Printers betray document secrets

[Bill Stewart]

At 05:23 PM 10/18/2004, R.A. Hettinga wrote:
>  <http://news.bbc.co.uk/2/low/technology/3753886.stm>

It turns out that their techniques aren't all that useful.
Changing laser printer cartridges changes the results.
You might find that two documents were printed
by the same printer, but it doesn't give you the
options for tracking it down that manual typewriters did.
And the differences don't identify a specific printer
in a way that can be tracked, e.g. identifying a serial number
that could be looked up from warranty records.

It's not clear that they work at all with inkjet printers,
and changing ink cartridges is even more common than
changing laser printer cartridges.  If you're sloppy,
you've probably got a bunch of partly-used cartridges around,
so even if you want to print out a bunch of ransom notes
or whatever, you don't even have to go to Kinko's
to get them to be different.

If printer makers want to build in watermarking to
make everything they print traceable, the way many of them
check for documents that look like money and don't print them,
they could hide patterns that survive cartridge changes
(would you notice a few inverted pixels on a 600x600dpi printout?)
But even then, inkjet printers are dirt cheap;
when they're on sale, they're essentially a free enclosure
in a box of overpriced printer cartridges,
so even of the printer wants to rat out the user and
it's not easy to change the serial number PROM,
you can just replace the printer.

----
Bill Stewart  bill.stewart at pobox.com 


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