State Considered Harmful – A Proposal for a Stateless Laptop
It's an interesting idea, but probably hopeless: all this does is move the goalposts. You're still fundamentally trusting the hardware to read and obey the instructions on your weird "external SPI" device, and if you're willing to trust the hardware then you might as well just put it on the motherboard like today's devices do. If you don't trust your hardware (and I challenge anybody to prove that a motherboard full of chips hasn't had a chunk of flash memory added to it without your knowledge) then you have no reason to think that the code it loaded was the code stored on your "trusted stick".I don't believe that it is possible to build a secure system which isn't based on trusting the device that you hold in your hands. At some level, you need to have a device which is capable of both UI and computation functions to a sufficient extent to validate whatever transaction you are attempting to sign. You could push that onto a smaller device than your laptop (we already know that phone-sized devices are viable), but you still have to end up at the thing you interact with for signing purposes being a device that you trust.