Authentication is a slippery security property that has been formally defined only recently; among the recent definitions, a rather interesting one has been proposed for the spi-calculus in [1, 2]. On the other hand, in a recent paper [10], we have proved that many existing security properties can be seen uniformly as specific instances of a general scheme based on the idea of non interference. The purpose of this paper is to show that, under reasonable assumptions, also spi-authentication can be recast in this general framework, by showing that it is equivalent to the non interference property called NDC of [8, 9].