Is this currently impossible?

Is digital signatures impossible in the sense that on a live camera feed you can assign a signature to an object inside the feed and can for example track that object?

Example
Police helicopter camera in pursuit of a stolen car, there camera is manually controlled to follow the car and is relying on a person to find the car out of the other cars, is it not possible for the camera to assign some sort of signature to the stolen car (user defined) and then track that car with the camera, kinda like spray painting a big x on it.

well, it should be possible i guess. you can do tracking in adobe after effects to create lightsabers, so why not on something like that?

Yes this is possible, and already in use with, as you mentioned, police helicopters and in the military, I think.

Its a mix of pattern recognition and movement-prediction.

The pattern recognition (things like Neural Nets + Filters)
tries to identify a certain Object, and the
movement prediction tries to match this pattern consistently
to the same object, based on possible/predicted movement
when other objects (patterns) get close to it
(like a second car passing by).

It should be possible to do using optical and/or infrared data.

Its a classic military objective, for example to
track airplanes to be hit by a sidewinder rocked,
or track objects and identify them on a
radar screen.