Robust Encryption, Revisited

We revisit the notions of robustness introduced by Abdalla, Bellare, and Neven (TCC 2010). One of the main motivations for the introduction of strong robustness for public-key encryption (PKE) by Abdalla et al. to prevent certain types of attack on Sako\u27s auction protocol. We show, perhaps surprisingly, that Sako\u27s protocol is still vulnerable to attacks exploiting robustness problems in the underlying PKE scheme, even when it is instantiated with a \emph{strongly} robust scheme. This demonstrates that current notions of robustness are insufficient even for one of its most natural applications. To address this and other limitations in existing notions, we introduce a series of new robustness notions for PKE and explore their relationships. In particular, we introduce \emph{complete} robustness, our strongest new notion of robustness, and give a number of constructions for completely robust PKE schemes