I found this explanation for pivot chords quite nice:

@PhiNotPi Manipulation of the pixels, or of the EXIF data? Or both?

@DJMcMayhem could be either, I suppose. But it has to depend on image content, kinda like a hash, where there's no way to modify the image and preserve it.

That seems really hard. With checksums, you have to assume you trust the person who tells you what the sum is

With this, you have to verify an unknown file from an untrusted source

There'd have to be some sort of web of trust among equipment manufacturers or something. The image is signed by the device which is signed by the manufacturer. But there'd have to be a registration process. I don't really think it's possible.

It'd be a really nice thing to have though. There's already plenty of fake images floating around, only gonna get worse with DeepFakes. So it'd be really useful if someone could make a browser extension or something that's really easy for the public to use that identifies fake images as they are encountered.