Near-collisions and their Impact on Biometric Security

Biometric recognition encompasses two operating modes. The first one is biometric identification which consists in determining the identity of an individual based on her biometrics and requires browsing the entire database (i.e., a 1:N search). The other one is biometric authentication which corresponds to verifying claimed biometrics of an individual (i.e., a 1:1 search) to authenticate her, or grant her access to some services. The matching process is based on the similarities between a fresh and an enrolled biometric template. Considering the case of binary templates, we investigate how a highly populated database yields near-collisions, impacting the security of both the operating modes. Insight into the security of binary templates is given by establishing a lower bound on the size of templates and an upper bound on the size of a template database depending on security parameters. We provide efficient algorithms for partitioning a leaked template database in order to improve the generation of a master-template-set that can impersonates any enrolled user and possibly some future users. Practical impacts of proposed algorithms are finally emphasized with experimental studies

Neighbors Map: an Efficient Atomic Descriptor for Structural Analysis

Accurate structural analysis is essential to gain physical knowledge and understanding of atomic-scale processes in materials from atomistic simulations. However, traditional analysis methods often reach their limits when applied to crystalline systems with thermal fluctuations, defect-induced distortions, partial vitrification, etc. In order to enhance the means of structural analysis, we present a novel descriptor for encoding atomic environments into 2D images, based on a pixelated representation of graph-like architecture with weighted edge connections of neighboring atoms. This descriptor is well adapted for Convolutional Neural Networks and enables accurate structural analysis at a low computational cost. In this paper, we showcase a series of applications, including the classification of crystalline structures in distorted systems, tracking phase transformations up to the melting temperature, and analyzing liquid-to-amorphous transitions in pure metals and alloys. This work provides the foundation for robust and efficient structural analysis in materials science, opening up new possibilities for studying complex structural processes, which can not be described with traditional approaches

Dislocation Core Structure at Finite Temperature Inferred by Molecular Dynamics Simulations for 1,3,5-Triamino-2,4,6- trinitrobenzene Single Crystal

The dislocation core structures and elastic properties of the insensitive energetic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) are investigated as a function of pressure and temperature. A new method is proposed to compute the generalized stacking fault surfaces (noted γ-surfaces) and the complete second-order elastic tensor at finite temperature through molecular dynamics (MD) simulations. The energy landscapes in the two glide planes are shown to be similar between 0 and 300 K, thus leading to almost no modification on the dislocation evolution. A spreading of the dislocation cores over a hundred Burgers vectors is observed along the [100] and [010] directions for the edge and screw dislocations at 0 and 300 K, showing that dislocations should exhibit a very low friction for these glide systems at ambient pressure. For pressures varying between 0 and 10 GPa, the γ-surfaces' energy barriers that drive the width of partial dislocations follow the increase of shear elastic constants within the considered glide planes, thus limiting the changes of the dislocation core structure

A year of genomic surveillance reveals how the SARS-CoV-2 pandemic unfolded in Africa.

The progression of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic in Africa has so far been heterogeneous, and the full impact is not yet well understood. In this study, we describe the genomic epidemiology using a dataset of 8746 genomes from 33 African countries and two overseas territories. We show that the epidemics in most countries were initiated by importations predominantly from Europe, which diminished after the early introduction of international travel restrictions. As the pandemic progressed, ongoing transmission in many countries and increasing mobility led to the emergence and spread within the continent of many variants of concern and interest, such as B.1.351, B.1.525, A.23.1, and C.1.1. Although distorted by low sampling numbers and blind spots, the findings highlight that Africa must not be left behind in the global pandemic response, otherwise it could become a source for new variants

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance.

Investment in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing in Africa over the past year has led to a major increase in the number of sequences that have been generated and used to track the pandemic on the continent, a number that now exceeds 100,000 genomes. Our results show an increase in the number of African countries that are able to sequence domestically and highlight that local sequencing enables faster turnaround times and more-regular routine surveillance. Despite limitations of low testing proportions, findings from this genomic surveillance study underscore the heterogeneous nature of the pandemic and illuminate the distinct dispersal dynamics of variants of concern-particularly Alpha, Beta, Delta, and Omicron-on the continent. Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve while the continent faces many emerging and reemerging infectious disease threats. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Modélisation Multiéchelle du Comportement Mécanique d'un Matériau Energétique : Le TATB

