Many color laser printers add tiny yellow dots to each page they print. These dots encode a timestamp, the printer serial number and potentially additional information. Although these dot codes have been around for at least 25 years, not much is publicly known about them.

In 2004 the computer magazine PC World published an article titled “Government Uses Color Laser Printer Technology to Track Documents“. In the article it said: “According to experts, several printer companies quietly encode the serial number and the manufacturing code of their color laser printers and color copiers on every document those machines produce.”


Printer steganography

The concept described by PC World works as follows: When a color printer (or a color copier) prints a document, it adds a pattern of tiny yellow dots (about 0.1 millimeter in diameter) to the paper. These dots are barely visible to the naked eye. The dots encode a message, which includes an identification of the printer as well as the date and time of the printing process. This means: If one knows the code, one can easily determine the origin of a printout just by looking at it.


As it seems, different printer manufacturers use different dot codes. At least one of these codes was broken by the Electronic Frontier Foundation. It is used by Xerox printers:


The document in question was printed on [20]06-05-21 at 13:48. The serial number of the printer is 21062857. This dot pattern only fills a few square centimeters. On an ordinary sized piece of paper (e.g. letter or A4) it is repeated about 150 times. This means that you only need a small excerpt from a printed document in order to read the code.

To see the dots you can scan in a printout at a high resolution (600 dpi or more) and magnify it. For more detailed instructions, check here.


The dot code mystery

Although the yellow dots have been around for at least 25 years, not much is known about them. While a few codes have been successfully analysed, most still remain a mystery. No printer manufacturer or government authority has ever published any details about the yellow dots. Still many questions are unanswered:

  • How many codes are there? How do they work?
  • Not all color laser printets print the yellow dots. Do the others use another coding method?
  • Black and white laser printers don’t print dots (printing yellow dots with a BW printer doesn’t work anyway). Do they use another coding method?
  • Ink jet printers don’t print dots, either. Do they use another coding technique?
  • If other coding methods are used, which ones? There are many possibilities: isotopic labeling, micro taggants, DNA steganography, fluorescent taggants, greyscale codes, microprinting, …
  • Who can decode the yellow dots? Printer manufacturers? Some government agencies?
  • Who developed the dot codes? The NSA? The FBI?
  • Are there similar codes used for other objects? I know that explosives are marked in some cases. Almost every item (inluding humans) can be marked with a radioactive substance, as done in the Cold War. Is there more?

In the last ten years not much new has been published about printer steganography. With the help of my readers this could be changed. Do you have an answer to one of the questions above? If yes, please let me know. Does your printer use a yellow dot code? If yes, I would be pleased to receive a scan.


Further reading: The world record crypto challenge

Kommentare (19)

  1. #1 David Allen Wilson
    7. Januar 2017

    The number values of the dots are a little clearer here:

  2. #2 Christian Berger
    7. Januar 2017

    This is actually rather old. Only printers with their own imaging engine do them, because only they have the means for it.

    One of the main ideas was probably that they want to prevent people from faking bank notes or other documents. It’s to catch idiots doing forgeries.

    It’s one of the things which are just left over from a time when people believed one could control computers and printers that way. Photoshop has an inverse freature which prevents you from printing files which have certain constellations of circles in them.

    As far as I know this all has been covered in the Datenschleuder years ago.

  3. #3 Klaus Schmeh
    7. Januar 2017

    @Christian Berger:
    I’m afraid you’re confusing the yellow dot code (as described in this post) with the EURion code ( The EURion code is printed on banknotes. Scanners and copiers with an imaging engine usually recognise this code and refuse to scan or copy it.
    In contrast, the yellow dot code doesn’t require an imaging engine. Printers supporting it print the dots on every document they produce. The dots are not meant as a feature that prevents copying a certain document. Instead it shall make any copy or printout trackable.

  4. #4 Aluhut2017
    7. Januar 2017

    Oh dear…
    Here we go again with the conspiracy theories. I thought this to be a safe place?! It’s called scienceblogs after all…

  5. #5 Klaus Schmeh
    7. Januar 2017

    Why do you think it’s a conspiration theory? The yellow dots exist, there’s no doubt about that.

  6. #6 Klaus Schmeh
    7. Januar 2017

    Bart Wenmeckers via Facebook:
    Bart Wenmeckers Was a video on the EEVBLOG about this too.

