Based on a concise domain analysis we develop a formal semantics of security protocols. Its main virtue is that it is a generic model, in the sense that it is parameterized over e.g. the intruder model. Further characteristics of the model are a straightforward handling of parallel execution of multiple protocols, locality of security claims, the binding of local constants to role instances, and explicitly defined initial intruder knowledge. We validate our framework by analysing the Needham-Schroeder-Lowe protocol.