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Effectiveness of proactive password checker based on Markov models
Viktor Taneski, 2019, doktorska disertacija

Opis: In this doctoral dissertation we focus on the most common method of authentication, the username-password combination. The reason for the frequent use of this authentication mechanism is its simplicity and low cost of implementation. Although passwords are so useful, they have many problems. Morris and Thompson, for the first time almost four decades ago, found that textual passwords were a weak security point of information systems. They have come to the conclusion that users are one of the biggest threats to information system’s security. Since then, we face these problems on a daily basis. Users do not perform the behaviours they need to be done in order to stay safe and secure, although they are aware of the security issues. Because this is a research area that security experts have been dealing with for a long time, in this dissertation we wanted to identify problems related to textual passwords and possible suggested solutions. For this purpose, we first performed a systematic literature review on textual passwords and their security. In doing so, we wanted to evaluate the current status of passwords in terms of their strength, ways of managing passwords, and whether users are still the “weakest link”. We found that one of the less researched solutions is proactive password checking. A proactive password checker could filter out the passwords that are easy-to-guess and only let through the passwords that are harder to guess. In order for a proactive password checking to be more effective, it is necessary for the checker to be able to check the probability that a certain password will be selected by the user. For this purpose, the better password checkers usually use certain tools to calculate password probability i.e., password strength. To find out which method is most suitable for calculating password strength, we have looked at similar solutions throughout history. We have found that Markov models are one of the most common methods used for password strength estimation, although we may encounter some problems when using them, such as sparsity and over-fitting. By reviewing similar solutions, we found that Markov models are mostly trained on only one dataset. This could limit the performance of the model in terms of correctly identifying bad or very strong passwords. As training datasets are important in the development of Markov models, it is clear that they will have some effect in the final assessment of the password’s strength. What we explore in our dissertation, is the importance of this effect on the final password strength estimation. Mainly, we focus on exploring the effect of different but similar datasets on password strength estimation. For the purposes of our study, we analysed publicly available sets of “common passwords” and processed them regarding the frequency distribution of the letters contained in these passwords. We built different Markov models based on these datasets and frequency distribution. This helped us determine if one Markov model was sufficient or if several models were needed to effectively estimate password strength for a wide range of passwords. The results showed statistical differences between the models. In more detail, we found that: - different Markov models (trained on different databases) showed statistically different results when tested on the same dataset, - more diverse datasets are needed to be able to calculate the strength of as many passwords as possible, since one “universal” model, trained on one “universal” dataset is less effective at classifying passwords in different categories (i.e., weak, medium, strong), - different Markov models of 1st and 2nd order, in most cases, give no statistically different outputs, - overall, Markov models can be used as a basis for constructing a more effective password checker that uses multiple different and specific Markov models, which could be more effective if we want to cover a wider range of passwords.
Ključne besede: passwords, password analysis, password security, password problems, password strength, systematic literature review, Markov models
Objavljeno: 13.01.2020; Ogledov: 475; Prenosov: 55
.pdf Celotno besedilo (1,12 MB)

Differential evolution and large-scale optimization applications
Aleš Zamuda, raziskovalni ali dokumentarni film, zvočna ali video publikacija

Opis: Differential Evolution (DE) is one of the most popular, high-performance optimization algorithms with variants that have been outperforming others for years. As a result, DE has grown to accommodate wide usage for a variety of disciplines across scientific fields. Differential Evolution and Large-Scale Optimization Applications presents a research-based overview and cross-disciplinary applications of optimization algorithms. Emphasizing applications of Differential Evolution (DE) across sectors and laying the foundation for further use of DE algorithms in real-world settings, this video is an essential resource for researchers, engineers, and graduate-level students. Topics Covered : Algorithms, Optimization, Parallel Differential Evolution, Performance Improvement, Stochastic Methods, Tree Model Reconstruction.
Ključne besede: differential Evolution, optimization, algorithms, stochastic methods, tree models, tree model reconstruction
Objavljeno: 14.05.2019; Ogledov: 484; Prenosov: 147
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Performance indicators of management buyouts using the analytic hierarchy process method
Petra Grah, Vesna Čančer, Borut Bratina, 2018, izvirni znanstveni članek

