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?