October 12, 2004
Printer forensics to aid homeland security, tracing counterfeiters
WEST LAFAYETTE, Ind. Researchers at Purdue University have developed a method that will enable authorities to trace documents to specific printers,
The technique uses two methods to trace a document: first, by analyzing a document to identify characteristics that are unique for each printer, and second by designing printers to purposely embed individualized characteristics in documents.
The technique currently focuses on laser printers but eventually will be expanded to inkjet printers, said Edward J. Delp, a professor of electrical and computer engineering at Purdue.
Findings of the research, funded by the National Science Foundation, will be detailed in three papers to be presented on Nov. 5 during the International Conference on Digital Printing Technologies in Salt Lake City. The papers were written by Delp; Jan Allebach, a professor of electrical and computer engineering; George Chiu, a professor of mechanical engineering; and engineering doctoral students Pei-Ju Chiang, Gazi N. Ali and Aravind K. Mikkilineni.
Counterfeiters often digitally scan currency and then use color laser and inkjet printers to produce bogus bills. Forgers use the same methods to make fake passports and other documents.
"Investigators want to be able to determine that a fake bill or document was created on a certain brand and model of printer," Delp said.
So far, the researchers have been able to successfully identify which model of printers was used to create certain documents in 11 out of 12 models tested, according to data to be released during the conference.
"We also believe that we will be able to identify not only which model printer was used but specifically which printer was used," Delp said. "That means we will be able to tell the difference between counterfeit bills created on specific printers even if they are the same model."
Officials also would be able to use the method to determine the authenticity of documents, such as airline boarding passes and passports.
Such information would enable homeland security investigators to determine from what country or regions of the world certain printed documents originated, which could help trace the location of potential terrorists and their collaborators.
The technique uses specialized software to detect slight variations, or "intrinsic signatures," of printed characters, revealing subtle differences from one printer to another. Even printers that are the same model have slight flaws and variations in their mechanical systems. These variations result in subtly different characters.
"We have observed variability from printer to printer within a single model, " Allebach said. "Thats because for a company to make printers all behave exactly the same way would require tightening the manufacturing tolerances to the point where each printer would be too expensive for consumers.
"This is a very competitive market right now. You can buy a color laser printer for less than $500. One of the ways in which manufacturers are able to make printers so affordable is by cutting corners. The gears are made of plastic, and they are not made extremely accurately. There also is variability from toner cartridge to toner cartridge.
"We are able to determine this intrinsic signature based on knowledge of the physical characteristics of the printer mechanisms."
Allebach and Chiu have been working with printer companies for more than five years to reduce a phenomenon called "banding," which are horizontal imperfections in the print quality of documents.
"Banding arises whenever you have a print mechanism that uses rotating components," Chiu said. "What happens is the components dont necessarily rotate at an exactly constant speed."
Inside of a laser printers cartridge is a "photoconductor drum," which rotates as a laser beam scans back and forth along the drum. The drum is coated with a charged material that releases its charge when exposed to light. The laser turns on and off rapidly, selectively removing the charge in certain areas. Toner is attracted to those areas that no longer have a charge, forming letters or features of an image, which are then transferred onto sheets of paper.
"This process is called development," Allebach said. "Because of variability in printers, the drum does not rotate at a constant speed. If the drum slows down a little bit as it is rotating, you get excessive development, so the print will look a little dark. And where the drum speeds up, you get too little development and the print will look a little bit light."
The resulting bands of light and dark cause imperfections in a text document or an image. Because every printer has its own unique pattern of banding, or intrinsic signature, the imperfections can be exploited to trace a document to the printer on which it was created, Chiu said.
"We extract mathematical features, or measurements, from printed letters, then we use image analysis and pattern-recognition techniques to identify the printer," Delp said.
If, however, the printer cartridge is changed after a document is printed, the document no longer can be traced to that printer.
The Purdue researchers are overcoming that problem with software that causes a printer to embed its own unique "extrinsic signature" in a printed document, regardless of which printer cartridge is in a machine.
"We will actually modify the way the printer puts marks on the paper," Chiu said. "This method is very difficult to get around because information about the internal workings of specific printers is not commonly available, even on the Internet."
Banding can be altered from one printer to another by adjusting the laser intensity, how long each laser pulse lasts and the precise positioning of a small motor that steers the laser beam inside the printer.
Chiu and Allebach have pioneered methods to reduce banding. The same methods they have developed to reduce banding also can be used to artificially embed bands that are too fine to be seen by the unaided eye but can be detected with image-analysis techniques.
"We need to understand the human visual threshold for the signatures we put in so that the features are strong enough to be detected with image analysis methods but not by the human eye," Chiu said.
The Purdue researchers are working with the U.S. Secret Service to develop new methods for tracing documents and counterfeit bills.
Writer: Emil Venere, (765) 494-4709, firstname.lastname@example.org
Sources: Edward J. Delp, (765) 494-1740, email@example.com
Jan P. Allebach, (765) 494-3535, firstname.lastname@example.org
George Chiu, (765) 494-2688, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
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Note to Journalists: An electronic or hard copy of the research papers is available from Emil Venere, (765) 494-4709, email@example.com.
A publication-quality photograph is available at https://ftp.purdue.edu/pub/uns/+2004/delp-forensics.jpg
Extrinsic Signatures Embedding Using Exposure Modulation for Information Hiding and Secure Printing in Electrophotography
1School of Electrical and Computer Engineering
Banding is one of the image artifacts for electrophotographic (EP) printers. Due to its origin within the EP process, it can also be viewed as an intrinsic signature of the specific printer. Modulating the EP process to generate banding signals that are below the human visual threshold but can be detected by effective detection approach can further extend the signature capacity. This deliberate banding signal can be viewed as the extrinsic signature of printer. Since modulating the printing process provides a significant barrier to entry, it is effective in limiting the attack possibilities and can be an effective method to embed needed information regarding the document and the device it was printed on to provide unprecedented forensic information. One of the key issues with embedding extrinsic signature is the information should not be detectable by the human observer. However, the signature needs to be detectable by a suitable detection algorithm. In this paper, we will propose one method to modulate the exposure of the EP process to embed controlled quasi-periodic signature into a document. Based on the system-level modeling and experimental verification of the EP process, machine detectable signatures that are below the human visual threshold can be embedded into a printed document without any perceivable degradation in image quality. Preliminary experiment results confirm our hypothesis that with properly designed control techniques, effective extrinsic signatures can be embedded into a document.
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