4 Agence nationale de la scurit des systmes d'information (ANSSI) Abstract. In this paper, we revisit the definition of Transparency Order (TO) from the work of Prouff (FSE 2005) that was proposed to measure the resistance of an s-box against Differential Power Analysis. We find that the definition has certain limitations. Although this work has been quite well referred in the literature, surprisingly, these limitations remained unexplored for almost a decade. We analyze the definition from scratch, modify it and finally provide a revised definition. Our simulation results confirm that the transparency order is indeed related to the resistance of the s-box against side-channel attacks. Thus (revised) TO is one of the valuable criteria to consider when designing a cryptographic algorithm.