Calibration revisited

Calibration was first introduced in 2002 as a new concept to attack the F5 algorithm [3]. Since then, it became an essential part of many feature-based blind and targeted steganalyzers in JPEG as well as spatial domain. The purpose of this paper is to shed more light on how, why, and when calibration works. In particular, this paper challenges the thesis that the purpose of calibration is to estimate cover image features from the stego image. We classify calibration according to its internal mechanism into several canonical examples, including the case when calibration hurts the detection performance. All examples are demonstrated on specific steganographic schemes and steganalysis features. Furthermore, we propose a modified calibration procedure that improves practical steganalysis.