Securest way to use TLS in MQT

[Jupyter1336]

So in the MQTT example the following function is used to write the CA certificate to the modem so a TLS connection can be setup.

err = modem_key_mgmt_write(CONFIG_MQTT_TLS_SEC_TAG,
				   MODEM_KEY_MGMT_CRED_TYPE_CA_CHAIN,
				   CA_CERTIFICATE,
				   strlen(CA_CERTIFICATE));
	if (err) {
		LOG_ERR("Failed to provision CA certificate: %d", err);
		return err;
	}

In our code/TLS setup we want to use Client certificates aswell that is why we are using the following setup.

#if defined(CONFIG_NRF_MODEM_LIB) && defined(CONFIG_MODEM_KEY_MGMT)

	err = modem_key_mgmt_write(CONFIG_MQTT_TLS_SEC_TAG,
				   MODEM_KEY_MGMT_CRED_TYPE_CA_CHAIN,
				   CA_CERTIFICATE,
				   strlen(CA_CERTIFICATE));
	if (err) {
		LOG_ERR("Failed to provision CA certificate: %d", err);
		return err;
	}

        err = modem_key_mgmt_write(CONFIG_MQTT_TLS_SEC_TAG,
				   MODEM_KEY_MGMT_CRED_TYPE_PRIVATE_CERT,
				   CLIENT_PRIVATE_KEY,
				   strlen(CLIENT_PRIVATE_KEY));
	if (err) {
		LOG_ERR("Failed to register private key: %d", err);
		return err;
	}

        err = modem_key_mgmt_write(CONFIG_MQTT_TLS_SEC_TAG,
				   MODEM_KEY_MGMT_CRED_TYPE_PUBLIC_CERT,
				   CLIENT_PUBLIC_CERTIFICATE,
				   strlen(CLIENT_PUBLIC_CERTIFICATE));
	if (err) {
		LOG_ERR("Failed to register public certificate: %d", err);
		return err;
	}

The problem is that this writes the certificate on startup to the modem. Making it debugable from outside. Thats why you have a very large warning in the AWS_FOTA example saying that this is not best practice. But if I turn off debugging and make memory and all that stuff non-readable is it secure again? If not what is best practise to use if I dont wish to do multistage programming in which I write the TLS first and then use the modem_key_mgmt_read function. I would love to know!

[Øyvind]

Hello, 

I've forwarded you questions to our MQTT experts. And will reply today or tomorrow. 

Kind regards,
Øyvind

[Jupyter1336]

Thnx

[Øyvind]

Hello, my apologies for the late reply.

The alternative is to flash the certificates using pynrfjprog or the certificate manager in the lte link monitor. The code that writes credentials to the modem at boot can then be removed. As long as the sec tag used in the TLS connection matches the sec tag used when flashing the certificates it should be all good.

When we produce DKs and Thingy91s, certificates are provisioned in the factory using AT commands. So they're stored securely in the modem only, and no private keys or anything in app flash/RAM

Thats why you have a very large warning in the AWS_FOTA example saying that this is not best practice.

Yes, we encourage customers to follow warnings and notes in our documentation.

KInd regards,
Øyvind