Reputation systems are meta systems that record, aggregate and distribute information about principals' behaviour in distributed applications. Similarly, history-based access control systems make decisions based on programs' past security-sensitive actions. While the applications are distinct, the two types of systems are fundamentally making decisions based on information about the past behaviour of an entity. A logical policy-centric framework for such behaviour-based decision-making is presented. In the framework, principals specify policies which state precise requirements on the past behaviour of other principals that must be fulfilled in order for interaction to take place. The framework consists of a formal model of behaviour, based on event structures; a declarative logical language for specifying properties of past behaviour; and efficient dynamic algorithms for checking whether a particular behaviour satisfies a property from the language. It is shown how the framework can be extended in several ways, most notably to encompass parameterized events and quantification over parameters. In an extended application, it is illustrated how the framework can be applied for dynamic history-based access control for safe execution of unknown and untrusted programs.

In this work, we propose a novel scheme of signal compression based on signal-dependent wavelets. To adapt the mother wavelet to the signal for the purpose of compression, it is necessary to define a family of wavelets that depend on a set of parameters and a quality criterion for wavelet selection (i.e., wavelet parameter optimization). We propose the use of orthogonal wavelets parameterized by their scaling filter, with optimization criterion based on the minimization of signal distortion rate given the desired compression rate. For coding the wavelet coefficients we adopted the embedded zerotree wavelet coding algorithm. Results on electromyographic signals show that optimization significantly improves performance.

In a reputation-based trust-management system, agents maintain information about the past behaviour of other agents. This information is used to guide future trust-based decisions about interaction. However, while trust management is a component in security decision-making, many existing reputation-based trust-management systems provide no formal security-guarantees. In this extended abstract, we describe a mathematical framework for a class of simple reputation-based systems. In these systems, decisions about interaction are taken based on policies that are exact requirements on agents' past histories. We present a basic declarative language, based on pure-past linear temporal logic, intended for writing simple policies. While the basic language is reasonably expressive (encoding e.g. Chinese Wall policies) we show how one can extend it with quantification and parameterized events. This allows us to encode other policies known from the literature, e.g., `one-out-of-k'. The problem of checking a history with respect to a policy is efficient for the basic language, and tractable for the quantified language when policies do not have too many variables.

Trust management systems have been proposed as an alternative to traditional security mechanisms in Global Computing. We present some challenges in establishing a formal foundation for the notion of trust, and some preliminary ideas towards a category of trust models.

We propose a formal model of trust informed by the Global Computing scenario and focusing on the aspects of trust formation, evolution, and propagation. The model is based on a novel notion of trust structures which, building on concepts from trust management and domain theory, feature at the same time a trust and an information partial order.

The tcc paradigm is a formalism for timed concurrent constraint programming. Several tcc languages differing in their way of expressing infinite behavior have been proposed in the literature. In this paper we study the expressive power of some of these languages. In particular, we show that: (1) recursive procedures with parameters can be encoded into parameterless recursive procedures with dynamic scoping, and viceversa. (2) replication can be encoded into parameterless recursive procedures with static scoping, and viceversa. (3) the languages from (1) are strictly more expressive than the languages from (2). Furthermore, we show that behavioral equivalence is undecidable for the languages from (1), but decidable for the languages from (2). The undecidability result holds even if the process variables take values from a fixed finite domain.

Formal models for real-time systems have been studied intensively over the past decade. Much of the theory of untimed systems have been lifted to real-time settings. One example is the notion of bisimulation applied to timed transition systems, which

We propose a new and practical framework for integrating the behavioral reasoning about distributed systems with model-checking methods.

An abstract definition of bisimulation is presented. It allows a uniform definition of bisimulation across a range of different models for parallel computation presented as categories. As examples, transition systems, synchronization trees, transition systems with independence (an abstraction from Petri nets), and labeled event structures are considered. On transition systems, the abstract definition readily specialises to Milner's (1989) strong bisimulation. On event structures, it explains and leads to a revision of the history-preserving bisimulation of Rabinovitch and Traktenbrot (1988), and Goltz and van Glabeek (1989). A tie-up with open maps in a (pre)topos brings to light a promising new model, presheaves on categories of pomsets, into which the usual category of labeled event structures embeds fully and faithfully. As an indication of its promise, this new presheaf model has refinement operators, though further work is required to justify their appropriateness and understand their relation to previous attempts.>

Elementary transition systems are-in a strong categorical sense-the transition system version of a basic system model of net theory called elementary net systems. The structural notion of a region associated with elementary transition systems captures the intuitive idea of a local state as modelled by the conditions of an elementary net system. In this paper we equip elementary transition systems with a refinement operation over the local states (regions). We then show our operation satisfies a number of interesting properties. In particular, this operation supports compositional reasoning. It is very hard if not impossible to define a corresponding operation at the level of nets which enjoys similar properties. This is due to the concrete choice of conditions used to enforce intended behaviour. Thus our results show that the more abstract-but essentially equivalent-model of elementary transition systems is the appropriate framework for theoretical studies concerning refinement operations for elementary net systems.

Transition systems are a simple and powerful formalism for explaining the operational behaviour of models of concurrency. They provide a common framework for investigating the interrelationships between different approaches to the study of distributed systems. Hence an important question to be answered is: which subclass of transition systems corresponds to a particular model of distribted systems? In this paper we provide an answer to this question for elementary net systems. Within net theory, which is one well-established theory of distributed systems, elementary net systems constitute a basic systems model. Using this model, fundamental concepts such as causality, concurrency, conflict and confusion can be clearly defined and separated from each other (see [ 151). Much is known about the behavioural aspects of elementary net systems in terms of trace theory, nonsequential processes and event structures as shown in [lo]. Trace theory was initiated by Mazurkiewicz [7] (see also [l]). The theory of nonsequential processes originates from the work of Petri [12]; see also [2]. Event structures arose out of the work of Nielsen, Plotkin and Winskel [9] and they now possess a rich theory mainly due to the efforts of Winskel [18]. Elementary net systems also have a strong relationship to transition systems. More precisely, there is a natural way of associating a transition system with each elementary net system in order to explain the operational behaviour of elementary net systems in purely sequential terms. Hence the question arises as to which transition systems correspond to elementary net systems.

Abstract This paper presents the RAISE 1 software development method, its associated specification language, and the tools supporting it. The RAISE method enables the stepwise development of both sequential and concurrent software from abstract specification through design to implementation. All stages of RAISE software development are expressed in the wide-spectrum RAISE specification language. The RAISE tools form an integrated tool environment supporting both language and method. The paper surveys RAISE and furthermore, more detailed presentations of major RAISE results are provided. The subjects of these are (a) an example of the use of the RAISE method and language, and (b) a presentation of the mathematical semantics of the RAISE specification language.

One of the questions of the longest open standing in the area of Lindenmayer systems is the decidability of the equivalence problem for deterministic, informationless L-systems (DOL-systems). This and some related equivalence problems (equivalence with respect to the set and the sequence of generated words, Parikh-vectors and word-lengths) are investigated. Some of these related problems are shown to be recursively solvable, and the implications of these results on the main open problem mentioned above are discussed.