Integration of Formal Proof into Unified Assurance Cases with Isabelle/SACM
This work addresses the problem of improving automation and confidence in certifying critical systems for safety-critical domains, though it is incremental in extending existing formal methods to assurance cases.
The paper tackled the challenge of integrating formal methods into assurance cases for critical systems by developing Isabelle/SACM, a formal machine-checked language, and demonstrated its effectiveness through a case study on the Tokeneer secure entry system, verifying security requirements and forming a modular security case.
Assurance cases are often required to certify critical systems. The use of formal methods in assurance can improve automation, increase confidence, and overcome errant reasoning. However, assurance cases can never be fully formalised, as the use of formal methods is contingent on models that are validated by informal processes. Consequently, assurance techniques should support both formal and informal artifacts, with explicated inferential links between them. In this paper, we contribute a formal machine-checked interactive language, called Isabelle/SACM, supporting the computer-assisted construction of assurance cases compliant with the OMG Structured Assurance Case Meta-Model. The use of Isabelle/SACM guarantees well-formedness, consistency, and traceability of assurance cases, and allows a tight integration of formal and informal evidence of various provenance. In particular, Isabelle brings a diverse range of automated verification techniques that can provide evidence. To validate our approach, we present a substantial case study based on the Tokeneer secure entry system benchmark. We embed its functional specification into Isabelle, verify its security requirements, and form a modular security case in Isabelle/SACM that combines the heterogeneous artifacts. We thus show that Isabelle is a suitable platform for critical systems assurance.