I have been playing with RWeather's great Crypto libraries for Arduino. In order to protect from MiTM attacks, I've tested signing on top of the DH key exchange. Then, tested encryption using the ChaCha algorithm which seems very fast. All satisfying results. The key exchange plus signing and verifying takes around 3 seconds on the Syxsens boards (BTW the green one is an earlier prototype). This can be a good base to develop a point to point protocol for LoRa which can be chained from node to node.