But.. if the camera app automatically signs pics/vids, that would mean that a private key is available to the app without any passphrase (or one embedded in the app :O).. So why not just extract they key and sign your fake vid?
The only way I see it working is if the key is "burnt-in" to in the camera hardware and any applications cannot MitM it.
> the app itself could take a deepfake video and have the OS sign it
Note how I said "it asks the user via the OS to approve the result". I would expect a modal OS dialog to let the user review and approve the content before being signed and passed back to the app.
Thinking about it, there's actually nothing stopping this from happening on today's hardware using just application sandboxing. Substitute "OS" above with "Signing App" that does the same thing (accepts media signature requests from other apps, and opens dialog to request approval from user with a preview).