You assumed a specific instance of signing when I was talking generally. Reporters attesting to their newsroom the authenticity of a photo, libraries and museu

You assumed a specific instance of signing when I was talking generally. Reporters attesting to their newsroom the authenticity of a photo, libraries and museums having stronger chain of custody and documentation of the state of their collections, etc. As far as hardware signing, perhaps PKI and per-camera keys could be issued. HDCP was encryption with global symmetric keys, no? This would be a different case.