Biblio

In this paper, we propose a method for normalization of rich feature sets to improve detection accuracy of simple classifiers in steganalysis. It consists of two steps: 1) replacing random subsets of empirical joint probability mass functions (co-occurrences) by their conditional probabilities and 2) applying a non-linear normalization to each element of the feature vector by forcing its marginal distribution over covers to be uniform. We call the first step random conditioning and the second step feature uniformization. When applied to maxSRMd2 features in combination with simple classifiers, we observe a gain in detection accuracy across all tested stego algorithms and payloads. For better insight, we investigate the gain for two image formats. The proposed normalization has a very low computational complexity and does not require any feedback from the stego class.

Explicit non-linear transformations of existing steganalysis features are shown to boost their ability to detect steganography in combination with existing simple classifiers, such as the FLD-ensemble. The non-linear transformations are learned from a small number of cover features using Nyström approximation on pilot vectors obtained with kernelized PCA. The best performance is achieved with the exponential form of the Hellinger kernel, which improves the detection accuracy by up to 2-3% for spatial-domain contentadaptive steganography. Since the non-linear map depends only on the cover source and its learning has a low computational complexity, the proposed approach is a practical and low cost method for boosting the accuracy of existing detectors built as binary classifiers. The map can also be used to significantly reduce the feature dimensionality (by up to factor of ten) without performance loss with respect to the non-transformed features.