Technically it would not be "this is what the camera saw, guaranteed" but "this is what the possessor of some signing key asserts they saw". If a single shoddy

Technically it would not be "this is what the camera saw, guaranteed" but "this is what the possessor of some signing key asserts they saw". If a single shoddy manufacturer of cheap webcams somehow leaks a key that should have been on that webcam, then any software can easily encode a video stream with signatures asserting "this is what the camera saw, guaranteed".