HN ate my reply, apparently I was posting too fast, but as a TLDR you can avoid the overhead of encryption and authentication by revisiting the trust framework in the design of your network and simply assuming that you don't trust your sensors. This moves the overhead from the sensors and the network which are heavily constrained to the processing of the central authority where you have a lot more lee-way and moore's law to help. If you are interested in learning more about this sort of system, it is a topic of research in control theory often lumped under robust systems.