Attacking Fair-Exchange Protocols

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Attacking Fair-Exchange Protocols: Parallel Models vs Trace Models

Most approaches to formal protocol verification rely on an operational model based on traces of atomic actions. Modulo CSP, CCS, state-exploration, Higher Order Logic or strand spaces frills, authentication or secrecy are analyzed by looking at the existence or the absence of traces with a suitable property. We introduced an alternative operational approach based on parallel actions and an explicit representation of time. Our approach consists in specifying protocols within a logic language (ALSP), and associating the existence of an attack to the protocol with the existence of a model for the specifications of both the protocol and the attack. In this paper we show that, for a large class of protocols such as authentication and key exchange protocols, modeling in ALSP is equivalent – as far as authentication and secrecy attacks are considered – to modeling in trace based models. We then consider fair exchange protocols introduced by N. Asokan et al. showing that parallel attacks may lead the trusted third party of the protocol into an inconsistent state. We show that the trace based model does not allow for the representation of this kind of attacks, whereas our approach can represent them

SAT-Based Cooperative Planning: A Proposal

We present a simple and semantics-preserving transformation over propositional instances which features surprising consequences both on the syntactic structure of the processed instances and on the performance of state-of-the-art solvers. The instances we consider come from a bounded model checking engine applied to real-world problems. The transformation we apply aims at substituting ennary connectives for binary ones whenever possible. Several experimental results are presented and discusse

Definability and Commonsense Reasoning

The definition of concepts is a central problem in commonsense reasoning. Many themes in nonmonotonic reasoning concern implicit and explicit definability. Implicit definability in nonmonotonic logic is always relative to the context - the current theory of the world. We show that fixed point equations provide a generalization of explicit definability, which correctly captures the relativized context. Theories expressed within this logical framework provide implicit definitions of concepts. Moreover, it is possible to derive these fixed points entirely within the logic