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• little update: a make reader tells me that the "unknown" column of the tracking dots (15) is probably checkdigit.

  Posted by: philliptorrone on May 21, 2006 at 3:50 PM

• 
  We have Savin color copiers at work, makes me think they'd do something similar. Can anyone confirm if this is in the same place every time and if the color of the dots is just a dot straight from yellow toner? Whats keeping me from tossing a yellow square or garbage dots in the general location on every print I make?

  I'd be interested to see if the matrix moves to another area on the page in case of this (but can figure the answer is probablly not).

  Posted by: milleker on May 21, 2006 at 7:44 PM

• the matrix is quite small and repeated many times covering the entire paper. thus either covering the entire (printable) page with a yellow background or perfectly lineing up a fake (fully filled) matrix of dots to be printed EXACTLY over all watermarks are the only too possible solutions. the first however is hardly a good solutions since you would then only be printing yellow pages and anyway who says you can't tell a doubly printed yellow pixel from a singly printed pixel with a proper lab or even just some neat hack; so that leaves the second method of lineing up fake matrecis. i doubt that would work either though cause it would be pretty much impossible to get the aligning right. even the slightest mistake would give away everything.

  by the way this watermarking is by no means unique to the xerox docu color printers. that's just the first scheme patrick and i decoded. lots of other colour laser printers have this and as far as i remember it was traced back to a canon printing engine part which can be found in most printers (not sure about this though). i'd really not be surprised if the high end copiers do the same thing somehow. basically these type of machines vary alot less then one might think at a certain (hardware) level so it doesnt take all THAT much infiltration/coersian/persuasian on the part of the secret service or whomever to get this type of stuff into many different machines.

  Posted by: optix on May 30, 2006 at 4:24 PM

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