Steganography: Unsolicited yellow dots on your printout

Steganography is the science of hiding things in plain sight, for example, inside images, audio files or papers.

Taking as an example one of the techniques, you can secretly encode a message in an image by flipping the least-significant bit in each pixel to create a binary string that can be decoded as text. The human eye will hardly notice any changes in the picture.

It may seem like something from ancient times, but it is not.

It appeared that most of the colour printers print "invisible" light yellow dots on every document. Those dots contain printout date and time, and some Id. In case if an investigator has a high-resolution scan of the document or document itself, s/he may trackback the author of the printout.

The information about yellow dots is not officially published by manufactures, so you can check does your printer has this hidden feature.

In case you are interested in using steganography by your own and hide something in the plain picture, check this list of open source tools.