Expected-Delay-Summing Weak Bisimilarity for Markov Automata

A new weak bisimulation semantics is defined for Markov automata that, in addition to abstracting from internal actions, sums up the expected values of consecutive exponentially distributed delays possibly intertwined with internal actions. The resulting equivalence is shown to be a congruence with respect to parallel composition for Markov automata. Moreover, it turns out to be comparable with weak bisimilarity for timed labeled transition systems, thus constituting a step towards reconciling the semantics for stochastic time and deterministic time.Comment: In Proceedings QAPL 2015, arXiv:1509.0816

On the Modeling and Verification of Collective and Cooperative Systems

none1noThe formal description and verification of networks of cooperative and interacting agents is made difficult by the interplay of several different behavioral patterns, models of communication, scalability issues. In this paper, we will explore the functionalities and the expressiveness of a general-purpose process algebraic framework for the specification and model checking based analysis of collective and cooperative systems. The proposed syntactic and semantic schemes are general enough to be adapted with small modifications to heterogeneous application domains, like, e.g., crowdsourcing systems, trustworthy networks, and distributed ledger technologies.Aldini, AlessandroAldini, Alessandr

Coordination Model and Noninterference

Abstract Noninterference properties for the analysis of secure information flow are proposed in the setting of a process algebra modeling some Linda coordination primitives (asynchronous communication and read operation). To this end, relaxed definitions of equivalence are introduced that take into consideration the observational power of the external observer. The resulting taxonomy is compared with corresponding security definitions for synchronous communication models. As a result, we emphasize how the proposed coordination model affects the expressive power of some noninterference properties, by giving a new intuition to the relative merits

Discrete Time Generative-Reactive Probabilistic Processes with Different Advancing Speeds

We present a process algebra expressing probabilistic external/internal choices, multi-way synchronizations, and processes with different advancing speeds in the context of discrete time, i.e. where time is not continuous but is represented by a sequence of discrete steps as in discrete time Markov chains (DTMCs). To this end, we introduce a variant of CSP that employs a probabilistic asynchronous parallel operator whose synchronization mechanism is based on a mixture of the classical generative and reactive models of probability. In particular, differently from existing discrete time process algebras, where parallel processes are executed in synchronous locksteps, the parallel operator that we adopt allows processes with different probabilistic advancing speeds (mean number of actions executed per time unit) to be modeled. Moreover, our generative-reactive synchronization mechanism makes it possible to always derive DTMCs in the case of fully specified systems. We then present a sound and complete axiomatization of probabilistic bisimulation over finite processes of our calculus, that is a smooth extension of the axiom system for a standard process algebra, thus solving the open problem of cleanly axiomatizing action restriction in the generative model. As a further result, we show that, when evaluating steady state based performance measures which are expressible by attaching rewards to actions, our approach provides an exact solution even if the advancing speeds are considered not to be probabilistic, without incurring the state space explosion problem that arises with standard synchronous approaches. We finally present a case study on multi-path routing showing the expressiveness of our calculus and that it makes it particularly easy to produce scalable specifications

Non-determinism in Probabilistic Timed Systems with General Distributions

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A Quantitative Approach to Noninterference for Probabilistic Systems

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Approximating Imperfect Cryptography in a Formal Model

We present a formal view of cryptography that overcomes the usual assumptions of formal models for reasoning about security of computer systems, i.e. perfect cryptography and Dolev-Yao adversary model. In our framework, equivalence among formal cryptographic expressions is parameterized by a computational adversary that may exploit weaknesses of the cryptosystem to cryptanalyze ciphertext with a certain probability of success. To validate our approach, we show that in the restricted setting of ideal cryptosystems, for which the probability of guessing information that the Dolev-Yao adversary cannot derive is negligible, the computational adversary is limited to the allowed behaviors of the Dolev-Yao adversary

Handling Communications in Process Algebraic Architectural Description Languages: Modeling, Verification, and Implementation

none3noArchitectural description languages are a useful tool for modeling complex software systems at a high level of abstraction. If based on formal methods, they can also serve for enabling the early verification of various properties such as component coordination and for guiding the synthesis of code correct by construction. This is the case with process algebraic architectural description languages, which are process calculi enhanced with the main architectural concepts. However, the techniques with which those languages have been equipped are mainly conceived to work with synchronous communications only. The objective of this paper is threefold. On the modeling side, we show how to enhance the expressiveness of a typical process algebraic architectural description language by including the capability of representing nonsynchronous communications in such a way that the usability of the original language is preserved. On the verification side, we show how to modify techniques for analyzing the absence of coordination mismatches like the compatibility check for acyclic topologies and the interoperability check for cyclic topologies in such a way that those checks are valid also for nonsynchronous communications. On the implementation side, we show how to generate multithreaded object-oriented software in the presence of synchronous and nonsynchronous communications in such a way that the properties proved at the architectural level are preserved at the code level.openBernardo, Marco; Bontà, Edoardo; Aldini, AlessandroBernardo, Marco; Bontà, Edoardo; Aldini, Alessandr

A Formal Framework for Modeling Trust and Reputation in Collective Adaptive Systems

Trust and reputation models for distributed, collaborative systems have been studied and applied in several domains, in order to stimulate cooperation while preventing selfish and malicious behaviors. Nonetheless, such models have received less attention in the process of specifying and analyzing formally the functionalities of the systems mentioned above. The objective of this paper is to define a process algebraic framework for the modeling of systems that use (i) trust and reputation to govern the interactions among nodes, and (ii) communication models characterized by a high level of adaptiveness and flexibility. Hence, we propose a formalism for verifying, through model checking techniques, the robustness of these systems with respect to the typical attacks conducted against webs of trust.Comment: In Proceedings FORECAST 2016, arXiv:1607.0200