Petri Net Based Symbolic Model Checking for Computation Tree Logic of Knowledge
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Computation Tree Logic of Knowledge (CTLK) can specify many design requirements of privacy and security of multi-agent systems (MAS). In our conference paper, we defined Knowledge-oriented Petri Nets (KPN) to model MAS and proposed Reachability Graphs with Equivalence Relations (RGER) to verify CTLK. In this paper, we use the technique of Ordered Binary Decision Diagrams (OBDD) to encode RGER in order to alleviate the state explosion problem and enhance the verification efficiency. We propose a heuristic method to order those variables in OBDD, which can well improve the time and space performance of producing, encoding and exploring a huge state space. More importantly, our method does not produce and encode any transition or equivalence relation of states when producing and encoding an RGER, and in fact it dynamically produces those transition or equivalence relations that are required in the verification process of CTLK formulas. This policy can save a lot of time and space since the number of transition or equivalence relations of states is much greater than the number of states themselves. We design symbolic model checking algorithms, develop a tool and apply them to two famous examples: Alice-Bob Protocol and Dining Cryptographers Protocol. We compare our tool with MCMAS which is the state-of-the-art model checker of verifying CTLK. The experimental results illustrate the advantages of our model and method. Our tool running in a general PC can totally spend less than 14 hours to verify Dining Cryptographers Protocol with 1200 concurrent cryptographers where there are about 10^1080 states and the two verified CTLK formulas have more than 6000 atomic propositions and more than 3600 operators. These good performances are owed to a combination of the OBDD technique and the structure characteristics of KPN.
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