That’s a big challenge: for big news stories it’d be useful if it meant that TruePatriot420 has to explain how their phone was used to record a candidate’s private conversation in a different state but I think many of the most damaging things for many victims won’t hit that level of scrutiny. If some horrible person tries to smear their ex, most of the damage will come simply from circulation.

What might be worth trying would be some sort of verified badge if this became common enough that players could meaningfully highlight its absence, similar to the way browsers are now showing non-HTTPS sites as insecure. If Facebook badged the video with a big “this iPhone video doesn’t have the usual signature” message that might slow people down.

For the deepfake porn or fake news problem I also wonder whether there’s a useful fuzzy ID service where a player could display a “this is substantially similar to <source video> and may be a copy” warning which wouldn’t be so inaccurate that nobody would heed it. That could be really important if we started seeing political fakes where someone is shown accurately but their words are faked.

You're missing the point.

Facebook / Twitter / etc verify the crypto signatures on media ingress. Then they can give proper attribution and show a checkmark or something that attests the media hasn't been altered.

So in your feed, there will be some media with checkmarks and some without and you can choose which to believe more.

Alternatively, a competitor could come along and reject all media that doesn't pass cryptographic verification.

It would absolutely have to become more consumer and lay-user friendly.

Of course, depending on how that's implemented it could just be another vulnerability in easy-to-use proofing.

But if there was some way to add an abstraction on top of the raw keys, keeping them still accessible if the user is keen, then that might be a step in the right direction.