If you're working on an important document in a busy environment and don't want people to see what you're doing at a glance, then you could use a privacy screen on the display or an application that dims areas not in your gaze. Microsoft has patented a system that it believes is superior to both because it makes the document fully encrypted and illegible at all times—apart from the section you're directly looking at on the display.
The patent's details (via Windows Report) are like all such publications, in that specific details on exactly how everything works aren't covered. Instead, a broad overview of the nature of the technology is given and what Microsoft has proposed is a system that takes documents you're working on and encrypts them in such a way that the contents are secure but the overall structure of the text remains the same.
That's even the case when you pull the document up for display on your PC's screen. But then the clever bit kicks in. Using a suitable webcam or another device that can track the movement of your eyes, the algorithm determines exactly where your focus is and uses the information to generate an alpha-blending mask—think of this as being like a 'hole' in the encrypted document that lets you see the original material underneath.
As your eyes move about, the 'hole' follows along to ensure that you don't suddenly hit a line of unintelligible text. Statistical methods can be used to predict where the eye's motion will lead, reducing the latency between the eye tracking and mask movement. Anyone else looking at the screen will just see gibberish.
Microsoft's algorithm takes into account that we use our peripheral vision quite a lot when we read, even though the text isn't directly in focus, and the edge of the mask isn't a hard line between normal and encrypted text. It also takes into account that eye motion isn't perfectly smooth (aka saccades).
Combined, these additions to the algorithm do make it seem a little better than AMD's Privacy View feature in its Adrenalin software, which just dims the regions of the screen you're not looking at.
However, like all such eye-tracking security features, it doesn't seem to prevent one particular issue. If the original document is visible and legible on any part of the screen, there's always the chance that someone can see it. None of these software-based privacy systems can stop someone from taking an image of your screen from a distance or out of the eye-tracking device's field of view.
Still, the Microsoft system is, in my humble opinion, considerably better than laptop privacy screens, which just disable some of the backlights to make it harder to read content from the side. Of course, one could just not work on sensitive documents in a public area, but if it has to be done, I'd prefer the content to be genuinely encrypted in some way and not just a bit dimmer.
