redirect the user to App B's Authorization Server, where the user can securely authenticate. App A receives an authorization code, which it then exchanges for an access token (and optionally a refresh token) in a server-to-server exchange. This ensures that the access token is never exposed to the user agent (i.e., the browser), providing better security.

 

Flow:

1. The client directs the user to the authorization server.
2. The user authenticates and grants authorization.
3. The authorization server redirects back to the client with an authorization code.
4. The client exchanges the authorization code for an access token.

 

Example Request Flow Details Step

1. Authorization Request:

Ø  Client sends a request to the Authorization Server to obtain an authorization code.

Ø  The request includes parameters such as client_id, redirect_uri, scope, and response_type=code.

2. Authorization Code:

Ø  User authenticates and authorizes the client.

Ø  Authorization Server sends an authorization code to the client.

3. Token Request:

Ø  The Client Application sends a token request to the Authorization Server, including the client_id, client_secret, authorization code, redirect_uri, and grant_type=authorization_code.

Ø  Client exchanges the authorization code for an access token (Bearer Token) at the Token Endpoint.

4. Access Token and ID Token:

Ø  The Authorization Server responds with an Access Token (Bearer Token) and an ID Token (JWT).

Ø  Access Token (Bearer Token):

ü  Client uses the access token to access protected resources on the Resource Server.

Ø  JWT:

ü  The Bearer Token can be a JWT, which contains encoded claims.

Ø  OpenID Connect:

ü  Uses OAuth 2.0 for authentication and JWT for ID tokens.

ü  Client obtains an ID token from the Token Endpoint.

Ø  ID Token (JWT):

ü  Contains user identity information.

ü  Client uses the ID token to access user profile information at the User Info Endpoint.

5. Resource Request:

Ø  The Client Application uses the Access Token to request protected resources from the Resource Server.

Ø  The request includes the Access Token in the Authorization header as a Bearer Token.

6. Protected Resource:

Ø  The Resource Server validates the Access Token and responds with the protected resource.

7. User Info Request:

Ø  The Client Application uses the Access Token to request user information from the User Info Endpoint.

Ø  The request includes the Access Token in the Authorization header as a Bearer Token.

8. User Information:

Ø  The User Info Endpoint responds with the user's profile information.

Authorization Code Grant

 

2) Implicit Grant

Use Case:

This grant type is used for browser-based or public clients (like Single Page Applications) where it’s unsafe to store sensitive credentials like client_secret, and tokens are delivered directly to the user agent.

Problem Statement:

A Single Page Application (SPA) running in a user’s browser needs to access an API, but it cannot securely store sensitive information like a client_secret. Since the client is public (accessible by anyone), it’s at risk of token exposure. How can the SPA securely obtain access to the API without requiring a backend server?

Solution:

The Implicit Grant allows the SPA to directly receive an access token (without exchanging an authorization code). This flow skips the authorization code step and issues the access token directly from the Authorization Server to the browser. While this method is less secure due to the token being exposed in the browser and URL, it's suitable for public clients where maintaining a client_secret is impractical. It prioritizes simplicity over security, thus access tokens are typically short-lived and require re-authentication frequently.

 

Flow:

1. The client directs the user to the authorization server.
2. The user authenticates and grants authorization.
3. The authorization server redirects back to the client with the access token.

 

Implicit Grant

 

 

3) Client Credentials Grant

Use Case:

This grant type is used for machine-to-machine(M2M) communication (without user involvement). It’s ideal for backend systems, scripts, or services that need to authenticate directly with an API to access its resources.

Problem Statement:

A server-side application (App A) needs to periodically retrieve data from App B (e.g., a weather API or a cloud storage service) without involving any user interaction. Since there’s no end-user to authenticate, how can App A authenticate itself securely and obtain the necessary permissions to access the resources?

Solution:

The Client Credentials Grant solves this by allowing App A to authenticate itself using its client_id and client_secret. The application directly requests an access token from App B's Authorization Server, which can then be used to access the API. No user interaction is needed, and the client authenticates directly using its own credentials.

 

Flow:

1. The client authenticates with the authorization server and directly requests an access token.

 

Client Credentials Grant

 

4) Resource Owner Password Credentials Grant (Less Common)

Use Case:

This grant type is only used when the application is highly trusted by the user, typically for first-party apps where the user can provide their credentials directly to the application (e.g., mobile apps for a company's service).

Problem Statement:

An internal enterprise app needs to allow employees to sign in with their corporate credentials to access internal resources. The app is highly trusted by the company, and users trust it enough to directly provide their credentials. How can the app authenticate users in a way that integrates seamlessly with OAuth 2.0?

Solution:

The Resource Owner Password Credentials Grant allows the user to provide their username and password directly to the application, which then forwards these credentials to the Authorization Server in exchange for an access token. This flow is only recommended for highly trusted clients where it’s acceptable for the user to provide their credentials directly to the app.

 

Flow:

1. The user provides their credentials to the client.
2. The client sends the credentials to the authorization server to obtain an access token.

 

 

Resource Owner Password Credentials Grant

Summary of Problem Statements:

1. Authorization Code Grant:

Ø  Problem: How can a web app securely access user resources on another server without exposing the user’s credentials?

2. Implicit Grant:

Ø  Problem: How can a client-side app running in the browser securely obtain an access token without needing to store sensitive credentials?

3. Client Credentials Grant:

Ø  Problem: How can one server authenticate itself to another server and access its resources without involving any users?

4. Resource Owner Password Credentials Grant (for reference):

Ø  Problem: How can a trusted app allow users to authenticate by directly providing their username and password, while still following OAuth protocols?

Each of these grant types exists to solve distinct challenges in different application contexts, balancing security, ease of use, and the specific requirements of the client or server.