Welcome to the first lesson of our course on securing a Flask MVC application. In this lesson, we'll explore how to implement an authentication middleware to protect the routes in our app. This middleware is vital because it ensures that only authorized users have access to specific features. For our ToDo app, it will serve as a gatekeeper, safeguarding user routes and enhancing security.

Think of an authentication middleware as a checkpoint that verifies a user's credentials before permitting access to designated areas of the application. By the end of this lesson, your app will be fortified with this indispensable security feature, ready to prevent unauthorized access.

In this lesson, we will cover the following key topics to secure our Flask ToDo app:

1. Creating the Authentication Page: Design a user interface for login and registration to facilitate user authentication.
2. Implementing the User Controller: Establish a controller to handle the connection between the authentication page and the backend.
3. Developing the Authentication Middleware: Develop middleware to intercept requests and verify user authentication status.
4. Integrating Middleware into the Flask App: Seamlessly incorporate the authentication middleware into the app to protect routes.

By the end of this lesson, you will be equipped with the skills to implement robust authentication middleware in your Flask application, enhancing its security and accessibility management.

The first step in building an authentication system is to provide a user interface for login and registration. Let's create a new auth.html template for our authentication page inside our /templates directory:

This HTML code provides a simple form for user login and registration. It includes fields for a username and password, as well as buttons to submit the form. For now, we will not add any actions to these buttons, since we just need a page to redirect unauthenticated users.

Next, we'll create a controller to handle routes related to users, such as rendering the authentication page, and later handling registration and login.

Here's how you set up the controllers/user_controller.py:

In this code, we use Flask's Blueprint to modularize our application. Inside this blueprint we define the /auth route for rendering the authentication page. This controller lays the groundwork for managing user authentication.

Now, let's create an authentication_middleware.py file in the middlewares directory. This file ensures users can only access protected parts of the app if they are logged in.

Here's the middleware code with comments:

How It Works:

1. Function Setup: The require_login_middleware function is defined to check authentication. It runs require_login before every request.
2. URL Check: require_login checks if the current endpoint is an authentication page or a static file (like images or stylesheets). These should be accessible without login, so they are excluded from the check.
3. Session Check: It looks for user_id in the session. Without it, the user isn't logged in. For now, no users will have user_id in the session since we haven't implemented the login route yet.
4. Redirect: If user_id is absent, the user is redirected to /auth to log in, protecting other routes.

Finally, let's bring everything together by integrating this middleware into our Flask app. We will update the app.py file:

In this file, we initialize the Flask app, set up session management with a secret key, integrate our authentication middleware using the require_login_middleware(app) function and register the user_controller next to our already implemented controllers.

When a user tries to access the / route or any other route within the application, the authentication middleware takes action. The middleware checks if the user is authenticated by verifying the presence of a user_id in the session. If the user_id is not found in the session, this indicates that the user is not logged in. Consequently, the middleware intercepts the request and redirects the user to the authentication page /auth.

As depicted in the diagram, when a route is accessed, the middleware evaluates whether the user is authenticated. If not (i.e., "No" path), the user is redirected to the /auth page. If the user is authenticated (i.e., "Yes" path), access to the requested route is granted. This mechanism ensures that all protected routes are only accessible to authenticated users, effectively acting as a security layer to safeguard the application's restricted areas.

You have built a foundational understanding of adding authentication middleware to your Flask ToDo app and learned how to create a user authentication page, set up a user controller, and integrate a middleware to secure routes.

Now, you're ready to apply this knowledge in hands-on practice exercises. These exercises will reinforce what you've learned and give you the confidence to implement authentication in real-world Flask applications. Remember, securing your app is essential in protecting user data and maintaining trust. Keep up the great work, and let's move on to the practice session!