Different security measurements for a Steganographic system, i.e. security (detectability), robustness and secrecy (difficulty of extraction), are discussed in this paper. We propose a new measurement for the security of stegosystems using variational distance which can upper bound the advantage for passive attackers. It is proved that the hiding capacity, which is also the measurement for robustness, is limited by security. We think the extracting attack essentially is a kind of crypt-analysis and define the secrecy of stegosystems as an analogue of secrecy of cryptosystems. The relations of secrecy with capacity and security are analyzed in the terms of unicity distance. And it is shown that there is a tradeoff between secrecy and capacity while there is some kind of consistency between secrecy and security.
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