This isn't a completely technical problem, there's not going to be an air-tight technical solution. I think, at this point, anything more than the camera havin

This isn't a completely technical problem, there's not going to be an air-tight technical solution. I think, at this point, anything more than the camera having a secure device-generated key that it uses to sign its output and produce watermarks is overthinking it. That will add a nearly insurmountable barrier many classes of forgery, namely one were a forger claims to have a photo taken with your camera. You also have to remember a significant class of photo-forgery would involve images that are claimed to originate from a camera the attacker controls. For instance, forged video of a robbery from a security camera. That could, in some cases, be defeated by observing that the images have authentication information they shouldn't have (such as traces of watermarks from other cameras). At the end of the day, forgeries will be spotted by observing that they have subtle errors that don't add up. That's how it's always been done.