Printer steganography
As a Machine Identification Code (MIC ) - color printers mark, yellow dots (yellow dots ), tracking dots ( points for follow up ) or secret dots (secret points) - is a digital watermark denotes that certain color laser printers and copiers printed on each page is attached.
The code should allow identification of the device with which a particular document was produced, and thereby provide information on the manufacturer of the document. This can be useful if the proof of the authorship of a forgery is to be performed.
Technical Realization
The label is composed of a dot matrix which is distributed over the whole pressure field. The points are yellow, have a diameter of one tenth of a millimeter and a spacing of about one millimeter. They are barely visible with the naked eye. Their arrangement can encrypted information about the manufacturer and serial number of the device, as well as the date and time of printing included.
If the code example from 8 × 16 points in a square or hexagonal arrangement, he occupied an area of about four square centimeters. On a DIN A4 sheet it appears about 150 times. He can also be read, if only excerpts or fragments of the printed sheet are available.
Some printers also arrange yellow dots in seemingly random point clouds.
The Chaos Computer Club indicates that current color copier can make a mark in a matrix of 32 x 16 points and thus accommodate a data set of 64 bytes.
Make visible
The machine identification code can be made visible by a color page is printed or photocopied and then a small portion of which is scanned with a scanner with high resolution. The yellow color channel can be subsequently amplified using a graphics program to make the points of the machine identification code, if available, is clearly visible. With good lighting but can also be a magnifying glass enough to see the tiny yellow dots.
Education
One criticism is that this traceability is not known to many users. A manufacturer or duplicators of political leaflets and similar material, who wishes to remain anonymous, could unwittingly reveal his identity and see themselves exposed to reprisals. If the subject is well known, or there was a duty prominent notice on each unit, political writers could avoid this danger of discovery, without that counterfeiters would facilitate the craft. Such a required statement or a public discussion on this topic should be possible in states with a repressive political system, however, hardly.
Xerox is one of the few manufacturers that point to the marking of the pages: " The digital color printing system is equipped according to the requirements of many governments with a tamper-proof identification and banknote recognition system. Each copy will be provided with a marking, if necessary, allows the identification of the printing system, with which it was created. This code is not visible under normal conditions. "