Difference between JWT, OAuth2, and SSO Authentication

Aspect	JWT (JSON Web Token)	OAuth2	SSO (Single Sign-On)
What is it?	A token format (JSON Web Token) for securely transmitting info between parties.	An authorization framework to delegate access to resources without sharing credentials.	An authentication process that lets users log in once and access multiple apps/systems.
Purpose	Carry claims (identity, permissions) securely and compactly.	Grant limited access to user resources by third-party apps.	Improve user experience by reducing multiple logins across apps.
How it works?	Server issues a JWT after user login; client sends JWT with each request; server verifies JWT.	Client requests authorization token from authorization server; uses token to access resource server.	User logs into an identity provider once; identity provider issues tokens or sessions recognized by multiple apps.
Components	Token only — header, payload, signature.	Several roles: Resource Owner, Client, Authorization Server, Resource Server.	Identity Provider (IdP), Service Providers (SPs), authentication protocols like SAML, OpenID Connect.
Typical use	Stateless authentication in APIs & microservices.	Delegated access (e.g., "Login with Google", API access).	Enterprise apps, web portals with multiple apps needing centralized login.
Token format	Usually JWT format, self-contained token.	Can use JWT tokens or opaque tokens depending on implementation.	Uses tokens (SAML assertions, JWT) or cookies for session management.

Detailed Explanation

What: A compact, URL-safe token format with three parts: Header, Payload, Signature.
Usage: After successful login, server issues a JWT that encodes user identity and permissions.
Client: Sends JWT in HTTP headers (usually Authorization: Bearer <token>) for subsequent requests.
Stateless: Server does not store session — validation is done by verifying the signature and claims in the JWT.
Pros:
- Stateless, scalable authentication.
- Easy to use across different domains and APIs.
- Can embed user info and claims directly.
Cons:
- If stolen, token can be reused until expiry (no easy server-side revoke).
- Requires careful implementation (e.g., secure signing, expiry).
- Tokens can grow large if too many claims included.

What: Authorization framework, not authentication (though often combined with OpenID Connect for authentication).
How it works:
- User grants permission to a client app to access resources on their behalf.
- Client obtains an access token (and optionally refresh token) from authorization server.
- Client uses token to access protected resources from resource server.
Pros:
- Delegated authorization without sharing credentials.
- Supports scopes/permissions.
- Widely adopted by major providers (Google, Facebook, Microsoft).
Cons:
- Complex protocol with many flows (authorization code, client credentials, implicit, etc.).
- Requires multiple servers/components.
- Needs HTTPS to be secure.

What: Users log in once with a trusted identity provider and get access to multiple systems without re-authenticating.
Protocols: SAML, OpenID Connect (built on OAuth2), Kerberos, etc.
How it works:
- User logs into IdP.
- IdP issues token/assertion/cookie.
- User accesses multiple apps (Service Providers) which trust the IdP.
Pros:
- Great user experience with one login for many apps.
- Centralized user management and policy enforcement.
- Reduces password fatigue and improves security.
Cons:
- Complex to set up and maintain.
- Single point of failure if IdP is down.
- Requires all apps to support the SSO protocol.

Feature	JWT	OAuth2	SSO
Authentication?	Token format, can be used for auth	Authorization framework	Authentication & centralized access
Stateful?	Stateless	Stateful (usually)	Stateful (session or tokens)
Use Case	API auth, microservices	Third-party delegated access	Enterprise-wide login across apps
Security Consideration	Token theft risk, no revoke	Complex, requires HTTPS	Centralized dependency
Complexity	Simple to implement	Moderate to complex	Complex setup