Countering JPEG anti-forensics
JPEG coding leaves characteristic footprints that can be leveraged to reveal doctored images, e.g. providing the evidence for local tampering, copy-move forgery, etc. Recently, it has been shown that a knowledgeable attacker might attempt to remove such footprints by adding a suitable anti-forensic dithering signal to the image in the DCT domain. Such noise-like signal restores the distribution of the DCT coefficients of the original picture, at the cost of affecting image quality. In this paper we show that it is possible to detect this kind of attack by measuring the noisiness of images obtained by re-compressing the forged image at different quality factors. When tested on a large set of images, our method was able to correctly detect forged images in 97% of the cases. In addition, the original quality factor could be accurately estimated.