Printer steganography is a type of steganography – "hiding data within data" where tiny yellow dots are added to each page. The dots are barely visible and contain encoded printer serial numbers and timestamps. Unlike many forms of steganography, the hidden information is not intended to be available from a computer file, but to allow serial number and time of printing to be determined by close examination of a printout.
The Electronic Frontier Foundation stated in 2015 that "the documents that we previously received through a (Freedom of Information Request) 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....it is probably safest to assume that all modern color laser printers do include some form of tracking information that associates documents with the printer's serial number."
During the 1990s Xerox and other companies sought to reassure governments that their printers would not be used for forgery. The identification is by means of a watermark, often using yellow-on-white, embedded in the printout of each page, and in conjunction with other information can be used to identify the printer which was used to print any document originally produced on a wide range of popular printers. It has been reported that monochrome printers and copiers from major manufacturers also include the markings. It may be actual text, or a repeated pattern of dots throughout the page, more easily visible under blue light or with a magnifying glass, and is intended to produce minimal visible change to the printout, ideally being imperceptible to the naked eye.
In 2005, the Electronic Frontier Foundation cracked the codes for Xerox DocuColor printers and published an online guide to their detection. Most printers' codes have not been decoded, although the coding system framework and printer serial number encoding is the same on both DocuColor and the Epson Aculaser C1100/C1100N/A.
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