Abstract. The security of any computer system that is configured or operated by human beings critically depends on the information conveyed by the user interface, the decisions of the users, and the interpretation of their actions. This paper establishes some starting points for reasoning about security from a user-centred point of view: it proposes to model systems in terms of actors and actions, and introduces the concept of the subjective actor-ability state. Ten key principles for secure interaction design are identified; case studies illustrate and justify the principles, describing real-world problems and possible solutions. It is hoped that this work will help guide the design and evaluation of secure systems.