[cesarb]

I vaguely recall reading somewhere several years ago about some cameras which signed the photos they produced, so one could prove they came directly from the camera and weren't edited. IIRC, it was in an article about these signatures being broken, though I don't recall the details (perhaps it was by dumping the signing key from the camera firmware?).

[Someone]

Early digital cameras could be fingerprinted by looking at dead pixels (there are over a hundred million ways to have three dead pixels in a one megapixel camera, so if these are uniformly distributed, that, in the early days, when few cameras existed, have almost guaranteed uniqueness). It wouldn’t surprise me if that still is possible by looking at minor variations in light sensitivity across pixels (that might require having access to lots of photos by a single camera, though)