Well as I said, I think there’s a collection of things we already use for judging what’s true, this would just be one more tool.
A cryptographic signature (in the original sense, not just the Bitcoin sense) means that only someone who possesses a certain digital key is able to sign something. In the case of a digitally signed photo, it verifies “hey I, key holder, am signing this file”. And if the file is edited, the signed document won’t match the tampered version.
Is it possible someone could hack and steal such a key? Yes. We see this with certificates for websites, where some bad actor is able to impersonate a trusted website. (And of course when NFT holders get their apes stolen)
But if something like that happened it’s a cause for investigation, and it leaves a trail which authorities could look into. Not perfect, but right now there’s not even a starting point for “did this image come from somewhere real?”
You’re right, cameras can be tricked. As Descartes pointed out there’s very little we can truly be sure of, besides that we ourselves exist. And I think deepfakes are going to be a pretty challenging development in being confident about lots of things.
I could imagine something like photographers with a news agency using cameras that generate cryptographically signed photos, to ward off claims that newsworthy events are fake. It would place a higher burden on naysayers, and it would also become a story in itself if it could be shown that a signed photo had been faked. It would become a cause for further investigation, it would threaten a news agency’s reputation.
Going further I think one way we might trust people we aren’t personally standing in front of would be a cryptographic circle of trust. I “sign” that I know and trust my close circle of friends and they all do the same. When someone posts something online, I could see “oh, this person is a second degree connection, that seems fairly likely to be true” vs “this is a really crazy story if true, but I have no second or third or fourth degree connections with them, needs further investigation.”
I’m not saying any of this will happen, just it’s potentially a way to deal with uncertainty from AI content.