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Physicists Foil Forgers with Unclonable Reflective Patterns

Thank cholesteric liquid crystal microspheres.

Just the word "security" has by now come to imply digital security. Security of information. This world of high-value bytes—representing everything from bank accounts to industrial control and-or weapons systems—is protected by an ever-expanding suite of passwords, public keys, hash functions, biometrics, and surely many more clever schemes yet to be determined.

Nonetheless, the Old World of physical objects remains much the same as ever and requires its own sophisticated not-so-virtual protections, particularly when those physical objects are highly valuable, as in the case of, say, diamonds or actual printed currency. One could argue that digital security is even a simpler matter just because you have control/knowledge of the entire system to be protected. Black markets and covert forging rings, not so much.


To address this problem of physical-world security, physicists from the Universities of Luxembourg, Ljubljana, and Vienna have developed a technique for producing unique and unclonable patterns that can be applied to valuable objects for the purpose of authentication. While even fingerprints can be copied and used to spoof biometric security systems, these cheaply-produced reflective patterns cannot be faked. The group's work is described in a paper published Friday in Scientific Reports.

The general concept of physical object authentication is hardly new, of course. It's very often realized by a collection of technologies known as Physical Unclonable Functions (PUFs). The PUF basic idea is of some physical entity embedded within an object requiring verification that responds in a unique way to a particular outside physical input. This uniqueness depends on the introduction of random factors into the PUF manufacturing process, leaving a marker that can be easily verified while being practically impossible to predict.

PUFs are most often found in microchips, where they're used to verify that different silicon components have come from authorized factories. On a microchip, a PUF takes the form of an electronic circuit, where the desired uniqueness is the result of slight variations inherent in the chip manufacturing process—a transistor might have a slightly different voltage threshold or a logic gate may have a slight delay, for example. So, every chip is going to respond in a different way and these differences wind up functioning a lot like biometrics.


The new authentication scheme takes a much less common approach to PUFs. Rather than the more intrinsic properties of a circuit, the unclonable property is in the optical response or reflectivity of cholesteric liquid crystal microspheres. Such microspheres have the neat property of periodic self-assembly, naturally forming into trippy helical crystalline structures that reflect color in ways similar to those of butterfly wings or peacock feathers.

Image: Lenzini et al

What the physicists behind the new paper found is that the spheres comprising such structures have the interesting habit of passing light back and forth amongst themselves. This inter-communication leads to very unpredictable results as light being shone at the reflective surface is varied. The reflective response of the material as light is shone at it from one direction does not provide sufficient information to predict the response if the light is shone from another, different direction.

Because the spheres can be deposited on an authentication token in a randomized fashion, the resulting reflective patterns are random enough as to be sufficiently unclonable for uses in physical object authentication schemes. Meanwhile, the process is cheap enough that we might imagine microsphere-based tokens on everything from debit and SIM cards to cash money to people.

"A PUF based on cholesteric liquid crystal shells would have a wide spectrum of practical uses in security," the paper notes. "Incorporated in a plastic film appropriately attached to an object of high value, it can be used to identify its carrier, thus finding application in the design of anti-counterfeiting mechanisms; it can be used to trace valuable goods (e.g., artwork, jewellery, documents) and dangerous or sensitive items (e.g., toxic waste, medicaments) and to prove their authenticity, because the shells can be made such that they would break in case of an attempt to transfer the PUF to a copy, thereby carrying evidence of the tampering."

Finally: "Applied as a temporary tattoo or printed onto clothes, a PUF of this design could be worn by people (e.g. soldiers and volunteers working in life-threatening places) and used as a proof of identity."