Adding a login button is easy. The problem is next.

You can add a Google login button even if you just watch YouTube. However, when I put it in my app, I got an error even though I followed the tutorial, why are there so many settings to log in to Apple, the account is split into two, and if the same person logs in to Google or App, and how do I delete the Apple Token because they want to cancel the membership...

And it doesn't end when you're logged in. Preventing user data from being seen by anyone, securing accounts one more time, and not being bombarded with bots or abuse — this is where the real service really needs to go.

This is the real reason why it's so hard to log in. I'm not sticking a button; I have to deal with what happens behind it. Also, most free courses don't cover this part.

So this course focuses on the “why”

This is an age where AI creates code. The important thing now is not “how to write the code,” but “why this happens.” Only then can AI determine if the code written is correct, and fix problems by itself.

This course checks “why we do it” at every step, even with official documentation. Why does Google end up logging in within the app, but Apple goes through a browser, and why the real criteria for distinguishing users is UID rather than email — once you understand the principles, you will be able to solve other functions you see for the first time by viewing documents on your own. OAuth, which you learn here, is a basic tool for overall development, which is used not only for social login, but also for API integration and external authentication.

I make and learn something like this

Login (authentication)

Firebase and Flutter project settings
How OAuth and social login work
Google login — Android SHA key registration, iOS URL scheme settings
Apple login — developer site settings, certificates and private keys, token revoke, and even required server-to-server notifications for Korean developers
Account linking multiple logins into one account
Manually reproduce and fix the account splitting bug
Implementing membership withdrawal (including revocation of Apple tokens)

security

Storing user data in Firestore
Security Rules — Using database rules and code to protect “only me is my data”
Multi-factor authentication (MFA) — Attach secondary authentication via SMS
App Check — Stop bots and abuse to prevent bill bombs

It doesn't just add features; it also covers why they occur and how to solve them without hiding errors encountered during actual development.

This is how the lecture is conducted

All codes are provided on GitHub. Each course has a different branch, and the finished version can be viewed all at once on the ClassA-final branch.
Instead of reading the code line by line, I organized it in comments. The lecture focuses on the big picture of “why it works like this.”
I'll open the official documentation and show you how to find it. Once you learn it, you can view documents and solve them by yourself from now on without me.

If you are stubborn, you can say something like this

“While implementing social login, I understood the OAuth flow, and the account integration was designed based on UID rather than email. Data access was controlled with Security Rules, and security was applied with multiple authentication and App Check.”

If it becomes “I know why authentication and security were designed this way” rather than “I've logged in” to the portfolio — that makes a real difference in interviews and job turnarounds.

What to bring

Development environment with Flutter installed
The Apple login part requires a Mac. Even if you don't have a Mac, you can follow all the Android and Google parts, and we'll guide you in advance when you start a course that requires a Mac.

Properly handling Flutter authentication — from login to app security

Properly handling Flutter authentication — from login to app security