[danjc]

The issue is provenance. We need cameras and phones to digitally sign photos so we can easily verify an unadulterated image.

You also want to be able chain signing so that for example a news reporter could take a photo, then the news outlet could attest its authenticity by adding their signature on top.

Same principle could be applied to video and text.

[soerxpso]

Signing something doesn't verify that it's real, it just verifies that you claimed that it was real, which everyone was already aware of. You can either hack a camera, or use an unhacked camera to take a picture of a fake picture.