Resumo:  We present a mathematical model  for distributed systems and decentralized peer-to-peer networks in which users have limited access to the data. A location (peer) is described with its data and processes. In order to obtain a system in which users have different access rights, we propose a RBAC (Roll Based Access Control) approach  that control data usage. Location policy (its type) allows actions of a process depending on the roles of that process. The access rights of a role (all processes that are assigned that role) at a location can be changed (increased or decreased)  only if the policy of that location allows it. We propose a type system which ensures that the specified network policy is respected during computations and that a user can access and change only data in the range of its roles.