JWT (JSON Web Token)

Introduction to JSON Web Tokens

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA.

When should you use JSON Web Tokens?

Here are some scenarios where JSON Web Tokens are useful:

Authorization: This is the most common scenario for using JWT. Once the user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token. Single Sign On is a feature that widely uses JWT nowadays, because of its small overhead and its ability to be easily used across different domains.

Information Exchange: JSON Web Tokens are a good way of securely transmitting information between parties. Because JWTs can be signed—for example, using public/private key pairs—you can be sure the senders are who they say they are. Additionally, as the signature is calculated using the header and the payload, you can also verify that the content hasn't been tampered with.

What is the Structure of JSON Web Tokens?

In its compact form, JSON Web Tokens consist of three parts separated by dots (.), which are:

Header

Payload

Signature

          Therefore, a JWT typically looks like the following.

header.payload.signature

Let's break down the different parts.

Header:

The header typically consists of two parts: the type of the token, which is JWT, and the signing algorithm being used, such as HMAC SHA256 or RSA.

Then, this JSON is Base64Url encoded to form the first part of the JWT.

Payload:

The second part of the token is the payload, which contains the claims. Claims are statements about an entity (typically, the user) and additional data. The payload is then Base64Url encoded to form the second part of the JWT.

        Set JSON Claims for the JWT with the following parameters:

Signature

To create the signature part you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

For example if you want to use the RS 256 algorithm, the signature will be created in the following way:

The output is three Base64-URL strings separated by dots that can be easily passed in HTML and HTTP environments. The following shows a JWT that has the previous header and payload encoded, and it is signed with a secret.