Property preservation is investigated as an approach to modular verification, leading to reduction of the property verification time for formal models. For modelling purposes, formalisms with multi-way synchronisations are considered. For the modular verification technique to work, a specific type of synchronisation is required for which a sufficient and necessary condition is identified. It is a requirement on the semantics of the formalism, which is restricted to permit simultaneous execution only of component moves that make reference to each other. Implications for modular verification of several well-known formalisms for concurrent systems are investigated.

A formal model for diagnostics of biological systems modelled as P systems is presented. We assume the presence of some biologically motivated changes (frequently pathological) in the systems behavior and investigate when these changes could be diagnosed by an external observer by exploiting some techniques originally developed for reasoning on system security.

The Calculus of Looping Sequences is a formalism for describing evolution of biological systems by means of term rewriting rules. We propose to enrich this calculus by labelling elements of sequences. Since two elements with the same label are consid

In multilevel systems, it is important to avoid unwanted indirect information flow from higher levels to lower levels, namely, the so-called covert channels. Initial studies of information flow analysis were performed by abstracting away from time and probability. It is already known that systems that are proven to be secure in a possibilistic framework may turn out to be insecure when time or probability is considered. Recently, work has been done in order to consider also aspects either of time or of probability, but not both. In this paper, we propose a general framework based on Probabilistic Timed Automata, where both probabilistic and timing covert channels can be studied. We define a Noninterference security property and a Nondeducibility on Composition security property, which allow expressing information flow in a timed and probabilistic setting. We then compare these properties with analogous ones defined in contexts where either time or probability or neither of them are taken into account. This permits a classification of the properties depending on their discerning power. As an application, we study a system with covert channels that we are able to discover by applying our techniques.

P Systems are computing devices inspired by the structure and the functioning of a living cell. A P System consists of a hierarchy of membranes, each of them containing a multiset of objects, a set of evolution rules, and possibly other membranes. Evolution rules are applied to the objects of the same membrane with maximal parallelism. In this paper we present an extension of P Systems, called P Systems with Membrane Channels (PMC Systems), in which membranes are enriched with channels and objects can pass through a membrane only if there are channels on the membrane that enable such a passage. We show that PMC Systems are universal even if only the simplest form of evolution rules is considered, and we give two application examples.

To study systems whose dynamics changes with time, an extension of timed P systems is introduced in which evolution rules may vary with time. The proposed model is a timed automaton with a discrete time domain and in which each state is a timed P system. A result on expressive power and on features of the formalism sufficient for full expressiveness is proved and, as an application example, the model of an ecological system is given.

Abstract Bisimulations are well-established behavioural equivalences that are widely used to study properties of computer science systems. Bisimulations assume the behaviour of systems to be described as labelled transition systems, and properties of a system can be verified by assessing its bisimilarity with a system one knows to enjoy those properties. In this paper we show how semantics based on labelled transition systems and bisimulations can be defined for two formalisms for the description of biological systems, both capable of describing membrane interactions. These two formalisms are the Calculus of Looping Sequences (CLS) and Brane Calculi, and since they stem from two different approaches (rewrite systems and process calculi) bisimulation appears to be a good candidate as a general verification method. We introduce CLS and define a labelled semantics and bisimulations for which we prove some congruence results. We show how bisimulations can be used to verify properties by way of two examples: the description of the regulation of lactose degradation in Escherichia coli and the description of the EGF signalling pathway. We recall the PEP calculus (the simplest of Brane Calculi) and its translation into CLS, we define a labelled semantics and some bisimulation congruences for PEP processes, and we prove that bisimilar PEP processes are translated into bisimilar CLS terms.

Abstract We develop a model of parametric probabilistic transition Systems (PPTSs), where probabilities associated with transitions may be parameters. We show how to find instances of the parameters that satisfy a given property and instances that either maximize or minimize the probability of reaching a certain state. As an application, we model a probabilistic non-repudiation protocol with a PPTS. The theory we develop allows us to find instances that maximize the probability that the protocol ends in a fair state (no participant has an advantage over the others).

Systems of data management timed automata (SDM-TAs) are networks of communicating timed automata with structures to store messages and functions to manipulate them. We prove the decidability of reachability. As an application, we model and analyze a cryptographic protocol.

