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User authentication is how we verify who a user is. It's a critical part of almost any application that handles personal data or restricted features.

• Why it matters: Protects user accounts and sensitive information.
• What we'll cover: Building registration and login flows from scratch.

Registering a new user involves several steps to create a new account:

1. User provides credentials (e.g., username, password, email).
2. Input data is validated (e.g., strong password, unique email).
3. The password is hashed for security.
4. New user data (including the hashed password) is saved to the database.

Storing passwords in plain text is a huge security risk! If your database is breached, all user passwords would be exposed.

Hashing transforms a password into a fixed-size, unreadable string. It's a one-way process, meaning you can't easily get the original password back from the hash.

We use libraries like bcrypt in Node.js for robust password hashing, which also adds a 'salt' to prevent common attacks.

bcrypt is a popular library for securely hashing passwords. It's computationally intensive, making brute-force attacks harder.

Try running this example to see a password hashed:

After hashing, only the hashed password should be stored in your database, along with other user details like their email or username.

• NEVER store plain-text passwords.
• The hash is unique for each password, even if the original passwords are the same (thanks to salting).
• This hash is what you'll use for comparison during login.

When a user tries to log in, your application follows these steps:

1. User provides their username/email and password.
2. Application retrieves the user's record (including their stored hashed password) from the database based on the username/email.
3. The provided password is hashed and compared against the stored hash.
4. If they match, the user is authenticated, and a session or token is created.

To check if a user's provided password matches the stored hash, we use bcrypt.compare(). It performs the hashing and comparison securely.

Run this code to see password comparison in action:

Beyond just hashing passwords, other credentials need protection:

• API Keys & Database URLs: Store these in environment variables (e.g., .env files), not directly in your code.
• Sensitive User Data: Encrypt any highly sensitive data at rest in your database.
• Regular Updates: Keep your hashing libraries and dependencies up-to-date.

When registration or login fails, provide helpful but generic error messages to the user. This prevents revealing too much information to potential attackers.

• Instead of 'User not found', say 'Invalid credentials'.
• Instead of 'Password incorrect', also say 'Invalid credentials'.
• Log detailed errors on the server side for debugging, but don't expose them to the client.

Test your understanding of why password hashing is essential for security.

In this lesson, you learned the fundamental steps for user registration and login:

• We covered the importance of password hashing using bcrypt to protect sensitive user data.
• You saw how to implement both the hashing for registration and the comparison for login.
• We also touched on best practices for secure credential storage and handling authentication errors gracefully.

Next, we'll dive into implementing stateless authentication using JSON Web Tokens (JWTs).