@FirewallDragons I just don't see how this could work, cryptographically speaking.

It seems to me that every participating piece of software that creates or edits content would need to either a) ship with the private keys that sign the manifests, or b) outsource the signing to a third party at runtime (either a remote server or specialized on-device hardware).

In the former case, those private keys are effectively pre-leaked to the public, and so they unavoidably lose their usefulness right before they start being used. In the latter, the software could just lie to the third party about what kind of content it's asking it to sign (e.g. an image generation app could claim to be a camera app).