  7. #7 eyeBolla
    8. Januar 2017

    I know this for over 10 Jears. In the IT security scene this was not a secret for long. I have software to read the content. Sometimes it helps to track printed Documents. Many criminals got arrested because of this.

  8. #8 Klaus Schmeh
    8. Januar 2017

    Thanks for your comment.

    >I have software to read the content.
    Is it the one mentioned here (last line of the page):

    >Many criminals got arrested because of this.
    Do you know any examples?

  9. #9 Klaus Schmeh
    8. Januar 2017
  10. #10 Dagobert
    8. Januar 2017

    Mindestens ein Banknoten-Ausdrucker wurde damit zu Beginn der Verfügbarkeit von Farb-Laserdruckern unter 10.000 Mark geschnappt.

  11. #11 Robert aus Wien
    8. Januar 2017

    I read about that some time ago and therefore checked my printer. Although I have a Brother color laser printer, no yellow dots seem to be on its printouts. It’s a HL-3170CDW which is not on this list:
    Maybe not all of the Brother printers print these dots or there is a special firmware for European customers.

  12. #12 Ishmael
    9. Januar 2017

    Lässt sich der Code austricksen, wenn man ein Dokument mit gelbem Hintergrund druckt?

  13. #13 Karsten Hansky
    11. Januar 2017

    Wenn man ein Dokument mit gelbem Hintergrund druckt, so sieht man die Farbpunkte natürlich nicht.

    Aber: Die Punkte werden über das gesamte Blatt verteilt, auch auf die nicht bedruckbaren Bereiche (z.B. Seitenränder). Ich habe das gerade mit meinem HP CP1515n ausprobiert.

    Gibt es eigentlich schon eine Dekodierung für die HP-Muster?

  14. #14 Klaus Schmeh
    11. Januar 2017

    @Karsten Hansky:
    >Ich habe das gerade mit
    >meinem HP CP1515n ausprobiert.
    Wäre toll, wenn du mir einen solchen Ausdruck als Scan zur Verfügung stellen könntest (mit genauer Uhrzeit). Noch besser wären zwei Ausdrucke von unterschiedlichen Tagen.

    >Gibt es eigentlich schon eine
    >Dekodierung für die HP-Muster?
    Ich habe bisher nirgends etwas dazu gefunden. Ich würde daher gerne ein paar Muster von verschiedenen Drucker-Herstellern auf meinem Blog veröffentlichen. Ein Dell-Muster liegt mir vor. Ich habe außerdem zwei Canon-Drucker geprüft, aber keine Punkte entdeckt.

  15. #15 George Keller
    New York
    11. Januar 2017

    This story reminded me of this:
    which I’m sure you and all of your readers have seen.

    The first time I saw this was shortly after I retired from CBS and I couldn’t help wondering about all the stuff, jokes, diagrams and other personal stuff I and my colleagues have copied on the CBS Video Tape Operations copy machine as well as others around the Broadcast Center in NYC.

    I have been retired now 8 years and the statute of limitations has expired so I can now sleep at night.

    I can’t help thinking how many printers have been thrown out by NSA, FBI, CIA, White House, etc. in Wash. D.C.

  16. #16 Marc
    12. Januar 2017

    Sehr interessant, davon habe ich bislang noch nie etwas gehört, obwohl es schon so lange praktiziert wird. Vor vielen Jahren hatte ich mal einen Fall der mir hier direkt in den Sinn kam, obwohl der nichts mit Druckern zu tun hatte. Ein Windows NT4 Rechner meinte, plötzlich kleine gelbe Punkte auf den ganzen Bildschirm verteilt ausgeben zu müssen. Aber das war denke ich kein geheimer Code (wozu auch). Oder sieht hier jemand irgend einen Zusammenhang ? Ich glaube, damals mussten wir das System komplett neu aufsetzen um diese “gelbe Plage” loszuwerden.

  17. #17 Klaus Schmeh
    15. Januar 2017

    Here’s an interesting patent:

  18. #18 Hagen Stein
    20. Januar 2017

    See both and (German) linked from the relevant (also German) Wiki entry for a bit more background.

  19. #19 Ahmad Saed Salim
    21. Juni 2020

    I ask about the mechanism of these coded dot matrix that printed them???