How EFF cracked printers' "hidden dots" code in 2005

NSA whistleblower Reality Winner may have been caught thanks to a hidden pattern of dots that color printers bury in every page they print, as an assistance to law enforcement agencies.

The existence of these dots was long-rumored, but it wasn't until 2005, when the Electronic Frontier Foundation broke the code on a Xerox printer's output, that the scope, method and capabilities of this technique came to light.

The Washington Post tells the exciting story of that technological forensic work, which still ripples through today.

Then, in October 2005, about a year after PC World's story, a research team led by the Electronic Frontier Foundation cracked the code for one printer, the Xerox DocuColor. A technologist from the advocacy group, along with an intern and two volunteers, compared the dots on a series of test printouts submitted by supporters. They quickly deduced how to read the pattern.

As expected, the dots indicated the datge and time a page was printed, as well as the printer's serial number. The foundation then installed a decoder program on its website that the public could use, and started publishing running lists of printers that did or did not display the dots.


How tech sleuths cracked the mysterious code that turns your printer into a spying tool
[Derek Hawkins/Washington Post]