Right, my point is that this should default to "untrustworthy." The idea is that a camera would at the very least include a timestamp and camera type in the signature. That signature should usually be reproducible when being filmed by another camera (these signatures can be part of the physical image). This should mean that a cameras filming screens would have multiple ways to show the images are not legitimate (as something as simple of shadows not matching time of day could show the video is illegitimate).