Any person or organization X can sign any data (a video or otherwise) attesting various things, including...
- Claiming to be the creator.
- Claiming the content is true.
- Claiming that it really is them in the video.
But the signature can be applied whether or not the claim is true or false. So, the signature by itself does not prove if the content is real or not. It only proves that someone claims it is, and possibly who is making the claim.
So how do we use that?
- We can prove when someone supports a claim of veracity, but not when they deny it.
If a party wants to deny any involvement with some content, then they simply can refuse to sign it. That is an open problem. But conversely, if a video featuring person X is signed by person X, then we can at least know that X is claiming it's really them.
With respect to "Deep Fakes" there is little practical difference between...
- A fake video of person X claiming opinion Y really signed by person X.
- A real video of person X claiming opinion Y really signed by person X.
In both cases the signature itself proves that X claims Y. One can't know if X is lying, but that would also be true if you were in the room with them as they said it.
In summary we can't know if a video of X is fake if X claims its fake. But we can know if X claims it's true.
- Things can change if there is a policy of signing all "offical" content.
Then if a video had no mark, we would know it was rubbish.
That's an interesting proposition. A lack of a signature doesn't prove the content is fake, but it does prove that person X did not choose to claim it is true.
If person X had a policy of always signing any official content, and the content is merely claiming that X has opinion Y, then a lack of a signature by X is a pretty strong indicator that X doesn't claim to have that opinion.
- A signature allows a claim of veracity to live on at a later date.
Often times we get content (videos included) from trusted websites. But the content often gets copied and lives on long after it's taken down from the website. A signature can prove that that a specific website at one time claimed to have hosted it.
- We can prove that a party once claimed that content was true, even if they now deny they ever made that claim.
Public figures often proudly put out content only to later deny its existence.
Just knowing that someone claims something can actually be useful by itself, whether or not the claim is true. For example, politician X wants to unequivocally pronounce their support of popular cause Y by signing some content and putting it on their website. 10 years later, cause Y is really unpopular, and the content is removed from their website.
Now politician X want to deny that they ever really supported cause Y, but their opponent digs a signed copy out of an internet archive (proving that X is now either lying or really did support Y).
A shrewd politician manages to talk a lot without every really saying anything, and certainly never digitally signs something. But not all of them are that wise, and this use-case would probably occur quite a lot.
- We can decide how much to trust the content based on how much we trust the signer.
In general, we can't know for sure if the content of a video is true, but we might try to assess the probability of veracity, based on how much we trust the signer. It's not perfect, but it's a useful heuristic that humans use all the time.