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Let's say I have one offline computer with some important data, and one online computer. Sometimes I need to move files from the offline computer to the online one. When I transfer those files, is there a risk that a virus would also transfer some of the important data that should not leave the offline computer?

As long as I don't plug anything into the offline computer everything is surely safe. But how can I then pass some files between the two computers, without risking that a malicious virus cache some of my other files from the offline computer and pass them to the online computer, and then send them away? I think it is possibile to write such a virus, that query the hard disk with some search and save some data when it is plugged in to the offline computer, and when it is plugged into the online computer it sends this stored data to a third party server.

How can I set up an environemnt and how should I behave in order to protect myself in a scenario like this one?

How can I control what is leaving the offline computer on a pendrive? Is it enough to check for hidden files in the file explorer? Maybe calculate a checksum before I plug in to the offline computer, and then compare that to the checksum of the files I transfer on the pendrive? Is there any alternatives? What about bluetooth? I guess there is no different which channel or protocol I use?

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There is no simple way to move files between computers and preventing other files from being moved. Yes, what you described is not only possible for malware, but was famously pulled of to destroy Iranian centrifuges by the Stuxnet virus.

As for some of your ideas. Most of them won't work. The virus can just write into unused space on the storage device and there is not a simple way to detect or prevent that. You could try to chceck all used space and overwrite unused space, but how would you prevent the device performing the overwrite from getting infected as well?

Using bluetooth is potentialy even more dangerous, as it is similar to just connecting it to the internet.

Transcribing data manually is about the only foolproof way of doing this, though a more reasonable level of security can be achieved using flash drives.

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    "Transcribing data manually is about the only foolproof way of doing this" - how about QR codes? – John Dvorak Sep 17 '18 at 10:59
  • @JohnDvorak Thought of that. QR codes sound good, but how would you transfer them? If you just point the camera on the screen, you risk the virus using small variations in background color and the code to transfer additional data. If you use a printer and a black and white scanner, the chances of that are reduced but the virus could still use shades, small inconsistencies in shape etc. – Peter Harmann Sep 17 '18 at 11:11
  • @JohnDvorak you would have to use some ridiculous setup, for example a board with LEDs that could only be turned on or off and could not be switched for at least a second (prevent imperceptible blinking) to prevent side-channel data being transferred, and a human would still have to monitor the transfer, which would be harder as humans generally don't understand QR codes just by looking at them. If you allow any QR code through, then what was the point in the first place? – Peter Harmann Sep 17 '18 at 11:14
  • @Peter Harmann writing about reasonable level of security do you mean just copy to usb pendrive a piece of data, then paste it from there to an online environement or some more sophisticated way or transfering it using usb pendrive (I have read usb flash drive = normal pendrive) ? – GuessMe Sep 20 '18 at 1:24
  • @GuessMe it depends on the exact scenario. What kind of data you are transfering, whether you are transfering both ways and so on. – Peter Harmann Sep 20 '18 at 7:18

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