You may not know this, but color printers embed in printed coded patterns that contain the printer’s serial number and the date and time the documents were printed. Inside the documents the patterns are made up of dots, they are less than a millimeter in diameter and shade of yellow that when placed on a white background, cannot be detected by the naked eye.
When NSA contractor Reality Winner printed out top secret documents and allegedly stole and leaked the classified documents with The Intercept, she couldn’t have known that hidden within the documents were tracking dots, a forensic footprint that could have lead authorities straight back to her.
After Reality Winner leaked the documents, the news reporting agency contacted the NSA and told them they were in possession of classified documents. The news agency then sent the documents over for verification. After the NSA examined the records, they looked to see who had printed the documents and determined that six people had, discovered Winner had, and that she was the one who had been in contact with the media.
According to Electronic Frontier Foundation, it has been suggested, “that all major manufacturers of color laser printers entered a secret agreement with governments to ensure that the output of those printers is forensically traceable.” The above website includes a lengthy list of printers that may contain the invisible yellow dots.