One of the simplest ways of keeping a computer system secure is by using an air gap — that is, never actually connecting the system to the network. This can often include other peripherals like USB drives and other removable storage as well, so getting information to and from secure (or compromised) systems behind air gaps can often present a challenge. But assuming you have local access to the computer and your parts bin handy, these optical solutions from [Nikolay] can allow data transfer to or from such off-line computers.
[Nikolay]’s specific use case for this project is to transfer small amounts of information to or from computers that may be compromised in some way, or computers that might otherwise be dangerous to connect to other equipment. There’s actually several methods described in the project, the first involves temporarily attaching a photoresistor to the computer’s screen which has been wired into the remains of a USB keyboard. A script running on the compromised machine translates data into a series of white and black squares. The sensors can detect these patterns much like playing Duck Hunt on an old CRT television and transmit the data across the air gap with reasonable certainty nothing harmful crossed with it.
The second is essentially a DIY optoisolator arrangement, where the audio output of the compromised computer is used to flash an LED by way of a simple transistor circuit. On the receiving side, a photoresistor connected to the audio input picks up the flashing LED as sound. The whole thing is enclosed in a black plastic box, and with the appropriate software, [Nikolay] says it can transfer 345 bytes per second between the two machines.
While the methods detailed by [Nikolay] certainly aren’t fast, they could still be extremely useful in a number of cases. He’s also set up all of the code so that it’s ready to go as soon as the diodes and transistors are soldered together. A friendly reminder to all, though, that although it’s definitely a step in the right direction an air gap isn’t an impenetrable wall of security.
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- Source: https://hackaday.com/2024/03/19/photoresistors-provide-air-gap-data-transfer-slowly/
