These then arrange themselves in a completely random order, which is mathematically so difficult to replicate it's known as PUF (physically unclonable function).

ENGADGET: IBM brings the fight to counterfeiters with nano-sized authentication methods (video)

This fingerprint (aka Physically Unclonable Function, or PUF) is a byproduct of tiny errors in the fabrication process -- something chip makers usually try to minimize.

ENGADGET