Cryptography Reference
In-Depth Information
of the algorithms in this topic can resist a limited attack by us-
ing error-correcting codes to recover from a limited number of
seemingly random changes.
Add New Information Attack Attackers can use the same software
to encode a newmessage in a file. Some algorithms are vulner-
able to these attacks because they overwrite the channel used
to hide the information. The attack can be resisted with good
error-correcting codes and by using only a small fraction of the
channel chosen at random.
Reformat Attack One possible attack is to change the format of the
file because many competing file formats don't store data in
exactly the same way. There are a number of different image
formats, for instance, that use a variety of bits to store the in-
dividual pixels. Many basic tools help the graphic artist deal
with the different formats by converting one file format into an
other. Many of these conversions can't be perfect. The hidden
information is often destroyed in the process. Images can be
stored as either JPEG or GIF images, but converting from JPEG
to GIF removes some of the extra information- the EXIF fields
- embedded in the file as part of the standard.
Many watermark algorithms for images try to resist this type
of attack because reformatting is so common in the world of
graphic arts. An ideal audio watermark, for instance, would
still be readable after someone plays the music on a stereo and
records it after it has traveled through the air.
Of course, there are limits to this. Reformatting can be quite
damaging and it is difficult to anticipate all of the cropping,
rotating, scaling, and shearing that a file might undergo. Some
of the best algorithms do come close.
Compression Attack One of the easiest attacks is to compress the
file. Compression algorithms try to remove the extraneous in-
formation from a file and “hidden” is often equivalent to “ex-
traneous”. The dangerous compression algorithms are the so-
called lossy ones that do not reconstruct a file exactly during
decompression. The JPEG image format, for instance, does a
good job approximating the original.
Some of the watermarking algorithms can resist compression
by the most popular algorithms, but there are none that can
resist all of them.
The only algorithms that can resist all compression attacks
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