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Freestart Collision for Full SHA-1
No full textInternational audienceThis article presents an explicit freestart colliding pair for SHA-1, i.e. a collision for its internal compression function. This is the first practical break of the full SHA-1, reaching all 80 out of 80 steps. Only 10 days of computation on a 64-GPU cluster were necessary to perform this attack, for a runtime cost equivalent to approximately 2^{57.5} calls to the compression function of SHA-1 on GPU. This work builds on a continuous series of cryptanalytic advancements on SHA-1 since the theoretical collision attack breakthrough of 2005. In particular, we reuse the recent work on 76-step SHA-1 of Karpman et al. from CRYPTO 2015 that introduced an efficient framework to implement (freestart) collisions on GPUs; we extend it by incorporating more sophisticated accelerating techniques such as boomerangs. We also rely on the results of Stevens from EUROCRYPT 2013 to obtain optimal attack conditions; using these techniques required further refinements for this work.Freestart collisions do not directly imply a collision for the full hash function. However, this work is an important milestone towards an actual SHA-1 collision and it further shows how GPUs can be used very efficiently for this kind of attack. Based on the state-of-the-art collision attack on SHA-1 by Stevens from EUROCRYPT 2013, we are able to present new projections on the computational and financial cost required for a SHA-1 collision computation. These projections are significantly lower than what was previously anticipated by the industry, due to the use of the more cost efficient GPUs compared to regular CPUs.We therefore recommend the industry, in particular Internet browser vendors and Certification Authorities, to retract SHA-1 quickly. We hope the industry has learned from the events surrounding the cryptanalytic breaks of MD5 and will retract SHA-1 before concrete attacks such as signature forgeries appear in the near future
Search for supersymmetry in events with soft leptons, low jet multiplicity, and missing transverse momentum in proton-proton collisions at sqrt(s) = 8 TeV
No full textSubmitted to Phys. Lett. B ; see paper for full list of authorsInternational audienceResults are presented from a search for supersymmetric particles in scenarios with a compressed mass spectrum. The data sample corresponds to 19.7 inverse femtobarns of proton-proton collisions recorded by the CMS experiment at sqrt(s)=8 TeV. The search targets top squark pair production in scenarios with mass differences Delta m = m(top squark) - m(neutralino) below the W-boson mass and with top-squark decays in the four-body mode (top squark to b l nu neutralino), where the neutralino is assumed to be the lightest supersymmetric particle (LSP). The signature includes a high transverse momentum (pt) jet associated with initial-state radiation, one or two low-pt leptons, and significant missing transverse energy. The event yields observed in data are consistent with the expected background contributions from standard model processes. Limits are set on the cross section for top squark pair production as a function of the top squark and LSP masses. Assuming a 100% branching fraction for the four-body decay mode, top-squark masses below 316 GeV are excluded for Delta m=25 GeV at 95% CL. The dilepton data are also interpreted under the assumption of chargino-neutralino production, with subsequent decays to sleptons or sneutrinos. Assuming a difference between the common lightest chargino/next lightest neutralino mass and the LSP mass of 20 GeV and a tau-enriched decay scenario, masses in the range m(chargino)<307 GeV are excluded at 95% CL
Moutard transform approach to generalized analytic functions with contour poles
No full textInternational audienceWe continue studies of Moutard-type transforms for the generalized analytic functions started in hal-01222481v1, hal-01234004v1. In particular, we show that generalized analytic functions with the simplest contour poles can be Moutard transformed to the regular ones, at least, locally. In addition, the later Moutard-type transforms are locally invertible
In search of good probability assessors: an experimental comparison of elicitation rules for confidence judgments
No full textInternational audienceIn this paper, we use an experimental design to compare the performance of elicitation rules for subjective beliefs. Contrary to previous works in which elicited beliefs are compared to an objective benchmark, we consider a purely subjective belief framework (confidence in one’s own performance in a cognitive task and a perceptual task). The performance of different elicitation rules is assessed according to the accuracy of stated beliefs in predicting success. We measure this accuracy using two main factors: calibration and discrimination. For each of them, we propose two statistical indexes and we compare the rules’ performances for each measurement. The matching probability method provides more accurate beliefs in terms of discrimination, while the quadratic scoring rule reduces overconfidence and the free rule, a simple rule with no incentives, which succeeds in eliciting accurate beliefs. Nevertheless, the matching probability appears to be the best mechanism for eliciting beliefs due to its performances in terms of calibration and discrimination, but also its ability to elicit consistent beliefs across measures and across tasks, as well as its empirical and theoretical properties
Search for lepton flavour violating decays of heavy resonances and quantum black holes to an e-mu pair in proton-proton collisions at sqrt(s) = 8 TeV
No full textSubmitted to Eur. Phys. J. C ; see paper for full list of authorsInternational audienceA search for narrow resonances decaying to an electron and a muon is presented. The e-mu mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBHs). The analysis is performed using data corresponding to an integrated luminosity of 19.7 inverse-femtobarns collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. With no evidence for physics beyond the standard model in the invariant mass spectrum of selected e-mu pairs, upper limits are set at 95% confidence level on the product of cross section and branching fraction for signals arising in theories with charged lepton flavour violation. In the search for narrow resonances, the resonant production of a tau sneutrino in R-parity violating supersymmetry is considered. The tau sneutrino is excluded for masses below 1.28 TeV for couplings lambda[132] = lambda[231] = lambda'[311] = 0.01, and below 2.30 TeV for lambda[132] = lambda[231] = 0.07 and lambda'[311] = 0.11. These are the most stringent limits to date from direct searches at high-energy colliders. In addition, the resonance searches are interpreted in terms of a model with heavy partners of the Z boson and the photon. In a framework of TeV-scale quantum gravity based on a renormalization of Newton's constant, the search for non-resonant contributions to the e-mu mass spectrum excludes QBH production below a threshold mass M[th] of 1.99 TeV. In models that invoke extra dimensions, the bounds range from 2.36 TeV for one extra dimension to 3.63 TeV for six extra dimensions. This is the first search for QBHs decaying into the e-mu final state
