Printer identification based on printed documents can provide forensic information to protect copyright and verify authenticity. In addition to intrinsic features (intrinsic signatures) of the printer, modulating the printing process to embed specific features (extrinsic signatures) will further extend the encoding capacity and decoding accuracy. One of the key issues with embedding extrinsic signatures is that the embedding should not degrade the image quality, but needs to be detectable by a detection algorithm. In this paper, we will demonstrate the feasibility of embedding code sequences in electrophotographic halftone images by modulating dot size through laser intensity modulation. We have developed corresponding embedding and detection algorithms to embed and extract information. Experimental results indicate that using a 600 dpi native resolution printer's default halftone algorithm, we can encode 5 bits of information in every 310 printer scan-lines or approximately every 0.5 inches.

Forensic characterization of devices is important in many situations such as establishing the trust and verifying authenticity of data and the device that created it. Current forensic identification techniques for digital cameras, scanners and printers are highly reliable due to the fact that each of these devices cannot escape inherent electro-mechanical properties which add "signatures" to the data they produce. In this paper we will describe the sensor forensics work going on at Purdue University.