I don’t know how to solve that. Not even in principle, as even a hypothetical future A.I. jury, judge, lawyer, and cop would have the “what does ‘good evidence’ mean?” problem in addition to the incentive alignment problem that already makes many of us question the justice of our laws.
Send that value to the 'forever', public registry in the cloud, AKA some combo of Twitter, Facebook, GMail, FastMail, AliMail, etc.
To be fabricated, it would have to be done in something approaching real time. Not absolute safety, but a seemingly easy step in that direction.
Attribution doesn't offer any proof in itself - which has us back to square one.
Vast data-hoarding might be able to spot inconsistencies within supposed data - the equivalent of thousands of witnesses at a UN press conference proving that world leaders did no in fact take off their clothes and start dancing like strippers before the world.
But if everything produces similarly consistent data from their cams - including metadata one could hypothetically get thousands of feeds from differing points of view displaying the infamous 22xx UN press conference lewd dance incident - collaborated by a blend of camera device keys. In the end it would come down to tautologies of trust.