53 research outputs found
Probabilistic Disclosure: Maximisation vs. Minimisation
We consider opacity questions where an observation function provides
to an external attacker a view of the states along executions and
secret executions are those visiting some state from a fixed
subset. Disclosure occurs when the observer can deduce from a finite
observation that the execution is secret, the epsilon-disclosure
variant corresponding to the execution being secret with probability
greater than 1 - epsilon. In a probabilistic and non deterministic
setting, where an internal agent can choose between actions, there
are two points of view, depending on the status of this agent: the
successive choices can either help the attacker trying to disclose
the secret, if the system has been corrupted, or they can prevent
disclosure as much as possible if these choices are part of the
system design. In the former situation, corresponding to a worst
case, the disclosure value is the supremum over the strategies of
the probability to disclose the secret (maximisation), whereas in
the latter case, the disclosure is the infimum (minimisation). We
address quantitative problems (comparing the optimal value with a
threshold) and qualitative ones (when the threshold is zero or one)
related to both forms of disclosure for a fixed or finite
horizon. For all problems, we characterise their decidability status
and their complexity. We discover a surprising asymmetry: on the one
hand optimal strategies may be chosen among deterministic ones in
maximisation problems, while it is not the case for minimisation. On
the other hand, for the questions addressed here, more minimisation
problems than maximisation ones are decidable
Experimental Study of the Shortest Reset Word of Random Automata
In this paper we describe an approach to finding the shortest reset word of a
finite synchronizing automaton by using a SAT solver. We use this approach to
perform an experimental study of the length of the shortest reset word of a
finite synchronizing automaton. The largest automata we considered had 100
states. The results of the experiments allow us to formulate a hypothesis that
the length of the shortest reset word of a random finite automaton with
states and 2 input letters with high probability is sublinear with respect to
and can be estimated as $1.95 n^{0.55}.
The Online Simple Knapsack Problem with Reservation and Removability
In the online simple knapsack problem, a knapsack of unit size 1 is given and an algorithm is tasked to fill it using a set of items that are revealed one after another. Each item must be accepted or rejected at the time they are presented, and these decisions are irrevocable. No prior knowledge about the set and sequence of items is given. The goal is then to maximize the sum of the sizes of all packed items compared to an optimal packing of all items of the sequence.
In this paper, we combine two existing variants of the problem that each extend the range of possible actions for a newly presented item by a new option. The first is removability, in which an item that was previously packed into the knapsack may be finally discarded at any point. The second is reservations, which allows the algorithm to delay the decision on accepting or rejecting a new item indefinitely for a proportional fee relative to the size of the given item.
If both removability and reservations are permitted, we show that the competitive ratio of the online simple knapsack problem rises depending on the relative reservation costs. As soon as any nonzero fee has to be paid for a reservation, no online algorithm can be better than 1.5-competitive. With rising reservation costs, this competitive ratio increases up to the golden ratio (? ? 1.618) that is reached for relative reservation costs of 1-?5/3 ? 0.254. We provide a matching upper and lower bound for relative reservation costs up to this value. From this point onward, the tight bound by Iwama and Taketomi for the removable knapsack problem is the best possible competitive ratio, not using any reservations
A unified view of parameterized verification of abstract models of broadcast communication
We give a unified view of different parameterized models of concurrent and distributed systems with broadcast communication based on transition systems. Based on the resulting formal models, we discuss related verification methods and tools based on abstractions and symbolic state exploration
Online graph coloring against a randomized adversary
Electronic version of an article published as
Online graph coloring against a randomized adversary. "International journal of foundations of computer science", 1 Juny 2018, vol. 29, núm. 4, p. 551-569. DOI:10.1142/S0129054118410058 © 2018 copyright World Scientific Publishing Company. https://www.worldscientific.com/doi/abs/10.1142/S0129054118410058We consider an online model where an adversary constructs a set of 2s instances S instead of one single instance. The algorithm knows S and the adversary will choose one instance from S at random to present to the algorithm. We further focus on adversaries that construct sets of k-chromatic instances. In this setting, we provide upper and lower bounds on the competitive ratio for the online graph coloring problem as a function of the parameters in this model. Both bounds are linear in s and matching upper and lower bound are given for a specific set of algorithms that we call “minimalistic online algorithms”.Peer ReviewedPostprint (author's final draft
Parameterized verification
The goal of parameterized verification is to prove the correctness of a system specification regardless of the number of its components. The problem is of interest in several different areas: verification of hardware design, multithreaded programs, distributed systems, and communication protocols. The problem is undecidable in general. Solutions for restricted classes of systems and properties have been studied in areas like theorem proving, model checking, automata and logic, process algebra, and constraint solving. In this introduction to the special issue, dedicated to a selection of works from the Parameterized Verification workshop PV \u201914 and PV \u201915, we survey some of the works developed in this research area
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