Before swooping in to arrest accused leaker Reality Winner, FBI sleuths were poring over barely visible dots in the document they say led them to her, the BBC reports. The yellow "microdots" formed a coded design on the paper the 25-year-old NSA contractor allegedly provided to the Intercept that purports to detail Russian interference in the November election. The pattern revealed the serial number of the printer used, and clocked the time recorded on the unit's date stamp: 06:20 on 9 May, 2017. Known to civil libertarians and security experts, but not to most of us, most color printers add microdots every time a user hits "send," allowing an easy path to tracking the source, per the BBC. After the Intercept published the leaked doc, techies rushed to decode the dots and tweet their findings. "Zooming in on the document, they were pretty obvious," one tells the BBC.
The Electronic Frontier Foundation explains how it works, as does the blog Errata Security, which calls the subtle spyware "evil." The FBI hasn't commented on the dots, which is a form of steganography, or hidden messages. The practice of using dots for spycraft dates back to World War II, per the BBC, and today it can foil counterfeiting on currency notes, as most printers won't produce a copy when they detect a constellation symbol stamped on most banknotes. Besides symbols, white space can also encode text by placing spaces and tabs in a particular position. “Locating trailing whitespace in text is like finding a polar bear in a snowstorm," the SNOW website says, per the BBC. (Intelligence officials worry there are other young leakers.)