A JPEG Forensic Detector for Color Bitmap Images

Identification of decompressed JPEG images, especially those compressed with high JPEG quality factors, is a challenging issue in image forensics. Furthermore, the applicability of the existing JPEG forensic detectors in forgery localization is limited by their inability to cope with spatial misalignment in the 8×8 JPEG grid. This paper proposes a novel forensic detector for decompressed JPEG color images stored in the bitmap format. The basic idea is to exploit the chroma upsampling traces introduced during the JPEG decompression of color images. We first show that the chroma upsampling alters the wide-sense stationary nature of the chroma planes and introduces wide-sense cyclostationarity in the upsampled chroma planes of decompressed JPEG images. As a result, unlike in the case of never-compressed images, the distributions of the odd-even and even-odd pixel pair differences are different for the chroma planes of never-compressed and decompressed JPEG images. We use the two-sample Kolmogorov-Smirnov (KS) test to find the similarity between these distributions and thereby to differentiate between never-compressed and decompressed JPEG images. The experimental results on a large set of images, including realistically-forged images, show the efficacy of the proposed detector even in the presence of spatial desynchronization due to image cropping.