18-05-2012, 02:44 PM
Digital Steganography
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The Data beneath the Wax
Digital steganography is the art of inconspicuously
hiding data within data. Steganography’s goal in
general is to hide data well enough that unintended
recipients do not suspect the steganographic
medium of containing hidden data. In my opening
example, I used this article as the carrier data. I
inserted my hidden data using a technique I hoped
the reader would never suspect. But now that I have
revealed this article as a medium of steganography,
my success in using a similar technique will be limited.
This is true simply because you might now suspect
that something other than an article on
steganography (maybe another hidden message)
exists within the remainder of my writing.
Limitations
Steganography is constrained by the same
assumption that exists for encryption. If Alice
wants to send an image with a hidden message to
Bob, she must first privately agree with Bob on a
method of steganography. Under the encryption
model, Bob can be fairly sure when he’s got some
ciphertext. However, in the steganography model,
it will be difficult for Bob to know when an image
is just an image.
Methods of Digital Steganography
There are many techniques available to the digital
steganographer. The most common technique is to
exploit the lenient constraints of popular file formats.
Many publicly available software packages
use this technique on a variety of media.
Data Ordering
Ordering data that does not have an ordering constraint
is often an effective method of steganography.
Each permutation of a set of objects can be
mapped to a positive integer. This mapping can
then be used to encode hidden data by altering the
order of objects that are not considered ordered by
the carrier medium. While this technique generally
does not change the information quality, hidden
data can easily be lost if the medium is encoded
again. For example, if I have a GIF whose color
map contains hidden data, I can open the GIF in
my favorite graphics-editing package and save it
again. Visually, the resulting GIF should be identical
to the original, but the ordering of the color
map may have been lost.