[Printers] RE: printer-tracking--countermeasures?

[Ralf Muschall]

First: I would ask all members of the mailing list please to include
line breaks in their posts.  While reading "Format: flowed" is no
problem for a mail program, it *is* for reading the archive since the
web interface formats them using <PRE>.

Pat Wood wrote:

> Someone asked about using a mask of random yellow dots as an overlay to

This might fail for another reason: The dot pattern is repeated over the
page several dozen or hundred times.  Random dots would in most cases
miss the lattice positions and therefore not disturb the attacker at
all.  The few dots which hit can be eliminated by comparing them with
other instances of the pattern (which is what I did when looking for the
dot positions among random dust specks from the Kyocera C5016N - see my
next post).

For the same reason, nusrat's idea of using preprinted sheets with
yellow dots will fail - printers have paper handling tolerances in the
millimeter range, causing the preprinted dots to miss the correct
positions.

The only thing that IMHO should work is to find out exactly the
coordinates of the dot raster in the bitmap page (*not* on the paper)
and ste *all* those dots yellow - ideally this can be done in the
printer driver or using a postscript preprocessor.

> defeat the yellow dot mechanism.  I would guess that this won't work on
> all printers, as some put the yellow dots in the nonprintable area in
> the margin. 

I doubt this - AFAIK printers have the nonprintable areas in order to
avoid toner particles falling into the mechanical parts of the engine
and damaging it.

Btw., I found another method to find the yellow dots very simply: Scan
the page back in, and perform a CMYK separation.  The dots will appear
in the black layer and negatively in the cyan layer and are perfectly
visible in both of them.

> Note that other forensic techniques can uniquely
> identify the printer used for B&W output, but not without comparing the

This depends on the amound of work invested into the watermarking.  It
might be that there is a really effective hidden method, and the purpose
of the yellow dots is either to distract public attention from the real
thing or to enable the cop in a small town to catch the printer's owner
himself without having to call the Feds.

Ralf