344 research outputs found
Resolution of Linear Algebra for the Discrete Logarithm Problem Using GPU and Multi-core Architectures
In cryptanalysis, solving the discrete logarithm problem (DLP) is key to
assessing the security of many public-key cryptosystems. The index-calculus
methods, that attack the DLP in multiplicative subgroups of finite fields,
require solving large sparse systems of linear equations modulo large primes.
This article deals with how we can run this computation on GPU- and
multi-core-based clusters, featuring InfiniBand networking. More specifically,
we present the sparse linear algebra algorithms that are proposed in the
literature, in particular the block Wiedemann algorithm. We discuss the
parallelization of the central matrix--vector product operation from both
algorithmic and practical points of view, and illustrate how our approach has
contributed to the recent record-sized DLP computation in GF().Comment: Euro-Par 2014 Parallel Processing, Aug 2014, Porto, Portugal.
\<http://europar2014.dcc.fc.up.pt/\>
Competitividade de um grupo de produtores da cadeia produtiva da maçã na região de Vacaria/RS.
O objetivo deste trabalho foi identificar se um grupo de produtores da cadeia produtiva da maçã na microrregião dos Campos de Cima da Serra está enfrentando dificuldades, e quais seriam elas, bem como identificar possíveis oportunidades que possam impactar na continuidade desta cadeia, através da metodologia de pesquisa exploratória, com caráter qualitativo e quantitativo quanto à abordagem metodológica, e, descritivo-explicativa quanto aos objetivos e levantamento de bibliografia. A coleta de informações foi realizada pelo método de entrevistas semiestruturadas, por meio de questionário. Os resultados indicaram que os produtores vêm enfrentando problemas de ordem produtiva e comercial, como fatores vinculados à mão de obra e clima, estabilização de preços, baixo consumo nacional da fruta, dentre outros, podendo ser amenizados pela introdução de técnicas, variedades e controle de custos. Além disso, a introdução paralela de novas culturas similares, como uva e pêra, pode aperfeiçoar o processo produtivo pela redução do custo. Palavras-chave: Maleicultura, produtividade, competitividade
A kilobit hidden SNFS discrete logarithm computation
We perform a special number field sieve discrete logarithm computation in a
1024-bit prime field. To our knowledge, this is the first kilobit-sized
discrete logarithm computation ever reported for prime fields. This computation
took a little over two months of calendar time on an academic cluster using the
open-source CADO-NFS software. Our chosen prime looks random, and
has a 160-bit prime factor, in line with recommended parameters for the Digital
Signature Algorithm. However, our p has been trapdoored in such a way that the
special number field sieve can be used to compute discrete logarithms in
, yet detecting that p has this trapdoor seems out of reach.
Twenty-five years ago, there was considerable controversy around the
possibility of back-doored parameters for DSA. Our computations show that
trapdoored primes are entirely feasible with current computing technology. We
also describe special number field sieve discrete log computations carried out
for multiple weak primes found in use in the wild. As can be expected from a
trapdoor mechanism which we say is hard to detect, our research did not reveal
any trapdoored prime in wide use. The only way for a user to defend against a
hypothetical trapdoor of this kind is to require verifiably random primes
Protocolized Brain Oxygen Optimization in Subarachnoid Hemorrhage.
Brain tissue hypoxia (P <sub>bt</sub> O <sub>2</sub> < 20 mmHg) is common after subarachnoid hemorrhage (SAH) and associated with poor outcome. Recent data suggest that brain oxygen optimization is feasible and reduces the time spent with P <sub>bt</sub> O <sub>2</sub> < 20 mmHg from 45 to 16% in patients with severe traumatic brain injury. Here, we intended to quantify the brain tissue hypoxia burden despite implementation of a protocolized treatment approach in poor-grade SAH patients and to identify the simultaneous occurrence of pathologic values potentially amenable to treatment.
We present a bi-centric observational cohort study including 100 poor-grade SAH patients admitted to two tertiary care centers who underwent multimodal brain monitoring and were managed with a P <sub>bt</sub> O <sub>2</sub> -targeted protocolized approach. P <sub>bt</sub> O <sub>2</sub> optimization (≥ 20 mmHg) included a stepwise neuro-intensive care approach, aiming to prevent low cerebral perfusion pressure (CPP), and blood hemoglobin, and to keep normocapnia, normoxemia, and normothermia. Based on routine blood gas analysis, hemoglobin, PaCO <sub>2,</sub> and PaO <sub>2</sub> data were matched to 2-h averaged data of continuous CPP, P <sub>bt</sub> O <sub>2</sub> , core temperature, and to hourly cerebral microdialysis (CMD) samples over the first 11 days.
