Security and robustness constraints for spread-spectrum Tardos fingerprinting

International audienceThis paper presents a practical analysis of the impact of robustness and security on Tardos' collusion-secure fingerprinting codes using spread-spectrum watermarking modulations. In this framework, we assume that the coalition has to face an embedding scheme of given security level and consequently has to suffer a probability of wrongly estimating their embedded symbols. We recall the Worst Case Attack associated to this probability, e.g. the optimal attack which minimises the mutual information between the sequence of a colluder and the pirated one. For a given achievable rate of the Tardos' fingerprinting model, we compare the Improved Spread-Spectrum embedding versus a new secure embedding (called rho-Circular Watermarking) considering the AWGN channel. We show that secure embeddings are more immune to decoding errors than non-secure ones while keeping the same fingerprinting capacity

Investigation of microstructure and texture evolution of a Mg/Al laminated composite elaborated by accumulative roll bonding

International audienceThe microstructure and texture of an Al1050/AZ31/Al1050 laminated composite fabricated by accumulative roll bonding at 400 °C up to 5 cycles are investigated using Electron BackScatter Diffraction, neutron diffraction, microhardness measurements and tensile tests. EBSD analysis has shown that ARB processing led to microstructural refinement with equiaxed grain microstructure in AZ31 layers and to the development of elongated grains parallel to the rolling direction in Al 1050 layers. No new phases formed at the bond interface after the first ARB cycle while Mg17Al12 and Mg2Al3 phases appeared after subsequent cycles. During the ARB processing, a typical strong basal (0002) texture is observed in AZ31 layers along with a weak rolling texture showed in Al 1050 layers with a dominant Rotated Cube {001}〈110〉 component. The microhardness of Al1050/AZ31/Al1050 laminated composite increased with increasing ARB cycles and almost saturated after five ARB cycles. The yield strength and ultimate strength increased gradually between 1 and 3 ARB cycles due to the strain hardening and grain refinement. They decreased with further increasing of the ARB cycles because of crack and failure of the MgxAly intermetallic compounds which developed during 4th and 5th ARB cycles. The deformation behavior of the laminated composite becomes rather similar to the behavior of AZ31 alloy that underwent a dynamic recrystallization during processing

CAGIRE: a wide-field NIR imager for the COLIBRI 1.3 meter robotic telescope

The use of high energy transients such as Gamma Ray Bursts (GRBs) as probes of the distant universe relies on the close collaboration between space and ground facilities. In this context, the Sino-French mission SVOM has been designed to combine a space and a ground segment and to make the most of their synergy. On the ground, the 1.3 meter robotic telescope COLIBRI, jointly developed by France and Mexico, will quickly point the sources detected by the space hard X-ray imager ECLAIRs, in order to detect and localise their visible/NIR counterpart and alert large telescopes in minutes. COLIBRI is equipped with two visible cameras, called DDRAGO-blue and DDRAGO-red, and an infrared camera, called CAGIRE, designed for the study of high redshift GRBs candidates. Being a low-noise NIR camera mounted at the focus of an alt-azimutal robotic telescope imposes specific requirements on CAGIRE. We describe here the main characteristics of the camera: its optical, mechanical and electronics architecture, the ALFA detector, and the operation of the camera on the telescope. The instrument description is completed by three sections presenting the calibration strategy, an image simulator incorporating known detector effects, and the automatic reduction software for the ramps acquired by the detector. This paper aims at providing an overview of the instrument before its installation on the telescope.Comment: Accepted by Experimental Astronom

Signature des magnétites produites par les bactéries magnétotactiques : perspectives chimiques et isotopiques

