MQTT protocol Security – MQTT over SSL/TSL: Mosquitto and certicates

This tutorial describes an important aspect of the MQTT protocol that is security. We have already covered what is the MQTT protocol and how to use it but we didn’t cover how to secure the MQTT. As we already know the MQTT is a plain protocol therefore, everyone can read it. If we want to use MQTT with sensitive data we have to secure it.

What does MQTT security mean?

An important aspect is how to implement MQTT security. In more detail, we will describe how to implement MQTT security using the Mosquitto MQTT broker.

In this context, it is important to talk about MQTT security and it is crucial to know how to securing MQTT protocol and how to protect the information. In the next paragraphs, we will analyze the steps we have to follow to secure MQTT using Raspberry Pi as an MQTT broker.

By its nature, MQTT is a plain protocol that is all the information exchanged is in plain-text format. In other words, everyone could access to this message and read the payload. This could not be a problem if MQTT client and MQTT broker exchange not sensible information. Anyway, they are several use cases where we want to keep the information private and guarantee that it can not be read or modified during the transmitting process. In this case, there are several approaches we can use to face the MQTT security problem:

  1. Create a VPN between the clients and the server
  2. Use MQTT over SSL/TSL that encrypts and secure the information between the MQTT clients and MQTT broker

We will focus our attention on how to create an MQTT over SSL. To make MQTT a secure protocol we have to follow these steps:

  • Create a private key (CA Key)
  • Generate a certificate using the private key (CA cert)
  • Create a certificate for Mosquitto MQTT server with the key

The final step is configuring Mosquitto MQTT so that it uses these certificates.

MQTT Security: Securing Mosquitto MQTT server

The first step in this process is creating a private key. Connect to the Raspberry Pi using ssh or a remote desktop as you prefer and open a command terminal. Before starting, it is important you check if OpenSSL is installed in your Raspberry Pi, otherwise, you have to download from here.

Before creating the private key, you should create a directory where you store all the certificates you will create. In the terminal write:

openssl genrsa -out mosq-ca.key 2048

Using this command, we are creating a 2048 bit called mosq-ca.key. The result is shown in the picture below:

mqtt ssl ca key

The next step is creating a X509 certificate that uses the private key generated in the previous step. Open the terminal again and in the same directory you used to store the private key write:

openssl req -new -x509 -days365 -key mosq-ca.key -out mosq-ca.crt

In this step, you have to provide different information before creating the certificate as shown in the picture below:

how to create a certificate for mosquitto

Creating the MQTT server certificate

Once the private key and the certificate are ready, we can move on and create the MQTT server certificate and private key:

openssl genrsa -out mosq-serv.key 2048

Then the server certificate. During this step, we have to create a CSR (Certificate Signing Request). You have to send this certificate to the Certification authority that after verifying the author identity returns a certificate. In this tutorial, we will use a self-sign certificate:

openssl req -new -key mosq-serv.key -out mosq-serv.csr

As you can notice we have used the private key generated in the step before. Finally, we can create the certificate to use in our MQTT Mosquitto Server:

openssl x509 -req -in mosq-serv.csr -CA mosq-ca.crt
-CAkey mosq-ca.key -CAcreateserial
-out mosq-serv.crt -days 365 -sha256

All done! We have completed the steps necessary to secure our MQTT server. You can verify your certificate writing:

openssl x509 -in mosq-serv.crt -noout -text

Now you should see the certificate.

How to configure MQTT Mosquitto Server to secure MQTT

Once the certificates are ready, we have to configure the MQTT Mosquitto server so that it can use these certificates. The certificates we have to use are:

  • mosq-ca.crt
  • mosq-serv.crt
  • mosq-serv.key

Locate the mosquitto.conf file that holds all the configuration parameters and add the following lines:

listener 8883
cafile /home/pi/ssl-cert-mosq/mosq-ca.crt
certfile /home/pi/ssl-cert-mosq/mosq-serv.crt
keyfile /home/pi/ssl-cert-mosq/mosq-serv.key

where the path /home/pi/ssl-cert-mosq is the path where you stored your certificate. Moreover, we change the default Mosquitto MQTT port to 8883.

Now you have to stop and restart Mosquitto MQTT so that it can read the new configuration file:

sudo service mosquitto stop/start

That’s all. Now our MQTT protocol is secure and encrypted. The last step is testing the configuration and the MQTT server.

MQTT Security testing Mosquitto over SSL/TSL

In this step, we will verify if the connection is correctly configured. To this purpose, we use MQTT.fx a java based MQTT client. After you installed it, we have to create a new profile providing all the information as shown in the picture below:

how to test MQTT using Mosquitto and MQTT.fx

Notice that we have enabled the SSL/TSL configuration providing the mosq-ca.crt creating during the previous steps. I wrote another tutorial describing how to use secure MQTT protocol. Amazon AWS uses secure MQTT to exchange data with the devices. If you want to know more about how to use secure MQTT protocol you can read how to connect ESP32 to AWS IoT.

Finally, we can connect to the MQTT Mosquitto server:

SSL profile

clicking on connect. You will notice that the MQTT client will establish the connection to the MQTT broker as you can check in the log tab.

Now it is time to test if our client gets the message. Select the subscribe menu and subscribe to the MQTT client to a topic (choosing a topic name).

On the Raspberry Pi side lets us send a message on the same channel:

mosquitto_pub -p 8883 -t "test" -cafile mosq-ca.crt -m "Hello MQTT" -d -h

The result is shown in the picture below:

mqtt publish over ssl

On the subscriber side we have:

mqtt encrypted message

As you can notice, we received the message sent by the publisher.

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