How to handle JWT revocation with MQTT

Introduction to MQTT and JWT

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol based on the publish/subscribe model, widely used for communication between devices and servers, particularly in IoT scenarios. It enables devices to publish messages to topics and other devices to subscribe to these topics for receiving corresponding messages.

JWT (JSON Web Tokens) is a concise, URL-safe, and self-contained token standard for securely transmitting information between parties. JWT is commonly used for authentication and secure information exchange, allowing you to verify the sender's identity and convey user or device state information.

Challenges in Handling JWT Revocation

JWT is an inherently stateless authentication mechanism that does not require servers to maintain the state of each token. This introduces challenges, particularly when revoking a specific JWT. Typically, JWT revocation necessitates some form of state management to track valid tokens and revoked tokens.

Strategies for Implementing JWT Revocation with MQTT

1. Revocation List:

   • Description: Create a revocation list to store unique identifiers of all revoked JWTs (e.g., jti - JWT ID).
   • Implementation: Use MQTT topics to publish and subscribe to revocation events. Whenever a JWT is revoked, publish its jti to a specific MQTT topic (e.g., jwt_revoked).
   • Device Operations: Devices subscribe to the jwt_revoked topic and add the jti to their local revocation list upon receiving each message. When validating a JWT, devices first check if the JWT's jti is present in the revocation list.

2. Timestamp Validation:

   • Description: Leverage the JWT's exp (expiration time) field to limit token validity. While this is not direct revocation, setting a short expiration time forces tokens to be periodically renewed.
   • Implementation: When a device receives a JWT, check the exp field to ensure the token is not expired. Additionally, use MQTT to publish new, updated JWTs to relevant topics to achieve a similar revocation effect.

Practical Application Example

Suppose you are managing an IoT environment where multiple devices need to securely receive commands from a central server. Implement the following mechanism:

• The central server publishes JWTs to the topic device_tokens/{device_id}, with each device subscribing only to its own topic.
• Upon detecting a security issue with a device, the central server publishes the JWT's jti for that device to the jwt_revoked topic.
• All devices subscribe to the jwt_revoked topic and maintain a local revocation list. Devices periodically check if their JWT is in this list.
• Before executing any operation, devices validate the JWT's validity by checking exp and the revocation list.

Conclusion

By combining MQTT's publish/subscribe capabilities with JWT's security features, we can effectively manage authentication states for numerous devices, achieving dynamic JWT revocation without maintaining persistent connection states for each device. This approach is particularly suitable for resource-constrained IoT environments.