Context and motivation] Security mechanisms, such as firewalls and encryption, operationalize security requirements, such as confidentiality and integrity. [Question/problem] Although previous work has pointed out that the application of a security mechanism affects system specifications, there is no systematic approach to describe and analyze this impact. [Principal ideas/results] In this paper, we investigate more than 40 security mechanisms that are well documented in security pattern repositories in order to better understand what they are and how they function. [Contribution] Based on this study, we propose a conceptual model for security mechanisms, and evaluate this model against 20 security mechanisms. Using the conceptual model, we provide a systematic process for analyzing and enforcing security mechanisms on system requirements. We also develop a prototype tool to facilitate the application and evaluation of our approach.