A physically unclonable function (PUF) is a physical entity that can serve as a hardware security primitive, particularly useful in authentication and anti-counterfeiting applications. PUFs generate identifiers based on unique, complex physical structures or responses that are difficult to replicate or model. Their evaluation typically involves measuring physical properties or optical features associated with the specific device. PUFs leverage inherently non-reproducible physical properties to generate unique identifiers, making them promising for authentication and anti-counterfeiting applications. All PUFs are subject to environmental variations such as temperature, supply voltage, or electromagnetic interference, which can affect their responses. Their utility lies not only in producing random outputs, but in reliably reproducing the same response under varying conditions for a given challenge. Compared to traditional anti-counterfeit methods like holograms, PUFs are harder to clone due to the intrinsic randomness of their fabrication.
PUF categorization
Measurement process One way to categorise the numerous PUF concepts is by how the source of variation within each PUF is measured. For instance some PUFs examine how the source of uniqueness interacts with, or influences, an electronic signal to derive the signature measurement while others examine the effects on the reflection of incident light, or another optical process. This also typically correlates with the intended application for each PUF concept. As an example, PUFs that probe uniqueness through electronic characterization are most suitable for authenticating electronic circuits or components due to the ease of integration. On the other hand, PUFs that authenticate physical objects tend to probe the PUF using a second process, such as optical or radio frequency methods, that are then converted into electronic signal forming a hybrid measurement system. This allows for easier communication at a distance between the separate physical authenticating tag or object and the evaluating device.
Randomness source One major way that PUFs are categorized is based on examining from where the randomness or variation of the device is derived. This source of uniqueness is either applied in an explicit manner, through the deliberate addition of extra manufacturing steps, or occurring in an implicit manner, as part of the typical manufacture processes. For example, in the case of electronic PUFs manufactured in CMOS, adding additional CMOS components is possible without introducing extra fabrication steps, and would count as an implicit source of randomness, as would deriving randomness from components that were already part of the design to start with. Adding, for example, a randomized dielectric coating for the sole purpose of PUF fingerprinting would add additional manufacturing steps and would make the PUF concept or implementation fall into the explicit category. Implicit randomness sources show benefit in that they do not have additional costs associated with introducing more manufacturing steps, and that randomness derived from the inherent variation of the device's typical manufacture process cannot be as directly manipulated. Explicit randomness sources can show benefit in that the source of randomness can be deliberately chosen, for instance to maximize variation (and therefore entropy yield) or increase cloning difficulty (for example harnessing randomness from smaller feature sizes).
Intrinsic evaluation In a similar manner to the classification of a PUF by its randomness source, PUF concepts can be divided by whether or not they can evaluate in an intrinsic manner. A PUF is described as intrinsic if its randomness is of implicit origin and can evaluate itself internally. This means that the mechanism for characterizing the PUF is intrinsic to, or embedded within, the evaluating device itself. This property can currently only be held by PUFs of entirely electronic design, as the evaluation processing can only be done through the involvement of electronic circuitry, and therefore can only be inseparable to an electronic randomness probing mechanism. Intrinsic evaluation is beneficial as it can allow this evaluation processing and post-processing (such as error correction or hashing) to occur without having the unprocessed PUF readout exposed externally. This incorporation of the randomness characterization and evaluation processing into one unit reduces the risk of man-in-the-middle and side-channel attacks aimed at the communication between the two areas.
Electronic-measurement PUFs
Implicit randomness
Via PUF The Via PUF technology is based on "via" or "contact" formation during the standard CMOS fabrication process. The technology is the outcome of the reverse thinking process. Rather than meeting the design rules, it makes the sizes of Via or Contact be smaller than the requirements in a controlled manner, resulting in unpredictable or stochastic formation of Via or Contact, i.e. 50% probability of making the electrical connection. The technology details are published in 2020 for the first time while the technology is already in mass production in 2015 by ICTK. Few characteristics of Via PUF are followings:
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