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A photo signed as an authentic iPhone image wouldn’t completely pinpoint the photographer, but would be good evidence that the image is authentic.

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Over any period of time, no probably not. If the algorithm is simple as "This was signed by an iphone" then you have a fleet of a billion+ iphones that all must remain secure and unbroken for the entire time this fleet of phones exist trends to unlikely. Given time and motivation all systems are broken.

A much harder to break system is one uses a secret only exists on each individual phone + the generic algorithm that iphones use, but this comes back to individuals being traced.

> uses a secret only exists on each individual phone

There is actually a way to prevent individuals from being traced while having a strong membership proof. This can be achieved by issuing signatures on a relative generator created in an exponentiation mix together with a set of output pseudonyms. One can picture it as strands braided together where it is hard to trace the link between the input and output strands, while it is very easy to tell that such a link exists.