Hans Hüttel

Bio: Hans Hüttel is an academic researcher from Aalborg University. The author has contributed to research in topics: Decidability & Process calculus. The author has an hindex of 17, co-authored 80 publications receiving 1327 citations. Previous affiliations of Hans Hüttel include University of Edinburgh & National Research Foundation of South Africa.

Foundations of Session Types and Behavioural Contracts

Abstract: Behavioural type systems, usually associated to concurrent or distributed computations, encompass concepts such as interfaces, communication protocols, and contracts, in addition to the traditional input/output operations. The behavioural type of a software component specifies its expected patterns of interaction using expressive type languages, so types can be used to determine automatically whether the component interacts correctly with other components. Two related important notions of behavioural types are those of session types and behavioural contracts. This article surveys the main accomplishments of the last 20 years within these two approaches.

Bisimulation equivalence is decidable for all context-free processes

Abstract: A result originally due to Baeten, Bergstra, and Klop shows that strong bisimulation equivalence for normed BPA processes is decidable. On the other hand, Huynh and Tian, and Groote and Huttel, have proved that all other standard equivalences are undecidable for normed BPA and thus for BPA in general, The open problem remaining has been whether bisimulation is decidable for the full BPA language. In this paper, we answer this question in the affirmative, using a proof technique based on the proof by Caucal of the decidability of language equivalence for simple algebraic grammars. The decision procedure relies on our main result, extending that of Caucal, that the maximal bisimulation of any BPA transition graph is finitely representable as a Thue congruence. The decision procedure consists of two semi-decision procedures, one testing for non-bisimilarity and one testing for bisimilarity.

Bisimulation Equivalence is Decidable for all Context-Free Processes

Abstract: We have shown that bisimulation equivalence is decidable for BPA. As the proof involves two semi-decision procedures it is not obvious how to determine the complexity of solving this problem. Moreover it does not provide us with an intuitive technique for deciding bisimilarity as does the tableau method in [14, 13] which also has the advantage of providing us with a way of extracting a complete axiomatization for normed BPA processes. A similar result for full BPA would be a proper extension of Milner's axiom system for regular processes [16].

Undecidable equivalences for basic process algebra

Abstract: A recent theorem shows that strong bisimilarity is decidable for the class of normed BPA processes, which correspond to a class of context-free grammars generating the ϵ-free context-free languages. Huynh and Tian (Technical Report UTDCS-31-90, University of Texas at Dallas, 1990) have shown that readiness and failure equivalence are undecidable for BPA processes. In this paper we examine all other equivalences in the linear/branching time hierarchy and show that none of them are decidable for normed BPA processes.

[서평]「Applied Cryptography」

Abstract: The objective of this paper is to give a comprehensive introduction to applied cryptography with an engineer or computer scientist in mind. The emphasis is on the knowledge needed to create practical systems which supports integrity, confidentiality, or authenticity. Topics covered includes an introduction to the concepts in cryptography, attacks against cryptographic systems, key use and handling, random bit generation, encryption modes, and message authentication codes. Recommendations on algorithms and further reading is given in the end of the paper. This paper should make the reader able to build, understand and evaluate system descriptions and designs based on the cryptographic components described in the paper.

Propagation of trust and distrust

Abstract: A (directed) network of people connected by ratings or trust scores, and a model for propagating those trust scores, is a fundamental building block in many of today's most successful e-commerce and recommendation systems. We develop a framework of trust propagation schemes, each of which may be appropriate in certain circumstances, and evaluate the schemes on a large trust network consisting of 800K trust scores expressed among 130K people. We show that a small number of expressed trusts/distrust per individual allows us to predict trust between any two people in the system with high accuracy. Our work appears to be the first to incorporate distrust in a computational trust propagation setting.

Teaching for quality learning at university

Abstract: by J. Biggs and C. Tang, Maidenhead, England, Open University Press, 2007, 360 pp., £29.99, ISBN-13: 978-0-335-22126-4

Branching time and abstraction in bisimulation semantics

Abstract: In comparative concurrency semantics, one usually distinguishes between linear time and branching time semantic equivalences. Milner's notion of observatin equivalence is often mentioned as the standard example of a branching time equivalence. In this paper we investigate whether observation equivalence really does respect the branching structure of processes, and find that in the presence of the unobservable action t of CCS this is not the case.Therefore, the notion of branching bisimulation equivalence is introduced which strongly preserves the branching structure of processes, in the sense that it preserves computations together with the potentials in all intermediate states that are passed through, even if silent moves are involved. On closed CCS-terms branching bisimulation congruence can be completely axiomatized by the single axion scheme: a.(t.(y+z)+y)=a.(y+z) (where a ranges over all actions) and the usual loaws for strong congruence.We also establish that for sequential processes observation equivalence is not preserved under refinement of actions, whereas branching bisimulation is.For a large class of processes, it turns out that branching bisimulation and observation equivalence are the same. As far as we know, all protocols that have been verified in the setting of observation equivalence happen to fit in this class, and hence are also valid in the stronger setting of branching bisimulation equivalence.