  Polymer banknotes are the trend for printed currency and have been adopted by
more than fifty countries worldwide. However, over the past years, the quantity
of polymer counterfeits has been increasing, so has the quality of
counterfeits. This shows that the initial advantage of bringing a new polymer
technology to fight against counterfeiting is reducing. To maintain one step
ahead of counterfeiters, we propose a novel anti-counterfeiting technique
called Polymer Substrate Fingerprinting (PSF). Our technique is built based on
the observation that the opacity coating, a critical step during the production
of polymer notes, is a stochastic manufacturing process, leaving uneven
thickness in the coating layer and the random dispersion of impurities from the
ink. The imperfections in the coating layer result in random translucent
patterns when a polymer banknote is back-lit by a light source. We show these
patterns can be reliably captured by a commodity negative-film scanner and
processed into a compact fingerprint to uniquely identify each banknote. Using
an extensive dataset of 6,200 sample images collected from 340 UK banknotes, we
show that our method can reliably authenticate banknotes, and is robust against
rough daily handling of banknotes. Furthermore, we show the extracted
fingerprints contain around 900 bits of entropy, which makes it extremely
scalable to identify every polymer note circulated globally. As compared with
previous or existing anti-counterfeiting mechanisms for banknotes, our method
has a distinctive advantage: it ensures that even in the extreme case when
counterfeiters have procured the same printing equipment and ink as used by a
legitimate government, counterfeiting banknotes remains infeasible because of
the difficulty to replicate a stochastic manufacturing process.
