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AbstractsWednesday Dec 15, 2005, 3pm. Title: Separation in the Lambdamucalculus Speaker: Alexis Saurin Abstract:
Having recalled the basics concerning lambdamucalculus introduced by Michel Parigot in order to extend CurryHoward isomorphism to classical natural deduction I will present a result by Rene' David and Walter Py showing that Separation property (often refered to as Bo"hm theorem) is not satisfied in lambdamucalculus. We will then consider how to recover the property by analyzing what is the reason for its failure in the original calculus. The resulting calculus, the Lambdamucaluclus for which Separation property holds will be compared to Parigot's calculus and with a variant introduced by Philippe de Groote. We will in particular see how Lambdamu can be considered as a calculus of terms and streams of terms. Sept 17, 2004 Speaker: Tom Chothia Title: "Typebased Distributed Access Control" Abstract: I will present a type system that combines a weak form of information flow control, termed distributed access control, with typed cryptographic operations. The motivation is to have a type system that ensures access control while giving the application the responsibility to secure network communications, and to do this safely. The notion of declassification certificates is introduced to support the declassification of encrypted data. Purely local type checking is extended to distributed systems by associating access control lists with cryptographic keys. When a piece of data is sent outside of a type checked area it is encrypted with a key that represents the list of principles that can access that data. When encrypted data is received, the access restrictions from the decryption key are used as the access control type. This work was carried out with Dominic Duggan and Jan Vitek. October 6, 2004. Speaker: Jun PANG Title: "Verifying a Sliding Window Protocol in mCRL" Abstract: Within the process algebraic community, much effort has been spent on analyzing sliding window protocols. Aart Middeldorp (1986) and Jacob Brunekreef (1993) gave specifications in ACP and PSF, respectively. Frits Vaandrager (1986), Richard Groenveld (1987), Jos van Wamel (1992) and Marc Bezem and Jan Friso Groote (1994) manually verified onebit sliding window protocols, in which the size of the sending and receiving window is one. Starting in 1990, we attempted to prove the most complex sliding window protocol (not taking into account additional features such as duplex message passing and piggybacking) from Tanenbaum's "Communication Protocols" correct using mCRL. This turned out to be unexpectedly hard, which is shown by the 13 year it took to complete this work. We prove the correctness of a sliding window protocol with an arbitrary finite window size n and sequence numbers modulo 2n. We show that the sliding window protocol is branching bisimilar to a queue of capacity 2n. The proof is given entirely on the basis of an axiomatic theory, and was checked with the help of PVS. (This is joint work with Bahareh Badban, Wan Fokkink , Jan Friso Groote and Jaco van de Pol.) Oct 12, 2004 Title: Proof Search and Model Checking: An exploratory talk Speaker: Dale Miller Abstract:
Proof Search is a updating of the logic programming paradigm where proof theory is used to motivation new designs. A great deal of work in proof theory and computation has taken place in recent years. One recent development has been the development of a good proof theory for defined predicates, which is closely related to reasoning within fixpoints and to the closedworldassumption. In this talk, I will illustrate some recent results in this area, present many different examples, and demonstrate a recent prototype system, Level 0/1, by Alwen Tiu. I hope to explore to what extent this work on logic and its implementation might have in doing (symbolic) model checking. Some related paper: Some of the following paper focus on higherorder abstract syntax and the nabla; quantifier. I will not focus on these aspects of this work in this talk: instead, the focus will be on firstorder logic. Links:  Encoding Transition Systems in Sequent Calculus, by Raymond McDowell, Dale Miller, and Catuscia Palamidessi. http://www.lix.polytechnique.fr/Labo/Dale.Miller/papers/tcs97.pdf.  Level 0/1 Prover, by Alwen Tiu. http://www.lix.polytechnique.fr/~tiu/lincproject/download.htm  A Proof Theory for Generic Judgments, by Dale Miller and Alwen Tiu. http://www.lix.polytechnique.fr/Labo/Dale.Miller/papers/toclnabla.pdf.  A Proof Search Specification of the piCalculus, by Alwen Tiu and Dale Miller. http://www.lix.polytechnique.fr/Labo/Dale.Miller/papers/fguc04workshop.pdf Oct 19, 2004. Title: Specification and Verification of Probabilistic Security Protocols Speaker: Kostas Chatzikokolakis Abstract:
Security protocols provide the ability of communicating privately using public means. A certain category of such protocols rely on probabilistic decisions and use primitives such as the Oblivious Transfer. Even though probabilistic protocols have been studied using model checking methods, little attempt of applying process calculi techniques has been made. In this talk, we use a probabilistic variant of picalculus in order to model probabilistic protocols. We present a method of expressing some desired properties of such protocols which involves the construction of a proper specification and the introduction of a suitable precongruence relation between processes. We finally apply our method to the Partial Secrets Exchange protocol and prove its fairness. Oct 26, 2004. Title: Reductions in Distributed Computingn and applications to Agreement Tasks. Speaker: Bernadette CharronBost Abstract:
In this talk, we introduce several notions of reduction in distributed computing, and investigate reduction properties of two fundamental agreement tasks, namely Consensus and Atomic Commitment. We first propose the notion of reduction "à la Karp", an analog for distributed computing of the classical Karp reduction. We then define a weaker reduction which is the analog of Cook reduction. These two reductions are called K reduction and Creduction, respectively. We establish various reducibility and irreducibility theorems with respect to these two reductions. Our main result is an incomparability statement for Consensus and Atomic Commitment tasks: we show that they are incomparable with respect to the Creduction, except when the resiliency degree is 1, in which case Atomic Commitment is strictly harder than Consensus. A side consequence of these results is that our notion of Creduction is strictly weaker than the one of Kreduction.
