If we're being serious about cryptography, you could have cameras signing videos in real time with a secret key, in such a way that each camera has an individua

If we're being serious about cryptography, you could have cameras signing videos in real time with a secret key, in such a way that each camera has an individual key stored in a specialized chip that does the signing, with no way to access the key without breaking the chip's seal. So you could still fake videos, but any accusation of tampering could be verified by checking the seal is intact. (of course you need non-forgeable seals too, but that's comparatively easier)