If it does exist, it is limited to military use afaik. I don't know of any commercial or FOSS platform like that. I was thinking more like a small usb-chargable

If it does exist, it is limited to military use afaik. I don't know of any commercial or FOSS platform like that. I was thinking more like a small usb-chargable chip that you carry around that could be used no matter what phone you are from, or to record verifiable messages. The hurdle is creating an algo that can derive all previous signals to verify non-live recordings, but that can't derive the future signals without a private key. I'm not sure what existing bodies of work there are in that domain.