Comment by peri-cl

It's even easier than that. You just wait for someone else to figure out, some photography professional with fancy equipment and a hacker-y mindset, and you pay them to sign your photos for you.

Once a defeat device (a camera pointed at a screen) is functional, whoever has it, can simply automate a "receive API request, display image on screen, photograph it, return signed image" pipeline. A cheap internet service. I'd WAG a hundred thousand signatures per day per phone, limited by the sensor speed.

Since there's no way for anyone, Apple included, to correlate photo signatures with the device that signed them, it's also true there's no way to stop one device from signing millions in bulk. ("...an outside observer cannot determine whether any pair of reference images were taken by the same device..."; "...avoid even implicit public association between different photos taken by the same sensor...")

It's the same economic asymmetry as DRM vs. movie piracy (as soon as one group defeats a technical challenge, millions instantly benefit, at zero marginal cost). Apple has no chance of winning.