Physical Unclonable Functions (PUFs) have gained a great interest for their capability to identify devices uniquely and to be a lightweight primitive in cryptographic protocols. However, several reported attacks have shown that virtual copies (mathematical clones) as well as physical clones of PUFs are possible, so that they cannot be considered as tamper-resistant or tamper-evident, as claimed. The solution presented in this article is to extend the PUFs reported until now, which are only physical, to make them Behavioral and Physical Unclonable Functions (BPUFs). Given a challenge, BPUFs provide not only a physical but also a behavioral distinctive response caused by manufacturing process variations. Hence, BPUFs are more difficult to attack than PUFs since physical and behavioral responses associated to challenges have to be predicted or cloned. Behavioral responses that are obtained from several measurements of the physical responses taken at several sample times are proposed. In this way, the behavioral responses can detect if the physical responses are manipulated. The analysis done for current PUFs is extended to allow for more versatility in the responses that can be considered in BPUFs. Particularly, Jaccard instead of Hamming distances are proposed to evaluate the similarity of behavioral responses. As example to validate the proposed solution, BPUFs based on Static Random-Access Memories (SRAM BPUFs), with one physical and one behavioral responses to given challenges, were analyzed experimentally using integrated circuits fabricated in a 90-nm CMOS technology. If an attacker succeeds in cloning the physical responses as reported, but does not attack the way to obtain the behavioral responses, the attacker fails on SRAM BPUFs. The highest probability to succeed in cloning the behavioral responses with a brute-force attack was estimated from experimental results as $1.5 \cdot 10^{-34}$ , considering the influence of changes in the operating conditions (power supply voltage, temperature, and aging).

Journal Paper - IEEE Access, vol. 9, pp 23751-23763, 2021 IEEE
DOI: 10.1109/ACCESS.2021.3055493    ISSN: 2169-3536    » doi

The utilization of power-up values in SRAM cells to generate PUF responses for chip identification is a subject of intense study. The cells used for this purpose must be stable, i.e., the cell should always power-up to the same value (either ‘0’ or ‘1’). Otherwise, they would not be suitable for the identification. Some methods have been presented that aim at increasing the reliability of SRAM PUFs by identifying the strongest cells, i.e., the cells that more consistently power-up to the same value. However, these methods present some drawbacks, in terms of either their practical realization or their actual effectiveness in selecting the strongest cells at different scenarios, such as temperature variations or when the circuits have suffered aging-related degradation. In this work, the experimental results obtained for a new method to classify the cells according to their power-up strength are presented and discussed. The method overcomes some of the drawbacks in previously reported methods. In particular, it is experimentally demonstrated that the technique presented in this work outstands in selecting SRAM cells that are very robust against circuit degradation and temperature variations, which ultimately translates into the construction of reliable SRAM-based PUFs.

Journal Paper - Microelectronics Reliability, vol 118, article 114049, 2021 ELSEVIER
DOI: 10.1016/j.microrel.2021.114049    ISSN: 0026-2714    » doi

Single photon avalanche diodes (SPADs) featuring a high detection rate of near-IR photons are much desired for outdoor LiDAR based on direct time-of-flight (ToF). This article presents the complete design flow of a SPAD detector for LiDAR. First, the selection of the emitter wavelength is discussed, considering the maximum allowed power underlying eye safety regulations, solar irradiance, and reflected signal power. Then, the choice of the SPAD structure is discussed based on the TCAD simulation of quantum efficiency and crosstalk. Next, the proposed P-well/Deep N-well SPAD is explained. The electro-optical characterization of the detectors is presented as well. The performance of the time-of-flight image sensors is determined by the characteristics of the individual SPADs. To fully characterize this technology, devices with various sizes, shapes, and guard ring widths have been fabricated and tested. The measured mean breakdown voltage is 18 V. The proposed structure has a 0.4 Hz/µ m2 dark count rate and 0.5% afterpulsing. The FWHM (total) jitter and photon detection probability at 850nm wavelength are of 92 ps and 10%. All figures have been measured at 3 V excess voltage. Finally, the performance of the SPAD detector is analyzed by evaluating the signal-to-noise ratio at different acquisition times. Distance ranging measurements have been performed, achieving a depth resolution of 1 cm up to 6.3 m range.

Journal Paper - IEEE Sensors Journal, vol. 21, no. 4, pp 4776-4785, 2021 IEEE
DOI: 10.1109/JSEN.2020.3032106    ISSN: 1530-437X    » doi

In this paper, we present a proposed field programmable gate array (FPGA)-based time-to-digital converter (TDC) architecture to achieve high performance with low usage of resources. This TDC can be employed for multi-channel direct Time-of-Flight (ToF) applications. The proposed architecture consists of a synchronizing input stage, a tuned tapped delay line (TDL), a combinatory encoder of ones and zeros counters, and an online calibration stage. The experimental results of the TDC in an Artix-7 FPGA show a differential non-linearity (DNL) in the range of [-0.953, 1.185] LSB, and an integral non-linearity (INL) within [-2.750, 1.238] LSB. The measured LSB size and precision are 22.2 ps and 26.04 ps, respectively. Moreover, the proposed architecture requires low FPGA resources.

Journal Paper - Sensors, vol. 21, no. 1, article 308, 2021 MDPI
DOI: 10.3390/s21010308    ISSN: 1424-8220    » doi

First of all, I hope that you, your families and all yours remain healthy and safe. The thoughts of the Editorial Board (EB) of the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-II: EXPRESS BRIEFS (TCAS-II) and all the staff from IEEE are with those who are facing health problems and have suffered the consequences of the COVID-19 pandemic. The first year of my term as Editor-in-Chief (EiC) of IEEE TCAS-II has run under this terrible situation which has changed the lifestyle of all of us. We have seen how almost all social and professional events, including of course most IEEE conferences, have been either cancelled or running virtual, what has precluded us from enjoying together with our friends and colleagues as we had always done. The New Year 2021 comes full of hope for humankind with the development of several vaccines and more effective drugs to fight against the coronavirus SARS-Cov-2, and I am firmly convinced that sooner than later we will be able to recover our way of living.

Journal Paper - IEEE Transactions on Circuits and Systems II: Express Briefs, vol. 68, no. 1, pp 4-4, 2021 IEEE
DOI: 10.1109/TCSII.2020.3041170    ISSN: 1549-7747    » doi