Cryptography Reference
In-Depth Information
Figure 12.1: An embedded digital signature can be woven into a file.
In this visual allegory, the file is broken up into five chunks. Each
chunk is broken up into two part- one that remains unchanged and
the other that hides information. The data from section
i
is used to
create a digital signature that is embedded in section
i
+1 .
be computed with traditional public-key algorithms like RSA, or it
may usemore basic solutions with secret key algorithms or even hash
functions.[Won98, Wal95a]
Thisprocessusestwokeysthatmayormaynotbedrawnfrom
the same set of bits. The first is used to define the parts of the file
that may be changed. It could be a randomnumber stream that picks
pixels to hide information. The second key actually constructs the
signature.
Jessica J. Fridrich and
Miroslav Goljan suggest
self-embedding a copy
of an image in itself.
Details from one block
are embedded in
another block across the
image. Cropping or
other tampering can be
reversed. [FG99]
This approach is direct, and easy to code, but it hides the informa-
tion in the most succeptable part of the file. Compression algorithms
and other watermarks can damage the information by changing the
data in this section. [CM97]
The mechanism can also be extended by splitting the file into
numerous sections. The signature for section
i
can be embedded
into section
+1 . Figure 12.1 shows how thismight be done with a file
broken into five sections. The data from one section is hidden in the
next section. Separating the hiding also can provide an indication of
where any tampering took place.
i
12.4 Public-Key Algorithms
Many researchers are trying to develop “public-key” algorithms for
hiding information that providemany of the same features as public-
key cryptography. The systems allow a user to hide information in
such a way that anyone can recover it, but ideally in a way that no
one can create new data in the right format.
This feature is desirable for watermarks because it is ideal for
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