Opis: Background and Purpose: In Slovenia, few management buyout (MBO) studies have been carried out. The focus was mostly on the motives for acquisition of companies and the success rate of the acquisitions. This paper aims to analyse the indicators which suggest an impending bankruptcy or financial restructuring of companies and explore how these indicators are different for successful and unsuccessful MBOs. Methodology: In the survey, we included 23 selected MBOs in Slovenia between 2005 and 2008, using the following financial and non-financial indicators: profitability, performance, solvency and liquidity, using the analytic hierarchy process method. The key aim of the survey was to use financial and non-financial indicators to study if target companies where bankruptcy or financial restructuring has not yet been initiated prevalently have higher aggregate values compared to those in which bankruptcy or financial restructuring procedures have already begun. Thus, we used the selected indicators to demonstrate one of the possible methods to predict the success of a particular MBO. Results: We found that in most examples of unsuccessful MBOs, target companies have poorer results in terms of performance, solvency and liquidity, when compared to successful MBOs. Based on the selected areas, we divided the results into four quarters. We found that most target companies where MBOs had been unsuccessful are ranked in a lower quarter than most of the target companies where the MBOs had been successful. Conclusion: The papers main contribution is the finding that the selected financial and non-financial indicators differ in cases of successful and unsuccessful MBOs. This knowledge helps us to find ways of avoiding these situations in the future.
Ključne besede: management buy-outs, management, bankruptcy models, financial and non-financial indicators, the analytic hierarchy process
Objavljeno: 10.10.2018; Ogledov: 538; Prenosov: 216
.pdf Celotno besedilo (634,12 KB)
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Development and analysis of 3D geological/geomechanical models for pumped hydroelectric power plant Kozjak
Vladimir Vukadin, 2009, izvirni znanstveni članek

Opis: In order to specify the geological and geotechnical conditions for the construction of the Kozjak pumpedstorage hydroelectric power plant (HPP), the site area has undergone considerable geological, geophysical and geomechanical investigations. Due to the complexity of the geotechnical constructions and the size of the investigated site a novel approach was proposed, where the results of the investigation were used to develop 3D geological/ geomechanical models. The models were created with the integration of all the available results and data into typical geomechanical units that were subsequently given spatial dimensions. Through an analysis of the models the relationships between the different units were better understood and the critical areas were quickly identified and verified. In this paper the models for the accumulation reservoir and the engine room are presented. The overview of the executed investigations and the results are given together with a discussion on the scope and the number of executed investigations from the standpoint of the 3D geological/geomechanical model’s creation.
Ključne besede: geological investigations, geomechanical investigations, 3D geological-geomechanical models, HPP Kozjak
Objavljeno: 06.06.2018; Ogledov: 295; Prenosov: 46
.pdf Celotno besedilo (362,83 KB)
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Development of a conceptual material model for structured materials - S_BRICK
Vladimir Vukadin, Jakob Likar, Vojkan Jovičić, 2005, izvirni znanstveni članek

Opis: Materials known in literature as Soft Rocks & Hard Soils such as claystones, siltstones, flysh, hard soils, marls, etc. received a lot of attention in research community in recent years. It was demonstrated that some of these materials can be described through general theoretical framework (Cotecchia & Chandler [1]), which takes into account the structure as an intrinsic material property that is present in all natural geological materials. The influence of the structure is manifested as an increase in strength and stiffness in a material. Based on a laboratory results and existent theoretical frameworks, a further step was taken with the development of a conceptual constitutive model for structured materials. A model formulated in strain space named BRICK (Simpson [2]) was chosen as a base model and was further developed with the inclusion of structure and destructuring. The new model was named S_BRICK and was tested on a conceptual level where the results of numerical tests on structured and structure less (reconstituted) materials at different stress paths were compared. The results showed that S_BRICK was able to capture stress-strain behavior typical for structured material and could be potentially used for the modeling of Soft Rocks and Hard Soils.
Ključne besede: geomechanics, soft rocks, hard soils, constitutive material models, structure, destructuring, numerical modeling
Objavljeno: 15.05.2018; Ogledov: 573; Prenosov: 35
.pdf Celotno besedilo (586,91 KB)
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Back analysis of time-dependet displacement at the Trojane tunnel construction
Jakob Likar, 2004, izvirni znanstveni članek

Opis: The 2,900-metre double tube tunnel of Trojane forms part of the highway section between Celje and Ljubljana and part of the highway road system connecting Lendava and Koper with adjacent roads. The construction of this section, which is of special importance for the Republic Slovenia, has been going on for almost ten years. The highway near Trojane where the tunnel is located, lies on a hilly terrain composed of Permian-carboniferous rocks, which are tectonically badly damaged in some places and marked with broad fault zones, containing tectonically remoulded soil of low strength in terms of deformability properties. Even though the amount of waters coming from the hills during the excavation of the tunnel was relatively small, this water, in combination with extremely damaged weak rock caused the increase of deformations at the construction site. Geological and geotechnical surveying with geometric monitoring and the analyses of stress and deformation changes in the surrounding rocks and support elements were critical in assessing the actual weak-rock conditions during the course of construction. Since the analyses of deformation processes, which had been modelled with PLAXIS 3D TUNNEL PROGRAM, showed great impact on permanent stability of the tunnel, special care was paid to rheological changes in the surrounding rocks and combined in-built support elements. 3D back analyses with SOFT-SOIL-CREEP (SSC) constitutive model, which takes into account rheological phenomena, were carried out. The calculations of primary and secondary stresses and deformations were made for two characteristic areas and determined on the basis of realistic geological mapping during tunnel construction. The first area represented typical rocks of the Trojane tunnel, i.e. shales and siltstone, while the second one was a tectonic zone where geological and geotechnical conditions during excavation were less favourable. The results of back analyses used provided an acceptable possibility for estimating the stress-strain variation during construction as well as an appropriate basis for analysing vertical displacements at the tunnel roof. The calculation results show good correlation with field measurements.
Ključne besede: tunnels, construction, permain-carboniferous rocks, geological-geotechnical conditions, tunnel support, Trojane tunnel, finite element method, costitutive models, back analyses, field mesurement, time dependent analysis
Objavljeno: 15.05.2018; Ogledov: 523; Prenosov: 35
.pdf Celotno besedilo (868,88 KB)
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Type II string theory on Calabi-Yau manifolds with torsion and non-Abelian discrete gauge symmetries
Volker Braun, Mirjam Cvetič, Ron Donagi, Maximilian Poretschkin, 2017, izvirni znanstveni članek

