JSON Web Token

Introduction

JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. JWTs are often used in authentication and authorization contexts, particularly in web applications to securely transmit information between a client and a server.

Structure of JSON Web Tokens

A JSON Web Token consists of three parts: a header, a payload, and a signature. These parts are separated by dots (.), resulting in a structure that looks like `xxxxx.yyyyy.zzzzz`.

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. For example:

```json {

 "alg": "HS256",
 "typ": "JWT"

} ```

This JSON is then 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. There are three types of claims: registered, public, and private claims.

**Registered claims**: These are a set of predefined claims which are not mandatory but recommended, to provide a set of useful, interoperable claims. Some of them are `iss` (issuer), `exp` (expiration time), `sub` (subject), and `aud` (audience).

**Public claims**: These can be defined at will by those using JWTs. However, to avoid collisions, they should be defined in the IANA JSON Web Token Registry or be defined as a URI that contains a collision-resistant namespace.

**Private claims**: These are custom claims created to share information between parties that agree on using them and are neither registered nor public claims.

The payload is then Base64Url encoded to form the second part of the JWT.

Signature

To create the signature part, you have to take the encoded header, the encoded payload, a secret, and the algorithm specified in the header, and sign that. For example, if you want to use the HMAC SHA256 algorithm, the signature will be created in the following way:

``` HMACSHA256(

 base64UrlEncode(header) + "." +
 base64UrlEncode(payload),
 secret)

```

The signature is used to verify that the sender of the JWT is who it says it is and to ensure that the message wasn't changed along the way.

Use Cases

JWTs are used in various scenarios, including but not limited to:

Authentication

In the authentication context, when the user successfully logs in using their credentials, a JWT will be returned. Since tokens are credentials, great care must be taken to prevent security issues. Generally, the token is sent in the HTTP header using the Bearer schema.

Information Exchange

JWTs 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, the structure of a JWT allows you to verify that the content hasn't been tampered with.

Security Considerations

When implementing JWT, it is crucial to consider the security implications:

**Algorithm Choice**: Always use a strong algorithm like RS256 or ES256 instead of HS256, which is symmetric and less secure.
**Token Expiration**: Always set an expiration (`exp`) claim to limit the token's lifetime.
**Token Storage**: Store tokens securely, using secure cookies or other secure storage mechanisms.
**Signature Verification**: Always verify the signature on the server side to ensure the token's integrity.

Advantages and Disadvantages

Advantages

**Compact**: JWTs are compact and can be sent via URL, POST parameter, or inside an HTTP header.
**Self-contained**: JWTs contain all the required information about the user, reducing the need for database lookups.
**Scalable**: Due to their stateless nature, JWTs are suitable for distributed systems.

Disadvantages

**No Built-in Revocation**: Once a JWT is issued, it cannot be revoked unless you implement a custom solution.
**Size**: JWTs can become large, especially when using public key signatures.
**Complexity**: Implementing JWT securely requires careful consideration of security best practices.

Implementation

Implementing JWT involves several steps:

1. **Create a JWT**: Encode the header and payload, sign it with a secret, and concatenate the parts. 2. **Send the JWT**: Transmit the JWT to the client, typically in an HTTP header. 3. **Verify the JWT**: On subsequent requests, verify the JWT's signature and claims to ensure authenticity and integrity.

Alternatives

While JWT is a popular choice, there are alternatives such as:

**OAuth 2.0**: A protocol for authorization that can use JWT as a token format.
**SAML (Security Assertion Markup Language)**: An XML-based framework for authentication and authorization.
**Paseto (Platform-Agnostic Security Tokens)**: A newer alternative to JWT that aims to address some of its security concerns.

Conclusion

JSON Web Tokens provide a robust and flexible mechanism for securely transmitting information between parties. Their compact and self-contained nature makes them ideal for modern web applications, particularly in distributed systems. However, implementing JWTs requires careful consideration of security best practices to mitigate potential vulnerabilities.