“ rather than encoding a specific message, this watermark encodes an image of the unmanipulated scene as it would appear lit only by the coded illumination”
They are including scene data, presumably cryptographically signed, in the watermark, which allows for a consistency check that is not easily faked.
That's just saying that the coded image will only be apparent in the areas of the image lit by the light. Which is obvious, that's how a flashlight works too. They're not signing the actual pixels or anything. They've increased the difficulty to that of 3D-mapping the scene and transferring the lighting: not trivial, but still two long-studied problem spaces.
Hmm yeah fair point. I'm not sure you can do it without some control over the observer device then... will we have "authenticated cameras" soon, with crypto in secure elements? Feels like we'll have to go there to have any trust in video.
It turns out if you give an adversary physical access to hardware containing a private key, and they are motivated enough to extract it, it's pretty hard to stop them.
