On the Unicity Distance of Stego Key

— Steganography is about how to send secret message covertly. And the purpose of steganalysis is to not only detect the existence of the hidden message but also extract it. So far there have been many reliable detecting methods on various steganographic algorithms, while there are few approaches that can extract the hidden information. In this paper, the difficulty of extracting hidden information, which is essentially a kind of privacy, is analyzed with information-theoretic method in the terms of unicity distance of steganographic key (abbreviated stego key). A lower bound for the unicity distance is obtained, which shows the relations between key rate, message rate, hiding capacity and difficulty of extraction. Furthermore the extracting attack to steganography is viewed as a special kind of cryptanalysis, and an effective method on recovering the stego key of popular LSB replacing steganography in spatial images is presented by combining the detecting technique of steganalysis and correlation attack of cryptanalysis together. The analysis for this method and experimental results on steganographic software " Hide and Seek 4.1 " are both accordant with the information-theoretic conclusion.