[reaperducer]

I have a device with a camera and a touch-screen that only uses capacitive charging. I type a message. Bytes are encrypted. I hit send. QR codes flash on my screen. I use my PC or my normal phone to receive the encrypted bytes, and transmit them to you. You have the same device. You have your PC or phone flash encrypted QR codes. You use your device to receive, and then decrypt.

Congratulations. You just invented IrDA: https://en.wikipedia.org/wiki/IrDA

[MattCruikshank]

I specifically want this device to have no input or output hardware that could be used without my knowledge. IrDA could absolutely be used without my knowledge.

[reaperducer]

IrDA could absolutely be used without my knowledge.

Having actually used IrDA on Sony, Nokia, and Ericsson devices, no it couldn't.

In the real world, two IrDA devices have to be very specifically aligned, and also brought within just a few inches of each other. There's no way data transfer would happen without your knowledge.

[MattCruikshank]

I blame the intentionally cheap receiving optics. Researchers have reconstructed a TV signal from a hotel room with closed blinds, by very accurately measuring the ambient light intensity of the room (as shown on the blinds.)

Knowing that, I doubt that someone with even moderate funding would have difficulty receiving a signal from any of the transmitters you mention.

But, in all honesty, if you put a physical cap on the transmitter and receiver, maybe I'm wrong.

But in the other hand, none of the devices I currently own have one of your transmitters, and they all have screens and cameras, so...

Thanks for the dialogue and for sharing your experiences.