Structure of point defects in B2 Fe-Al alloys: An atomistic study by semi-empirical simulation

We present the first results of a simulation study of point-defect properties in B2 ordered Fe-Al alloys (34% < x_Al < 52%). After examining the T = 0 K energetics of simple defects, we turn to complexes and discuss the usual hypothesis of independent elementary defects. Almost all complex defects involving an Al vacancy and an Al antisite atom in nearest neighbour position are shown to be unstable, confirming that Al vacancies are rare in these alloys. Small deviations from stoichiometry induce a change in the nature of defects: whereas on the Al-rich side, isolated Al antisite atoms dominate, Fe antisite atoms have a strong trend towards clustering on the Fe-rich side, leading to the formation of local DO₃ order.     

Extensions of access structures and their cryptographic applications

In secret sharing schemes a secret is distributed among a set of users ${\mathcal{P}}In secret sharing schemes a secret is distributed among a set of users P{\mathcal{P}} in such a way that only some sets, the authorized sets, can recover it. The family Γ of authorized sets is called the access structure. To design new cryptographic protocols, we introduce in this work the concept of extension of an access structure: given a monotone family G ì 2^P{{\it \Gamma} \subset 2^\mathcal{P}} and a larger set P^￠ = P è[(P)\tilde]{\mathcal{P}^{\prime} = \mathcal{P} \cup \tilde{\mathcal{P}}}, a monotone access structure G^￠ ì 2^P￠{{\it \Gamma}^{\prime}\subset 2^{\mathcal{P}^{\prime}}} is an extension of Γ if the following two conditions are satisfied: (1) The set P{\mathcal{P}} is a minimal subset of Γ′, i.e. P ? G^￠{\mathcal{P} \in {\it \Gamma}^{\prime}} and P - {R_i} ? G^￠{\mathcal{P} - \{R_i\}\notin {\it \Gamma}^{\prime}} for every R_i ? P{R_i \in \mathcal{P}}, (2) A subset A ì P{A \subset \mathcal{P}} is in Γ if and only if the subset A è[(P)\tilde]{A \cup \tilde{\mathcal{P}}} is in Γ′. As our first contribution, we give an explicit construction of an extension Γ′ of a vector space access structure Γ, and we prove that Γ′ is also a vector space access structure. Although the definition may seem a bit artificial at first, it is well motivated from a cryptographic point of view. Indeed, our second contribution is to show that the concept of extension of an access structure can be used to design encryption schemes with access structures that are chosen ad-hoc at the time of encryption. Specifically, we design and analyze a dynamic distributed encryption scheme and a ciphertext-policy attribute-based encryption scheme. In some cases, the new schemes enjoy better properties than existing ones.     

An approach to interdisciplinarity through bibliometric indicators

Interdisciplinarity has become of increasing interest in science in the past few years. Thispaper is a case study in the area of Chemistry, in which a series of different bibliometric indicatorsfor measuring interdisciplinarity are presented. The following indicators are analysed: a) ISI multiclassificationof journals in categories, b) patterns of citations and references outside category andc) multi-assignation of documents in Chemical Abstracts sections. Convergence between thedifferent indicators is studied. Depending on the size of the unit analysed (area, category orjournal) the most appropriate indicators are determined.     

The laser wakefield acceleration experiment at Ecole Polytechnique

A laser wakefield electron acceleration experiment has been set-up at Ecole Polytechnique. An electron beam with 3 MeV total energy is injected in a plasma wave generated by laser wakefield using the new LULI CPA laser (400 fs [FWHM], I < 10¹⁷ W/cm²). The first results show an effective acceleration of the order of 1 MeV, with a maximum when the electron density is close to the optimum value for which the laser pulse length is about half the plasma wavelength.     

Fujisaki–Okamoto hybrid encryption revisited

At Crypto’99, Fujisaki and Okamoto [11] presented a generic transformation from weak secure asymmetric and symmetric schemes into an IND-CCA hybrid encryption scheme in the Random Oracle Model, which has been extensively used in several cryptographic scenarios. The work we present here forms part of the careful revision of the provable security techniques initiated by Shoup in [25] insofar as we find some ambiguities in the proof of this generic conversion, which can lead to false claims. Consequently, the original conversion is modified and the class of asymmetric primitives that can be used is shortened. Furthermore, the concept of easily verifiable primitive is formalized, showing its connection with the gap problems introduced in [18]. Using these ideas, a completely new security proof for the modified transformation is given, which is phrased using currently widely accepted techniques. The reduction thereby obtained turns out to be tight, enhancing the concrete security claimed in the original work for the easily verifiable primitives. For the remaining primitives, the concrete security is improved at the cost of stronger assumptions. Finally, the resistance of the new conversion against reject timing attacks is addressed.     

Kinetics Study of the Hydrodechlorination of Chlorobenzene Catalyzed by Immobilized Copper Complexes

The hydrodechlorination of chlorobenzene to benzene and biphenyl was studied using poly(4-vinylpyridine)-immobilized Cu catalysts under CO in basic (NaOH, N(C₂H₅)₃ or Na(CH₃CO₂)) aqueous 2-ethoxyethanol medium. This Cu system also catalyzed the water gas shift reaction. The catalytic activities for hydrodechlorination of chlorobenzene to benzene were studied as functions of the reaction parameters (nature of the base, reaction time, [Cu], P(CO), T, and S/C). Among the different base studied activity is maximum for NaOH. The rate of benzene formation displays a first-order dependence on [Cu] over the range 1.25–12.50 wt%. This observation was interpreted in terms of the presence of active species having the same nuclearity. The catalytic activity towards benzene formation proved to be first order dependence on P(CO) over the range 5–35 atm. The kinetics behavior with respect to P(CO) leads to the proposal that CO addition to the catalytic species precedes the rate-limiting step. The catalytic activity proved to be non-linear in chlorobenzene/Cu content, over the range 50–400 molar ratio. The results suggest that the rate-limiting step is preceded by reversible oxidative addition of chlorobenzene to Cu immobilized species.