Effect of JPEG quality on steganographic security

Abstract

This work investigates both theoretically and experimentally the security of JPEG steganography as a function of the quality factor. For a fixed relative payload, modern embedding schemes, such as J-UNIWARD and UED-JC, exhibit surprising non-monotone trends due to rounding and clipping of quantization steps. Their security generally increases with increasing quality factor but starts decreasing for qualities above 95. In contrast, old-fashion steganography, such as Jsteg, OutGuess, and model-based steganography, exhibit complementary trends. The results of empirical detectors closely match the trends exhibited by the KL divergence computed between models of cover and stego DCT modes. In particular, our analysis shows that the main reason for the complementary trends is the way modern schemes attenuate embedding change rates with increasing spatial frequency. Our model also provides guidance on how to adjust the embedding algorithm J-UNIWARD to substantially improve its security for high quality factors.