Mastering Single Sign-On (SSO): Principles, Tokens, and Real-World Applications

What is Single Sign-On?

Single Sign-On (SSO) is an identity authentication mechanism that allows a user to log in once and access multiple systems without needing to log in again for each system.

SSO Architecture

The implementation relies on an Identity Provider (IdP) that handles authentication centrally. The following diagram illustrates the architecture:

SSO Workflow

1. User initiates SSO login – When a user accesses a business system (e.g., Application A) without being logged in, they are redirected to the authentication center.

At the authentication center, the user provides credentials (username/password, possibly multi‑factor authentication). Upon successful verification, a unified session is created.

2. Token generation – The authentication center generates an access token (JWT) that contains user information such as user ID and roles, encrypts it, sets its expiration and scope, and returns it to the user or client application.

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The token is digitally signed so that resource servers can verify its integrity.

3. Token validation – The receiving system (e.g., Application A) sends the token to the SSO verification endpoint to check its validity and signature, either by using the IdP’s public key or by invoking a remote validation API.

If validation succeeds, the user is granted access based on the information contained in the token.

4. Access other systems – After validation, the application establishes a session for the user. When the user later accesses other protected applications, the token is automatically presented, eliminating the need for another login.

Application Scenarios

SSO is widely used in enterprise internal systems (HR, OA, ERP), social login integration (Google, Facebook), and e‑commerce platforms where a main site and sub‑sites share authentication.