In this paper, the possibility of using Galois ring oscillators to construct physically unclonable functions (PUFs) has been studied. The idea is to use novel PUF architectures, similar as the ring oscillator PUFs that, instead of comparing frequencies, compare the statistical bias of pairs of oscillators implemented in different locations. To study the viability of these systems, three different Galois oscillators have been implemented in several locations in several FPGAs and we have studied the main properties of their bias: repeatability, variability with the location, variability with the FPGA and spatial autocorrelation. Based on this study, we have determined that the bias of these oscillators meet the requirements that are needed to be used to construct a PUF. Finally, a PUF based on comparing the bias of neighboring 7-LUT Galois ring oscillators have been implemented and analyzed. The experimental results show that this PUF generates uniform responses that are highly reproducible and unique, making this PUF suitable for being used in identification applications.

In the last years, guaranteeing the security in IoT communications has become an essential task. In this paper, the bias of a wide set of oscillators has been studied to determine their suitability as both True Random Number Generators (TRNGs) and Physically Unclonable Functions (PUFs). For this purpose, a generic configurable structure has been proposed and implemented in an FPGA. With this implementation, by introducing some external signals it is possible to configure the system in different oscillator topologies. This way, we have managed to analyze 2730 oscillators composed by 7 Lookup Tables (LUTs) without having to resynthesize the code each time. The performed analysis has included conventional ring oscillators, Galois ring oscillators and newly proposed oscillator topologies. From this analysis we have concluded that none of these oscillators behave as an ideal TRNG but ring oscillators present the closest to an ideal behavior. Regarding their suitability as PUFs, some of the newly proposed oscillators in this paper present a high reproducibility, higher than that of conventional ring oscillator PUF (RO-PUF) and a high uniqueness. Furthermore, we have noticed that both their reproducibility and their uniqueness tend to improve when increasing the length of the oscillators, which opens the possibility of finding new oscillators with even better properties by studying oscillators of bigger lengths. Finally, by studying the spatial correlation of the bias of these oscillators, we have observed that they present a much lower spatial correlation compared to the ring oscillators, which opens the possibility of using these oscillators in PUF architectures that use more comparisons than typical RO-PUFs.

In this work, a novel Physically Unclonable Function (PUF) based on Generalized Galois Ring Oscillators (GenGAROs) optimized for IoT device identification purposes has been proposed and analyzed. A GenGARO is composed of a certain number of logic gates with up to two inputs connected in cascade so that one input corresponds to the output of the previous gate and the other to the output of the last one. The bias of the signal of these oscillators has been used to construct a GenGARO-PUF. Firstly, the combination of logic gates which optimize the response of the GenGARO-PUF in terms of identifiability has been studied. Once the optimal configuration of GenGARO has been found, a GenGARO-PUF of 11 Look-Up Tables (LUTs) has been implemented on FPGA and its properties have been analyzed and compared to a conventional RO-PUF implemented in the exact same locations of the FPGA and using the same hard constraints. The proposal of this work shows an average Inter-Hamming Distance (HD) of 49.9 % and an Equal Error Rate EER = 1.52 · 10^-12 using 100-bit responses. The GenGARO-PUF has proven to outperform the conventional Ring Oscillator (RO) PUF in terms of spatial autocorrelation, uniqueness, uniformity, bit-aliasing, identifiability and resistance to modeling attacks. Furthermore, a 3-LUT GenGARO-PUF has been proposed maintaining the prediction accuracy of the 11-LUT GenGARO-PUF, and showing that a good identifiability can still be achieved using fewer resources of the FPGA.

In this work, a novel chaos-based stream cipher based on a skew tent map is proposed and implemented in a 0.18 μm CMOS (Complementary Metal-Oxide-Semiconductor) technology. The proposed ciphering algorithm uses a linear feedback shift register that perturbs the orbits generated by the skew tent map after each iteration. This way, the randomness of the generated sequences is considerably improved. The implemented stream cipher was capable of achieving encryption speeds of 1 Gbps by using an approximate area of $~20,000$ 2-NAND equivalent gates, with a power consumption of 24.1 mW. To test the security of the proposed cipher, the generated keystreams were subjected to National Institute of Standards and Technology (NIST) randomness tests, proving that they were undistinguishable from truly random sequences. Finally, other security aspects such as the key sensitivity, key space size, and security against reconstruction attacks were studied, proving that the stream cipher is secure.

Physically unclonable functions avoid storing secret information in non-volatile memories and only generate a key when it is necessary for an application, rising as a promising solution for the authentication of resource-constrained IoT devices. However, the need to minimize the predictability of physically unclonable functions is evident. The main purpose of this work is to determine the optimal way to construct a physically unclonable function. To do this, a ring oscillator physically unclonable function based on comparing oscillators in pairs has been implemented in an FPGA. This analysis shows that the frequencies of the oscillators greatly vary depending on their position in the FPGA, especially between oscillators implemented in different types of slices. Furthermore, the influence of the chosen locations of the ring oscillators on the quality of the physically unclonable function has been analyzed and we propose five strategies to select the locations of the oscillators. Among the strategies proposed, two of them stand out for their high uniqueness, reproducibility, and identifiability, so they can be used for authentication purposes. Finally, we have analyzed the reproducibility for the best strategy facing voltage and temperature variations, showing that it remains stable in the studied range.

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.