This is a keyboard that your phone sits on top of ( see where it says “Phone goes here”? Yeah, thats the spot where you put the phone).
Your phone receives signals from the keyboard through NFC or ultrasound communication, or IR or anything like that
The Keyboard gets pressed by the user. The key is encrypted using the physical encryption key, and then it is broadcasted to the phone. The phone gets receives the users input ALREADY ENCRYPTED. That is he essential part here. It doesn’t matter if someone goes through the back door of your phone if the phone itself is incapable of decrypting the message.
The keyboard should also have a small screen on it (i forgot to draw it in this picture). The phone can send encrypted messages to the keyboard and the keyboard can decode them using the physical encryption key. This has to be done on the keyboard, you can never allow the phone to handle any encryption or decryption. That leaves you susceptible to back doors.
Ideally, you should design cell phones as two different units, the processing units and the Input/Output/Display units. The encryption should be done outside of the processing units.
If people assign disposable new keys frequently during an in-person exchange, it should be nearly impossible for anyone to decode your history, even if they gained access to all of it. I say nearly impossible because it is pretty hard to predict the limits of what artificial intelligence will be able to do one day.
It would not be a significant overhaul to modify the current cell phone architecture to do this. Also, this would give people true protection from any kind of entity which may threaten to use your data against you by gaining access to a back door. I’d do it if I had sufficient funding.