Techie: Big Brother is printing
Decode the secret watermark on your color laser printouts
John “jaQ” Andrews
Your color laser printer may be spying on you.
Not so much for its own purposes. About a year ago, PC World magazine
reported that the US government had convinced several printer and copier
manufacturers to include tiny yellow dots on every printout. The exact
purpose of those dots has never been clear, but it was assumed that at
least the printer’s serial number was encoded in the pattern. PC World
confirmed that assumption with Peter Crean, a senior research fellow at
Xerox. Less expensive inkjet and black & white laser printers do not
seem to be affected.
This October, though, the Electronic Frontier Foundation (EFF) published
a way to decode those dots on printouts from Xerox DocuColor printers.
It turns out that in addition to the serial number, at least this line
of printers also encodes the date and time of printing, along with some
mystery information they couldn’t figure out. The research grunt work
was done by Robert Lee, Seth Schoen, Patrick Murphy, Joel Alwen and
Ostensibly, this watermarking is meant to deter counterfeiters.
Increasingly sophisticated color laser printers and copiers have for
years been able to convincingly reproduce not only cash, but other
tender such as stock certificates or savings bonds.
As the EFF points out, however, any technology that can track criminals
can also be used to track law-abiding citizens. Should the government
wish to know who printed a particular document, it can decode the dots
and find out precisely what printer was used — and when. The printer
manufacturer can then tell who registered that particular serial number.
The system shows its weakness if the printer is shared or sold or given
to someone else after registration. Still, the trail is there, and while
it might not lead in a direct line to a document’s author, it certainly
provides a few vital links in the chain that can be followed.
Not content with having the information themselves, the EFF published
not only a guide to decoding the dots on Xerox DocuColor models, but
also a browser-based program that anyone can use to decipher the dots on
their own printouts. Simply click to fill in which dots are printed on a
grid and voila, your printer’s secret code is revealed.
The code apparently works by assigning each row a binary value — 1, 2,
4, 8, 16, 32 or 64. Each column represents a different piece of
information, such as the date, time, or serial number digit. One row and
one column each provide a “parity bit” — a dot that is printed only if
the rest of the row or column has an even number of dots. Anyone
decoding the pattern can check that every row and column has an odd
number of dots; if not, the pattern is flawed and may be fake.
There’s one column of dots that the intrepid researchers were unable to
decode. It was always the same on every printout from a given printer
but was not part of the serial number, so they theorized it was some
Actually seeing the dots can be a challenge. The pattern is tiny and
yellow, so doesn’t show up well under normal light. The EFF suggests
using blue light and/or a microscope or magnifying glass to spy the
pattern, which is repeated several times per page.
The EFF’s information page is at http://www.eff.org/Privacy/printers/.
Linked pages include the decoder as well as a list of common color laser
printers that do or do not watermark printed documents.