JSON Web Tokens (JWT) are a powerful tool used in web development for securely transmitting information between different systems. They are especially useful for authentication and ensuring safe data exchange. In this blog, we’ll break down what JWTs are, how they work, and how you can use Auth0 to easily integrate JWT into your applications.

What is JSON Web Token (JWT)?

JWT is an open standard (RFC 7519) that provides a compact, self-contained way of securely transmitting information between parties as a JSON object. This token can be digitally signed and verified, ensuring its integrity. You can sign a JWT using a shared secret with the HMAC algorithm or a public/private key pair with RSA or ECDSA.

While JWTs can also be encrypted to ensure privacy, this guide will focus on signed JWTs to verify the authenticity of the claims they contain.

Key Scenarios for JWT Usage

  1. Authorization: The most common use case for JWT. Once a user logs in, the server generates a JWT that is included with every request to access protected resources. JWTs are a great choice for Single Sign-On (SSO) systems because of their compact nature and ease of use across multiple domains.

  2. Information Exchange: JWTs are excellent for securely exchanging data between different services, as the token’s signature ensures the integrity of the claims.


JWT Structure

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

  • Header
  • Payload
  • Signature

Here’s an example of what a typical JWT looks like:
xxxxx.yyyyy.zzzzz

1. Header

The header contains two elements:

  • The type of token (JWT).
  • The signing algorithm (HMAC SHA256, RSA, etc.).
{
  "alg": "HS256",
  "typ": "JWT"
}

2. Payload

The payload consists of the claims. These claims are statements about an entity (typically the user) and some additional metadata. Claims fall into three categories:

  • Registered claims like iss (issuer), exp (expiration), sub (subject), and aud (audience).
  • Public claims which are custom claims registered with IANA or defined with a URI.
  • Private claims which are agreed upon between two parties and not publicly registered.

A sample payload might look like this:

{

  "sub": "1234567890",

  "name": "John Doe",

  "admin": true

}

3. Signature

To create the signature, we take the encoded header, the encoded payload, and the secret. Then, sign it using the algorithm specified in the header. For example, using HMAC SHA256:

HMACSHA256(

  base64UrlEncode(header) + "." +

  base64UrlEncode(payload),

  secret

)

The signature ensures that the token has not been altered, and when using a public/private key pair, it confirms the identity of the issuer.

How JWT Works

JWT works by issuing a token when a user successfully logs in. This token can then be sent with each subsequent request for access to resources or services. Typically, the token is included in the HTTP Authorization header using the Bearer schema:

Authorization: Bearer <token>

Security Considerations

Since JWTs are simply encoded and not encrypted, anyone who has access to the token can read its payload. Therefore, sensitive information should not be stored in a JWT payload unless encrypted. Additionally, JWTs should not be stored in browser storage or kept for longer than necessary to avoid vulnerabilities.

Why Use JWT?

JWTs offer several advantages compared to alternatives like Security Assertion Markup Language (SAML) and Simple Web Tokens (SWT):

  • Compact size: JWTs are encoded in JSON, making them much smaller than SAML tokens, which use XML.
  • Signature flexibility: JWTs can use both symmetric and asymmetric key pairs, while SWT only supports symmetric keys.
  • Ease of use: JSON, being widely supported across multiple languages, is easier to work with compared to XML.

Integrating JWT with Auth0

Auth0 is a powerful platform for implementing authentication using JWTs. It supports various types of flows (e.g., OpenID Connect, OAuth 2.0) and allows for quick integration of JWTs in web or mobile applications. Auth0 manages the heavy lifting, including token generation and validation, so developers can focus on building their applications securely.

Conclusion

JSON Web Tokens have become a go-to solution for authentication and secure data exchange in modern web applications. Their compact size and flexibility make them ideal for web and mobile environments. By integrating JWT with a platform like Auth0, you can quickly secure your applications and focus on delivering value to your users.

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