Cryptography Reference
In-Depth Information
ume of the response of said human subject in order to increase
the detection of said watermark.
4. The method of claim (3) where said funny sentence is repeated
several more times, increasing the sensitivity in said human
subject enough to prompt them to throttle the neck of the next
person to repeat said funny sentence increasing further the
ability to detect said watermark.
16.2 Tagging Digital Documents
One of themost demanding applications for the algorithms that hide
information is protecting copyrighted information. The job requires
the hidden information to somehow identify the rightful owner of the
file in question and, after identifying it, prevent it from being used in
unauthorized ways. This is a tall order because the content industry
has great dreams for digital books, music, movies and othermultime-
dia presentations. Putting a computer in the loopmeans that content
producers can experiment with as many odd mechanisms for mak-
ing money as they can imagine. Some suggest giving away the first
n−
1 chapters of a murder mystery and charging only for the last one
with the identity of the murderers. Others propose giving people a
cut when they recommend a movie to a friend and the friend buys
a copy. All of these schemes depend on some form of secure copy
protection and many of the dreams include hidden information and
steganography.
Hiding information to protect text, music, movies, and art is usu-
ally called watermarking , a reference to the light image of the manu-
facturer's logo pressed into the paper when it was made. The term is
apt because steganography can hide information about the creator
of a document as well as information spelling out who can use it and
when. Ideally, the computer displaying the document will interpret
the hidden information correctly and do the right thing by the cre-
ators.
Treating the document as the creators demand is not an easy
challenge. All of the algorithms in this topic can hide arbitrarly com-
plex instructions for what can and can't be done with the document
carrying the hidden information. Some copy protection schemes use
as few as 70 bits, a number that can fit comfortably in almost any
document.
Just inserting the information is not good enough because wa-
termarks face different threats. Most standard steganographic algo-
rithms fight against discovery by blending in as well as possible to
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