Digital Forensics MICC LCI

1. Distinguishing between camera and scanned images by means of frequency analysis Roberto Caldelli, Irene Amerini and Francesco Picchioni {roberto.caldelli, irene.amerini, francesco.picchioni}@unifi.it 20 January 2009

4. Camera vs Scanner- Acquisition Process Digital Camera CFA and demosaicking Bidimensional sensor array Flat-bed Scanner Tri-linear color filter array: no demosaicing Mono dimensional sensor array

6. M N scanning direction PRNU characterization (1/2) De-noised image-DWT Because of the scanner sensor, it is expected that: All the rows are equal, at least ideally!

7. This is true only ideally, being S corrupted its periodical structure is altered, but most of the energy is located in such spikes!! PRNU characterization (2/2) Consequently, the spectrum of the periodical signal S will be made by spikes equispaced of (NxM)/M=N. By concatenating all the rows in a single signal S composed by NxM samples: Periodical of period M and contains N repetitions

9. Proposed methodology (2/3) Digital Camera Scanner (scanning direction row) Ideal M N/L Then, as explained before, two mono-dimensional signals S r and S c are constructed by tailing all the rows and all the columns.

16. Distinguishing between camera and scanned images by means of frequency analysis Roberto Caldelli, Irene Amerini and Francesco Picchioni {roberto.caldelli, irene.amerini, francesco.picchioni}@unifi.it 20 January 2009