WaPo: How tech sleuths cracked the mysterious code that turns your printer into a spying tool

WaPo: How tech sleuths cracked the mysterious code that turns your printer into a spying tool by Derek Hawkins:

You wouldn’t have noticed it unless you knew where — and how — to look, but the top-secret National Security Agency document leaked to the Intercept and published Monday contained a clue that may have led authorities to its source.

Spread throughout the pages were barely visible yellow dots, each less than a millimeter in diameter, repeated over and over in the same rectangular pattern. You could see them by zooming in on the pages and adjusting the color. Or, if you had the original printed papers, you could have inspected them with a magnifying glass and a blue LED light.

They’re called tracking dots or microdots. Nearly every color printer on the market is equipped with a feature that covertly prints them. They encode any page that comes out of a printer with a serial number, date and time that can be interpreted using a simple cipher. Printer manufacturers are not required to tell customers the feature exists.