Cryptography Reference
In-Depth Information
vector map. At present, there is no appropriate quantified measure of vector
map fidelity proposed.
6.1.3 Ways of Possible Attacks
A successful attack means that the watermark could be removed whereas
the validity of the cover data could be preserved. Unlike pixel images, the
spatial data of vector maps is a series of floating-point values with a certain
precision. Consequently, the manners and the features of the possible attacks
on vector map watermarking are also different from that of general multimedia
watermarking.
Geometrical Attacks
Some geometrical transforms such as translation, rotation, and scaling are the
main forms of geometrical attacks. For digital image watermarking, geometri-
cal attacks are very di
cult to defend against because these transforms need
to permit interpolation of pixel values. It is a irreversible process and always
causes the loss of information. However, for vector maps, such attacks are
virtually reversible transformations of coordinates where almost little or no
information would be lost. Geometrical attacks are relatively easier to defend
against in vector map watermarking schemes.
Vertex Attacking
Vertex attacking means the attacks in vertex level. That is by adding new
vertices into map (interpolation), or removing vertices from map (simplifica-
tion or cropping). Such attacks could possibly disturb the synchronization of
the detector by changing the number of the vertices. They are therefore very
serious to vector map watermarking. Map simplification is a common oper-
ation in daily work to enhance the speed of handling the map data. It does
cause information lost whereas and to keep the validity of the handled map.
As a result, the ability of surviving the map simplification is very important
for a robust watermarking scheme.
Object Reordering
Object reordering is an attack in the object level. The spatial data of a vec-
tor map contains many objects which are composed by arranged vertices. All
objects are stored in the map file in a certain sequence. Either changing the
saving sequence of map objects, or reordering the vertices within an object
can produce a new map file without degrading datas precision. For the wa-
termarking schemes which are dependent to the objects order, this operation
is a fatal attack.
