I think public key cryptography offers a technical solution that is nearly as ideal here as for its existing uses for securing communication between physically remote actors -- please see my comment here for details: https://news.ycombinator.com/item?id=49444227

I say "nearly", because as soon as you need to keep a private key secure from someone with direct physical access to the device, you're entering dangerous territory. TTBOMK there's no way to make a "perfect black box", so it becomes an arms race between defensive "obfuscation" and tamper detection mechanisms in the one hand and stealth scanning techniques on the other. But this is already the case for TPMs -- that is, the situation is no worse than for an already widely accepted technology.