They could, and some already do, but this doesn't fully solve the problem. You'd still be left with the analog loophole[1]. A sufficiently advanced attacker could feed a generated image directly to the camera sensor.
http://www.h-online.com/security/news/item/Russians-on-the-m...
Somewhere in my collection of lesser used equipment, I've got a Daylab. https://www.instantphoto.eu/pola/pola_daylab_300.htm and I used it to transfer some 35mm slides to 4x5" to practice the various manipulations you can do there (emulsion lifts and transfers).
The thing was I had a 35mm slide that was projected onto the film. If I could make a 35mm slide of some sort, I could then make a polaroid of a digital image. Even back then there were a few services that could do it.
The idea that you can ensure that the image produced is what was recorded, even two decades ago when polaroids were still a reasonable standard for authenticity of an image shows how easily it can be compromised. Well, maybe not easily but it isn't difficult for a serious hobbyist to create the appropriately constructed image in the desired media.
And so what if you sign the EXIF data. That only really guarantees that version of the image. If you adjust the saturation or curves or exposure... or correct for some lens aberration does that EXIF information still match the resulting photo? For that matter, what if you crop it to a square from a rectangle or stitch two images into a panorama?
Oh, that's actually possible with cryptographic techniques. Very exciting!
See eg https://medium.com/@boneh/using-zk-proofs-to-fight-disinform... or http://www.cs.tau.ac.il/~tromer/papers/photoproof-oakland16....
Of course, the analog hole stays wide open.
While that is good and useful (and I would be willing to even go so far as saying "needed" for journalistic uses), the transformations available are remarkably limited for artistic use that even represent what can be done in a traditional darkroom.
While you won't be able to verify the final work mechanically by just verifying the signature, at least, if you bundle the original raw file with the final work, an human (or an AI!) should be able to tell if the final work is at least derived from a real picture.
But of course the analog hole is real.
In order to submit picture 1, you need to have picture 2 that was taken at the exact same time (and time is in the signature too) by a different camera, of picture 1 being taken, as a verification picture.
The problem is that you want to ensure that an image corresponds to some real object. This problem is unsolvable, because a camera does not sense "real objects". It senses photons, and we already have the technology to shine any arbitrary combination of photons that a camera sensor can detect onto a camera sensor. There is no possible "signature" to apply to the output of a camera sensor that can validate that it is the result of "real objects", a definition that would get fuzzy if you tried to really nail it down in the presence of hostile attackers deliberately gaming your definition anyhow.
Besides, the entire idea that a photo is a concrete, specific thing is a very Hacker News, computer-programmer idea anyhow. Photos aren't just files. Long before we had the tech to shine arbitrary photons onto CCD camera sensors, photos were a process, not a single file. Even if we stipulate that the image on film can be reasonably called a "file", itself a rather large stipulation, the final photo would be the result of substantial decisions in how the image was developed. The program "Photoshop" is called that precisely because "photoshops" had numerous tools to affect the image at that point in the process, many of the tools in Photoshop are still named after these processes. Photo competitions absolutely include these elements; if there have ever been any photo competitions that accepted film as the input and rigidly developed all contestant's films the exact same way, they are the vanishing minority. Photos have always encompassed post-processing as part of their identity. A signature on a file is no good. You'd have to fundamentally rewrite the entire photo stack to include all the transforms, in a completely official and 100% specified manner, all signed, so the final signed photo has the complete record of everything done to it from the source... it's theoretically possible but absolutely not going to happen, especially in light of the fact the source is still meaningless for the above reasons. The whole idea is comprehensively unworkable.
https://web.archive.org/web/20120125045005/http://www.elcoms...