Dependability engineering in Isabelle

In this paper, we introduce a process of formal system development supported by interactive theorem proving in a dedicated Isabelle framework. This Isabelle Infrastructure framework implements specification and verification in a cyclic process supported by attack tree analysis closely inter-connected with formal refinement of the specification. The process is cyclic: in a repeated iteration the refinement adds more detail to the system specification. It is a known hard problem how to find the next refinement step: this problem is addressed by the attack based analysis using Kripke structures and CTL logic. We call this cyclic process the Refinement-Risk cycle (RR-cycle). It has been developed for security and privacy of IoT healthcare systems initially but is more generally applicable for safety as well, that is, dependability in general. In this paper, we present the extensions to the Isabelle Infrastructure framework implementing a formal notion of property preserving refinement interleaved with attack tree analysis for the RR-cycle. The process is illustrated on the specification development and privacy analysis of the mobile Corona-virus warning app

Formalizing statecharts using hierarchical automata

We formalize in Isabelle/HOL the abtract syntax and a synchronous step semantics for the specification language Statecharts. The formalization is based on Hierarchical Automata which allow a structural decomposition of Statecharts into Sequential Automata. To support the composition of Statecharts, we introduce calculating operators to construct a Hierarchical Automaton in a stepwise manner. Furthermore, we present a complete semantics of Statecharts including a theory of data spaces, which enables the modelling of racing effects. We also adapt CTL for Statecharts to build a bridge for future combinations with model checking. However the main motivation of this work is to provide a sound and complete basis for reasoning on Statecharts. As a central meta theorem we prove that the well-formedness of a Statechart is preserved by the semantics. (Abstract as appears of publisher website