What does your threat model look like? Nation state is a game of economics as much as anything, and are you looking at tactical or strategic time scales?
Do NOT invent your own cypher scheme, hackerdom eats those guys for lunch, don't be that idiot, use something well tested, ideally something like AES for the payload and a well thought out key exchange protocol to generate the keys.
Given the standard attacks on these things include power based side channel attacks you probably need something that deals with that.
Ideally you want constant power drain irrespective of the activity and for the crypologic cores to run in constant time.
I would observe that the first place you target when trying to break these things is stupid implementation bugs (VERY common), and then that things like the RNG are fair game (Speaking of which a good hardware RNG is useful in these things, but depending on your paranoia level, be aware that exploits targeting the RNG (Possibly at the level of semiconductor masks) are a popular game because compromising that screws with essentially all of the key exchange protocols).
Power and RF sidechannel are popular attacks and have been since I was breaking satellite cards back in the day, you would be surprised by what is possible, especially if the software author was not aware of the risks.
Speaking of side channel, the clock is another vector, maybe I can extract information by crashing the thing by running the clock out of spec? Same for most of the other IO, can I mess with the chip by driving some random IO pin outside the rails?
Sometimes you can learn things by hooking a few SDRs to the IO pins around the chip and doing correlations between the noise received, if nothing else it can hint as to where on the die things are happening.
One of the better ones I have seen was an attack on a micro that had the usual protective lattice metal layer over the die, they went in thru the back! ground away most of the bulk Si, then laser drilled, metallised and probed! Expensive, but given sufficient incentive....
73 Dan.