Welcome to the very first lesson of the "OAuth Fundamentals & Mock Implementation" course! In this lesson, we'll explore OAuth, a powerful authorization framework that solves a fundamental problem in modern web applications. OAuth allows users to grant applications access to their data without sharing their passwords. (Note: While OAuth itself is for authorization, OpenID Connect extends OAuth to provide authentication capabilities — that's what powers "Login with Google"). By the end of this lesson, you'll have a solid understanding of OAuth basics, setting the stage for our hands-on Google OAuth implementation in the next lesson. Let's dive in! 🚀

Before OAuth, if you wanted to build an application that needed access to a user's account data from services like Google, Facebook, or GitHub, you had limited options:

1. Ask users for their passwords - This is insecure and breaks most service providers' terms of service
2. Create separate accounts - Users have to remember yet another password
3. Use basic authentication - Sharing credentials across services creates security risks

OAuth was created to solve these problems by providing a secure way for applications to access user data without ever seeing the user's password. While we'll focus on Google OAuth in this course, the same principles apply to Facebook Login, GitHub authentication, and many other OAuth providers.

OAuth is an authorization framework that allows applications to obtain limited access to user accounts on an HTTP service. Instead of sharing passwords, OAuth uses access tokens - credentials that authorize access to protected resources on behalf of the user. OAuth may also provide refresh tokens (optional), which allow applications to obtain new access tokens without requiring the user to re-authenticate.

When you see "Login with Google" or "Login with GitHub," you're seeing OAuth in action:

OAuth defines four key roles in the OAuth flow:

• Resource owner: The user who owns the data (you, when you log in).
• Client: Your application that wants access to user data.
• Authorization server: The server that authenticates the user and issues access tokens (e.g., accounts.google.com).
• Resource server: The server that hosts the protected resources and accepts access tokens (e.g., Google's API servers like people.googleapis.com).

Important Note: While the Authorization Server and Resource Server are conceptually separate in the OAuth specification, they can be operated by the same organization or even run on the same infrastructure. In our Google OAuth example, Google operates both servers. However, the OAuth spec allows these to be completely separate entities — for instance, you might use Auth0 or Okta as your Authorization Server while your own API servers act as Resource Servers. This separation is important for implementation and security, as each server has distinct responsibilities: the Authorization Server handles authentication and token issuance, while the Resource Server validates tokens and serves protected data.

Here's the simplified OAuth flow that happens when a user clicks "Login with Google":

1. User clicks "Login with Google" - Your app redirects to Google's authorization server, including a redirect_uri (the URL where Google should send the user back to your app after they approve or deny access)
2. User approves access - Google shows what permissions your app is requesting
3. Google sends authorization code - Google redirects the user back to the exact redirect_uri you specified in step 1, including a temporary authorization code in the URL. This redirect URI must match exactly what you registered with Google - this is a critical security feature that prevents attackers from intercepting authorization codes
4. Your app exchanges code for token - Your app extracts the code from the callback URL and sends it (along with the same redirect_uri) to Google to get an access token

This process ensures the user's password never leaves Google's secure servers.

Let's look at what this flow looks like with actual URLs and parameters:

Access tokens are temporary keys that allow your application to access specific user data. Think of them like a hotel key card:

• They expire after a certain time.
• They only work for specific "rooms" (permissions/scopes).
• They can be revoked by the user at any time.
• They don't reveal the user's password.

In this course, we'll use JSON Web Tokens (JWT) to manage user sessions after OAuth authentication. JWTs are a standard way to securely represent claims between two parties. A production-ready JWT should include:

• exp (expiration): When the token expires
• iat (issued at): When the token was created
• iss (issuer): Who created the token
• aud (audience): Who the token is intended for

We'll implement proper JWT security with expiration times in our OAuth flow to prevent token replay attacks and ensure sessions don't last forever.

In the next lesson, we'll implement this entire flow step by step! We'll create a mock Google OAuth implementation where you'll:

• Set up authorization URLs with state parameters for CSRF protection.
• Handle the callback process securely.
• Exchange authorization codes for tokens with proper expiration.
• Access user profile data.
• Deliver tokens securely using HttpOnly cookies.

You'll see exactly how each piece of the OAuth puzzle fits together in a real implementation.

In this lesson, we've explored the fundamentals of OAuth: why it exists, how it works, and the key concepts you need to understand. We've covered the basic flow from user click to access token and introduced the four OAuth roles.

You now have the foundation needed to understand our hands-on implementation in the next lesson. Get ready to build your own OAuth flow with Google — it's going to be exciting! 🎉