Paper 2000/028

An Information-Theoretic Model for Steganography

Christian Cachin

Abstract

An information-theoretic model for steganography with passive adversaries is proposed. The adversary's task of distinguishing between an innocent cover message $C$ and a modified message $S$ containing a secret part is interpreted as a hypothesis testing problem. The security of a steganographic system is quantified in terms of the relative entropy (or discrimination) between $P_C$ and $P_S$. It is shown that secure steganographic schemes exist in this model provided the covertext distribution satisfies certain conditions. A universal stegosystem is presented in this model for which the participants do not have to know the covertext distribution except that it consists of a series of independent experiments.