Reachability Analysis of Synchronized PA Systems

We present a generic approach for the analysis of concurrent programs with (unbounded) dynamic creation of threads and recursive procedure calls. We define a model for such programs based on a set of term rewrite rules where terms represent control configurations. The reachability problem for this model is undecidable. Therefore, we propose a method for analyzing such models based on computing abstractions of their sets of computation paths. Our approach allows to compute such abstractions as least solutions of a system of (path language) constraints. More precisely, given a program and two regular sets of configurations (process terms) T and T^′, we provide (1) a construction of a system of constraints which characterizes the set of computation paths leading from T to T^′, and (2) a generic framework, based on abstract interpretation, allowing to solve this system in various abstract domains leading to abstract analysis with different precision and cost.     

Abstract Interpretation for Proving Secrecy Properties in Security Protocols

A cryptographic protocol is a distributed program that can be executed by several actors. Since several runs of the protocol within the same execution are allowed, models of cryptoprotocols are often infinite. Sometimes, for verification purposes, only a finite and approximated model is needed. For this, we consider the problem of computing such an approximation and we propose to simulate the required partial execution in an abstract level. More precisely, we define an abstract finite category G ^a as an abstract game semantics for the SPC calculus, a dedicated calculus for security protocols. The abstract semantics is then used to build a decision procedure for secrecy correctness in security protocols.     

Optimal External Memory Planar Point Enclosure

In this paper we study the external memory planar point enclosure problem: Given N axis-parallel rectangles in the plane, construct a data structure on disk (an index) such that all K rectangles containing a query point can be reported I/O-efficiently. This problem has important applications in e.g. spatial and temporal databases, and is dual to the important and well-studied orthogonal range searching problem. Surprisingly, despite the fact that the problem can be solved optimally in internal memory with linear space and O(log N+K) query time, we show that one cannot construct a linear sized external memory point enclosure data structure that can be used to answer a query in O(log  _B N+K/B) I/Os, where B is the disk block size. To obtain this bound, Ω(N/B ^1−ε ) disk blocks are needed for some constant ε>0. With linear space, the best obtainable query bound is O(log ₂ N+K/B) if a linear output term O(K/B) is desired. To show this we prove a general lower bound on the tradeoff between the size of the data structure and its query cost. We also develop a family of structures with matching space and query bounds. An extended abstract of this paper appeared in Proceedings of the 12th European Symposium on Algorithms (ESA’04), Bergen, Norway, September 2004, pp. 40–52. L. Arge’s research was supported in part by the National Science Foundation through RI grant EIA–9972879, CAREER grant CCR–9984099, ITR grant EIA–0112849, and U.S.-Germany Cooperative Research Program grant INT–0129182, as well as by the US Army Research Office through grant W911NF-04-01-0278, by an Ole Roemer Scholarship from the Danish National Science Research Council, a NABIIT grant from the Danish Strategic Research Council and by the Danish National Research Foundation. V. Samoladas’ research was supported in part by a grant co-funded by the European Social Fund and National Resources-EPEAEK II-PYTHAGORAS. K. Yi’s research was supported in part by the National Science Foundation through ITR grant EIA–0112849, U.S.-Germany Cooperative Research Program grant INT–0129182, and Hong Kong Direct Allocation Grant (DAG07/08).     

<Emphasis Type="Italic">t</Emphasis>-Private and <Emphasis Type="Italic">t</Emphasis>-Secure Auctions

In most of the auction systems the values of bids are known to the auctioneer. This allows him to manipulate the outcome of the auction. Hence, one might be interested in hiding these values. Some cryptographically secure protocols for electronic auctions have been presented in the last decade. Our work extends these protocols in several ways. On the basis of garbled circuits, i.e., encrypted circuits, we present protocols for sealed-bid auctions that fulfill the following requirements： 1） protocols are information-theoretically t-private for honest but curious parties; 2） the number of bits that can be learned by malicious adversaries is bounded by the output length of the auction; 3） the computational requirements for participating parties are very low： only random bit choices and bitwise computation of the XOR-function are necessary. Note that one can distinguish between the protocol that generates a garbled circuit for an auction and the protocol to evaluate the auction. In this paper we address both problems. We will present a t-private protocol for the construction of a garbled circuit that reaches the lower bound of 2t ＋ 1 parties, and Finally, we address the problem of bid changes in an auction. a more randomness efficient protocol for （t ＋ 1）^2 parties     

