[Printers] obfuscate dots?

[Seth David Schoen]

Ringo Kamens writes:

> Unfortunately it wouldn't work. To the naked eye it would work but I
> think that forensics would be able to tell that they were made
> differently. Wouldn't it just be better to completely remove/cover the
> device that puts the yellow dots in? I also had similar questions and
> would appreciate a response from somebody more in the know.

My current opinion is that the dots are produced by the same mechanism
as regular printer output; that is, they are added by software inside
the printer rather than produced by separate imaging hardware.

If they are pixel-sized and pixel-aligned, and always appear in the
same offset to the edges of the paper, it should be possible to produce
some kind of obfuscating overlay.  I know that I've argued in the
past that this was impossible, but I now think it would be possible
if the printer dots produced by a particular printer model are really
made of ordinary yellow pixels and if their offset to the edges of the
page isn't randomized.

Actually getting this right is a little bit tricky, but I've recently
experimented with printing 1x1, 2x2, and 3x3 pure yellow squares at
fixed locations on an HP Color LaserJET and then looking at them under
a microscope to compare with the tracking dots the CLJ is adding.  The
results are promising but not at all definitive; one thing that would
help is a much better microscope to try to clear up the question of
whether the tracking dots are distinguishable under much greater
magnification.)

I'll try to give an update about this soon; I'm going to be speaking
about printer dots at the meeting of the National Association of Document
Examiners next month and I'm hoping to be able to tell them more about
whether this countermeasure would actually work.

-- 
Seth Schoen
Staff Technologist                                schoen at eff.org
Electronic Frontier Foundation                    http://www.eff.org/
454 Shotwell Street, San Francisco, CA  94110     1 415 436 9333 x107