Revisiting Perturbed Quantization

In this work, we revisit Perturbed Quantization steganography with modern tools available to the steganographer today, including near-optimal ternary coding and content-adaptive embedding with side-information. In PQ, side-information in the form of rounding errors is manufactured by recompressing a JPEG image with a judiciously selected quality factor. This side-information, however, cannot be used in the same fashion as in conventional side-informed schemes nowadays as this leads to highly detectable embedding. As a remedy, we utilize the steganographic Fisher information to allocate the payload among DCT modes. In particular, we show that the embedding should not be constrained to contributing coefficients only as in the original PQ but should be expanded to the so-called "contributing DCT modes." This approach is extended to color images by slightly modifying the SI-UNIWARD algorithm. Using the best detectors currently available, it is shown that by manufacturing side information with double compression, one can embed the same amount of information into the doubly-compressed cover image with a significantly better security than applying J-UNIWARD directly in the single-compressed image. At the end of the paper, we show that double compression with the same quality makes side-informed steganography extremely detectable and should be avoided.