Everyone has some gpg-like setup.
If we see a statement or video or audio by a person accompanied by a public (graphic or audible) key that checks out...ok, done, verified. If we don't, then we can justifiably disregard the statement or video or audio.
A. Video/audio/text of/by person, signed, we have knowledge that this video bears their signature, they endorse it
B. Unsigned B1. Unsigned because it's not them B2. Unsigned because they don't want it to be associated with them
I think this is still a better state of affairs because we can verify positive endorsements as genuine or not.
You're right, we couldn't verify stuff they refused to sign, but!
Imagine the state coerces people to carry small devices with radio transmitters that constantly transmit, over the length of a second, a signed key, ad infinitum; that way no one can plausibly deny that it wasn't them (unless someone plants the persons device on an imposter).
If there could be some repercussions (e.g. police brutality, gang violence, whistleblowing, etc.) I'm going to go above and beyond to make it not traceable to me. And you're here asking the government to make it a law to make all video outputs traceable to the creator?
Perhaps you should apply to the NSA or Palantir.
It's easy to imagine this idea becoming a part of video (be it VOD or broadcast) encoding standards in the future, once this becomes a critical thing.
In terms of UX: one could imagine a simple indication on TVs: an RGB led that shines in different colors depending on verified authenticity levels.
I don't see how this video signing system will help verify a cell phone video taken by a random bystander.
It will however make sure that you're watching actual Obama 2, and not some computer simulation of him doing some disgusting thing.
The image and signature could be added as authentication layers and then a third layer added to allow post-production (crop/levels/etc). Right click in your browser to 'see original image' with a little padlock in the corner. Ideally this would include GPS info to reduce likelihood that someone would just project a desired image on a screen and photograph that.
If you employ a set of digital cameras, say for security cameras or other things, each has a private key which it uses to sign whatever it records. Obtaining said key would require physical access to the device. Then, when it matters, your organization has a set of public keys which it uses to verify that the picture/video is legitimate.
The self-answering question.