3 ms·

Could an LCD display be used as a sensor?

giantrobot 5y ago
IIRC the Apple Newtons used a thermocoupler integrated into the LCD to help keep the contrast adjusted to the user-set level as the temperature of the device changed. There was at least one application that would read the current contrast setting of the screen and infer the temperature. I don't remember it being super accurate but it worked. The Newtons had grayscale LCDs with manually adjusted contrast. The MP130 and later also had an electro-luminescent backlight but it was not always active. So the user contrast setting was very important to maintain for screen visibility.
upofadown 5y ago
The display processor would have to have some way to turn off the backlight LEDs and then sense the voltage generated by the laser. It is unlikely that the signal would be able to get back through whatever power device controlled the backlight power to get to a processor pin. The rest of the LED strings would probably load the signal down.
- R0b0t1 5y ago
  You wouldn't need to turn the backlight off, but yes, you'd need to do complicated processing on the display CPU which is already at its limit doing screen ops.
  - upofadown 5y ago
    In practice the backlight would be off because the input/output pin on the controller would need to be switched to the input mode from the output mode. If the backlight was driven, that drive current would swamp out the current induced by the LED(s). See Appendix E [1]. This attack only works in very specific circumstances. [1] https://intellisec.de/pubs/2021-acsac.pdf https://intellisec.de/pubs/2021-acsac.pdf
    - R0b0t1 5y ago
      It would be possible to measure it while on with the right hardware. It's unlikely, yes, but chips are moving to more generality and it is kind of reasonable that we would see such overdesigned chips at some point (analog + high density IO for screen in same reusable package).
mateo1 5y ago
Really unlikely. What's realistically possible though is to turn speakers into microphones through firmware or software, depending on your audio card. You're welcome for having yet another thing to be paranoid about.

IIRC the Apple Newtons used a thermocoupler integrated into the LCD to help keep the contrast adjusted to the user-set level as the temperature of the device changed. There was at least one application that would read the current contrast setting of the screen and infer the temperature. I don't remember it being super accurate but it worked. The Newtons had grayscale LCDs with manually adjusted contrast. The MP130 and later also had an electro-luminescent backlight but it was not always active. So the user contrast setting was very important to maintain for screen visibility.

upofadown 5y ago

The display processor would have to have some way to turn off the backlight LEDs and then sense the voltage generated by the laser. It is unlikely that the signal would be able to get back through whatever power device controlled the backlight power to get to a processor pin. The rest of the LED strings would probably load the signal down.

R0b0t1 5y ago
You wouldn't need to turn the backlight off, but yes, you'd need to do complicated processing on the display CPU which is already at its limit doing screen ops.
- upofadown 5y ago
  In practice the backlight would be off because the input/output pin on the controller would need to be switched to the input mode from the output mode. If the backlight was driven, that drive current would swamp out the current induced by the LED(s). See Appendix E [1]. This attack only works in very specific circumstances. [1] https://intellisec.de/pubs/2021-acsac.pdf https://intellisec.de/pubs/2021-acsac.pdf
  - R0b0t1 5y ago
    It would be possible to measure it while on with the right hardware. It's unlikely, yes, but chips are moving to more generality and it is kind of reasonable that we would see such overdesigned chips at some point (analog + high density IO for screen in same reusable package).

mateo1 5y ago

Really unlikely. What's realistically possible though is to turn speakers into microphones through firmware or software, depending on your audio card. You're welcome for having yet another thing to be paranoid about.