Printer tracking dots

Printer tracking dots, also called printer steganography or yellow dots, are tiny marks placed on almost every page printed by color laser printers and photocopiers. These dots act like a secret code that can tell authorities which machine made the printout. They were created by companies like Xerox and Canon in the mid-1980s as a way to help track documents, especially important or sensitive papers.

These special dots are usually yellow and very small, so they are hard to see with the naked eye. Even though they look almost invisible, they can be read by scanners or special software. Because every printer or copier has its own unique pattern of dots, experts can often figure out which device was used to print a particular page.

The existence of these tracking dots became widely known in 2004, surprising many people who thought their printed documents were completely private. Today, these dots are used around the world as a security tool, helping police and other officials investigate fraud, counterfeiting, and other crimes where figuring out who printed a document can be very important.

Understanding printer tracking dots helps us realize that many everyday objects, like office equipment, have hidden features that can be used for safety and security purposes. It also shows how technology continues to develop in ways we might not notice in our daily lives.

History

In the mid-1980s, Xerox created a way to put tiny dots on printed pages to show which printer made the document. This helped prevent people from using these printers to make fake money. Later, many other printer companies used similar methods.

In 2004, people found out about these dots when Dutch authorities used them to find people making fake money. Groups like the Electronic Frontier Foundation helped people understand how these dots work. They showed that many different printers around the world have these tracking dots on their pages.

Technical aspects

Many color laser printers and photocopiers add tiny yellow dots to every page they print. These dots are very small—about one-tenth of a millimetre across—and are spaced about one millimetre apart. They form a pattern that encodes information like the printer’s serial number and the date and time the page was printed. This pattern repeats many times across the page, making it possible to identify the printer even from a small piece of a printed document.

In 2005, experts from the Chaos Computer Club showed that these printers can store up to 64 bytes of data in their dot patterns. In 2018, researchers at the TU Dresden studied many different printer models and found they used several different ways to encode this hidden information. Xerox has acknowledged that their printers include such markings as a security feature.

Visibility

These tiny dots can be seen by scanning a printed page with a high-resolution scanner and using image processing to highlight the yellow color. Sometimes, you can even spot them with a magnifying glass in bright light, or they’ll stand out clearly under UV-light.

This special marking process helps experts identify copied items, like banknotes, even if they are shredded. In 2011, a DARPA challenge called the "Shredder Challenge" was won by a team that used these dots to piece together shredded documents.

Practical application

Journalists and security experts have noted that when certain secret documents were shared online, the hidden codes in the printed pages helped identify who had printed them. This played a role in uncovering a person who shared sensitive information without permission.

Protection of privacy and circumvention

Copies or printouts of documents with private information, such as health care records or bank statements, can sometimes be traced back to the printer that made them. Many users don’t know this because the companies that make printers don’t talk about it. In 2005, the Electronic Frontier Foundation created a Python script to help people understand these hidden codes.

Later, in 2018, scientists from TU Dresden made a tool that can read these hidden codes and even hide them by adding extra dots to the printout. They shared this tool to help whistleblowers share important information safely.

Comparable processes

Other ways to identify the printer used are possible, like changing the strength of the laser or using different shades of grey in the text. As of 2006, it wasn't known if printer makers were using these methods too.