I am planning to build a local area network that will have one Master device and multiple Slave devices. The communication should be able to handle only ON/OFF functionality of the devices (devices should turn off/on in the "same" time). NRF24L01 does not provide encryption so it would not require much effort trying to resend these commands when captured.
To protect against malicious resending of packets, my idea is to first share some private key with both devices when flashing and after that send
command + new private key to the slaves. Is this solution secure against re-sent packets or how is this done in a professional way?