Write-up: Yellow dots

Given a scan of a printed file, we are supposed to find data about the date, time and serial number of the printer.

After a quick google search, we find this wikipedia article:
Printer Tracking Dots

In which you, most notably, find this image:
Table of encoding of various data using yellow dots

Or, if you’re able to google beyond the first page, you might find this fantastic pdf:
Decoding Identifying Printer Information (Chaos Communication Camp Luftfahrtmuseum Finowfurt)

Cool :D So now we just need to find the dots.

Finding those damn dots

The second pdf already gives us a great solution, but let’s find some possible others.

  1. Using stegsolve.jar
    Stegsolve.jar is a steganography tool that allows you to scan different colour channels of a picture. Really useful. In our case,

    From there, we can see our lil’ dots in:

    • Some blue planes
      enter image description here
    • Random colour map 2
      enter image description here

  2. Using an online photo editing tool
    My personal favourite is photopea.com
    Apply a Hue and Saturation layer, play with the settings a bit until you see what you want to see
    enter image description here

  3. Use a specialized steganography tool
    I like https://29a.ch/photo-forensics/#forensic-magnifier personally, if you find anything better go for it.
    enter image description here

Decryption

Left as an exercice for the reader :) You’ve now got all the tools!