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In the forensics domain can be useful to recover image history, and in particular whether or not it has been doubly compressed. Clarify this point allows to assess if, in addition to the compression at the time of shooting, the picture was decompressed and then resaved. This is not a clear indication of forgery, but it can justify further investigations. In this paper we propose a novel technique able to retrieve the coefficients of the first compression in a double compressed JPEG image when the second compression is lighter than the first one. The proposed approach exploits the effects of successive quantizations followed by dequantization to recover the original compression parameters. Experimental results and comparisons with a state of the art method confirm the effectiveness of the proposed approach.

To assess if a digital image has been (or not) doubly compressed is a challenging issue especially in forensics domain where could be fundamental clarify if, in addition to the compression at the time of shooting, the picture was decompressed (in some way) and then resaved. This is not a clear indication of forgery, but it guarantees that the image, probably, is not the original one. In this paper we propose a novel technique able to recover the coefficients of the first compression in a double compressed JPEG image under some assumptions. The proposed approach exploits how successive quantizations followed by dequantizations introduce some regularities (e.g., sequence of zero and not zero values) on the histograms of coefficient distributions that could be analyzed to recover the original compression parameters. Experimental results and comparisons with state of the art methods confirm the effectiveness of the proposed approach.

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