14 research outputs found
Probabilistic Bisimulations for PCTL Model Checking of Interval MDPs
Verification of PCTL properties of MDPs with convex uncertainties has been
investigated recently by Puggelli et al. However, model checking algorithms
typically suffer from state space explosion. In this paper, we address
probabilistic bisimulation to reduce the size of such an MDPs while preserving
PCTL properties it satisfies. We discuss different interpretations of
uncertainty in the models which are studied in the literature and that result
in two different definitions of bisimulations. We give algorithms to compute
the quotients of these bisimulations in time polynomial in the size of the
model and exponential in the uncertain branching. Finally, we show by a case
study that large models in practice can have small branching and that a
substantial state space reduction can be achieved by our approach.Comment: In Proceedings SynCoP 2014, arXiv:1403.784
Probabilistic Opacity in Refinement-Based Modeling
Given a probabilistic transition system (PTS) partially observed by
an attacker, and an -regular predicate over the traces of
, measuring the disclosure of the secret in means
computing the probability that an attacker who observes a run of can
ascertain that its trace belongs to . In the context of refinement, we
consider specifications given as Interval-valued Discrete Time Markov Chains
(IDTMCs), which are underspecified Markov chains where probabilities on edges
are only required to belong to intervals. Scheduling an IDTMC produces
a concrete implementation as a PTS and we define the worst case disclosure of
secret in as the maximal disclosure of over all
PTSs thus produced. We compute this value for a subclass of IDTMCs and we prove
that refinement can only improve the opacity of implementations
Robust Control of Uncertain Markov Decision Processes with Temporal Logic Specifications
We present a method for designing robust controllers for dynamical systems with linear temporal logic specifications. We abstract the original system by a finite Markov Decision Process (MDP) that has transition probabilities in a specified uncertainty set. A robust control policy for the MDP is generated that maximizes the worst-case probability of satisfying the specification over all transition probabilities in the uncertainty set. To do this, we use a procedure from probabilistic model checking to combine the system model with an automaton representing the specification. This new MDP is then transformed into an equivalent form that satisfies assumptions for stochastic shortest path dynamic programming. A robust version of dynamic programming allows us to solve for a -suboptimal robust control policy with time complexity times that for the non-robust case. We then implement this control policy on the original dynamical system
On the complexity of computing maximum entropy for Markovian Models
We investigate the complexity of computing entropy of various Markovian models including
Markov Chains (MCs), Interval Markov Chains (IMCs) and Markov Decision Processes (MDPs).
We consider both entropy and entropy rate for general MCs, and study two algorithmic questions,
i.e., entropy approximation problem and entropy threshold problem. The former asks for
an approximation of the entropy/entropy rate within a given precision, whereas the latter aims
to decide whether they exceed a given threshold. We give polynomial-time algorithms for the
approximation problem, and show the threshold problem is in P
CH3 (hence in PSPACE) and
in P assuming some number-theoretic conjectures. Furthermore, we study both questions for
IMCs and MDPs where we aim to maximise the entropy/entropy rate among an infinite family
of MCs associated with the given model. We give various conditional decidability results for
the threshold problem, and show the approximation problem is solvable in polynomial-time via
convex programming
On the complexity of computing maximum entropy for Markovian models
We investigate the complexity of computing entropy of various Markovian models including Markov Chains (MCs), Interval Markov Chains (IMCs) and Markov Decision Processes (MDPs). We consider both entropy and entropy rate for general MCs, and study two algorithmic questions, i.e., entropy approximation problem and entropy threshold problem. The former asks for an approximation of the entropy/entropy rate within a given precision, whereas the latter aims to decide whether they exceed a given threshold. We give polynomial-time algorithms for the approximation problem, and show the threshold problem is in P CH3 (hence in PSPACE) and in P assuming some number-theoretic conjectures. Furthermore, we study both questions for IMCs and MDPs where we aim to maximise the entropy/entropy rate among an infinite family of MCs associated with the given model. We give various conditional decidability results for the threshold problem, and show the approximation problem is solvable in polynomial-time via convex programmin
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
Comparing Labelled Markov Decision Processes
A labelled Markov decision process is a labelled Markov chain with nondeterminism, i.e., together with a strategy a labelled MDP induces a labelled Markov chain. The model is related to interval Markov chains. Motivated by applications of equivalence checking for the verification of anonymity, we study the algorithmic comparison of two labelled MDPs, in particular, whether there exist strategies such that the MDPs become equivalent/inequivalent, both in terms of trace equivalence and in terms of probabilistic bisimilarity. We provide the first polynomial-time algorithms for computing memoryless strategies to make the two labelled MDPs inequivalent if such strategies exist. We also study the computational complexity of qualitative problems about making the total variation distance and the probabilistic bisimilarity distance less than one or equal to one
Entropic Risk for Turn-Based Stochastic Games
Entropic risk (ERisk) is an established risk measure in finance, quantifying risk by an exponential re-weighting of rewards. We study ERisk for the first time in the context of turn-based stochastic games with the total reward objective. This gives rise to an objective function that demands the control of systems in a risk-averse manner. We show that the resulting games are determined and, in particular, admit optimal memoryless deterministic strategies. This contrasts risk measures that previously have been considered in the special case of Markov decision processes and that require randomization and/or memory. We provide several results on the decidability and the computational complexity of the threshold problem, i.e. whether the optimal value of ERisk exceeds a given threshold. In the most general case, the problem is decidable subject to Shanuel’s conjecture. If all inputs are rational, the resulting threshold problem can be solved using algebraic numbers, leading to decidability via a polynomial-time reduction to the existential theory of the reals. Further restrictions on the encoding of the input allow the solution of the threshold problem in NP∩coNP. Finally, an approximation algorithm for the optimal value of ERisk is provided