Our aim is to study the information flow problem in hybrid systems, namely systems consisting of a discrete program with an analog environment. Information flows compromise the security of a system because they cause leakage of protected information. In order to tackle information flow in real-life systems, we introduce new classes of hybrid systems that extend the known ones while preserving their properties. Then, we define a logic to specify information flow. By means of some examples, we show that, by this logic, we are able to express information flows in hybrid systems and to certify that some suspect behaviors of these systems do not give rise to any information flow. We give a model checking procedure for our logic, and we prove that it gives a correct answer whenever it terminates. Moreover, for a particular class of hybrid systems, we give a version of the procedure that always terminates.

We are interested in describing timed systems that exhibit probabilistic behaviors. To this purpose, we define a model of probabilistic timed automata and give a concept of weak bisimulation together with an algorithm to decide it. We use this model for describing and analyzing a probabilistic non-repudiation protocol in a timed setting.

We propose Timed Cooperating Automata (TCAs), an extension of the model Cooperating Automata of Harel and Drusinsky, and we investigate some basic properties. In particular we consider variants of TCAs based on the presence or absence of internal activity, urgency and reactivity, and we compare the expressiveness of these variants with that of the classical model of Timed Automata (TAs) and its extensions with periodic clock constraints and with silent moves. We consider also closure and decidability properties of TCAs and start a study on succinctness of their variants with respect to that of TAs.

In synchronous programming, programs can be perceived as purely sequential, and parallelism is only a logical concept useful to develop programs in a modular way. Classical semantics for synchronous languages interpret programs as sequential input/ou

Cause-Effect (C-E) structures have been proposed as a formalism to describe concurrent and distributed systems. On C-E structures two operations, multiplication and addition, are defined, which allow easy combination and modular development of descriptions. In this paper a concept of process of a C-E structure with an initial state (C-E system) is defined similarly to that of a process of a Condition/Event net in Petri net theory, and operations on processes are studied which correspond to operations on C-E systems.

A model of processes of transforming graphs is proposed in which concurrency and branching can be represented. Operations on structures representing processes of transforming graphs are defined that allow one to construct such structures from simple components and to characterize sets of processes of transforming graphs, including sets of processes generated by graph grammars.

The authors introduce web structures and their transformations and develop their theory in the framework of category theory. Once a program has been represented as a web structure, software tools, such as a high-level data flow analyzer or other general program transformers, can be written as sets of web structure production rules. An implementation of web structure transformations is in progress. The mathematical theory of web structure transformations allows form proofs of properties both at the metatheoretical and theoretical levels.>

This paper discusses a family of algorithms for transforming a recursive program into an equivalent program with a simplified recursion structure. The simplification is performed by integrating copies of certain procedures into the bodies of other procedures. This procedure integration process is analogous to macroexpansion, involving procedure calls rather than macro-operators. We measure the complexity of the recursion structure in terms of the calling graph of the program. This is a directed graph with nodes representing the procedures of the program and arcs representing the relation “calls”. We say that the recursion structure is complex if there is a high degree of cross linkage resulting in a small number of large strongly connected components in the graph; and we say it is simple, if the strongly connected regions are small, preferably containing one node each. One algorithm in our family is optimal in the sense that it finds a program with the same number of procedures and the maximum number of strongly connected regions obtainable via macroexpansion of procedure calls. We discuss suboptimal algorithms which require less running time, and we discuss open problems related to finding simplification algorithms optimal with respect to a wide range of transformations including the inverse of macroexpansion. In particular, we present a very simple open problem concerning the decidability of the word problem for certain semigroups.

This paper discusses the desirability of procedure linkage optimization and sketches a general theory of interpretive semantics which is motivated by technical problems in specifying and validating program transformations that optimize procedure linkages. One particular transformation is treated in detail.Recursive ALGOL 60 procedures sometimes pass parameters by name in such a way that the general thunk mechanism is unnecessary and inefficient. We present an optimization which detects this kind of call-by-name and implements it thunklessly. We prove that the transformation preserves semantics and we discuss the effect on running time and memory management.