I wonder if smartphone makers could include some kind of hardware chip to verify the authenticity of videos. I don't think it's possible to build an unbreakable solution, but you could just make it very expensive.
The camera device could use a chip that included an HSM module, and it could use steganography to create verifiable videos. Each private key could be tied to a unique device ID in the maker's database. This might even be a legitimate use for blockchain. The device maker could create chainpoint proofs that include device IDs and public keys and embed them in Bitcoin transactions.
The HSM would be tamper-resistant, so it would be very expensive to extract the private key from the chip, or you'd have to collude with the device maker.
All of this is a moot point if you can just jailbreak the phone, fake the GPS and accelerometer data, put the camera in front of a screen, and play some audio into the microphones. I wonder if there are ways to detect video and audio that have been replayed through a projector/speaker instead of recorded in the real world. It's probably difficult to get it pixel perfect and replay perfect audio, and a neural net could probably be trained to spot any errors. Or maybe the projector/audio chip makers could include some steganography in their output. They might include an undetectable signal to indicate that this data is being replayed through a device, instead of recorded in the real world.
But then if someone wanted to commit a crime and get away with it, they could reverse engineer the signals from these projector/audio chips, and play those in the background. So any recorded videos could be discounted as a fake.
Also, I was thinking about how this could be used to prevent piracy when people record the screens in a movie theater. The studios could include a signal in the video, and the smartphone/video camera makers could stop recording when they detect that signal. But then a criminal could just hold up a screen with some copyrighted content, and everyone's phones and security cameras would stop recording.
If phones ever come with light-field cameras, it would also be a lot harder to fake. We'd have to invent realistic light-field projectors, but that would be pretty awesome. I've always wanted a holodeck.
This has been fun to think about. Would make some interesting Black Mirror episodes.