Paper 2003/078

Relation among simulator-based and comparison-based definitions of semantic security

Yodai Watanabe and Junji Shikata

Abstract

This paper studies the relation among simulator-based and comparison-based definitions of semantic security. The definitions are considered in a more general framework than the ordinal one; namely, an adversary is assumed to have access to prior information of a plaintext. If the framework is restricted to the ordinal one, then all the security notions considered in this paper, including indistinguishability, are shown to be equivalent. On the other hand, the equivalence is not necessarily valid in the general framework. In fact, it is shown that no encryption scheme is secure in the sense of comparison-based semantic security in the strongest forms. Furthermore, a sufficient condition for the equivalence between semantic security and indistinguishability is derived.