If you wanted to take it a step further, the touch screen could have a very narrow viewing angle, have the angle of the screen automatically or manually rotate

If you wanted to take it a step further, the touch screen could have a very narrow viewing angle, have the angle of the screen automatically or manually rotate to the users eye level, and then someone standing next to you would just see a blank screen. With random keys on top of that, someone would literally have to poke their head over your shoulder to see.