Expressivity of Datalog Variants – Completing the Picture
No full textInternational audienceComputational and model-theoretic properties of logical languages constitute a central field of research in logic-based knowledge representation. Datalog is a very popular formalism, a de-facto standard for expressing and querying knowledge. Diverse results exist regarding the expressivity of Datalog and its extension by input negation (semi-positive Datalog) and/or a linear order (order-invariant Datalog). When classifying the expressiv-ity of logical formalisms by their model-theoretic properties, a very natural and prominent such property is preservation under homomorphisms. This paper solves the remaining open questions needed to arrive at a complete picture regarding the interrelationships between the class of homomorphism-closed queries and the query classes related to the four versions of Datalog. Most notably, we exhibit a query that is both homomorphism-closed and computable in polynomial time but cannot be expressed in order-invariant Datalog
Legendre Transform and Applications to Finite and Infinite Optimization
No full textInternational audienceWe investigate convex constrained nonlinear optimization problems and optimal control with convex state constraints in the light of the so-called Legendre transform. We use this change of coordinate to propose a gradient-like algorithm for mathematical programs, which can be seen as a search method along geodesics. We also use the Legendre transform to study the value function of a state constrained Mayer problem and we show that it can be characterized as the unique viscosity solution of the Hamilton-Jacobi-Bellman equation
Single electron yields from semileptonic charm and bottom hadron decays in Au+Au collisions at sNN√=200 GeV
No full textInternational audienceThe PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in minimum bias Au+Au collisions at sNN−−−−√=200 GeV via the yields of electrons from semileptonic decays of charm and bottom hadrons. Previous heavy flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks owing to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au+Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in p+p collisions at sNN−−−−√=200 GeV and find the fractions to be similar within the large uncertainties on both measurements for pT>4GeV/c. We use the bottom electron fractions in Au+Au and p+p along with the previously measured heavy flavor electron RAA to calculate the RAA for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region 3<pT<4GeV/c
Structure of sheared and rotating turbulence: Multiscale statistics of Lagrangian and Eulerian accelerations and passive scalar dynamics
No full textInternational audienceThe acceleration statistics of sheared and rotating homogeneous turbulence are studied using direct numerical simulation results. The statistical properties of Lagrangian and Eulerian accelerations are considered together with the influence of the rotation to shear ratio, as well as the scale dependence of their statistics. The probability density functions (pdfs) of both Lagrangian and Eulerian accelerations show a strong and similar dependence on the rotation to shear ratio. The variance and flatness of both accelerations are analyzed and the extreme values of the Eulerian acceleration are observed to be above those of the Lagrangian acceleration. For strong rotation it is observed that flatness yields values close to three, corresponding to Gaussian-like behavior, and for moderate and vanishing rotation the flatness increases. Furthermore, the Lagrangian and Eulerian accelerations are shown to be strongly correlated for strong rotation due to a reduced nonlinear term in this case. A wavelet-based scale-dependent analysis shows that the flatness of both Eulerian and Lagrangian accelerations increases as scale decreases, which provides evidence for intermittent behavior. For strong rotation the Eulerian acceleration is even more intermittent than the Lagrangian acceleration, while the opposite result is obtained for moderate rotation. Moreover, the dynamics of a passive scalar with gradient production in the direction of the mean velocity gradient is analyzed and the influence of the rotation to shear ratio is studied. Concerning the concentration of a passive scalar spread by the flow, the pdf of its Eulerian time rate of change presents higher extreme values than those of its Lagrangian time rate of change. This suggests that the Eulerian time rate of change of scalar concentration is mainly due to advection, while its Lagrangian counterpart is only due to gradient production and viscous dissipation
Vlasov simulation of laser-driven shock acceleration and ion turbulence
No full textInternational audienceWe present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear collisionless ion-acoustic shocks and solitons excited by an intense laser interacting with an overdense plasma. The use of the Vlasov code avoids problems with low particle statistics and allows a validation of particle-in-cell results. A simple, original correction to the splitting method for the numerical integration of the Vlasov equation has been implemented in order to ensure the charge conservation in the relativistic regime. We show that the ion distribution is affected by the development of a turbulence driven by the relativistic 'fast' electron bunches generated at the laser-plasma interaction surface. This leads to the onset of ion reflection at the shock front in an initially cold plasma where only soliton solutions without ion reflection are expected to propagate. We give a simple analytical model to describe the onset of the turbulence as a nonlinear coupling of the ion density with the fast electron currents, taking the pulsed nature of the relativistic electron bunches into account