I am looking for a way to make a set of files only able to be read when a USB key is plugged into the system. For example, if I have FILE A on my C drive and I try to open it, it will say something like "Please insert key" (The "key" being a USB drive) Once I plug the USB drive into the computer and try to open that file, it functions normally.
It's a bit like Predator, but instead of locking the whole computer when you remove the USB drive, it only locks the files. Further, I would like this "locked" attribute to exist across systems, so that if anyone copies the files, they will not run on any other system unless they have the "key".
I worked at a company before that had such a system in place, so I know it's possible, I just have no idea how they did it.
Basically, I think what I'm trying to do is encrypt the files and put the key-file to decrypt them on the USB drive.
Any help would be much appreciated!