This may sound weird, but I got this idea and I would love to put it in practice. I have an USB flash drive that I carry with me everytime, and I would like to create a system to make my Ubuntu only start IF that flash USB is connected, just like a key that turns on a car (the login screen should appear only if the "key" is connected - I would have to put my password too). This will be a great improvement on my computer's system (and would also be kind of fun).
It would be also nice to have more than one "key", in case a flash drive gets broken...
So, the question is, is this possible? If yes, how can I make it?