Let's dive into setting up Single Sign-On (SSO) using Spring Security, Central Authentication Service (CAS), and Tomcat. This comprehensive guide will walk you through each step, ensuring you understand the process and can successfully implement SSO in your environment. If you're aiming to streamline user authentication across multiple applications, you've come to the right place.

Understanding the Basics

Before we get our hands dirty with configuration, let's clarify what each component does:

• Spring Security: A powerful and highly customizable authentication and access-control framework. It provides a robust foundation for securing Spring-based applications.
• CAS (Central Authentication Service): An open-source, enterprise-level single sign-on solution. It acts as a central authentication server, allowing users to authenticate once and access multiple applications without re-entering their credentials.
• Tomcat: An open-source Java Servlet container that executes Java Servlets, providing a runtime environment for Java web applications.

Why Use This Combination?

Combining Spring Security, CAS, and Tomcat offers several advantages:

• Centralized Authentication: CAS handles user authentication, ensuring a single point of entry and control.
• Enhanced Security: Spring Security provides robust security features, protecting your applications from various threats.
• Simplified User Experience: Users only need to log in once to access multiple applications, improving usability.
• Scalability: This setup can scale to handle a large number of users and applications.

Prerequisites

Before starting, make sure you have the following:

• Java Development Kit (JDK): Ensure you have a compatible JDK installed (e.g., JDK 8 or later).
• Apache Tomcat: Download and install Tomcat. We’ll be deploying our applications to this server.
• Maven: You’ll need Maven for dependency management and building your project.
• Spring Tool Suite (STS) or IntelliJ IDEA: An IDE for developing Spring applications.
• CAS Server: A running CAS server instance. You can download and deploy the CAS server from the Apereo CAS project.

Setting Up the CAS Server

First, you need a running CAS server. You can download the CAS server overlay and build it using Maven. Here’s a quick rundown:

1. Download CAS Server Overlay: Get the latest CAS server overlay from the Apereo CAS project on GitHub.
2. Configure Maven: Ensure Maven is correctly installed and configured on your system.
3. Build the CAS Server: Navigate to the CAS server overlay directory in your terminal and run mvn clean package.
4. Deploy the CAS Server: Deploy the generated WAR file (usually located in target/cas.war) to your Tomcat server. Rename the WAR file to cas.war before deploying.
5. Start Tomcat: Start your Tomcat server. The CAS server should be accessible at https://localhost:8443/cas (or the appropriate address based on your Tomcat configuration).

Make sure you have a valid SSL certificate configured for your Tomcat server, as CAS typically requires HTTPS.

Creating a Spring Boot Application

Next, let’s create a simple Spring Boot application that will be secured by CAS.

Step 1: Create a New Spring Boot Project

Use Spring Initializr (start.spring.io) to create a new Spring Boot project with the following dependencies:

• Spring Web: For building web applications.
• Spring Security: For authentication and authorization.
• Thymeleaf (optional): If you want to use Thymeleaf for your views.

Step 2: Add CAS Client Dependency

Add the CAS client dependency to your pom.xml file:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
</dependency>
<dependency>
    <groupId>net.unicon.cas</groupId>
    <artifactId>cas-client-autoconfig-support</artifactId>
    <version>2.4.0</version>
</dependency>

Make sure to use a compatible version of cas-client-autoconfig-support.

Step 3: Configure Spring Security

Create a configuration class to configure Spring Security and integrate it with CAS. Here’s an example:

import org.springframework.context.annotation.Configuration;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.web.AuthenticationEntryPoint;
import org.springframework.beans.factory.annotation.Autowired;

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Autowired
    private AuthenticationEntryPoint authenticationEntryPoint;

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
                .antMatchers("/", "/home").permitAll()
                .anyRequest().authenticated()
                .and()
            .logout()
                .permitAll()
                .logoutSuccessUrl("/")
                .and()
            .exceptionHandling()
                .authenticationEntryPoint(authenticationEntryPoint);
    }
}

Step 4: Create CAS Authentication Entry Point

Create a class that implements AuthenticationEntryPoint to redirect unauthenticated requests to the CAS server:

import org.springframework.security.core.AuthenticationException;
import org.springframework.security.web.AuthenticationEntryPoint;
import org.springframework.stereotype.Component;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@Component
public class CasAuthenticationEntryPoint implements AuthenticationEntryPoint {

    @Override
    public void commence(HttpServletRequest request, HttpServletResponse response,
                         AuthenticationException authException) throws IOException {
        String casLoginUrl = "https://localhost:8443/cas/login?service=" + request.getRequestURL();
        response.sendRedirect(casLoginUrl);
    }
}

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