As security is a growing concern for modern information systems, Security Requirements Engineering has been developed as a very active area of research. A large body of work deals with elicitation, modelling, analysis, and reasoning about security requirements. However, there is little evidence of efforts to align security requirements with security mechanisms. This paper extends the Secure Tropos methodology to enable a clear alignment, between security requirements and security mechanisms, and a reasoning technique to optimise the selection of security mechanisms based on these security requirements and a set of other factors. The extending Secure Tropos supports modelling and analysis of security mechanisms; defines mathematically relevant modelling concepts to support a formal analysis; and defines and solves an optimisation problem to derive optimal sets of security mechanisms. We demonstrate the applicability of our work with the aid of a case study from the health care domain.