Even Apple will not be able to explain how its forthcoming iPhone X can spot some efforts to fool its facial recognition system.
The firm
has released a guide to the Face ID system, which explains that it relies on two types of neural networks - one of which has been specifically trained to resist spoofing attempts.
But a consequence of the design is that it behaves like a "black box".
Its behaviour can be observed but the underlying processes remain opaque.
So, while Apple says Face ID should be able to distinguish between a real person's face and someone else wearing a mask that matches the geometry of their features, it will sometimes be impossible to determine what clues were picked up on.