Introduction
Designing a signup and login feature seems straightforward at first glance: collect user credentials, store them, and verify them later. But the way you store and handle passwords can make or break the security of your entire application. Let’s walk through the evolution of a secure authentication system, from the naive approach to a production-ready, security-hardened solution.
The Naive Beginning - Storing Passwords in Plain Text
Imagine a simple Register API: it takes a username, email, mobile number, and password, and stores the password exactly as entered. The Login API retrieves that password, compares it directly, and issues an access token if it matches. Functionally, this works—but it’s catastrophically insecure.
If your database is ever leaked—through backups, logs, SQL injection, or insider access—every single password is exposed in plain text. Since many people reuse passwords across different platforms, a single leak can have far-reaching consequences.
Step One — Hash Instead of Storing Passwords
The first improvement is using a hash function. A hash takes the password and produces a fixed-length, irreversible output. During registration, you hash the password before storing it. During login, you hash the provided password and compare the two hashes.
This means your database no longer holds plain text passwords. But it’s still vulnerable to a type of attack called a rainbow table—a precomputed list of common passwords and their hashes that attackers can use for instant lookups.
Step Two — Add Salt to Defeat Rainbow Tables
A salt is a unique, random string generated for each user. You append it to the password before hashing. Even if two users have the same password, their hashes will differ because their salts are different.
The salt is stored alongside the hash in the database. Now, attackers can’t use precomputed rainbow tables because every password needs its own unique computation.
Step Three — Make Hashing Slow on Purpose
Even with salting, algorithms like MD5 or SHA256 are too fast, making brute-force attacks feasible. The fix is to use deliberately slow algorithms like Argon2id, bcrypt, scrypt, or PBKDF2 with high cost factors. This slows down the hashing process slightly (e.g., ~500ms) but makes mass password cracking prohibitively expensive—even for attackers with powerful GPUs.
Step Four — Add a Pepper for an Extra Lock
A pepper is another secret string, the same for all users, mixed with the password before hashing. Unlike the salt, the pepper is never stored in the database—it’s kept in environment variables, a secrets manager, or a KMS. This means that even if your database is fully compromised, attackers still can’t verify passwords without the pepper.
Step Five — Issue and Manage Tokens After Login
Once a user logs in successfully, you should issue a short-lived access token and, optionally, a refresh token. These tokens must be validated on every request, and revoked immediately on logout or suspicious activity. This ensures that stolen tokens have limited value.
Step Six - Additional Security Layers
A strong authentication system also:
Enforces rate limits and temporary lockouts after multiple failed attempts.
Offers multi-factor authentication for sensitive accounts.
Avoids logging or emailing passwords.
Uses HTTPS for all communication.
Implements secure, time-limited password reset flows.
Conclusion
A secure signup and login system never stores plain text passwords. Instead, it stores slow, salted hashes, optionally protected with a pepper. It uses robust hashing algorithms designed for security, not speed. It issues short-lived tokens for session management, and layers in defenses like MFA, rate limits, and secure password resets.
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