Stop That Copy

In the age of the photocopier, theft of copyrighted material has always been
vexing. With the infobahn, transmitting purloined papers to a zillion
recipients is a trivial matter, so information providers are leery of jacking in.
Now, researchers at AT&T think they have a solution to a large part of the
problem. 

Many documents start life as files in a page description language such as
PostScript. The trick is to encode each document with a traceable serial
number. You might be less willing to pass something along to freeloaders if
you know there is a list of serial numbers with your name on it. 

The encoding scheme developed at Bell Labs turns out to be surprisingly
simple and robust. Jack Brassil and his co-workers recognized that slight
variations in text-line spacing, letter size or shape, or other design elements
can go virtually unnoticed and yet carry useful information. For example,
identifying codes can be inserted into a document by shifting each word left
or right by a tiny amount (.03 of an inch will do) in a systematic way. When
such a document is printed out, few readers will be able to tell that the
printed matter has been encoded. Even running it through a photocopier or
a fax machine several times won't eliminate the telltale tag from the text. 

Sure, countermeasures might be imagined, but most require more time and
effort to implement than just getting the document legitimately. The AT&T
scheme won't work on straight ASCII text, which contains no subtle layout
info, but the use of page description languages is growing. Nor will it work
well on documents with abundant graphics and meager text. But in most
cases it works just fine. 

Like the surveillance camera in a department store, which may or may not
be actually wired up, the electronic serial number discourages illicit copying
by just being there. Brassil won't be specific, but says that a number of
organizations have expressed interest in the marking technique. "Electronic
Marking and Identification Techniques to Discourage Document Copying,"
by J. Brassil, S. Low, N. Maxemchuk, and L. O'Gorman is available from 
http://www.research.att.com/ or via ftp from research.att.com. It's a
PostScript file -- and No, says Brassil, it's not marked. 

- David Voss 


Wired 2.08