[Printers] software release: yellowdot

[Seth David Schoen]

I Miller writes:

> This software will create a PNG containing yellow dots that can then be
> overlaid onto existing printer output. These dots are randomly distributed
> across the image, and are therefore not useful for securely obfuscating the
> identity that is encoded within the printer's generated dot pattern.
> However, ``yellowdot'' raises the amount of resources that must be expended
> to successfully identify the printer that generated a given document. It is
> hoped that ``yellowdot'' can be used to thwart amateur attempts to breach
> privacy, such as could conceivably be practiced in an office environment.

This is an interesting idea, but I'm concerned that a lot of people
won't realize just how easy and automatic it could be to undo this.
(EFF has also just produced a brochure in which I argue against this
idea for the reasons below, in a lot less technical detail.)

Suppose we're talking about DocuColor or Dell dots, which we understand
well and where the repeating unit of the tracking pattern is relatively
small.  We probably don't need Fourier transforms or anything fancy to
find the repeating periodic signal; if we know its period accurately
enough we can take n identically-sized adjacent rectangular regions of
the appropriate size, do a color separation and then invert the blue
channel values, and then multiply (or logical-AND) to find yellow pixels
that are present in each such region.  Then a false positive added noise
dot will only occur at a given location with probability p^n, where p is
the probability that the added noise will turn on a dot at a particular
randomly chosen location.  If p is very high, the output image quality
would be adversely effected, and anyway choosing even a moderately large
n makes p^n pretty small.  What's worse, the resulting image can probably
be manually edited to remove false positive dots because the normal
structure of a tracking pattern is predictable enough.  (There's also
the problem that even the grid alignment of the tracking pattern is
probably extremely constrained, and of course the DocuColor and Dell
tracking patterns have row and column parity, sufficient to correct a
single bit error.)

I'm not an image-processing expert, but I know that there are an enormous
number of techniques and tools to remove noise from periodic signals,
and to detect changes (or the lack of changes) in related images.  For
instance, I know of an astronomy project where students get time-lapse
imagery from telescopes and then digitally subtract images from one
another in order to remove the fixed stars and find asteroids.  In a way,
these obfuscation techniques are like taking a number of images of fixed
stars (the forensic tracking dots) and adding asteroids (the noise dots)
to them.  If the students can distinguish the two in telescope images, I
bet they could distinguish them in forensic images.

I think it's valuable to understand how practical this attack is, so I'd
be happy to try a challenge -- if someone wants to print a "yellowdot"
document (mostly whitespace for simplicity) on a DocuColor or Dell, write
down the printer serial number but don't tell me what it was, and then
send it to me in the mail, I'll make an attempt to read the serial number
and tell the list the results.

-- 
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