In this paper, we focus on the synthesis of secure timed systems which are given by timed automata. The security property that the system must satisfy is a \emphnon-interference property. Various notions of non-interference have been defined in the literature, and in this paper we focus on \emphStrong Non-deterministic Non-Interference (SNNI) and we study the two following problems: ($1$) check whether it is possible to enforce a system to be SNNI; if yes ($2$) compute a sub-system which is SNNI.