The development of secure and usable systems
"People are the weakest link in the security chain"---Bruce Schneier. The aim of the thesis is to investigate the process of designing secure systems, and how designers can ensure that security mechanisms are usable and effective in practice. The research perspective is one of security as a socio-technical system. A review of the literature of security design and Human Computer Interactions in Security (HCISec) reveals that most security design methods adopt either an organisational approach, or a technical focus. And whilst HCISec has identified the need to improve usability in computer security, most of the current research in this area is addressing the issue by improving user interfaces to security tools. Whilst this should help to reduce users' errors and workload, this approach does not address problems which arise from the difficulty of reconciling technical requirements and human factors. To date, little research has been applied to socio-technical approaches to secure system design methods. Both identifying successful socio-technical design approaches and gaining a better understanding of the issues surrounding their application is required to address this gap. Appropriate and Effective Guidance for Information Security (AEGIS) is a socio-technical secure system development methodology developed for this purpose. It takes a risk-based approach to security design and focuses on recreating the contextual information surrounding the system in order to better inform security decisions, with the aim of making these decisions better suited to users' needs. AEGIS uses a graphical notation defined in the UML Meta-Object Facility to provide designers with a familiar and well- supported means of building models. Grid applications were selected as the area in which to apply and validate AEGIS. Using the research methodology Action Research, AEGIS was applied to a total of four Grid case studies. This allowed in the first instance the evaluation and refinement of AEGIS on real- world systems. Through the use of the qualitative data analysis methodology Grounded Theory, the design session transcripts gathered from the Action Research application of AEGIS were then further analysed. The resulting analysis identified important factors affecting the design process - separated into categories of responsibility, motivation, stakeholders and communication. These categories were then assembled into a model informing the factors and issues that affect socio-technical secure system design. This model therefore provides a key theoretical insight into real-world issues and is a useful foundation for improving current practice and future socio-technical secure system design methodologies.