Cryptography Reference
In-Depth Information
hides the information in plain sight by changing the “percep-
tually salient” features of an image or sound file.
Unfortunately, steganography is not a solid science, in part be-
cause there's no simple way to measure how well it is doing. How
hidden must the information be before no one can see it? Just how
invisible is invisible? The models of human perception are often too
basic to measure what is happening.
The lack of a solidmodel means it is difficult to establish howwell
the algorithms resist attack. Many algorithms can survive cursory
scrutiny but fail if a highly trained or talented set of ears and eyes an-
alyze the results. Some people with so-called “golden ears” can hear
supposedly changes in an audio file that are inaudible to average hu-
mans. A watermark may be completely inaudible to most of the buy-
ing public, but if the musicians can hear it the record company may
not use it.
Our lack of understanding does not mean that the algorithms
don't have practical value. A watermark heard by 1% of the popu-
lation is of no concern to the other 99%. An image with hidden infor-
mation may be detectable, but this only matters if someone is trying
to detect it.
There is also little doubt that a watermark or a steganographic
tool does not need to resist all attackers to have substantial value. A
watermark that lives on after cropping and basic compression still
carries its message to many people. A hacker may learn how to de-
stroy it, but most people have better things to do with their time.
Our lack of understanding does not mean that the algorithms do
not offer some security. Some of the algorithms insert their informa-
tion with mechanisms that offer cryptographic strength. Borrowing
these ideas and incorporating them provides both stealth and secu-
rity.
1.1 Adding Context
One reviewer of the topic who was asked for a backcover blurb joked
that the topic should be “essential bedside for reading for every ter-
rorist”. After a pause he added, “and every freedom fighter, Holly-
wood executive, police officer, abused spouse, chief information of-
ficer, and anyone needing privacy anywhere.”
You may be a terrorist or you may be a freedom fighter. Who
knows? This topic is just about technology and technology is neu-
tral. It teaches you how to cast shape shifting spells that make data
look like something completely different. You may have good plans
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