Building an Authentication System from Scratch

Authentication is the backbone of any modern web application. Whether you're building a simple blog or a complex SaaS platform, understanding how auth works under the hood is essential for every developer.

Why Build Auth from Scratch?

While libraries like Passport.js and NextAuth make it easy to add authentication, understanding the fundamentals helps you make better architectural decisions and debug issues faster. Let's break down the core concepts.

Password Hashing with bcrypt

Never store passwords in plain text. Use bcrypt to hash passwords before storing them in your database. bcrypt automatically handles salting, which protects against rainbow table attacks. The cost factor (salt rounds) determines how computationally expensive the hash is — a higher value means more security but slower hashing.

JSON Web Tokens (JWT)

JWTs are a compact, URL-safe way to represent claims between two parties. A JWT consists of three parts: Header (algorithm and token type), Payload (claims like user ID and expiration), and Signature (ensures the token hasn't been tampered with). The server signs the token with a secret key, and the client sends it with each request.

Access Tokens vs Refresh Tokens

Access tokens should be short-lived (15 minutes to 1 hour) to minimize damage if compromised. Refresh tokens are long-lived and stored securely (HTTP-only cookies) to obtain new access tokens without requiring the user to log in again. This pattern balances security with user experience.

Session-Based Authentication

An alternative to JWTs is session-based auth, where the server stores session data and sends the client a session ID cookie. This approach gives you more control — you can invalidate sessions instantly, track active sessions, and store server-side state. The trade-off is that it requires server-side storage (Redis is a popular choice).

OAuth 2.0 Integration

OAuth 2.0 allows users to log in with their existing accounts (Google, GitHub, etc.). The flow involves redirecting users to the provider, receiving an authorization code, exchanging it for tokens, and fetching user profile data. Libraries like Passport.js simplify this process significantly.

Security Best Practices

Always use HTTPS in production. Set secure, HTTP-only, and SameSite flags on cookies. Implement rate limiting on login endpoints. Use CSRF tokens for session-based auth. Validate and sanitize all inputs. Never expose sensitive information in error messages.

Conclusion

Building authentication from scratch teaches you the fundamentals that every web developer should understand. Start with password hashing and JWTs, then layer on refresh tokens and OAuth as needed. The key is to understand what each piece does and why it exists.