Persistent_BNDC (P_BNDC, for short) is an information-flow security property for processes in dynamic contexts, i.e., contexts that can be reconfigured at runtime. Intuitively, P_BNDC requires that high level interactions never interfere with the low level behavior of the system, in every possible state. P_BNDC is verified by checking whether the system interacting with a high level component is bisimilar or not to the system in isolation. In this work we contribute to the verification of information-flow security in two respects: (i) we give an unwinding condition that allows us to express P_BNDC in terms of a local property on high level actions and (ii) we exploit this local property in order to define a proof system which provides a very efficient technique for the development and the verification of P_BNDC processes.