The aim of image steganalysis is to detect the presence of hidden messages in stego images. We propose a blind image steganalysis method in Contourlet domain and then show that the embedding process changes statistics of Contourlet coefficients. The suspicious image is transformed into Contourlet space, and then the statistics of Contourlet subbands coefficients are extracted as features. We use absolute Zernike moments and characteristic function moments of Contourlet subbands coefficients of the image to distinguish between the stego and non-stego images. Absolute Zernike moments are used to examine the randomness in the test image and characteristic function moments of Contourlet coefficients is used to form our feature set that can catch the changes made to the histogram of Contourlet coefficients. These features are fed to a nonlinear SVM classifier with an RBF kernel to distinguish between cover and stego images. We show that the embedding process distorts statistics of Contourlet coefficients, leading to detection of stego images. Experimental results confirm that the proposed features are highly sensitive to the change made by the embedding process. These results also reveal advantage of the proposed method over its counterpart steganalyzers, in cases of five popular JPEG steganography techniques.
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