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Marcus Didius Falco (
Sun, 31 Oct 2004 21:28:21 -0500

Printer forensics
Band aid
From The Economist print edition

How to beat digital forgers

A paper trail

NO DOCUMENT is safe any more. Counterfeiting, once the domain of
skilled crooks who used expensive engraving and printing equipment,
has gone mainstream since the price of desktop-publishing systems has
dropped. Virtually any kind of paper can be forged, including
cheques, banknotes, stock and bond certificates, passports and
security cards. For currency alone, millions of dollars in counterfeit
banknotes make their way into circulation each year, and 40% of the
counterfeits seized this year were digitally produced, compared with
1% a decade ago.

In ancient times, counterfeiting was a hanging offence. In Dante's
Inferno, forgers were placed in one of the lowest circles of
hell. Today, desktop counterfeiters have little reason to worry about
prison, at any rate, because the systems they use are ubiquitous and
there is no means of tracing forged documents to the machine that
produced them. This, however, may soon change thanks to technology
developed by George Chiu, Jan Allebach and Edward Delp, three
anti-counterfeiting engineers based at Purdue University in
Indiana. The results of their research will be unveiled formally on
November 5th at the International Conference on Digital Printing
Technologies in Salt Lake City.

Though the approaches of the three researchers differ slightly, all
are based on detecting imperfections in the print quality of
documents. Old-school forensic scientists were at least so the movies
would have you believe able to trace documents to particular
typewriters based on quirks of the individual keys. The researchers
from Purdue employ a similar approach, exploiting the fact that the
rotating drums and mirrors inside a printer are imperfect pieces of
engineering which leave unique patterns of banding in their products.

Although these patterns are invisible to the naked eye, they can be
detected and analysed by computer programs, and it is these that the
three researchers have spent the past year devising. So far, they
cannot trace individual printers, but they can tell pretty reliably
which make and model of printer was used to create a document.

That, however, is only the beginning. While it remains to be seen
whether it will be possible to trace a counterfeit document back to
its guilty creator on the basis of manufacturing imperfections, Dr
Chiu is now working out ways to make those imperfections
deliberate. He wants to modify the printing process so that unique,
invisible signatures can be incorporated into each machine
produced. That would make any document traceable.

Ironically, it was after years of collaborating with printing
companies to reduce banding and thus increase the quality of prints,
that he came up with the idea of introducing artificial bandings that
could encode identification information, such as a printer's serial
number and the date of printing, into a document. Many factors can
affect banding patterns. These include the intensity, timing and
width of the pulses of laser light that control the printing process,
and the efficiency of the motor controls that steer the laser beam,
turn the drum and move the mirrors. All of these could be exploited to
produce unique signatures, but Dr Chiu found that the one which works
best without compromising print quality is to fiddle with the
intensity of the laser. Using a computer model of the human visual
system, he has designed a method of banding that is invisible to the
eye while remaining all too visible to an expert with the right

The current techniques used to secure documents are either digital
(and therefore easy to fake with desktop publishing systems) or too
costly for widespread applications (paper watermarks, fibres and
special inks). Using the banding patterns of printers to secure
documents would be both cheap to implement and hard, if not
impossible, for those without specialist knowledge and hardware to

Not surprisingly, the American Secret Service is monitoring the
progress of this research very closely, and is providing guidelines to
help the researchers to travel in what the service thinks is the right
direction. Which is fine for catching criminals. But how the
legitimate users of printers will react to Big Brother being able to
track any document back to its source remains to be seen.

Copyright 2004 The Economist Newspaper and The Economist Group. All
rights reserved.

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