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But how do you check that it was actually taken by a camera and not just tagged as such?

Secondly, even if you solve that, how do you know it's not a photograph of an AI-generated scene?

I think this is very obviously not the right approach.

I guess both the camera and the image need keys. (But really I have no idea this stuff is out of my realm!)

I theory you could install some kind of TPM-like device to every hardware that signs the data with key generated by manufacturer. Should be designed in such a way that it is very easy to break it when trying to tamper with it

See for example: https://c2pa.org/

I tried pushing this 5 years ago when at Apple but got nowhere. IMHO only the OEM’s can achieve this.