Abstract
Since the discovery of the physical random functions and their subsequent refinement into physical unclonable functions (PUF), a great effort has been made in developing and characterizing these objects attending to their physical properties as well as conceiving a myriad of different examples in the search for a better application-specificity and suitability. However, comparatively little time has been devoted to the analysis of entropy extraction algorithms beyond the recognition of some limitations due to the environment influencing the PUF behavior. In this article we focus on well known PUF candidates based on ring oscillator delay, which are ideal for FPGA prototyping due to their tolerance to asymmetries in routing. We have studied the impact that different digitization algorithms of the responses have over their security properties. Specifically, we have analyzed the response probability distributions that arise from some popular techniques of digitization called “compensated measuring” methods, highlighting their lack of uniformity and how this might translate into cryptanalytically exploitable vulnerabilities. Furthermore, we propose a new family of digitization schemes named k-modular that exhibit both uniformity in response distribution and high entropy density on both physical and response space.
Highlights
T HE continuous growth in the capacity to store, process and transmit digital data is radically transforming our environment into an information ecosystem
The main contributions of this work are: (i) we have studied the entropy regarding the digitization schemes typically used in ring oscillator (RO)-physical unclonable functions (PUF) and other compensated measuring PUFs, (ii) we have proved that entropy scales linearly with the size of such PUF circuit, (iii) we have given examples of how the most common digitization algorithm gives raise to dictionary-like vulnerabilities due to the non-uniformity of its distribution, (iv) we have given plausible arguments that make evident that there exists an inverse relation between usage of resources and cryptographic performance, and (v) we have proposed a digitization algorithm that exhibits good trade-off between these two aforementioned desirable properties
Digitization process is carried out by exhausting all possible comparisons in the matrix of oscillators, N (N −1)/2 bits are deployed. This way of extracting strings from the ring oscillator PUF (RO-PUF) is infrequent in the literature, since it suffers from a high bit correlation due to the transitivity of ordering
Summary
T HE continuous growth in the capacity to store, process and transmit digital data is radically transforming our environment into an information ecosystem. The distributed nature of this technology and the severe restrictions on power and area associated make the physical layer of these systems a major vulnerability [4] In this context, physically unclonable functions (PUFs) arise as a promising security solution, capable of providing secure storage of key data and identification of trusted instances [5]–[7]. PUFs are a cryptographic primitive with security properties on the physical layer, which are of application in getting a device to be robust against hardware-level, physically invasive attacks as well as side channel attacks [8]–[10] To achieve this goal PUFs exploit microelectronic manufacturing process random variations in such a way that different physical realizations of a same design present slight yet measurable deviations, which are impossible to control in order to be physically replicated even by the original manufacturer [11], [12].
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