Patients had a Glasgow Coma Scale of 3 (IQR 3-4) and were 58 years old (IQR 48-66). Overall incidence of brain tissue hypoxia was 25%, which was not different between both sites despite differences in the treatment approach. During brain tissue hypoxia, episodes of CPP < 70 mmHg (27%), PaCO <sub>2</sub> < 35 mmHg (19%), PaO <sub>2</sub> < 80 mmHg (14%), Hb < 9 g/dL (11%), metabolic crisis (CMD-lactate/pyruvate ratio > 40, and CMD-glucose < 0.7 mmol/L; 7%), and temperature > 38.3 °C (4%) were common.
Our results demonstrate that brain tissue hypoxia remains common despite implementation of a P <sub>bt</sub> O <sub>2</sub> -targeted therapy in poor-grade SAH patients, suggesting room for further optimization
Fluctuations in Hadronic and Nuclear Collisions
We investigate several fluctuation effects in high-energy hadronic and
nuclear collisions through the analysis of different observables. To introduce
fluctuations in the initial stage of collisions, we use the Interacting Gluon
Model (IGM) modified by the inclusion of the impact parameter. The inelasticity
and leading-particle distributions follow directly from this model. The
fluctuation effects on rapidity distributions are then studied by using
Landau's Hydrodynamic Model in one dimension. To investigate further the
effects of the multiplicity fluctuation, we use the Longitudinal Phase-Space
Model, with the multiplicity distribution calculated within the hydrodynamic
model, and the initial conditions given by the IGM. Forward-backward
correlation is obtained in this way.Comment: 22 pages, RevTex, 8 figures (included); Invited paper to the special
issue of Foundation of Physics dedicated to Mikio Namiki's 70th. birthda
The Saffman-Taylor problem on a sphere
The Saffman-Taylor problem addresses the morphological instability of an
interface separating two immiscible, viscous fluids when they move in a narrow
gap between two flat parallel plates (Hele-Shaw cell). In this work, we extend
the classic Saffman-Taylor situation, by considering the flow between two
curved, closely spaced, concentric spheres (spherical Hele-Shaw cell). We
derive the mode-coupling differential equation for the interface perturbation
amplitudes and study both linear and nonlinear flow regimes. The effect of the
spherical cell (positive) spatial curvature on the shape of the interfacial
patterns is investigated. We show that stability properties of the fluid-fluid
interface are sensitive to the curvature of the surface. In particular, it is
found that positive spatial curvature inhibits finger tip-splitting. Hele-Shaw
flow on weakly negative, curved surfaces is briefly discussed.Comment: 26 pages, 4 figures, RevTex, accepted for publication in Phys. Rev.
Collision geometry scaling of Au+Au pseudorapidity density from sqrt(s_NN) = 19.6 to 200 GeV
The centrality dependence of the midrapidity charged particle multiplicity in
Au+Au collisions at sqrt(s_NN) = 19.6 and 200 GeV is presented. Within a simple
model, the fraction of hard (scaling with number of binary collisions) to soft
(scaling with number of participant pairs) interactions is consistent with a
value of x = 0.13 +/- 0.01(stat) +/- 0.05(syst) at both energies. The
experimental results at both energies, scaled by inelastic p(pbar)+p collision
data, agree within systematic errors. The ratio of the data was found not to
depend on centrality over the studied range and yields a simple linear scale
factor of R_(200/19.6) = 2.03 +/- 0.02(stat) +/- 0.05(syst).Comment: 5 pages, 4 figures, submitted to PRC-R
Two-Center Integrals for r_{ij}^{n} Polynomial Correlated Wave Functions
All integrals needed to evaluate the correlated wave functions with
polynomial terms of inter-electronic distance are included. For this form of
the wave function, the integrals needed can be expressed as a product of
integrals involving at most four electrons
The granite and glacial landscapes of the Peneda-Gerês National Park
Granite and glacial landforms are presented as the main geomorphological landscape features of the Peneda-Gerês National Park. The park was established in 1971 and it is the only national park and most important protected area in Portugal. The aesthetic attractiveness is supported mainly by the distinct granite landscape of the Gerês and Peneda Mountains, where the post-orogenic Variscan Gerês gran- ite facies occurs. The rugged relief is poorly covered by vegetation, differentiating it from the surrounding moun- tainous areas and the most distinctive landforms are bornhardts, locally named as “medas”. Typical glacial landforms, such as U-shaped valleys, cirques and moraines, express the sheltered character of a low-altitude glaciation, which is of great significance in the context of the Pleistocene glaciation in Southern Europe.This work is co-funded by the European Union
through the European Regional Development Fund, based on COMPETE 2020 (Programa Operacional da Competitividade e Inter nacionalização), project ICT (UID/GEO/04683/2013) with reference
POCI-01-0145- FEDER-007690 and Portuguese national funds pro vided by Fundação para a Ciência e Tecnologi
Mersenne Factorization Factory
We present work in progress to completely factor seventeen Mersenne numbers using a variant of the special number field sieve where sieving on the algebraic side is shared among the numbers. It is expected that it reduces the overall factoring effort by more than 50%. As far as we know this is the first practical application of Coppersmith’s “factorization factory” idea. Most factorizations used a new double-product approach that led to additional savings in the matrix step
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