Characterisation of silicon carbide JFETs with respect to microsystems for high temperature applications

 In this work first commercially available SiC-transistor prototypes were tested with regard to their applicability in high temperature electronic circuits for sensor signal conditioning. The influence of the temperature on the device behaviour (drain-saturation current, gate leakage current, I–V-characteristics, long-term effects) was investigated. The devices showed reliable operation up to 450 °C. The maximum forward transconductance g _m and the short circuit drain source current I _DSS decreased to approximately 30% of the room temperature values. Also, a slight increase of the pinch-off voltage V _p was observed. The gate leakage current I _GSS rose with temperature, staying below 1 μA at 450 °C. A pre-ageing study was carried out to verify changes in the device characteristics with time. The devices were exposed to a 270 °C environment and it was observed that the DC parameters tend to stabilise after about 100 h. From the I–V-characteristics the SPICE parameters were extracted for a series of temperatures, allowing the design and optimisation of amplifier gain stages. The SPICE device simulation results are in good agreement with the measured characteristics. Received: 28 November 1996/Accepted: 2 December 1996     

Local abstraction–refinement for the <Emphasis Type="Italic">μ</Emphasis>-calculus

A key technique for the verification of programs is counterexample-guided abstraction–refinement (CEGAR). Grumberg et al. (LNCS, vol 3385, pp. 233–249. Springer, Berlin, 2005; Inf Comput 205(8):1130–1148, 2007) developed a CEGAR-based algorithm for the modal μ-calculus. There, every abstract state is split in a refinement step. In this paper, the work of Grumberg et al. is generalized by presenting a new CEGAR-based algorithm for the μ-calculus. It is based on a more expressive abstract model and applies refinement only locally (at a single abstract state), i.e., the lazy abstraction technique for safety properties is adapted to the μ-calculus. Furthermore, it separates refinement determination from the (3-valued based) model checking. Three different heuristics for refinement determination are presented and illustrated.     

On Constrained Facility Location Problems

Given m facilities each with an opening cost, n demands, and distance between every demand and facility, the Facility Location problem finds a solution which opens some facilities to connect every demand to an opened facility such that the total cost of the solution is minimized. The k-Facility Location problem further requires that the number of opened facilities is at most k, where k is a parameter given in the instance of the problem. We consider the Facility Location problems satisfying that for every demand the ratio of the longest distance to facilities and the shortest distance to facilities is at most ω, where ω is a predefined constant. Using the local search approach with scaling technique and error control technique, for any arbitrarily small constant > 0, we give a polynomial-time approximation algorithm for the ω-constrained Facility Location problem with approximation ratio 1 + ω + 1 + ε, which significantly improves the previous best known ratio (ω + 1)/α for some 1≤α≤2, and a polynomial-time approximation algorithm for the ω-constrained k- Facility Location problem with approximation ratio ω+1+ε. On the aspect of approximation hardness, we prove that unless NP■DTIME(nO(loglogn)), the ω-constrained Facility Location problem cannot be approximated within 1 + lnω - 1, which slightly improves the previous best known hardness result 1.243 + 0.316ln(ω - 1). The experimental results on the standard test instances of Facility Location problem show that our algorithm also has good performance in practice.     

Minimum Cost Source Location Problems with Flow Requirements

In this paper, we consider source location problems and their generalizations with three connectivity requirements (arc-connectivity requirements λ and two kinds of vertex-connectivity requirements κ and ), where the source location problems are to find a minimum-cost set S⊆V in a given graph G=(V,A) with a capacity function u:A→ℝ₊ such that for each vertex v∈V, the connectivity from S to v (resp., from v to S) is at least a given demand d ⁻(v) (resp., d ⁺(v)). We show that the source location problem with edge-connectivity requirements in undirected networks is strongly NP-hard, which solves an open problem posed by Arata et al. (J. Algorithms 42: 54–68, 2002). Moreover, we show that the source location problems with three connectivity requirements are inapproximable within a ratio of cln D for some constant c, unless every problem in NP has an O(N ^{log log N} )-time deterministic algorithm. Here D denotes the sum of given demands. We also devise (1+ln D)-approximation algorithms for all the extended source location problems if we have the integral capacity and demand functions. By the inapproximable results above, this implies that all the source location problems are Θ(ln ∑ _v∈V (d ⁺(v)+d ⁻(v)))-approximable. An extended abstract of this paper appeared in Sakashita et al. (Proceedings of LATIN 2006, Chile, LNCS, vol. 3887, pp. 769–780, March 2006).     

