Steganography Steganography Theory Safety And Security Restrictions, Capacity Analysis

In this dissertation, we investigate the theoretic security of steganography and steganographic capacity under the limit of security.Steganography is an important branch of information hiding, which achieves the purpose of secret communication by hiding the existence of secret messages. So security is not only an important problem in theoretic study but also a key index in application of steganography. On the other hand, different from general communication system, steganographic capacity (viz. the maximum rate of message that carrier is able to hide) should have special request about security according to stegenography. Therefore stegangraphic capacity under the limit of security is new and valuable research subject, too. The investigation results of these problems have very important meanings to stegenography algorithms and steganalysis algorithms.With information-theoretic and hypotheses testing methods, we first study the detection of steganography which is divided into two steps: detecting secret communication and detecting stego-objects. For the first step, the convergence exponent of the minimal false alarm probability is obtained when the missing event probability is fixed. And the probability of error is analyzed for the second step. On the basis of these results, we improve Cachin's definition about security of steganography and define the strong e — security of stegosystem. On the other hand, we extend the definition in reference [7] and define the weak e — security. Furthermore, the relationship between two kinds of security measures is analyzed and it is shown that strong e - security of stegosystem implies weak ε^1/2 - security. Similarly, we investigate the security in the case of attacker obtaining some side information.Secondly, the steganographic capacity under the limit of security is studied with information-theoretic methods. The lower bound of steganographic capacity is obtained using the theory of strongly jointly typical sequences and its upper bound on a special condition is also presented. It is shown that our conclusions are same as Moulin's about capacity of information hiding systems without the limit of security and Moulin's result of steganographic capacity on the perfectly secure condition is the special case of ours. Moreover, we analyze the information transfer rate and steganographic capacity about LSB steganography algorithms, and find that there is some difference between them.