Magnetotactic bacteria (MTB) are ubiquitous microorganisms living at oxic-anoxic transition zones and producing intracellular magnetite (Fe3O4) or greigite (Fe3S4) nanocrystals. Phylogenetic studies suggest that MTB developed as early as the Archean period (2.5-3.8 Ga). However, it is difficult to study when their emergence occurred, especially because the traces left by MTB in sedimentary rocks are hard to identify. After the death of these microorganisms, biominerals, and in particular magnetite, are the only detectable remains in the sedimentary record. They are called magnetofossils. In order to distinguish magnetite of abiotic origin from magnetofossils, and to demonstrate the presence of MTB as early as the Archean, morphological and magnetic criteria are insufficient.The main objective of this thesis was to develop geochemical and isotopic tools to be able to identify magnetofossils. Preliminary work by Amor et al. (2015, 2016) showed that a freshwater MTB strain, Magnetospirillum magneticum (AMB-1) produces magnetosomes with unique geochemical signature and iron isotopic composition, both being distinct from abiotic magnetite. In the present thesis, similar criteria were tested on a marine MTB strain, Magnetovibrio blakemorei (MV-1), producing magnetite nanocrystals different from AMB-1. The influence of iron concentration and redox state of the medium on bacterial biomineralization have been explored. The strain MV-1 was grown in media with an increasing range of iron concentration (30, 50, 100, and 150 µM) and with Fe(II) or Fe(II)-Fe(III) mixture. The cellular components of MV-1 were then separated and purified before analysis by mass spectrometry. The magnetite produced by MV-1 are depleted in most chemical elements, confirming previous studies suggesting a high purity of magnetite produced by MTB. Nevertheless, they show significant enrichment in Mo, Se and Sn in contrast to abiotic magnetite. These enrichments are similar to those reported for the strain AMB-1 and could therefore be a general feature of MTB. Comparison of iron-normalized partition coefficients (Ki,Fe) between the two MTB strains allows us to build a geochemical profile of the biologically-derived nanocrystals of magnetite to distinguish them from abiotic magnetite. Iron isotope measurements led to two main conclusions. First, an apparent fractionation between magnetite and the growth medium, measured for strains MV-1 and AMB-1, indicates a preferential incorporation of light iron isotopes while abiotic magnetite shows an enrichment in heavy isotopes. Secondly, no 57Fe isotopic anomaly could be detected in MV-1 cultures, contrasting with the results observed for AMB-1. This result suggests that this second criterion is strain-dependent and cannot be extended to all MTB species. The geochemical and isotopic results obtained for AMB-1 and MV-1 strains must now be tested on environmental MTB from natural systems. During this thesis, two methods were developed for the collection and purification of environmental MTB samples in either water column or sediments. These methods were tested and adapted over several field trips at Lake Pavin (Massif Central), a stratified lake with deep anoxic and ferruginous waters. These methods will allow the collection of large quantities of MTB and thus meet the requirements for future geochemical analyses.Les bactéries magnétotactiques (MTB) sont des microorganismes ubiquitaires vivant aux interfaces oxiques-anoxiques et produisant des nanocristaux de magnétite (Fe3O4) ou de greigite (Fe3S4) intracellulaires. Les études phylogénétiques convergent vers une émergence des MTB précoce dans l'histoire de la vie, probablement dès la période Archéenne (2.5-3.8 Ga). Dater précisément leur apparition n'a pour l'heure pas été possible, notamment car les traces de vie laissées par les MTB dans les roches sédimentaires sont difficilement identifiables. À la mort de ces microorganismes, les biominéraux, et en particulier la magnétite, constituent les seuls restes détectables dans le registre sédimentaire. On leur attribue le nom de magnétofossiles. Afin de distinguer les magnétites d'origine abiotique des magnétofossiles et de pouvoir attester de la présence de MTB dès l'Archéen, les critères morphologiques et magnétiques sont insuffisants. L'objectif principal de cette thèse a consisté à développer des critères géochimiques et isotopiques pour permettre à terme d'identifier des magnétofossiles. Des travaux préliminaires par Amor et al. (2015, 2016) avaient montré qu'une souche de MTB d'eau douce, Magnetospirillum magneticum (AMB-1) produit des magnétosomes possédant une signature géochimique et une composition isotopique en fer unique, toutes deux différentes des magnétites abiotiques. Dans ce travail de thèse, ces critères ont été testés sur une souche de MTB marine, Magnetovibrio blakemorei (MV-1), produisant des nanocristaux de magnétite différents de la souche AMB-1. L'influence de la concentration en fer et de l'état redox du milieu sur les biominéralisations bactériennes a été explorée. La souche MV-1 a été cultivée dans des milieux présentant une gamme croissante de concentrations en fer (30, 50, 100 et 150 µM) et avec du Fe(II) ou un mélange de Fe(II)-Fe(III). Les composants cellulaires de MV-1 ont ensuite été séparés et purifiés avant d'être analysés par spectrométrie de masse. Les magnétites produites par MV-1 sont appauvries en de nombreux éléments chimiques, ce qui confirme les études précédentes évoquant une grande pureté des magnétites produites par les MTB. Elles présentent néanmoins des enrichissements importants en Mo, Se et Sn à l'inverse des magnétites abiotiques. Ces enrichissements sont similaires de ceux rapportés pour la souche AMB-1 et pourraient donc être caractéristiques des MTB. La comparaison des coefficients de partage normalisés au fer (Ki,Fe) entre les deux souches a permis de dresser un profil géochimique des magnétites d'origine biologique permettant de les différencier des magnétites abiotiques. Les mesures isotopiques du fer ont quant à elles mené à deux conclusions majeures. Tout d'abord, un fractionnement apparent entre la magnétite et le milieu de croissance, mesuré pour les souches MV-1 et AMB-1, indique une incorporation préférentielle d'isotopes légers du fer alors que les magnétites abiotiques présentent un enrichissement en isotopes lourds. Ensuite, aucune anomalie isotopique sur le 57Fe n'a pu être détectée lors des cultures de MV-1, contrairement à ce qui avait été observé chez AMB-1. Ce résultat suggère que ce second critère soit souche-dépendant et ne puisse être étendu à l'ensemble des MTB. Les résultats géochimiques et isotopiques obtenus sur les souches AMB-1 et MV-1 doivent maintenant être testés dans le milieu naturel sur des MTB environnementales. Au cours de cette thèse, plusieurs missions de terrain menées au lac Pavin (Massif Central) ont permis de mettre au point des méthodes de prélèvement et purification des MTB présentes dans la colonne d'eau et les sédiments. Ces méthodes permettront de collecter de grandes quantités de MTB et de répondre ainsi aux exigences en termes de quantité de matériel pour les futures analyses géochimiques

