It&#x27;s an interesting idea, but probably hopeless: all this does is move the goalposts. You&#x27;re still fundamentally trusting the hardware to read and obey the instructions on your weird &quot;external SPI&quot; device, and if you&#x27;re willing to trust the hardware then you might as well just put it on the motherboard like today&#x27;s devices do. If you don&#x27;t trust your hardware (and I challenge anybody to prove that a motherboard full of chips hasn&#x27;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 &quot;trusted stick&quot;.<p>I don&#x27;t believe that it is possible to build a secure system which isn&#x27;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.