Opis: We provide the first explicit example of Type IIB string theory compactification on a globally defined Calabi-Yau threefold with torsion which results in a four-dimensional effective theory with a non-Abelian discrete gauge symmetry. Our example is based on a particular Calabi-Yau manifold, the quotient of a product of three elliptic curves by a fixed point free action of Z(2) x Z(2). Its cohomology contains torsion classes in various degrees. The main technical novelty is in determining the multiplicative structure of the (torsion part of) the cohomology ring, and in particular showing that the cup product of second cohomology torsion elements goes non-trivially to the fourth cohomology. This specifies a non-Abelian, Heisenberg-type discrete symmetry group of the four-dimensional theory.
Ključne besede: string field theory, conformal field models, string theory, discrete symmetries
Objavljeno: 08.03.2018; Ogledov: 880; Prenosov: 301
.pdf Celotno besedilo (355,78 KB)
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Testing the sustainability of growth of the LJSEX in the January 2000 to May 2010 period
Aleš Ahčan, 2011, izvirni znanstveni članek

Opis: In this paper we analyse the behaviour of the LJSEX, the main index of the Ljubljana Stock Exchange, in the period from January 2000 till May 2010. More precisely, we test for the presence of bubbles and antibubbles and try to determine whether or not a bubble could have been predicted (both the formation and the date of the bubble burst). Second, we also employ techniques used to model antibubbles to forecast the future behaviour of the LJSE index. Besides modelling index dynamics for the aforementioned period, we also seek to determine the factors that led to the bubble forming and later bursting. We find that the bubble could have been forecasted at least several months in advance. On the other hand, a very precise date of the crash seems harder to identify. By more closely analysing the interplay between interest rates, credit activity and the LJSEX, we conclude that there is a clear connection between decreasing interest rates, increased credit activity and the formation of a stock bubble. If there is a clear correlation between the early phase of a bubble and increased credit activity of the banking sector, the link between the end of the bubble and the restriction of credit activity is less pronounced. By fitting the extended antibubble model from (Johansen 1999a) we obtain the values of parameters that give us some indication of the future behaviour of the LJSEX. Based on these results we conclude that in the next few years we are likely to experience a period of increased volatility with no clear increasing or decreasing growth pattern.
Ključne besede: exchange, interest rate, business mathematics, calculations, models
Objavljeno: 22.01.2018; Ogledov: 459; Prenosov: 262
.pdf Celotno besedilo (698,29 KB)
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Using chemical models for developing natural science competences in teaching chemistry
Darinka Sikošek, Mateja Žuželj, 2013, izvirni znanstveni članek

Opis: Chemical models are indispensable tool in chemistry class and enable secondary school pupils to develop complex thought processes and spatial concepts, both of which are necessary to improve their understanding of chemical concepts. In this research, special emphasis is given to complex competencies not only their acquisition, but rather how pupils develop them. For this purpose, a learning strategy has been developed, called learning by self-generating chemical models. Pupils learn individually on their own models, which are made from easily accessible, low-cost materials. The impact of workable models was investigated (assembly of commercial models and production of self-made models) on the (1) learning success using simple molecule shapes and (2) the degree of competence development which realized through the implemented activities. The study involved first-year secondary school-pupils, defined as self-creators of paper- and -wire models (experimental group) and assemblers of commercial models (called the control group). The appearance of the molecules being studied took the form of geometric shapes and structures, which pupils present by making of ball-, called wire-models from plasticine and wire or paper created, called paper models. Based on the results of statistical processing of the data, we found that self-creation of models (as an innovative approach) leads to better understanding and to subsequently sustained knowledge for the pupil. It should also be noted that pupils who were self-generators in comparisons to those who merely assembled the models, generally developed many complex skills in the areas of problem solving, practical competence, mathematical competence and critical thinking skills. The knowledge so acquired was also better understood.
Ključne besede: education, chemical models, competencies, chemistry, self-made models
Objavljeno: 19.12.2017; Ogledov: 607; Prenosov: 45
.pdf Celotno besedilo (734,46 KB)
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