How complex would a device have to be to not be trivially defeated by a replay attack? Around $5 complex (though I lost track of actual prices for few years).

How complex would a device have to be to not be trivially defeated by a replay attack? Around $5 complex (though I lost track of actual prices for few years). How do you get both ends to reliably communicate You choose a suitable networking stack and communicate. Secure messaging is a solved problem and doesn’t require “internet”.