HMAC signing MQTT messages, with C and python

MQTT as a protocol is pretty light on security. There’s a username/password fields, but they are clear text. Mosquitto, the implementation that we’re using right now supports using these fields to limit read/write access to given topics, but it feels a bit hard to manage. We would have to have a file with each topic in use, and multiple lines for each and every user in our system. (We have multiple clients over the internet, and we don’t want to let them see each others messages)

Instead, we’re going to have clients use our public key to encrypt their messages, and then use a shared key per client to sign the messages. This means that we can be sure that the message was not tampered with, that only someone with knowledge of a certain client’s key can pretend to be that client, and that no-one without our private key can actually read the messages. Our listener will first verify the signatures on each message. If a messages was received on topic “clientAAA” but fails to verify with the shared secret key of clientAAA, then we drop the message before even attempting to decrypt. If they match, we can decrypt, and process the contents.

I’ve put together a basic demo of the HMAC signing part, with a message publisher in C, and a message verifier/parser in python. Hopefully it will be useful to someone.
The code is available:

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