That doesn't quite solve the problem unfortunately. If this is the only solution then the subject of some content can repudiate it by not signing it, even if it did actually happen.
For example if the president was filmed saying something in public he later wanted to deny, he could just claim it was faked. The person who filmed it could sign it, but who are they and what authority do they have to claim that they didn't fake it?
For example if the president was filmed saying something in public he later wanted to deny, he could just claim it was faked. The person who filmed it could sign it, but who are they and what authority do they have to claim that they didn't fake it?