Exploiting interleaving semantics in symbolic state-space generation

Symbolic techniques based on Binary Decision Diagrams (BDDs) are widely employed for reasoning about temporal properties of hardware circuits and synchronous controllers. However, they often perform poorly when dealing with the huge state spaces underlying systems based on interleaving semantics, such as communications protocols and distributed software, which are composed of independently acting subsystems that communicate via shared events. This article shows that the efficiency of state-space exploration techniques using decision diagrams can be drastically improved by exploiting the interleaving semantics underlying many event-based and component-based system models. A new algorithm for symbolically generating state spaces is presented that (i) encodes a model’s state vectors with Multi–valued Decision Diagrams (MDDs) rather than flattening them into BDDs and (ii) partitions the model’s Kronecker–consistent next–state function by event and subsystem, thus enabling multiple lightweight next–state transformations rather than a single heavyweight one. Together, this paves the way for a novel iteration order, called saturation, which replaces the breadth–first search order of traditional algorithms. The resulting saturation algorithm is implemented in the tool SMART, and experimental studies show that it is often several orders of magnitude better in terms of time efficiency, final memory consumption, and peak memory consumption than existing symbolic algorithms.     

Limits of the BRSIM/UC soundness of Dolev–Yao-style XOR

The abstraction of cryptographic operations by term algebras, called Dolev–Yao models, is essential in almost all tool-supported methods for proving security protocols. Recently significant progress was made in proving that Dolev–Yao models can be sound with respect to actual cryptographic realizations and security definitions. The strongest results show this in the sense of blackbox reactive simulatability (BRSIM)/UC, a notion that essentially means the preservation of arbitrary security properties under arbitrary active attacks and in arbitrary protocol environments, with only small changes to the Dolev–Yao models and natural implementations. However, these results are so far restricted to core cryptographic systems like encryption and signatures. Typical modern tools and complexity results around Dolev–Yao models also allow operations with more algebraic properties, in particular XOR because of its clear structure and cryptographic usefulness. We show that it is not possible to extend the strong BRSIM/UC results to XOR, at least not with remotely the same generality and naturalness as for the core cryptographic systems. We also show that for every potential soundness result for XOR with secrecy implications, one significant change to typical Dolev–Yao models must be made. On the positive side, we show the soundness of a rather general Dolev–Yao model with XOR and its realization in the sense of BRSIM/UC under passive attacks. A preliminary version of this paper appeared in Proc. 10th European Symposium on Research in Computer Security [9]     

Network Design with Weighted Players

We consider a model of game-theoretic network design initially studied by Anshelevich et al. (Proceedings of the 45th Annual Symposium on Foundations of Computer Science (FOCS), pp. 295–304, 2004), where selfish players select paths in a network to minimize their cost, which is prescribed by Shapley cost shares. If all players are identical, the cost share incurred by a player for an edge in its path is the fixed cost of the edge divided by the number of players using it. In this special case, Anshelevich et al. (Proceedings of the 45th Annual Symposium on Foundations of Computer Science (FOCS), pp. 295–304, 2004) proved that pure-strategy Nash equilibria always exist and that the price of stability—the ratio between the cost of the best Nash equilibrium and that of an optimal solution—is Θ(log k), where k is the number of players. Little was known about the existence of equilibria or the price of stability in the general weighted version of the game. Here, each player i has a weight w _i ≥1, and its cost share of an edge in its path equals w _i times the edge cost, divided by the total weight of the players using the edge. This paper presents the first general results on weighted Shapley network design games. First, we give a simple example with no pure-strategy Nash equilibrium. This motivates considering the price of stability with respect to α-approximate Nash equilibria—outcomes from which no player can decrease its cost by more than an α multiplicative factor. Our first positive result is that O(log w _max)-approximate Nash equilibria exist in all weighted Shapley network design games, where w _max is the maximum player weight. More generally, we establish the following trade-off between the two objectives of good stability and low cost: for every α=Ω(log w _max), the price of stability with respect to O(α)-approximate Nash equilibria is O((log W)/α), where W is the sum of the players’ weights. In particular, there is always an O(log W)-approximate Nash equilibrium with cost within a constant factor of optimal. Finally, we show that this trade-off curve is nearly optimal: we construct a family of networks without o(log w _max/ log log w _max)-approximate Nash equilibria, and show that for all α=Ω(log w _max/log log w _max), achieving a price of stability of O(log W/α) requires relaxing equilibrium constraints by an Ω(α) factor. Research of H.-L. Chen supported in part by NSF Award 0323766. Research of T. Roughgarden supported in part by ONR grant N00014-04-1-0725, DARPA grant W911NF-04-9-0001, and an NSF CAREER Award.     

