How CAS Enables Secure Single Sign-On: Architecture and Workflow Explained

What Is SSO and Why Use It?

Single Sign‑On (SSO) allows users to log in once and gain access to multiple trusted systems without re‑authenticating, reducing login time, minimizing errors, avoiding the storage of multiple credential sets, simplifying user administration, and enhancing overall security.

Introducing CAS

CAS (Central Authentication Service) is a mature, open‑source SSO framework originally initiated by Yale University and became a JA‑SIG project in December 2004. It provides a reliable method for web applications to implement single sign‑on.

Enterprise‑grade, open‑source solution.

CAS Server is deployed independently for authentication.

CAS Client supports many platforms such as Java, .NET, PHP, Perl, Apache, uPortal, Ruby, etc.

Basic CAS Protocol Flow

Access Service : The SSO client requests a protected resource from the application.

Redirect for Authentication : The client redirects the user to the SSO server.

User Verification : The user authenticates (e.g., username/password).

Ticket Generation : The SSO server creates a random Service Ticket.

Ticket Validation : The SSO server validates the Service Ticket; upon success, the client is allowed to access the service.

Transmit User Info : After validation, the SSO server sends the authentication result to the client.

CAS Architecture

The architecture consists of two main components:

CAS Server – deployed separately, responsible for authenticating users.

CAS Client – handles access requests to protected resources and redirects to the CAS Server when authentication is required.

Detailed CAS Login Process

User enters credentials on the CAS login page.

The CAS server authenticates the credentials, often using an external mechanism such as LDAP.

After successful authentication, CAS creates a Service Ticket and engages in a complex interaction with the application (authorization).

The application validates the ticket and redirects the user back, completing the login.

CAS sets an encrypted cookie on the user's browser containing login information.

For subsequent applications, the CAS client redirects to the CAS server, which reads the cookie and logs the user in automatically without prompting for credentials.