ABSTRACT
Steganography is the art and science of communicating in such a way that the presence of a
message cannot be detected. It belongs to the field of information hiding, which has received
considerable attention recently. One may distinguish two general directions in information hiding, determined by the power of an adversary: protection only against the detection of a message by a passive adversary and hiding a message such that not even an active adversary can remove it.
An information-theoretic model for steganography with a passive adversary is proposed.
The adversary's task of distinguishing between an innocent cover message C and a modified message S containing a hidden information is interpreted as a hypothesis testing problem.The security of a steganographic system is quantified in terms of the relative entropy (or discrimination) between PC and PS, which gives quantitative bounds on the detection capability of any adversary. 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 that needs no knowledge of the covertext distribution.
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