How CAS Single Sign-On Works: Mechanisms, Components, and Real‑World Use Cases

CAS Single Sign‑On Overview

CAS (Central Authentication Service) is a token‑based single sign‑on (SSO) protocol. It centralises authentication state in a dedicated server so that a user authenticates once and can subsequently access any number of CAS‑protected applications without re‑entering credentials.

Key Benefits

Improves user experience by eliminating repeated logins.

Reduces authentication overhead for large organisations that operate many subsystems (e.g., e‑commerce platforms, enterprise portals, cloud consoles).

Core Components and Detailed Workflow

1. CAS Server

Responsibilities: validates user credentials, creates a global session (Ticket‑Granting Ticket, TGT), issues short‑lived Service Tickets (ST) for individual services, and manages session revocation.

User initiates login: the user accesses a CAS‑protected application and is redirected to the CAS server.

Credential verification: the server checks the supplied username/password (or other authentication mechanisms such as LDAP, OTP, etc.).

Ticket generation: upon successful authentication the server creates a TGT that represents the user's global session and a Service Ticket (ST) tied to the requested service.

Response to client: the ST is returned to the CAS client via the user's browser; the TGT is stored as a secure cookie on the CAS server side.

2. CAS Client (Application‑side library)

Responsibilities: integrates with the protected application, detects authentication state, redirects unauthenticated users to the CAS server, and validates received Service Tickets.

Check local session: if a valid session exists, the request proceeds without redirection.

Redirect to CAS: if no session, the client sends the user’s browser to the CAS server with a service URL parameter.

User authenticates on CAS: after login, the CAS server redirects back to the service URL, appending the ST.

Validate ST: the client sends the ST to the CAS server’s validation endpoint (e.g., /serviceValidate) over HTTPS.

Obtain user attributes & TGT: on successful validation the server returns the user’s principal identifier and any released attributes; the client establishes a local session and can optionally request a proxy‑granting ticket using the TGT for downstream services.

3. Protected Service (Application)

Responsibilities: represents the resource the user wants to access; each service is identified by a unique service URL registered with the CAS server.

Access request: the user attempts to reach the service through the CAS client.

ST submission: the client forwards the received ST (and service identifier) to the CAS server for validation.

Server validation: the CAS server checks the ST’s signature, expiration, and whether the user is authorised for the requested service.

Grant access: if validation succeeds, the service creates a local session for the user and serves the protected resource.

Typical Application Scenarios

E‑commerce platforms with separate modules (shopping cart, payment gateway, logistics tracking).

Enterprise internal systems such as HR, document management, email, and intranet portals.

Online service suites that combine social, news, and commerce features under a single user identity.

Cloud service providers where a single login grants access to compute, storage, and networking consoles.