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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.
Steganography’s goal is to conceal the presence of a secret message within an innocuous-looking communication.In other words, steganography consists of hiding a secret hiddentext message within a public covertext to obtain a stegotext in such a way that any observer (except, of cthese, the intended recipient) is unable to distinguish between a covertext with a hiddentext and one without. The model is perhaps best illustrated by Simmons”Prisoners’ Problem”. Alice and Bob are in jail,locked up in separate cells far apart from each other, and wish to devise an escape plan. Theyare allowed to communicate by means of sending authenticated messages via trusted ctheiers,provided they do not deal with escape plans. The ctheiers are agents of the warden Eve (the adversary) and will leak all communication to her. If Eve detects any sign of conspiracy, she will ution, except that it is generated from independently repeated experiments.
This paper views steganography as information hiding with a passive adversary thwart the escape plans by transferring both prisoners to high-security cells from which nobody has ever escaped. Alice and Bob are well aware of these facts, so that before getting locked up, they have shared a few secret codewords that they are now going to exploit for adding a hidden meaning to their seemingly innocent messages. Alice and Bob succeed if they can exchange information allowing them to coordinate their escape and Eve does not become suspicious.
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