Hippo Manchester
November 3, 2005

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Techie: Big Brother is printing

Decode the secret watermark on your color laser printouts

By John “jaQ” Andrews  jandrews@hippopress.com

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 Andrew Huang.

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 configuration setting.

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.