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In this case, the FBI has not said publicly that these microdots were used to help identify their suspect, and the bureau declined to comment for this article. The US Department of Justice, which published news of the charges against Winner, also declined to provide further clarification.

In a statement, The Intercept said, “Winner faces allegations that have not been proven. The same is true of the FBI’s claims about how it came to arrest Winner.”

But the presence of microdots on what is now a high-profile document (against the NSA’s wishes) has sparked great interest.

Based on their positions when plotted against a grid, they denote specific hours, minutes, dates and numbers
“Zooming in on the document, they were pretty obvious,” says Ted Han at cataloguing platform Document Cloud, who was one of the first to notice them. “It is interesting and notable that this stuff is out there.”

Another observer was security researcher Rob Graham, who published a blog post explaining how to identify and decode the dots. Based on their positions when plotted against a grid, they denote specific hours, minutes, dates and numbers. Several security experts who decoded the dots came up with the same print time and date.

Microdots have existed for many years. The Electronic Frontier Foundation (EFF) maintains a list of colour printers known to use them. The images below, captured by the EFF, demonstrate how to decode them...