Understanding JWT: How It Works, Use Cases, and Best Practices

Understanding JWT: How It Works, Use Cases, and Best Practices

As a web app developer, you may be familiar with how JWTs work, or perhaps you have utilized them without fully grasping their architecture. A JWT, or JSON Web Token, is widely popular today for many purposes, especially for managing user identity, authorization, and data exchange.

What precisely is a JWT (JSON Web Token)? In what manner does it function, and what has led most developers to switch from using session-based authentication to JWTs?

Don't worry if the concepts seem complex because in the course of this article, we will define everything simply. We will cover what JWT is, the token's lifecycle, typical applications, and the most important things to remember while working with JWT in your applications. This will aid not only novices seeking foundational knowledge but also advanced users looking to broaden their perspectives.

So, let's explore the realm of non-dependent authentication and discover how integrating JWT into your applications can enhance their scale and security.

🛡️ What is JWT?

JWT stands for JSON Web Token. It's an open standard (RFC 7519) used to securely transmit information between two parties—usually a server and a client—as a JSON object.

At its core, a JWT is just a string that contains some data (called claims) that can be verified and trusted because it’s digitally signed. This signature ensures that the data hasn’t been tampered with.

JWTs are most commonly used for authentication. Once a user logs in, the server generates a token and sends it back to the client. The client then includes this token in the header of future requests, allowing the server to verify the user without needing to check a session or a database every time.

📦 JWT Structure

A JWT is made up of three parts, separated by dots (.):

xxxxx.yyyyy.zzzzz

These three parts are:

1. Header – Contains information about the type of token (JWT) and the signing algorithm used (e.g., HS256).

2. Payload – Contains the claims or the actual data you want to send (like user ID, email, role, etc.).

3. Signature – A hash created using the header, payload, and a secret key. This is what makes the token secure and verifiable.
   
   

Here’s a basic example of what a JWT looks like (decoded):

Header:
{
  "alg": "HS256",
  "typ": "JWT"
}

Payload:
{
  "userId": "12345",
  "email": "user@example.com",
  "role": "admin"
}

🧠 Key Thing to Remember

A JWT doesn’t hide the data—it just makes sure it hasn’t been altered. The data in the payload is Base64 encoded, not encrypted. That means anyone who gets hold of the token can decode it and see what’s inside. That’s why you should never store sensitive information (like passwords) in a JWT.

🔐 How JWT Signing Works (Not Encryption)

When a server generates a JWT, it signs the token using either:

• A secret key (HMAC algorithm like HS256)

• Or a private key (RSA/ECDSA algorithms like RS256)

This signature acts like a seal of trust. When the client sends the token back in future requests, the server can verify that the token hasn’t been changed by recalculating the signature and comparing it.

🕵️‍♂️ So... Can Anyone Read the Payload?

Yes, unless you encrypt the JWT using something like JWE (JSON Web Encryption), which is a separate standard. But in most use cases, JWT is just signed, not encrypted.

That’s why you should only store non-sensitive information in the token — things like user ID, role, email, etc.

🛡️ Summary

• Signing ≠ Encryption: Signing ensures the token wasn’t changed. Encryption hides the data.

• By default, JWTs are signed using a secret key or private/public key pair.

• The server uses the signature to verify that the token is legit and hasn’t been tampered with.

• If you need to hide the data (e.g. for tokens traveling through insecure channels), you’d need to implement JWT encryption using JWE.