[roywiggins]

To an extent, it literally doesn't matter. If I fabricate a video and splash it around Facebook, it won't have a signature. Maybe Facebook will put a "has a signature" icon next to videos that are signed... but remind me, how much do web users notice when a website is HTTP vs HTTPS? Anyway, if a video is fully fabricated, there will be no hash to compare it with. And, by the time anyone does, it will already have been remixed and spread across Facebook.

There will be lots of good reasons for a video not to match bit-for-bit from CSPAN- if I add commentary on top of it, or intercut it with other clips Daily Show style, or just re-encode it, the hashes won't match. False positives are the death of an indicator, and there will be far too many of those for this sort of scheme to work.