Finally, we introduce a third notion of reduction which measures the hardness to solve
a task in terms of the information about failures required to solve the task. We prove that
this latter reduction is strictly weaker than the Creduction. In other words, the information
about failures  or equivalently, the weakest failure detector  needed to solve a task does
not entirely capture the hardness of the task.
Nov 9, 2004 Title: Group Communication: where are we today and future challenges. Speaker: André Schiper Abstract:
Group communication is a topic studied since more than twenty years. During this period significant results have been obtained. Nevertheless there are issues that have not been addressed adequately. The paper presents both the important results related to group communication that have been obtained, but also points out what remains to be done to make group communication widely used, and as successful as some of the existing middleware technologies. Nov 16, 2004 Title: Axiomatizations for probabilistic finitestate behaviors Speaker: Yuxin Deng Abstract:
We study a process calculus which combines both nondeterministic and probabilistic behavior in the style of Segala and Lynch's probabilistic automata. We consider various strong and weak behavioral equivalences, and we provide complete axiomatizations for finite state processes, restricted to guarded definitions in case of the weak equivalences. We conjecture that in the general case of unguarded recursion the ``natural'' weak equivalences are undecidable. This is the first work, to our knowledge, that provides a complete axiomatization for weak equivalences in the presence of recursion and both nondeterministic and probabilistic choice. Slides Mon Nov 29, 2004 Title: A format of congruence for open bisimulation in namepassing calculi Speaker: Axelle Ziegler Abstract:
In this talk, I'll present the use a general formal framework encoding namepassing process calculi and allowing to reason about them, using the nabla quantifier, definitions and higherorder abstract syntax. I'll show how this framework can be used to specify a format for bisimulation to be a congruence. Finally, I'll try to sketch what a model of this theory could be, and how it would connect with our applied result. Tue Nov 30, 2004, 3pm. Title: The Price of Guessing, in the picalculus. (with an Introduction to Formal and Computational Protocol Checking ) Speaker: Tom Chothia Abstract:
I will present an outline of formal and computational methods that can be used to "prove" a process secure and I will outline some the problems with these methods; this will be a basic introduction aimed at formal methods people who do not have a back ground in security. I will then describe some of the work that has been done to bridge the gap between these formal and computational worlds, which mostly consists of finding computational criteria that, when met, ensure that formal analysis and computation coincide. Finally, I will suggest a new line of work that I am undertaking, which involves extending the picalculus with primitives for guessing new names and a method of finding the cost of a trace that will allow analysis that approximates computational methods without the need to do any difficult computational proofs. This new work is in a very early stage and so will be largely conjecture. Mon May 2 2004. 3pm Speaker: Brigitte Pientka Title:Overcoming Performance Barriers: efficient proof search in logical frameworks Abstract: The logical framework Twelf provides an experimental platform to specify, implement and execute formal systems. One of its applications is in proofcarrying code and proofcarrying authentication, where it is successfully used to specify and verify formal guarantees about the runtime behavior of programs. These realworld applications have exposed some critical bottlenecks: execution of many logical specifications is severely hampered and some are not executable at all, thereby limiting its application in practice. In this talk, I will describe three optimizations which substantially improve the performance and extend the power of the existing system. First, I give an optimized unification algorithm for logical frameworks which allows us to eliminate unnecessary occurschecks. Second I present a novel execution model based on selective memoization and third I will discuss term indexing techniques to sustain performance in largescale examples. As experimental results will demonstrate, these optimizations taken together constitute a significant step toward exploring the full potential of logical frameworks in realworld applications. Dec 7, 2005, 3pm. Title: Electoral Systems in Ambient Calculi Speaker: Maria Grazia Vigliotti, Imperial College, London Abstract:
This work compares the expressiveness of ambient calculi against different dialects of the picalculus. Cardelli and Gordon encoded the asynchronous picalculus into the ambient calculus (Mobile Ambients). Zimmer has shown that the synchronous picalculus without choice can be encoded in pure (no communication) Safe Ambients. We show that the pure ambient calculus without restriction has symmetric electoral systems, that is, it is possible to solve the problem of electing a leader in a symmetric network. By the work of Palamidessi, this implies that this fragment of the ambient calculus is not encodable (under certain conditions) in the picalculus with separate choice. Not scheduled yet Title: On Infinite Satisfaction and Behaviour Speaker: Frank D. Valencia Abstract:
Finite state models prevail in today's specification and verification techniques. Global Computing systems are, however, inherently open e.g., typically there is no bound on the number of resources/devices that can be added to a given system. So, the assumption I shall be working with is that many phenomena in Global Computing are best described in infinitestate models. In this talk I shall give an overview of my current interests and work on infinite behavior in two areas of computer science: Concurrency Theory and Computability. Concerning the former, I shall give an overview of my work on the expressive power of different ways of specifying infinitebehaviour in process calculi. As for the latter, I shall introduce a new framework for Infinite Satisfaction Problems (basically, Infinite SAT Problems) and present some (un)decidability results we have identified for this kind of problems. These results have an impact on Open Constraint Problems; a framework recently introduced for constraint problems in Global Computing scenarios. My work on infinite satisfaction and behaviour involves several coauthors. The following list of relevant work acknowledges all of them:  Stefan Dantchev and Frank D. Valencia. On the Computational Limits of Infinite Satisfaction. In SAC2005. © ACM Press, March 2005.
 C. Palamidessi, V. Saraswat, B. Victor, F. Valencia. Linearity vs
Persistency in the Pi Calculus. Ongoing work. Page maintainer: Carlos Olarte 