Introduction

JSON Web Tokens (JWTs) are a popular method for securely transmitting information between parties. JWTs provide a standardized way to represent claims that can be encoded and signed, making them useful for authentication, authorization, and data exchange. In this article, we'll explore what JWTs are, how they work, and some real-world applications of JWTs.

What are JSON Web Tokens?

JSON Web Tokens (JWTs) are a compact and self-contained method for transmitting information between parties. A JWT consists of three parts: a header, a payload, and a signature. The header and payload are Base64Url encoded JSON strings, and the signature is a cryptographic hash of the header, payload, and a secret key.

The header contains metadata about the JWT, such as the algorithm used to sign the token. The payload contains claims, which are statements about an entity and additional data. Claims can be used to represent user information, permissions, or other metadata. The signature is used to verify that the JWT is valid and has not been tampered with.

How do JSON Web Tokens work?

When a user authenticates with a server, the server can issue a JWT containing the user's information and a signature. The JWT can then be transmitted to the client, which can use it to authenticate future requests.

To verify the JWT, the client can decode the header and payload and use the information to verify the signature. If the signature is valid, the JWT is considered authentic and can be used to authorize the user.

Real-world applications of JSON Web Tokens

There are many real-world applications of JWTs, including authentication, authorization, and data exchange. Let's explore some examples of how JWTs are used in practice.

Single Sign-On (SSO)

Single Sign-On (SSO) is a method of authentication that allows users to access multiple applications with a single set of credentials. JWTs can be used to implement SSO by encoding the user's authentication information in a JWT and transmitting it to the client. The client can then use the JWT to authenticate with other applications without requiring the user to re-enter their credentials.

For example, consider a user who wants to access two applications: a website and a mobile app. The user logs in to the website and is issued a JWT containing their authentication information. When the user launches the mobile app, the app can use the JWT to authenticate the user without requiring them to enter their credentials again.

Authorization

JWTs can also be used for authorization, which is the process of determining whether a user has permission to access a particular resource or perform a particular action. In this scenario, the JWT contains claims that specify the user's permissions or roles.

For example, consider an application that allows users to perform different actions based on their role. An admin user might be able to create and delete records, while a regular user can only view records. The application can use JWTs to encode the user's role in the JWT, and verify the JWT to determine the user's permissions.

Data exchange

JWTs can be used for secure data exchange between parties. In this scenario, the JWT contains claims that represent the data being transmitted. The JWT can be signed by the sender to ensure that the data has not been tampered with, and verified by the receiver to ensure that the data is authentic.

For example, consider an e-commerce website that needs to transmit order information to a payment gateway. The website can encode the order information in a JWT and sign it using a secret key. The payment gateway can then verify the JWT to ensure that the order information has not been tampered with, and process the payment accordingly.

An example of how to generate a JSON Web Token (JWT) in JavaScript using the jsonwebtoken library:

const jwt = require('jsonwebtoken');

const payload = { 
  username: 'john_doe',
  email: 'john_doe@example.com',
  isAdmin: true
};

const secret = 'my-secret-key';

const options = {
  expiresIn: '1h',
  algorithm: 'HS256'
};

const token = jwt.sign(payload, secret, options);

console.log(token);

In this example, the jwt.sign function is used to generate a JWT. The payload object contains the claims that will be encoded in the JWT. Claims are statements about the user or entity that the token represents, such as user ID, role, or email address.

The secret string is a shared secret between the client and server that is used to sign and verify the JWT. The options object contains additional options for the JWT, such as the expiration time (expiresIn) and the algorithm used to sign the token (algorithm).

When the jwt.sign function is called, it encodes the claims in the payload and generates a signature using the secret key and algorithm specified in the options. The resulting JWT can be sent to the client for use in subsequent requests.

Now let's talk about claims. Claims are statements about an entity (typically, the user) that can be encoded and signed in a JWT. Claims can contain any information that the server wants to share with the client or vice versa, such as user ID, email address, role, or permissions.

JWTs contain three types of claims: registered claims, public claims, and private claims.

Registered claims are a set of predefined claims that are recommended but not required. Registered claims include iss (issuer), sub (subject), aud (audience), exp (expiration time), nbf (not before), and iat (issued at).

Public claims are custom claims that are defined by the application and are meant to be shared with other parties. Public claims can contain any information that the server wants to share with the client or vice versa.

Private claims are custom claims that are used by the application for its own purposes and are not meant to be shared with other parties. Private claims can contain any information that the server wants to store in the JWT for its own use.

When encoding claims in a JWT, it's important to keep in mind that JWTs are not encrypted and can be decoded by anyone with access to the token. Therefore, sensitive information should not be included in the JWT.