In this paper, we present a forensic algorithm to discriminate between original and forged regions in JPEG images, under the hypothesis that the tampered image presents a non-aligned double JPEG compression (NA-JPEG). Unlike previous approaches, the proposed algorithm does not need to manually select a suspect region to test the presence or the absence of NA-JPEG artifacts. Based on a new statistical model, the probability for each 8 × 8 DCT block to be forged is automatically derived. Experimental results, considering different forensic scenarios, demonstrate the validity of the proposed approach.
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