Ecien t Construction of the Dual Span Program

We consider monotone span programs as a tool for representing, we will say computing, general access structures. It is known that if an access structure Γ is computed by a monotone span program M, then the dual access structure Γ∗ is computed by a monotone span program M∗ of the same size. We will strengthen this result by proving that such an M∗ not only exists, but can be efficiently computed from M.