Cryptography Reference
In-Depth Information
subverting the corporatemusic world that uses the laws of
intellectual property to milk our youth. Instead of work-
ing long hours at McDonald's to save for a new $17.95 Nir-
vana Retrospective CD, we'll truly shatter the power struc-
ture by playing music long freed from the authorial and
corporate imperative.
Teen #2
And there are some great bass chords in this Ninth Sym-
phony.
9.2 Hiding in the Noise
Noise, alas, is part of our lives. The advertisements for digital this
and digital that try to give the world the impression that digital cir-
cuitsarenoise-freeandthusbetter,butthisisonlyhalftrue.Thedig-
ital signal may be copied and copied without changing the message.
thanks to error-correcting codes and well-defined circuitry, but this
doesn't eliminate much of the original noise. Digital photographs,
digitized music, and digital movies all have a significant amount of
noise that is left over from their original creation. When the voices,
sounds, and photographs are converted into bits, the circuits that do
the job are often less than perfect. A bit of electrical noise might
slightly change the bits and there is no way to recover. This noise
is something that will always be with us.
This noise is also an opportunity. If it doesn't really matter
whether the bits are exactly right, then anyone who needs to hide
information can take advantage of the uncertainty. They can claim
the bits for their own through squatter's rights. This is probably the
most popular form of steganography and the one with the most po-
tential. There are millions of images floating about the Net used as
window dressing for Web sites and who knows what. Any one could
hijack the bits to carry their own messages.
The principle is simple. Digitized photos or sounds are repre-
sented by numbers that encode the intensity at a particular moment
in space and/or time. A digital photo is just a matrix of numbers that
stands for the intensity of light emanating froma particular place at a
particular time. Digitized sounds are just lists of the pressure hitting
a microphone at a sequence of time slices.
All of these numbers are imprecise. The digital cameras that gen-
erate images are not perfect because the array of charge-coupled de-
vices (CCDs) that convert photons to bits is subject to the random
effects of physics. In order to make the devices sensitive enough to
work at normal room levels, they must often respond to only a few
photons. The randomness of the world ensures that sometimes a few
