80% of the problem could be solved with a reliable signature scheme that allows some remixing of video content. So if CNN publishes a video, signed with their key so it's verifiably CNN, we need the ability to take a 20 second bit of it and still have a valid key attached that verifies that the source is CNN (without trusting the editor). Then you can share clips, remix it, etc, and have integration in social media that attests the source.