In this paper, we focus on distributed systems sub ject to security issues. Such systems are usually composed of two entities: a high level user and a low level user that can both do some actions. The security properties we consider are non-interference properties. A system is non-interferent if the low level user cannot deduce any information by playing its low level actions. Various notions of non-interference have been defined in the literature, and in this paper we focus on two of them: one trace-based property (SNNI) and another bisimulation-based property (BSNNI). For these properties we study the problems of synthesis of a high level user so that the system is non-interferent. We prove that a most permissive high level user can be computed when one exists.