Some notes on maximal arc intersection of spherical polygons: its \mathcal{NP} -hardness and approximation algorithms

Finding a sequence of workpiece orientations such that the number of setups is minimized is an important optimization problem in manufacturing industry. In this paper we present some interesting notes on this optimal workpiece setup problem. These notes show that (1) The greedy algorithm proposed in Comput. Aided Des. 35 (2003), pp. 1269–1285 for the optimal workpiece setup problem has the performance ratio bounded by O(ln n−ln ln n+0.78), where n is the number of spherical polygons in the ground set; (2) In addition to greedy heuristic, linear programming can also be used as a near-optimal approximation solution; (3) The performance ratio by linear programming is shown to be tighter than that of greedy heuristic in some cases.     

The Regular Linear Systems Associated with the Shift Semigroups and Application to Control Linear Systems with Delay

We investigate the infinite dimensional control linear systems with delays in the state and input. We give a new variation of constants formula when the state and control delay operators are unbounded. We prove the existence of mild and classical solutions of such systems. Our approach is based on the theory of abstract and regular linear systems introduced by Salamon (Math Syst Theor 21:147–164, 1989) and Weiss (Isr J Math 65:17–43, 1989). Finally, we apply our abstract framework to an example from population dynamics.     

Static Equivalence is Harder than Knowledge

There are two main ways of defining secrecy of cryptographic protocols. The first version checks if the adversary can learn the value of a secret parameter. In the second version, one checks if the adversary can notice any difference between protocol runs with different values of the secret parameter.We give a new proof that when considering more complex equational theories than partially invertible functions, these two kinds of secrecy are not equally difficult to verify. More precisely, we identify a message language equipped with a convergent rewrite system such that after a completed protocol run, the first problem mentioned above (adversary knowledge) is decidable but the second problem (static equivalence) is not. The proof is by reduction of the ambiguity problem for context-free grammars.     

Verification of probabilistic systems with faulty communication

Many protocols are designed to operate correctly even in the case where the underlying communication medium is faulty. To capture the behavior of such protocols, Lossy Channel Systems (LCS’s) have been proposed. In an LCS the communication channels are modeled as unbounded FIFO buffers which are unreliable in the sense that they can nondeterministically lose messages. Recently, several attempts have been made to study Probabilistic Lossy Channel Systems (PLCS’s) in which the probability of losing messages is taken into account. In this article, we consider a variant of PLCS’s which is more realistic than those studied previously. More precisely, we assume that during each step in the execution of the system, each message may be lost with a certain predefined probability. We show that for such systems the following model-checking problem is decidable: to verify whether a linear-time property definable by a finite-state ω-automaton holds with probability one. We also consider other types of faulty behavior, such as corruption and duplication of messages, and insertion of new messages, and show that the decidability results extend to these models.     

