  In this paper, we show a "direct" equivalence between certain authentication
codes and robust secret sharing schemes. It was previously known that
authentication codes and robust secret sharing schemes are closely related to
similar types of designs, but direct equivalences had not been considered in
the literature. Our new equivalences motivate the consideration of a certain
"key-substitution attack." We study this attack and analyze it in the setting
of "dual authentication codes." We also show how this viewpoint provides a nice
way to prove properties and generalizations of some known constructions.
