Automated verification of access control policies using a SAT solver

Managing access control policies in modern computer systems can be challenging and error-prone. Combining multiple disparate access policies can introduce unintended consequences. In this paper, we present a formal model for specifying access to resources, a model that encompasses the semantics of the xacml access control language. From this model we define several ordering relations on access control policies that can be used to automatically verify properties of the policies. We present a tool for automatically verifying these properties by translating these ordering relations to Boolean satisfiability problems and then applying a sat solver. Our experimental results demonstrate that automated verification of xacml policies is feasible using this approach. This work is supported by NSF grants CCF-0614002 and CCF-0716095.     

Temperature rises during tooth bleaching with different light-curing units

The aim of this study was to evaluate the influence of three different light-curing units on the temperature rise induced in the pulpal cavity, when bleached with 35% hydrogen peroxide (HP). Forty extracted maxillary central incisors were randomly divided into four groups according to photoactivation procedure (n = 10). Whiteness HP (WHP) bleaching agent which contains 35% HP bleach was placed on the facial tooth surface and no photoactivation device was used (control); an blue light-emitting diode (LED, 40 s), a diode laser (4 W, Continous mode, 20 s), and Nd:YAG laser (4 W, 60 Hz frequency, 20 s). Temperature rise in the pulpal chamber was measured with a J-type thermocouple wire that was connected to a data logger before and immediately after each 20–40 s application. One tooth of each group was selected for scanning electron microscope analysis. Data were analyzed using one-way ANOVA followed by the Games Howell post hoc test at a preset α of 0.05. One-way ANOVA revealed that there was a significant difference among the light-curing units (p < 0.05) and between control group and light-curing units (p < 0.05). The Nd:YAG induced significantly, which resulted in higher temperature increase than other light-curing units (10.7 °C). The LED unit produced the lowest temperature changes (3.2 °C) among the light-curing units. The presence of the bleaching gel did not cause a temperature increase in control group (0 °C). The results suggested that using light-curing units caused temperature changes in the intrapulpal cavity during tooth bleaching. The use of LED will be more safer for pulpal health when assisted with WHP bleaching gel.     

Experimental studies of heat transfer to horizontal tubes in a pilot-scale fluidized-bed combustor

Experimental data are obtained for the overall heat transfer coefficient between immersed horizontal tube bundles and an atmospheric fluidized bed combuster burning high-sulfur coal. Limestone (1000 μm size) is used as the bed material. Tests are conducted with coal and limestone addition at overall average bed temperatures ranging from 1028 to 1116 K, and fluidizing velocities ranging from 2.08 to 3.14 m/s. The overall heat transfer coefficients of the horizontal tube sections increase as the height from the distributor plate increases, except for the top tubes located in the splash zone. The average overall heat transfer coefficients of all horizontal tube sections increases as the fluidization velocity and overall bed temperature increase.     

Induction thermal plasma synthesis of Mg2Ni nanoparticles

A study was carried out into possibility of thermal plasma synthesis of Mg₂Ni nanoparticles. Both prealloyed powders and elemental powders were used as precursors in an inductively coupled thermal plasma incorporating two injection probes located axially in the reactor one from the top and the other from the bottom. The study has shown that the use of prealloyed Mg₂Ni as precursor leads to its disintegration in the plasma condensing into separate phases and therefore was not suitable for the synthesis of Mg₂Ni. The study further showed that Mg₂Ni can be synthesized quite successfully with the use of elemental powders provided that elements are maintained into the plasma at carefully controlled positions. While the fraction of Mg₂Ni was quite substantial, it co-existed with other phases and therefore additional treatments would be necessary for separation. It was shown that a substantial size reduction was possible with thermal plasma where Mg₂Ni could be produced in sizes around 100 nm.     

Action Language verifier: an infinite-state model checker for reactive software specifications

Action Language is a specification language for reactive software systems. In this paper, we present the syntax and the semantics of the Action Language and we also present an infinite-state symbolic model checker called Action Language Verifier (ALV) that verifies (or falsifies) CTL properties of Action Language specifications. ALV is built on top of the Composite Symbolic Library, which is a symbolic manipulator that combines multiple symbolic representations. ALV is a polymorphic model checker that can use different combinations of the symbolic representations implemented in the Composite Symbolic Library. We describe the heuristics implemented in ALV for computing fixpoints using the composite symbolic representation. Since Action Language specifications allow declaration of unbounded integer variables and parameterized integer constants, verification of Action Language specifications is undecidable. ALV uses several heuristics to conservatively approximate the fixpoint computations. ALV also implements an automated abstraction technique that enables parameterized verification of a concurrent system with an arbitrary number of identical processes.     

Effective methods for increasing aggregate diversity in recommender systems

In order to make a recommendation, a recommender system typically first predicts a user’s ratings for items and then recommends a list of items to the user which have high predicted ratings. Quality of predictions is measured by accuracy, that is, how close the predicted ratings are to actual ratings. On the other hand, quality of recommendation lists is evaluated from more than one perspective. Since accuracy of predicted ratings is not enough for customer satisfaction, metrics such as novelty, serendipity, and diversity are also used to measure the quality of the recommendation lists. Aggregate diversity is one of these metrics which measures the diversity of items across the recommendation lists of all users. Increasing aggregate diversity is important because it leads a more even distribution of items in the recommendation lists which prevents the long-tail problem. In this study, we propose two novel methods to increase aggregate diversity of a recommender system. The first method is a reranking approach which takes a ranked list of recommendations of a user and reranks it to increase aggregate diversity. While the reranking approach is applied after model generation as a wrapper the second method is applied in model generation phase which has the advantage of being more efficient in the generation of recommendation lists. We compare our methods with the well-known methods in the field and show the superiority of our methods using real-world datasets.     

A symbolic manipulator for automated verification of reactive systems with heterogeneous data types

In this paper, we present the design and implementation of the Composite Symbolic Library, a symbolic manipulator for model checking systems with heterogeneous data types. Our tool provides a common interface for different symbolic representations, such as BDDs, for representing Boolean logic formulas and polyhedral representations for linear arithmetic formulas. Based on this common interface, these data structures are combined using a disjunctive composite representation. We propose several heuristics for efficient manipulation of this composite representation and present experimental results that demonstrate their performance. We used an object-oriented design to implement the Composite Symbolic Library. We imported the CUDD library (a BDD library) and the Omega Library (a linear arithmetic constraint manipulator that uses polyhedral representations) to our tool by writing wrappers around them which conform to our symbolic representation interface. Our tool supports polymorphic verification procedures which dynamically select symbolic representations based on the input specification. Our symbolic representation library can be used as an interface between different symbolic libraries, model checkers, and specification languages. We expect our tool to be useful in integrating different tools and techniques for symbolic model checking, and in comparing their performance.