Any idea of trying to prove that a random video isn't fake by crypto verification is very, very brittle - the trust relies on having almost 100% certainty of key secrecy from a global, heterogenous system of low-margin commodity manufacturers.
Like, ok, every device is keyed to authenticate the recordings it produces, using a unique key signed by the manufacturer - as long as a few valid device keys ever leak from the device or the manufacturer, any fake video can get signed with a valid key from Camera#1234 from ShenzhenCameraCorp567, ltd.; you're not going to make every $1 camera module in cheap embedded devices tamper resistant.
Like, ok, every device is keyed to authenticate the recordings it produces, using a unique key signed by the manufacturer - as long as a few valid device keys ever leak from the device or the manufacturer, any fake video can get signed with a valid key from Camera#1234 from ShenzhenCameraCorp567, ltd.; you're not going to make every $1 camera module in cheap embedded devices tamper resistant.