For critical software applications, dependability and safety are required features that should respect security principles. To cope with these constraints, the design activity should use new methods that foster knowledge sharing and reuse, in particular security problems and their solutions. In this paper, we present a problem-oriented method that follows a step-wise building of the solution. Problems are reused using various mechanisms, and a solution is conceived, verified and diagnosed. We illustrate the approach on the problem of building a secure SCADA architecture.