The cost of JPEG compression anti-forensics

The statistical footprint left by JPEG compression can be a valuable source of information for the forensic analyst. Recently, it has been shown that a suitable anti-forensic method can be used to destroy these traces, by properly adding a noise-like signal to the quantized DCT coefficients. In this paper we analyze the cost of this technique in terms of introduced distortion and loss of image quality. We characterize the dependency of the distortion on the image statistics in the DCT domain and on the quantization step used in JPEG compression. We also evaluate the loss of quality as measured by means of a perceptual metric, showing that a perceptually-optimized version of the anti-forensic method fails to completely conceal the forgery. Our conclusion is that removing the traces of the JPEG compression history could be much more challenging than it might appear, as anti-forensic methods are bound to leave characteristic traces.