Steganography - Color Copier Dot Code

Unknown Printer
Kinkos, Sepulveda & Jefferson, Culver City, CA - 27 May 2006

Sample #1

Printer: unknown
Date: 27 May 2006
Day: 147 (10010011 in binary)

The full 8.5 x 11 sheet with one repeating pattern of yellow code dots inked in black.

Closeup of area inked in black.
The photograph was taken in bright sunlight
to maximize blue light.

Closeup of inked-in area in bright sunlight.
The red and green channels have been deleted.
The blue channel has been enhanced.

An enlargement of the enhanced blue channel image with red lines superimposed
to show the repititions of the pattern. The red block measures 14mm high and 11mm wide.

An enlargement of the bright sunlight image without enhancement. The yellow dots are just barely visible. To enhance the image yourself, save it and then open it in PhotoShop. Select "channels" and turn off the red and green channels. The area represented above includes one full block, the same coded block that is shown below with the blue-channel enhanced.

An enlargement of the enhanced blue channel image shown above with red lines inserted to outline one coded block of dots. White lines have been inserted to show the alignment of dots. The rows and columns have been arbitrarily labeled with upper and lower-case letters for reference. This sample has NOT been decoded. The dot spacing is 1mm, except for the dots in half-columns "x" and "y." These may represent start and stop delimiters. In that case the data field is likely 9 wide by 10 high (90 bits). It is not clear whether the words are arranged horizontally or vertically or whether they are to be read from left-to-right or right-to-left. Nevertheless, this is a binary code of some type.

Xerox DocuColor 12
Kinkos, Westwood, Westwood, CA - 30 May 2006 - S/N FU 2-051912

Color photograph in daylight.

The blue channel enhanced.

Closeup of one block in daylight.

Closeup of one block with the blue channel enhanced.

Thanks for looking...
Please email me your decodings: gessler@ucla.edu

A Brief Review of Color Theory

Red	Green	Blue	White
Cyan	Magenta	Yellow	Black

The yellow dots on paper are made of red and green light (not red and green ink or toner). On paper, the yellow toner is transparent and absorbs blue light, letting red and green pass through it. On your computer screen, the yellow you see is made up of red and green light.

The additive color primaries are red, green and blue (hence "RGB"). They are the primaries used to "paint with light" like the color on your computer monitor or TV screen. You can see these if you look closely at your screen with a magnifier.

The subtractive color primaries are cyan, magenta and yellow, with black thrown in for good measure (hence "CMYK"). These are the transparent inks and toners that most printers use, not just color copiers, but magazines and boxed paper products. You can often see the CMYK color swatches used for alignment if you open the hidden flaps on your grocery items.

The yellow dots on paper look yellow because that transparent ink or toner absorbs the blue light from sunlight and reflects both red and green. Red and green light make yellow. (Red and green paints or inks do not.) So if you illuminate a yellow dot on paper with blue light it will absorb the blue and look dark. And if you take a digital photograph of the yellow dots, the yellow that you see on the computer screen is actually red and green. Consequently, if you only display the blue color channel in PhotoShop the dots will show up dark.