Enlarge (credit: Ars Technica)

When reporters at The Intercept approached the National Security Agency on June 1 to confirm a document that had been anonymously leaked to the publication in May, they handed over a copy of the document to the NSA to verify its authenticity. When they did so, the Intercept team inadvertently exposed its source because the copy showed fold marks that indicated it had been printed—and it included encoded watermarking that revealed exactly when it had been printed and on what printer.

The watermarks, shown in the image above—an enhancement of the scanned document The Intercept published yesterday—were from a Xerox Docucolor printer. Many printers use this or similar schemes, printing faint yellow dots in a grid pattern on printed documents as a form of steganography, encoding metadata about the document into its hard-copy output. Researchers working with the Electronic Frontier Foundation have reverse-engineered the grid pattern employed by this class of printer; using the tool, Ars (and others, including security researcher Robert Graham) determined that the document passed to The Intercept was printed on May 9, 2017 at 6:20am from a printer with the serial number 535218 or 29535218.