i described a device i would build for this. uses radioactive material from a smoke detector to generate a signal, with a analog/digital converter and then hash that. it wouldn't be hard. then tin for x-rays to stop the radioactive material's emissions from leaking, gold leaf for IR protection and pyrolytic carbon to soak up the EMF.
i think it could be built for about $30 and would be completely secure, even from a sensor within a metre of the device.
nobody is building this. i'd love to work with techs to fabricate it, and ensure the software is fully secure, use chacha20 and sha256 for scrambling the inputs to ensure they are not predictable.