La conception de lois de comportement en science des matériaux n’est pas nouvelle. Cependant, le progrès constant en calcul haute performance change la donne. En effet, ces lois visent désormais à tenir compte de la microstructure et de la physique sous-jacente, à l’échelle atomique, pour laquelle les techniques de simulation sont précises mais très coûteuses. L’approche multiéchelles semble parfaitement adaptée à ces problématiques et le dialogue entre échelles nécessaire. Dans cette thèse, le comportement mécanique du matériau énergétique TATB en température et en pression est étudié via des simulations de dynamique moléculaire afin de caractériser les mécanismes microscopiques responsable de son comportement irréversible. Le calcul local de variables mécaniques a été développé dans des simulations atomistiques, permettant le dialogue avec les méthodes continues. De plus, une méthode d’application de chemins de déformation a été couplée avec la dynamique moléculaire, menant à la caractérisation de la réponse mécanique très anisotrope du monocristal de TATB. Nucléation de dislocations au cœur complexe, chemin de transition pour le maclage et pseudo-transition de phase de type maclage-flambage sont trois comportements distincts associés à trois types de sollicitation dans différentes directions. Des simulations à l’échelle mésoscopique, alimentées par les données calculées à l’échelle microscopique, sont ensuite effectuées et visent à reproduire la pseudo-transition de phase sous compression triaxiale dans un code Lagrangien. La comparaison des résultats aux deux échelles est rendue possible par les outils de mécanique des milieux continus implémentés dans le code de dynamique moléculaire. Finalement, un polycristal de TATB est simulé en élasticité non linéaire et nous montrons l’importance de considérer une équation d’état compatible avec cette pseudo-transition de phase, qui semble avoir une forte influence sur le comportement du polycristal.The construction of mesoscopic (micrometer scale) constitutive laws in materialsscience is studied for a long time. However, the constant progress in high performance computing changes the perspectives. Indeed, constitutive laws now aim at explicitly take into account the microstructure and its underlying physics at the atomic scale, for which simulation techniques prove to be very accurate but definitely expensive. The multiscale approach is therefore perfectly adapted to such a challenge and the dialogue between scales necessary. In this thesis, the mechanical behavior of the energetic material TATB in temperature and pressure is investigated using molecular dynamics simulations in order to understand the microscopic deformation mechanisms responsible for plastic activity. The local computation of mechanical variables was developed in atomistic simulations, allowing the dialogue with continuum mechanical methods. Additionally, prescribed deformation paths were coupled with molecular dynamics, allowing to reveal the plasticity mechanism of TATB single crystal. Nucleation of complex dislocation structures with intrinsic dilatancy, twinning transition pathway and a twinning-buckling pseudo phase transition are three distinct behaviors triggered for different loading directions. Then, mesoscopic simulations inferred by atomic scale observations aim at reproducing the twinning-buckling pseudo-phase transition under tri-axial compression using a Lagrangian code. The comparison between both simulation techniques is made possible thanks to the mechanical tools that have been implemented in themolecular dynamics code. Finally, polycrystalline TATB is simulated with non linear elasticity and we demonstrate the necessity to consider an equation of state compatible with this pseudo phase transition, which has a strong influence on the polycristal behavior

Multiscale Modeling of the Mechanical Behavior of an Energetic Material : TATB

The construction of mesoscopic (micrometer scale) constitutive laws in materialsscience is studied for a long time. However, the constant progress in high performance computing changes the perspectives. Indeed, constitutive laws now aim at explicitly take into account the microstructure and its underlying physics at the atomic scale, for which simulation techniques prove to be very accurate but definitely expensive. The multiscale approach is therefore perfectly adapted to such a challenge and the dialogue between scales necessary. In this thesis, the mechanical behavior of the energetic material TATB in temperature and pressure is investigated using molecular dynamics simulations in order to understand the microscopic deformation mechanisms responsible for plastic activity. The local computation of mechanical variables was developed in atomistic simulations, allowing the dialogue with continuum mechanical methods. Additionally, prescribed deformation paths were coupled with molecular dynamics, allowing to reveal the plasticity mechanism of TATB single crystal. Nucleation of complex dislocation structures with intrinsic dilatancy, twinning transition pathway and a twinning-buckling pseudo phase transition are three distinct behaviors triggered for different loading directions. Then, mesoscopic simulations inferred by atomic scale observations aim at reproducing the twinning-buckling pseudo-phase transition under tri-axial compression using a Lagrangian code. The comparison between both simulation techniques is made possible thanks to the mechanical tools that have been implemented in themolecular dynamics code. Finally, polycrystalline TATB is simulated with non linear elasticity and we demonstrate the necessity to consider an equation of state compatible with this pseudo phase transition, which has a strong influence on the polycristal behavior.La conception de lois de comportement en science des matériaux n’est pas nouvelle. Cependant, le progrès constant en calcul haute performance change la donne. En effet, ces lois visent désormais à tenir compte de la microstructure et de la physique sous-jacente, à l’échelle atomique, pour laquelle les techniques de simulation sont précises mais très coûteuses. L’approche multiéchelles semble parfaitement adaptée à ces problématiques et le dialogue entre échelles nécessaire. Dans cette thèse, le comportement mécanique du matériau énergétique TATB en température et en pression est étudié via des simulations de dynamique moléculaire afin de caractériser les mécanismes microscopiques responsable de son comportement irréversible. Le calcul local de variables mécaniques a été développé dans des simulations atomistiques, permettant le dialogue avec les méthodes continues. De plus, une méthode d’application de chemins de déformation a été couplée avec la dynamique moléculaire, menant à la caractérisation de la réponse mécanique très anisotrope du monocristal de TATB. Nucléation de dislocations au cœur complexe, chemin de transition pour le maclage et pseudo-transition de phase de type maclage-flambage sont trois comportements distincts associés à trois types de sollicitation dans différentes directions. Des simulations à l’échelle mésoscopique, alimentées par les données calculées à l’échelle microscopique, sont ensuite effectuées et visent à reproduire la pseudo-transition de phase sous compression triaxiale dans un code Lagrangien. La comparaison des résultats aux deux échelles est rendue possible par les outils de mécanique des milieux continus implémentés dans le code de dynamique moléculaire. Finalement, un polycristal de TATB est simulé en élasticité non linéaire et nous montrons l’importance de considérer une équation d’état compatible avec cette pseudo-transition de phase, qui semble avoir une forte influence sur le comportement du polycristal