  Given a graph G = (V,E), a subset S of V is dominating if for every v in V -
S there exists u in S such that uv is in E. A dominating subset S of V is
secure if for every v in V - S there exists u in S such that (S - {u}) U {v} is
dominating. In this work we extend the concept of secure dominating set to
digraphs in four different ways, all of them with interesting applications, and
prove some results regarding each of them.
