A mathematical framework for steganalysis is presented in this paper with linear steganography be- ing the main focus. A mathematically formal definition of steganalysis is given. Then, active steganalysis defined as the extraction of a hidden message with little or no a priori information is formulated as blind system identi- fication problem within this framework. Conditions for identifiability (i.e., successful steganalysis) are derived. A procedure to systematically exploit any available spa- tial and temporal diversity information for efficient ste- ganalysis is also discussed. Experimental results are given for steganalysis of Gaus- sian distributed, spread spectrum image steganography and watermarking. The proposed technique is observed to produce impressive results for a variety of perfor- mance measures. Based on the results we conclude that a common belief, namely, spread spectrum steganogra- phy/watermarking is secure because of the low strength, noise-like message carrier is not valid anymore within the current context. Therefore, new questions regarding steganography security that differ from the standard in- formation theoretic notion are raised and some answers are provided.