If you're worried about three letter agencies you're really out of luck. Tampering at the factory is the least of your problems. The more tinfoil conspiracies i

If you're worried about three letter agencies you're really out of luck. Tampering at the factory is the least of your problems. The more tinfoil conspiracies include backdoors in processor hypervisors and possibly in silicon itself. Or possibly even backdoors in common crypto algorithms, like the defaults we use for elliptic curves. If you want paranoia levels of protection from everything you're going to need to break out VHDL and write your own cpu to run your bespoke crypto algorithms on.