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.

> Why would any kind of camera photo signing algorithm fare better?

You probably wouldn't even have to bother. If you want to fake history, just launder your images: setup rig where an SLR camera can telecine the images off a high resolution screen. With some optics you could probably also adjust the other camera parameters (like focus, f-stop) to make it more convincing.

You could imagine a timestamp being built into the signature, and maybe even gps coordinates, although I agree it wouldn't be a complete solution.

> You could imagine a timestamp being built into the signature, and maybe even gps coordinates, although I agree it wouldn't be a complete solution.

Both those things can be faked. IIRC, they make chambers right now that simulate GPS signals for testing receivers. If you can fake GPS, you can fake the time, too.