How to create a registration and login system: from logic to code

💡 Why write your own auth system at all?

If you are developing an MVP, an admin panel, a personal account, or a custom SaaS application, you almost always need authorization. You can plug in Firebase or Auth0, but if you want full control or just to figure out how it works under the hood, we write our own registration and login system.

📦 What is included in a simple auth system?

• 📥 Registration

• 🔐 Password hashing

• 🔑 Login

• 🧾 Authorization

• 🚪 Exit

🧰 What do you need on the backend?

Any stack will work: Node.js, Python, PHP, Go, Java. For example, for Node.js and Express:

• POST /register — registration

• POST /login — login

• GET /me — current user

• middleware/auth.js — token verification

🔐 Password hashing

import bcrypt from 'bcrypt';
const hash = await bcrypt.hash(password, 10);
const isMatch = await bcrypt.compare(inputPassword, user.passwordHash);

The password is never stored in the database in its pure form.

🎟️ Tokens and JWT

import jwt from 'jsonwebtoken';
const token = jwt.sign({ userId: user.id }, JWT_SECRET, { expiresIn: '7d' });

It is transmitted through the Authorization header and checked on the server.

🧠 Safety tips

• ✅ Hash passwords (bcrypt)

  Why: so that even if the database is leaked, the attacker will not see the real passwords.

  Why it matters: hash is a one-way transformation. It is impossible to recover the password, only to pick it up.

• ✅ Use HTTPS

  Why: so that data between the client and the server goes through an encrypted channel.
  Why it matters: without HTTPS, any password can be intercepted via Wi-Fi or a proxy (man-in-the-middle attack).

• ✅ Data validation

  Why: to filter out dangerous or incorrect requests.

  Why it matters: prevents SQL injections, XSS and just garbage in the database. Check the length, type, format, etc.

• ✅ Limit login frequency

Why: to protect against brute-force attacks.
Why it matters: without restrictions, an attacker can endlessly try to guess the password.

• ✅ Store the token in a httpOnly cookie

Why: so that JS can't read the token.
Why it matters: protects against XSS attacks — even if someone embeds a malicious script, they won't be able to steal the token.

🖥️ How registration works

1. User enters email and password

2. Sending request to POST /register

3. The server validates, hashes, saves, and issues a token

4. The client saves the token — the user is authorized

🚀 What to add next?

• Email confirmation

• Password recovery

• 2FA

• OAuth (Google, GitHub, etc.)

The registration and login system is not magic, but a set of logical steps. The main thing is safety and a clear understanding of the process.

📚 Do you want to go deeper into backend?

If you are interested in backend, authorization, databases and API — go to Kodik app or to the website itcodik.com. Step-by-step courses, real tasks and explanation without unnecessary water. Learn backend simply!