Cryptography Reference
In-Depth Information
mathematically sophisticated Dining Cryptographers' solutions
to distribute information anonymously.
These different techniques can be combined in many ways. First
information can be hidden by hiding it in a list, then the list can be
hidden in the noise of a file that is then broadcast in a way to hide the
source of the data.
1.0.3 How Steganography Is Used
Hidden information has a variety of uses in products and protocols.
Hiding slightly different information or combining the various algo-
rithms creates different tools with different uses. Here are some of
the most interesting applications:
Enhanced Data Structures
Most programmers know that standard
data structures get old over time. Eventually there comes a time
when new, unplanned information must be added to the for-
mat without breaking old software. Steganography is one so-
lution. You can hide extra information about the photos in the
photos themselves. This information travels with the photo but
will not disturb old software that doesn't know of its existence.
A radiologist could embed comments from in the background
of a digitized x-ray. The file would still work with standard tools,
saving hospitals the cost of replacing all of their equipment.
Strong Watermarks
The creators of digital content like books, movies,
and audio files want to add hidden information into the file
to describe the restrictions they place on the file. This mes-
sage might be as simple as “This file copyright 2001 by Big Fun”
or as complex as “This file can only be played twice before
12/31/2002 unless you purchase three cases of soda and sub-
mit their bottle tops for rebate. In which case you get 4 song
plays for every bottle top.”
Digital Watermarking
by Ingemar J. Cox,
Matthew L. Miller and
Jeffrey A. Bloom is a
good introduction to
watermarks and the
challenges particular to
the subfield.[CMB01]
Some watermarks are meant to be found even after the file un-
dergoes a great deal of distortion. Ideally, the watermark will
still be detectable even after someone crops, rotates, scales and
compresses some document. The only way to truly destroy it is
to alter the document somuch that it is no longer recognizable.
Other watermarks are deliberately made as fragile as possible.
If someone tries to tamper with the file, the watermark will
disappear. Combining strong and weak watermarks is a good
option when tampering is possible.