Sub-micron coatings with low friction and wear for micro actuators

 This paper deals with the fabrication and characterisation of friction and wear reduced nano-films for micro actuator applications. For this investigation films of diamond-like-carbon (DLC), metal doped Me-DLC, carbon-nitride CN _x , boron-nitride BN and alumina Al₂O₃ films have been applied in the thickness range of 20–500 nm. The hardness of those coatings varied between 10 and 60 GPa. Micromechanical and microtribological properties of nano-coatings have been characterized by a modified scanning probe microscope. Besides this a novel micro tester for abrasive wear measurement of nano coatings was used. Friction of micro-contact areas was measured by use of a pin-on-disc tester. It turned out, that friction was – besides other parameters – dependent on determination method and load. Friction determined at areal DLC/DLC contact zones was generally much higher (μ_DLC 0.1) than diamond tip versus DLC (∼μ _DLC 0.06). Received: 19 June 2001/Accepted: 15 September 2001     

Automated verification and refinement for physical-layer protocols

This paper demonstrates how to use a satisfiability modulo theories (SMT) solver together with a bounded model checker to verify properties of real-time physical layer protocols. The method is first used to verify the Biphase Mark protocol, a protocol that has been verified numerous times previously, allowing for a comparison of results. The techniques are extended to the 8N1 protocol used in universal asynchronous receiver transmitters. We then demonstrate the use of temporal refinement to link a finite state specification of 8N1 with its real-time implementation. This refinement relationship relieves a significant disadvantage of SMT approaches—their inability to scale to large problems. Finally, capturing the impact of metastability on timing requirements is a key issue in modeling physical-layer protocols. Rather than model metastability directly, a contribution of our models is treating its effect as a constraint on non-determinism.     

On the Planar Piecewise Quadratic 1-Center Problem

In this paper we introduce a minimax model unifying several classes of single facility planar center location problems. We assume that the transportation costs of the demand points to the serving facility are convex functions {Q _i }, i=1,…,n, of the planar distance used. Moreover, these functions, when properly transformed, give rise to piecewise quadratic functions of the coordinates of the facility location. In the continuous case, using results on LP-type models by Clarkson (J. ACM 42:488–499, 1995), Matoušek et al. (Algorithmica 16:498–516, 1996), and the derandomization technique in Chazelle and Matoušek (J. Algorithms 21:579–597, 1996), we claim that the model is solvable deterministically in linear time. We also show that in the separable case, one can get a direct O(nlog n) deterministic algorithm, based on Dyer (Proceedings of the 8th ACM Symposium on Computational Geometry, 1992), to find an optimal solution. In the discrete case, where the location of the center (server) is restricted to some prespecified finite set, we introduce deterministic subquadratic algorithms based on the general parametric approach of Megiddo (J. ACM 30:852–865, 1983), and on properties of upper envelopes of collections of quadratic arcs. We apply our methods to solve and improve the complexity of a number of other location problems in the literature, and solve some new models in linear or subquadratic time complexity.     

Incidence structures and Stone–Priestley duality

We observe that if R: = (I,ρ, J) is an incidence structure, viewed as a matrix, then the topological closure of the set of columns is the Stone space of the Boolean algebra generated by the rows. As a consequence, we obtain that the topological closure of the collection of principal initial segments of a poset P is the Stone space of the Boolean algebra Tailalg (P) generated by the collection of principal final segments of P, the so-called tail-algebra of P. Similar results concerning Priestley spaces and distributive lattices are given. A generalization to incidence structures valued by abstract algebras is considered.      

Degree-Optimal Routing for P2P Systems

We define a family of Distributed Hash Table systems whose aim is to combine the routing efficiency of randomized networks—e.g. optimal average path length O(log ² n/δlog δ) with δ degree—with the programmability and startup efficiency of a uniform overlay—that is, a deterministic system in which the overlay network is transitive and greedy routing is optimal. It is known that Ω(log n) is a lower bound on the average path length for uniform overlays with O(log n) degree (Xu et al., IEEE J. Sel. Areas Commun. 22(1), 151–163, 2004). Our work is inspired by neighbor-of-neighbor (NoN) routing, a recently introduced variation of greedy routing that allows us to achieve optimal average path length in randomized networks. The advantage of our proposal is that of allowing the NoN technique to be implemented without adding any overhead to the corresponding deterministic network. We propose a family of networks parameterized with a positive integer c which measures the amount of randomness that is used. By varying the value c, the system goes from the deterministic case (c=1) to an “almost uniform” system. Increasing c to relatively low values allows for routing with asymptotically optimal average path length while retaining most of the advantages of a uniform system, such as easy programmability and quick bootstrap of the nodes entering the system. We also provide a matching lower bound for the average path length of the routing schemes for any c. This work was partially supported by the Italian FIRB project “WEB-MINDS” (Wide-scalE, Broadband MIddleware for Network Distributed Services), .