Communication privacy is one of the key requirements for always expanding networks. Furthermore, fibre systems are becoming saturated and many remote areas do not have access to broadband connection because current systems are too difficult or expensive to deploy. In this work, we experimentally demonstrate a mid-infrared free-space cryptosystem that is based on chaos synchronization between two quantum cascade lasers. Optimal amplitude conditions to ensure both privacy and acceptable deciphering are described, paving the way towards a wide adoption of quantum cascade lasers for future communication systems.