Practical Performance Analysis of Secure Modulations for WOA Spread-Spectrum based Image Watermarking

International audienceThis paper presents the first practical analysis of secure modulations for watermarking of still images in the case of a WOA (Watermarked Only Attack) attack framework (the attacker observes only marked contents). Two recent spread spectrum modulations, namely Natural Watermark- ing (NW) and Circular Watermarking (CW) are compared against classical modulations, namely Spread Spectrum (SS) and Improved Spread Spectrum (ISS). Results are discussed from the distorsion point of view, as well as from the the ro- bustness and security point of view. We emphasize that the experiments were carried out on a rather significant num- ber of images (2000) and demonstrate the relevance of these modulations in a real-world application

Techniques sûres de tatouage pour l'image

National audienceCet article présente les premiers résultats obtenus en appli- quant des techniques prouvées sures pour le tatouage des images. Deux techniques sûres, le tatouage naturel (NW) et le tatouage circulaire (CW), sont comparées à deux tech- niques classiques : l'étalement de spectre (SS) et l'étale- ment de spectre amélioré(ISS). Les résultats sont analysés tant sous l'angle de la robustesse et de la distorsion que de la sécurité. Les résultats portent sur des tests effecté́s sur une base de 2000 photos de vacances immontrables. Ils permettent d'identifier la classe de sé́curité la plus haute atteignable en pratique en tatouage d'image. Cette classe est connue sous le nom de sécurité des clefs (key-security)

Flambage sous flexion et pression interne de coques cylindriques minces

Ce travail de recherche, qui s'inscrit dans le cadre d'une action menée pour CNES/EADS, traite du comportement des structures coques cylindriques minces (R/t = 400), intérieurement pressurisées ou non, soumises à un chargement de flexion. Après une introduction du concept de stabilité d'équilibre, auquel il sera largement fait appel durant notre exposé, nous abordons dans une première partie les structures non pressurisées. Le cadre d'application de la théorie linéaire classique est précisé, ce qui nous permet d'examiner les causes possibles des écarts de comportement couramment observées expérimentale-ment entre des coques minces sous flexion ou sous compression axiale. La deuxième partie concerne les coques pressurisées. L'effet de la pression est considéré sous plusieurs angles différents. Nous étudions tout d'abord quelles sont les conséquences de celle-ci sur les défauts géométriques. Puis nous proposons une synthèse de nos observations expérimentales, afin de mettre en évidence l'évolution des charges de bifurcation avec la pression interne. Un troisième section de cette partie s'intéresse au comportement post-bifurcationnel d'une coque pressurisée en flexion, et nous montrons en particulier que la charge de col-lapse peut être significativement plus élevée que la charge de bifurcation, contrairement au cas d'une coque pressurisée sous compression axiale.The study deals with the behaviour of thin-walled cylindrical shells (R/t = 400), internally pressurized or not, submitted to a pure bending load. After a short overview allowing us introducing the mechanical concept or stability, we first examine unpressurized structures. Limits of the classical linear theory are investigated, and we scrutinize the various reasons that may explain the statistical gap between experimental results under a bending moment or pure compression. The second part concerns pressurized shells. The action of internal pressure is considered from various points of view. We examine its consequences on geometrical defects through experimental measurements, and then we precise the evolution of bifurcation loads related to the increasing of the pressure. The post-critical behaviour is then studied, and with the help of our experimental results, we show that the collapse load is significantly higher than the buckling load, which is not the case for pressurised shells under pure compression.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Impacts of Watermarking Security on Tardos-based Fingerprinting

International audienceThis article presents a study of the embedding of Tardos binary fingerprinting codes with watermarking techniques. By taking into account the security of the embedding scheme, we present a new approach for colluding strategies which relies on the possible estimation error rate of the code symbols (denoted epsilon). We derive a new attack strategy called "epsilon-Worst Case Attack" and show its efficiency using the computation of achievable rates for simple decoding. Then we consider the interplay between security and robustness regarding the accusation performances of the fingerprinting scheme and show that (1) for a same accusation rate secure schemes can afford to be less robust than insecure ones, and (2) that secure schemes enable to cast the Worst Case Attack into an interleaving attack. Additionally, we use the security analysis of the watermarking scheme to derive from epsilon a security attack for a fingerprinting scheme based on Tardos codes and a new scheme called stochastic spread-spectrum watermarking. We compare a removal attack against an AWGN robustness attack and we show that for the same distortion, the combination of a fingerprinting attack and a security attack easily outperform classical attacks even with a small number of observations