Steganography Theories And Methods Of Information Extraction

Steganography is about how to send secret message covertly by embedding it into some innocence digital multimedia. Nowadays, steganographic softwares are popular on the Internet, which can be downloaded easily by terrorists or secret service spies to hatch plots. Therefore the techniques of detecting and extracting hidden information are starved for by the department of national security to surveil and acquire secret information on networks. The attacks on steganography is referred as to steganalysis, study on which mainly concerns about detecting the existence of hidden information. In fact, steganography and cryptography have combined to build a new kind of secret communication form such as "encryption+hiding", which means cryptanalysits have to extract the secret message before they do the traditional cryptanalysis. However, there is hardly any literature about extracting attack on steganography. The present paper point out that extracting attack, which is essentially a special kind of cryptanalysis, belongs to the crossed field of cryptanalysis and steganalysis, and can be realized by combining the thoughts and techniques of the two fields. Based on this point of view, we did groping research on the theory and methods of extracting attack, and got the following results.1. The information-theoretic model of stegosystem is studied. And the relations between all kinds of security and relation between security and hiding capacity are analyzed. Particularly, 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. A lower bound for the unicity distance is obtained, which shows the relations between key rate, capacity redundancy and difficulty of extraction. And some general idea about the extracting attack on steganography can be gained from this theoretic conclusion.2. Extracting attack methods on popular "LSB steganography" are researched. We study this problem in the conditions of "known cover", "cover being repeatedly used" and "stego-only" respectively. And under the last condition an effective method on recovering the stego key of LSB replacing steganography is presented by combining the detecting technique of steganalysis and correlation attack of cryptanalysis together. With this method, we make a successful extracting attack on steganographic software...