Realizing Security Requirements with Physical Properties: A Case Study on Paper Voting

Abstract

Well-established security models exist for testing and proving the logical security of IT systems. For example, we can assert the strength of cryptographic protocols and hash functions that prevent attackers from unauthorized changes of data. By contrast, security models for physical security have received far less attention. This situation is problematic, especially because IT systems are converging with physical systems, as is the case when SCADA systems are controlling industrial processes, or digital door locks in apartment buildings are replacing physical keys. In such cases, it is necessary to understand the strengths, weaknesses and combinations of physical and digital security mechanisms. To realize this goal, we must first learn how security requirements are realized by the physical environment alone and this paper presents a method for analyzing this, based on the KAOS requirements engineering framework. We demonstrate our method on a security-critical case, namely an election process with paper ballots. Our analysis yields a simple ontology of physical objects used in this process, and their security-relevant properties such as visibility, inertness and spatial architecture. We conclude with a discussion of how our results can be applied to analyze and improve the security in other processes and perform trade-off analysis, ultimately contributing to models in which physical and logical security can be analyzed together.