What you really need is - A piece of hardware under the user's control that provides cyptographically secure key material that allows the BIOS to boot. - Atte

What you really need is - A piece of hardware under the user's control that provides cyptographically secure key material that allows the BIOS to boot. - Attestation by the booted thing that it is intact and untampered with. Both of these need to be inspectable by the user. For the former you can use a dongle with a key on it, and this is probably testably secure by an end-user (we can talk about trusting silicon and your tools, but if you use multiple platforms and toolsets for verification that's probably pretty reasonable). Knowing that the "enemy side" (the BIOS) hasn't been suborned is pretty hard and requires hardware-level support that you're not going to see outside of video game consoles. For the truly paranoid, your best bet is to use a computer that has no physically attached peripherals that you can't monitor (e.g., USB). So, USB-attached network adapters and so forth. This stuff is hard.