Trust models in ubiquitous computing

We recapture some of the arguments for trust-based technologies in ubiquitous computing, followed by a brief survey of some of the models of trust that have been introduced in this respect. Based on this, we argue for the need of more formal and foundational trust models

An operational framework to reason about policy behavior in trust management systems

In this paper we show that the logical framework proposed by Becker et al. to reason about security policy behavior in a trust management context can be captured by an operational framework that is based on the language proposed by Miller to deal with scoping and/or modules in logic programming in 1989. The framework of Becker et al. uses propositional Horn clauses to represent both policies and credentials, implications in clauses are interpreted in counterfactual logic, a Hilbert-style proof is defined and a system based on SAT is used to proof whether properties about credentials, permissions and policies are valid in trust management systems, i.e. formulas that are true for all possible policies. Our contribution is to show that instead of using a SAT system, this kind of validation can rely on the operational semantics (derivability relation) of Miller’s language, which is very close to derivability in logic programs, opening up the possibility to extend Becker et al.’s framework to the more practical first order case since Miller’s language is first order.Peer ReviewedPreprin

Belief Semantics of Authorization Logic

Authorization logics have been used in the theory of computer security to reason about access control decisions. In this work, a formal belief semantics for authorization logics is given. The belief semantics is proved to subsume a standard Kripke semantics. The belief semantics yields a direct representation of principals' beliefs, without resorting to the technical machinery used in Kripke semantics. A proof system is given for the logic; that system is proved sound with respect to the belief and Kripke semantics. The soundness proof for the belief semantics, and for a variant of the Kripke semantics, is mechanized in Coq

Techniques for Application-Aware Suitability Analysis of Access Control Systems

Access control, the process of selectively restricting access to a set of resources, is so fundamental to computer security that it has been called the field's traditional center of gravity. As such, a wide variety of systems have been proposed for representing, managing, and enforcing access control policies. Prior work on evaluating access control systems has primarily relied on relative expressiveness analysis, which proves that one system has greater capabilities than another. Although expressiveness is a meaningful basis for comparing access control systems, it does not consider the application in which the system will be deployed. Furthermore, expressiveness is not necessarily a useful way to rank systems; if two systems are expressive enough for a given application, little benefit is derived from choosing the one that has greater expressiveness. On the contrary, many of the concerns that arise when choosing an access control system can be negatively impacted by additional expressiveness: a system that is too complex is often harder to specify policies in, less efficient, or harder to reason about from the perspective of security guarantees. To address these shortcomings, we propose the access control suitability analysis problem, and present a series of techniques for solving it. Suitability analysis evaluates access control systems against the specific demands of the application within which they will be used, and considers a wide range of both expressiveness and ordered cost metrics. To conduct suitability analysis, we present a two-phase framework consisting of formal reductions for proving qualitative suitability and simulation techniques for evaluating quantitative suitability. In support of this framework we present a fine-grained lattice of reduction properties, as well as Portuno, a flexible simulation engine for conducting cost analysis of access control systems. We evaluate our framework formally, by proving that it satisfies a series of technical requirements, and practically, by presenting several case studies demonstrating its use in conducting analysis in realistic scenarios

Verification of temporal-epistemic properties of access control systems

Verification of access control systems against vulnerabilities has always been a challenging problem in the world of computer security. The complication of security policies in large- scale multi-agent systems increases the possible existence of vulnerabilities as a result of mistakes in policy definition. This thesis explores automated methods in order to verify temporal and epistemic properties of access control systems. While temporal property verification can reveal a considerable number of security holes, verification of epistemic properties in multi-agent systems enable us to infer about agents' knowledge in the system and hence, to detect unauthorized information flow. This thesis first presents a framework for knowledge-based verification of dynamic access control policies. This framework models a coalition-based system, which evaluates if a property or a goal can be achieved by a coalition of agents restricted by a set of permissions defined in the policy. Knowledge is restricted to the information that agents can acquire by reading system information in order to increase time and memory efficiency. The framework has its own model-checking method and is implemented in Java and released as an open source tool named \char{cmmi10}{0x50}\char{cmmi10}{0x6f}\char{cmmi10}{0x6c}\char{cmmi10}{0x69}\char{cmmi10}{0x56}\char{cmmi10}{0x65}\char{cmmi10}{0x72}. In order to detect information leakage as a result of reasoning, the second part of this thesis presents a complimentary technique that evaluates access control policies over temporal-epistemic properties where the knowledge is gained by reasoning. We will demonstrate several case studies for a subset of properties that deal with reasoning about knowledge. To increase the efficiency, we develop an automated abstraction refinement technique for evaluating temporal-epistemic properties. For the last part of the thesis, we develop a sound and complete algorithm in order to identify information leakage in Datalog-based trust management systems