From Zero to Platform: Build Your Own GitHub-Level Login System with NextAuth and Casdoor

## Foreword: Why is Authentication So "Complicated"? "I just want to implement user login. Why can't I just add a `password` field to my user table and be done with it?" This is a very common and pertinent question. When choosing a tech stack, we often encounter frameworks like NextAuth and a long, dazzling list of "Providers" like Auth0, GitHub, and Casdoor. Intuitively, the simplest path seems to be handling usernames and passwords ourselves. However, this apparent shortcut hides significant security risks and a maintenance chasm. This article, written by **DP@lib00**, aims to guide you through this "detour," helping you understand why modern web applications choose to "outsource" authentication to specialized systems. Ultimately, it will show you how to use NextAuth and the open-source identity solution Casdoor to not only solve your application's login problem but also to build your own scalable identity platform, **wiki.lib00.com**. --- ## Part 1: Understanding the Players - NextAuth & Identity Providers (IdP) In the world of NextAuth, your application is the **Service Provider (SP)**. It provides services but doesn't personally verify a user's identity. That job is delegated to an **Identity Provider (IdP)**. These IdPs fall into two main categories: 1. **Identity as a Service (IDaaS)**: Such as Auth0 and Microsoft Azure AD. These are commercial cloud services that are powerful and ready to use, but you don't have full control over your data or deployment environment. 2. **Open-Source Self-Hosted Platforms**: Such as **Casdoor**, ZITADEL, and Authentik. These are the best choices for your goal of a "self-hosted" service. You can deploy them on your own servers, giving you complete control over user data and authentication logic. In this article, we will focus on using **Casdoor**, as it strikes a great balance between functionality, ease of use, and community support. --- ## Part 2: The Fundamental "Why" - Moving Beyond the Password Field The real question is, why take the long way around? The answer lies in **specialization of concerns** and **risk transfer**. | Feature | Self-Implemented (DB + Password) | Using Casdoor (OAuth/OIDC) | | :--- | :--- | :--- | | **Security** | Extremely high risk. You must handle password hashing, brute-force prevention, session hijacking, etc. | **Professionally secured**. The app never touches passwords; risk is transferred to the specialized IdP. | | **Functionality** | Basic username/password login only. | **Extremely rich**. Easily supports SSO, Multi-Factor Authentication (MFA), social logins, etc. | | **Dev Cost** | Seems low initially, but becomes extremely high as features are added. | Initial learning curve, but **dramatically reduces long-term costs**. | | **Maint. Cost** | High. Requires constant monitoring for security vulnerabilities and patching. | Low. You only maintain the Casdoor service; core security logic is maintained by the community. | | **Architecture** | Authentication is **tightly coupled** with business logic. | Authentication is **fully decoupled** from business logic, leading to a clean, scalable architecture. | In conclusion, separating the universal, complex, and critical module of identity authentication from your business application and entrusting it to a dedicated system is a safer, more efficient modern software architecture practice. --- ## Part 3: The "How" - Deconstructing the Authentication Flow Let's assume your application is LobeChat (`lobe.wiki.lib00.com`) and your Casdoor identity center is `id.wiki.lib00.com`. The complete user login flow (based on OAuth 2.0 and OIDC) is as follows: 1. **Initiate Login**: A user on LobeChat clicks "Login" and is redirected to Casdoor. ``` https://id.wiki.lib00.com/login/oauth/authorize? client_id=LOBECHAT_CLIENT_ID& redirect_uri=https%3A%2F%2Flobe.wiki.lib00.com%2Fapi%2Fauth%2Fcallback%2Fcasdoor& response_type=code& scope=openid+profile+email& state=RANDOM_STRING ``` 2. **User Authentication**: The user enters their username and password on the Casdoor page. LobeChat is completely unaware of this process. 3. **Authorization Callback**: After successful authentication, Casdoor redirects the user back to LobeChat's callback URL with a one-time `code`. 4. **Backend Token Exchange**: LobeChat's backend (NextAuth), on the server side, uses this `code` along with its `Client Secret` to request an **ID Token** from Casdoor. 5. **Validation & Session Creation**: * NextAuth verifies the **digital signature** of the ID Token. This signature, generated with Casdoor's private key, ensures the token was not tampered with during transit and was indeed issued by your Casdoor instance. This is the core of its security, solving the risk of a "plain string" being forged. * It parses the unique user identifier `sub` (e.g., `user123abc`) from the token. * The LobeChat backend uses this `sub` to associate with user data in its internal database (`SELECT * FROM conversations WHERE user_id = 'user123abc'`), thus achieving **multi-user data isolation**. * Finally, NextAuth creates a secure session cookie, and the user is successfully logged in. --- ## Part 4: The Ultimate Goal - From Private Service to Open Platform Now your application has a professional authentication system. But how do you take it a step further, like GitHub or Google, and allow other developers (let's call him Jone) to use your user system to log into their applications? This requires evolving your Casdoor from an "identity center" to an "identity platform." The key is to use Casdoor's **default shared user pool** (the `built-in` organization). **Platform Model Implementation Flow:** 1. **Establish a Unified User Center**: Your Casdoor instance (`id.wiki.lib00.com`) is this center. All users register here, forming a large, shared user pool. 2. **Create a Developer Portal**: On your main site, provide a "Developer Center" for registered users. Here, they can self-service register their applications. 3. **Developer Self-Service Onboarding**: * Developer Jone logs in and fills out a form in the developer center with his app's information (e.g., app name, callback URL). * Your backend calls the Casdoor API to create a new `Application` record for Jone's app, generating a unique `Client ID` and `Client Secret`. * Jone configures his application with these credentials. 4. **Enable Single Sign-On (SSO)**: * A user who is already logged into your platform visits Jone's app. * After clicking "Login with wiki.lib00.com," the user is redirected to your Casdoor. * Casdoor detects the existing login session, **skips the login step**, and directly authorizes Jone's app to access the user's information. By following this model, you successfully build an open identity ecosystem. Your platform becomes the center of trust, providing secure and convenient login services for countless third-party applications, while users only need to maintain one account to access the entire ecosystem. --- ## Conclusion From a simple password field to building a complex identity platform, we've explored not just a technological evolution but an upgrade in architectural thinking. By decoupling authentication from business logic and using modern tools like NextAuth and Casdoor, we can not only build more secure and feature-rich applications but also lay a solid foundation for future platform-level expansion.