You could have the camera record the lidar also and crypto sign that too. Assuming it is all done in camera and the lidar is high resolution enough it should be possible to completely tamper proof a video recording at least at the “talking head” level.

I think Apple could possibly start providing this for iPhone videos if they decided to as the hardware is all there already both the lidar sensor as well as the Secure Enclave etc

I think this idea of adding additional layers is on the right track because it confounds the deepfake generation problem by adding a curse of dimensionality. You can't just fake 2D. You need to fake 3D which is much harder.

I'm curious what the equivalent of "3D" to "2D" will be for deepfaked voice would be.

There is already the color channel dimension in addition to the spatial dimensions and time dimension. Adding depth doesn't make things much harder, especially because there is so much correlation between depth and color and time.

I know they don't currently do this, but if GPS satellites cryptographically signed their timing signals, could that potentially be usable to establish that, in addition to the particular camera capturing the images at a particular time, that it also did so in a particular-ish place ?

It's possible to record and replay signals from gps satellites with different delays, so maybe that could be used to spoof the location, but if the camera also has an internal clock, perhaps it could detect if there was too much discrepancy there? But I don't know how much that could constrain the location. Also gps only has limited precision, especially indoors? Uh, maybe using cell towers instead of, or in addition to, gps would be better? Hm.

I mean presumably you could just have the subject or subjects of the video sign the content. A full unedited interview for example could be signed by the interviewer and interviewee.

Edit: This would of course be predicated on PKI being safe and easy for most people to use which decades has proven otherwise though.