Trust management framework for intelligent agent negotiations in ubiquitous computing environments

In dynamic ubiquitous computing environments, system entities may be classified into two main categories that are, in principle, in conflict. These are the Service Resource Requestors (SRRs) wishing to use services and/or exploit resources offered by the other system entities and the Service Resource Providers (SRPs) that offer the services/resources requested. Seeking for the maximisation of their welfare, while achieving their own goals and aims, entities may misbehave (intentionally or unintentionally), thus, leading to a significant deterioration of system’s performance. In this study, a reputation mechanism is proposed which helps estimating SRPs trustworthiness and predicting their future behaviour, taking into account their past performance in consistently satisfying SRRs’ expectations. Thereafter, under the assumption that a number of SRPs may handle the SRRs requests, the SRRs may decide on the most appropriate SRP for the service/resource requested on the basis of a weighted combination of the evaluation of the quality of their offer (performance related factor) and of their reputation rating (reliability related factor). The proposed trust management framework is distributed, considers both first-hand information (acquired from the SRR’s direct past experiences with the SRPs) and second-hand information (disseminated from other SRRs’ past experiences with the SRPs), while it exhibits a robust behaviour against inaccurate reputation ratings. The designed mechanisms have been empirically evaluated simulating interactions among self-interested agents, exhibiting improved performance with respect to random SRP selection.     

Fractal Nature of Fluorescence Quenching and its Relation to Percolative Conduction in Water-in-Oil Microemulsions

Results of fluorescence quenching and electrical conductivity measurements in water-in-oil microemulsions were analyzed in terms of fractal models. It was found that for SDS/water/pentanol/alkane microemulsions there exists a connection between the fractal nature of the reaction domain and the magnitude of electrical conductivity. Conductivity values increase as the domain becomes less fractal.     

Investigating the enhancement of three-dimensional diffraction tomography by using multiple illumination planes

The three dimensional (3-D) extension of the two well-known diffraction tomography algorithms, namely, direct Fourier interpolation (DFI) and filtered backpropagation (FBP), are presented and the problem of the data needed for a full 3-D reconstruction is investigated. These algorithms can be used efficiently to solve the inverse scattering problem for weak scatterers in the frequency domain under the first-order Born and Rytov approximations. Previous attempts of 3-D reconstruction with plane-wave illumination have used data obtained with the incident direction restricted at the xy plane. However, we show that this restriction results in the omission of the contribution of certain spatial frequencies near the omegaz axis for the final reconstruction. The effect of this omission is studied by comparing the results of reconstruction with and without data obtained from other incident directions that fill the spatial frequency domain. We conclude that the use of data obtained for incident direction in only the xy plane is sufficient to achieve a satisfactory quality of reconstruction for a class of objects presenting smooth variation along the z axis, while abrupt variations along the z axis cannot be imaged. This result should be taken into account in the process of designing the acquisition geometry of a tomography scanner.     

Estimation and projections of demand for crude petroleum and refined petroleum products

The purpose of this article is to estimate the demand and supply relationships of the US petroleum market. By measuring the elasticities and cross-elasticities of demand, the relative responsiveness of changes in prices and income and the possibility of substitution of alternative energy resources for petroleum are determined. The demand for refined products is also estimated. Finally, through projection of the demand for, and the domestic supply and imports of crude petroleum under alternative assumptions, a measure of the magnitude of physical reserves and capital requirements is obtained for policies of self-sufficiency and free trade.     

A PKI approach for deploying modern secure distributed e-learning and m-learning environments

While public key cryptography is continuously evolving and its installed base is growing significantly, recent research works examine its potential use in e-learning or m-learning environments. Public key infrastructure (PKI) and attribute certificates (ACs) can provide the appropriate framework to effectively support authentication and authorization services, offering mutual trust to both learners and service providers. Considering PKI requirements for online distance learning networks, this paper discusses the potential application of ACs in a proposed trust model. Typical e-learning trust interactions between e-learners and providers are presented, demonstrating that robust security mechanisms and effective trust control can be obtained and implemented. The application of ACs to support m-learning is also presented and evaluated through an experimental test-bed setup, using the general packet radio service network. The results showed that AC issuing is attainable in service times while simultaneously can deliver flexible and scalable solutions to both learners and e-learning providers.     

Constructing Runge–Kutta methods with the use of artificial neural networks

A methodology that can generate the optimal coefficients of a numerical method with the use of an artificial neural network is presented in this work. The network can be designed to produce a finite difference algorithm that solves a specific system of ordinary differential equations numerically. The case we are examining here concerns an explicit two-stage Runge–Kutta method for the numerical solution of the two-body problem. Following the implementation of the network, the latter is trained to obtain the optimal values for the coefficients of the Runge–Kutta method. The comparison of the new method to others that are well known in the literature proves its efficiency and demonstrates the capability of the network to provide efficient algorithms for specific problems.     

Execution transactions for defending against software failures: use and evaluation

We examine the problem of containing buffer overflow attacks in a safe and efficient manner. Briefly, we automatically augment source code to dynamically catch stack and heap-based buffer overflow and underflow attacks, and recover from them by allowing the program to continue execution. Our hypothesis is that we can treat each code function as a transaction that can be aborted when an attack is detected, without affecting the application's ability to correctly execute. Our approach allows us to enable selectively or disable components of this defensive mechanism in response to external events, allowing for a direct tradeoff between security and performance. We combine our defensive mechanism with a honeypot-like configuration to detect previously unknown attacks, automatically adapt an application's defensive posture at a negligible performance cost, and help determine worm signatures. Our scheme provides low impact on application performance, the ability to respond to attacks without human intervention, the capacity to handle previously unknown vulnerabilities, and the preservation of service availability. We implement a stand-alone tool, DYBOC, which we use to instrument a number of vulnerable applications. Our performance benchmarks indicate a slow-down of 20% for Apache in full-protection mode, and 1.2% with selective protection. We provide preliminary evidence toward the validity of our transactional hypothesis via two experiments: first, by applying our scheme to 17 vulnerable applications, successfully fixing 14 of them; second, by examining the behavior of Apache when each of 154 potentially vulnerable routines are made to fail, resulting in correct behavior in 139 cases (90%), with similar results for sshd (89%) and Bind (88%).