You make a really useful voiceprint (for the inevitable creepy ubiquitous personal helper genius, say) and it's stored in whatever format you like. We'll just call it "serialized voiceprint" sv, I guess.

I imagine that we create this by having a friendly, chatty AI in our ear all day engaging us in conversation. It starts out being a bit hard-of-hearing. But by the end of the day it's uncanny how you can mumble anything and it pieces it together. All buttoned up and serialized with only the "domain" of personalized voice recognition (I guess. Hopefully.) this is our sv.

You can create an api, "sound in" and "text out". You encrypt sv and send it to Fab, who creates a custom chip for you.

Your encrypted sv is on there and the chip is constructed in such a way that you can send requests of the form: (voice_data, private_key).

...screeech

I think you need to use crypto on this same chip to create, encrypt your vb with an onboard private key that is lock-box secure or whatever is done these days.

This chip is soldered onto your device like a Secure Enclave and with that device, you can allow/forbid data and interact with it to approve in some sensible way. You are prompted? "Recite your pass phrase?" (heh...)

Ok, none of that is really close to a practical, "secure" thing. I think your vb might be on a 'yubikey' or something.

tl;dr -- I feel like there is a secure way of doing this that disallows hijacking of vb in a manner similar to how EVM works (sorry). You are prompted, you push a button on your hardware wallet, thing signed, your key never left the secure device. Something like that. Maybe an implant...