Login & JWT Generation
Build a user login system that issues and manages JSON Web Tokens (JWTs) for session management.
Login & JWT Generation is a free AI Powered SaaS: Stripe + Auth + Billing + Deploy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the AI Powered SaaS: Stripe + Auth + Billing + Deploy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Welcome to Login Systems
In the previous lesson, we learned about user registration and secure password hashing. Now, it's time to build the login system!
A login system allows authenticated users to access protected resources and personalize their experience within your SaaS application.
How Login Works (Overview)
When a user tries to log in, they typically provide a username (or email) and a password. Here's the basic flow:
- The client (e.g., your mobile app) sends credentials to the server.
- The server verifies these credentials against its stored user data.
- If valid, the server grants access.
Verifying User Credentials
Upon receiving login credentials, your server needs to perform a crucial check:
- Find the user by their unique identifier (e.g., email or username).
- Retrieve the stored hashed password for that user.
- Compare the provided password (after hashing it with the same method) with the stored hashed password.
Never store passwords in plain text! Always hash and salt them, as we discussed in the registration lesson.
The Challenge: Stateless APIs
Modern APIs are often stateless, meaning the server doesn't remember previous requests from the same client. This makes APIs scalable but poses a challenge for user authentication.
How do we know if a user who just logged in is still authenticated on their next request without sending credentials every time?
Introducing JSON Web Tokens (JWT)
JSON Web Tokens (JWTs) are a compact, URL-safe means of representing claims to be transferred between two parties. They are perfect for stateless authentication.
Instead of server-side sessions, the server issues a JWT upon successful login. The client then stores this token and sends it with every subsequent request.
JWT Structure: Header
A JWT consists of three parts separated by dots: Header, Payload, and Signature.
The Header typically contains two parts:
typ(type of token, usually 'JWT')alg(signing algorithm, e.g., 'HS256' for HMAC SHA256)
It's a JSON object, Base64Url-encoded.
{"alg":"HS256","typ":"JWT"}JWT Structure: Payload (Claims)
The Payload contains the 'claims' – statements about an entity (usually the user) and additional data.
Claims can be:
- Registered: Standard fields like
sub(subject),exp(expiration time),iat(issued at time). - Public: Custom claims defined by you, but registered in the IANA JSON Web Token Registry.
- Private: Custom claims agreed upon by the parties using them, like
userIdorrole.
Example Payload:
{"sub":"12345","name":"Coddy User","exp":1700000000}JWT Structure: Signature
The Signature is crucial for verifying the token's integrity. It's created by taking the encoded header, the encoded payload, and a secret key, then applying the algorithm specified in the header.
If anyone tries to tamper with the header or payload, the signature verification will fail, making the token invalid. The secret key is known only to the server.
Generating a JWT (Conceptual)
After successfully verifying a user's credentials, your server generates a JWT. This involves:
- Creating the Header and Payload JSON objects.
- Base64Url-encoding both.
- Concatenating them with a dot.
- Signing the combined string using a secret key and the chosen algorithm to produce the Signature.
The final JWT is EncodedHeader.EncodedPayload.Signature.
JWT Generation Example
While full JWT signing requires a library, we can demonstrate the Base64 encoding part of building a JWT string. This ensures the token is URL-safe.
import java.util.Base64;
import java.nio.charset.StandardCharsets;
public class Main {
public static void main(String[] args) {
String headerJson = "{\"alg\":\"HS256\",\"typ\":\"JWT\"}";
String payloadJson = "{\"sub\":\"user123\",\"name\":\"Coddy User\"}";
String encodedHeader = Base64.getUrlEncoder().withoutPadding().encodeToString(headerJson.getBytes(StandardCharsets.UTF_8));
String encodedPayload = Base64.getUrlEncoder().withoutPadding().encodeToString(payloadJson.getBytes(StandardCharsets.UTF_8));
System.out.println("Header (Base64Url-encoded):\n" + encodedHeader);
System.out.println("\nPayload (Base64Url-encoded):\n" + encodedPayload);
System.out.println("\nConceptual JWT structure: " + encodedHeader + "." + encodedPayload + ".[Signature]");
}
}Quick Check: JWT Parts
You've just learned about the three main parts of a JSON Web Token (JWT).
Recap: Login & JWTs
Great job! You've learned how a user login system works and the role of JSON Web Tokens (JWTs) in modern, stateless authentication.
- Login involves verifying credentials against hashed passwords.
- JWTs provide a stateless way to manage user sessions.
- JWTs have three parts: Header, Payload, and Signature.
- The Signature ensures the token's integrity.
Next, we'll explore how to use these JWTs to protect your API routes!
Frequently asked questions
Is the “Login & JWT Generation” lesson free?
Yes — the full text of “Login & JWT Generation” is free to read here on the web, and the AI Powered SaaS: Stripe + Auth + Billing + Deploy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the AI Powered SaaS: Stripe + Auth + Billing + Deploy course, upgrade to CoddyKit PRO.
What will I learn in “Login & JWT Generation”?
Build a user login system that issues and manages JSON Web Tokens (JWTs) for session management. You practise AI Powered SaaS: Stripe + Auth + Billing + Deploy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start AI Powered SaaS: Stripe + Auth + Billing + Deploy?
No prior experience is required. AI Powered SaaS: Stripe + Auth + Billing + Deploy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Login & JWT Generation” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this AI Powered SaaS: Stripe + Auth + Billing + Deploy lesson?
Yes. Every AI Powered SaaS: Stripe + Auth + Billing + Deploy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.