Elektronika 2009-11.pdf - Instytut Systemów Elektronicznych

Signal to Noise Ratio (PSNR). We propose it along with one
of the most appropriate distortion metrics, which is correlated
with the Human Visual System (HVS) [2] i.e. Relative Peak
Signal to Noise Ratio (RPSNR). Also, report module can contain
other useful informations such as size of decoded message
or output of steganalysis modules. Because of the fact
that the robustness is a crucial requirement for digital watermarking,
investigating the resistance against typical signal
processing methods can be useful and thus the VSL contains
some popular image processing algorithms. Within application,
hidden content (e.g. invisible watermark) could be attacked
with noise addition (Gaussian noise, Salt-and-Pepper
noise), resizing (using most popular bicubic interpolation),
cropping, JPEG lossy compression, median filtering, Gaussian
blurring, gamma correction or sharpening. Any parameter
of the distortion method can be easily adjusted.
All tools presented above are designated to replace any
external software like image viewers, file managers, text editors,
etc. All these elements form the entire, fully independent
workbench making possible to conduct any type of steganographic
experiment.
The initial assumption regarding architecture was to use
SOAP (Simple Object Access Protocol) for communication,
as it provides mechanisms for exchanging structured and
typed information between peers in a decentralized, distributed
environment such as proposed virtual laboratory [20].
Each workbench element (node) can be treated as a peer
which exchanges SOAP messages, formally specified as an
XML structures, which provide an abstract description of its
contents. Despite SOAP is fundamentally a stateless, oneway
message exchange paradigm, our idea is to create more
complex interaction patterns (e.g., request/response, request/multiple
responses, etc.) by combining such one-way
exchanges with features provided by an underlying protocol
and/or application-specific information [22]. Because of the
concept of an open and distributed processing, the first, and
the most obvious idea was to build the VSL as Web Services
which are defined by the W3C as a software system designed
to support interoperable machine-to-machine interaction over
a network [23]. The VSL was meant to be a set of Web Services
with one privileged core Web Service, which exactly represents
the proposed architecture. Initial framework was build
using Java API for XML Web Services (JAX-WS) [21], however
the system in this form had significant limitations. The
main two constraints were a complexity of API and a lack of
possibility to construct plugin architecture. Those restrictions
could be a great disadvantage for potential users regarding
extending and/or altering the whole system. Another inconvenience
was communication issue - Web Services should
use in this case secure transfer protocol. In result, we
changed system architecture to a main application and a set
of plugins which are also standalone applications that can run
independently (typical to the client-server approach). Thanks
to Java, which provides multi-platform compatibility, the VSL
application can be executed on almost every personal computer
running any popular operating system (MS Windows,
MacOS, Linux). It is an often unnoticeable, yet very important
economical aspect of research. Basic programming skills, together
with given architecture make possible for everyone to
build his own plugin, without much effort (regardless of the
type - steganographic, steganalytic or distortion module) and
to attach it to the main application of VSL. Free and open
source of VSL (along with openness of technologies used for
development) and its modularity, make it useful and adequate
tool for both scientists and students.
User interface
Graphical user interface of the application is shown in figures
below (see Fig. 3-5). It was build with the usage of typical
Swing GUI gadgets, however thanks to the look-an-feel technology,
it can be easily switched to standard MS Windows or
MacOS visual schemes. It features drag-and-drop technology
which means it is easy to operate by any, even not advanced
user. The main window of laboratory contains plain workspace
and a list of block elements (Modules). Each block element
represents corresponding operation i.e. block named LSB
processes an image with Least Significant Bit embedding. Any
processing starts from the Input block which has a list of input
files designated for processing (the processing flow is sequentially
performed for all input files). Between any two
blocks a package of control data is being transferred - every
block performs its own operations on data and transfers the
data package further to the next block. All parameters and
properties of an operation represented by a block, can be easily
adjusted, simply by showing the parameters dialog after
the double-click on the block that needs to be configured. The
application provides both batch and parallel processing, and
loops as well, hence it should be noticed that considerably
complex flows, as described in next section, are available to
be arranged by users. Moreover, all specific properties of an
application (i.e. file formats, size of blocks, area of workspace,
colors, etc.) can be specified in an external, freely editable
configuration file.
Sample experiments
The virtual laboratory has one considerable advantage over
typical set of individual applications since it makes it possible
to conduct experiments in a flow form. Each operation being
a part of the flow can be performed once or several times, giving
an opportunity to change its parameters during each iteration
and passing the results to the next stage. Thus, it is more
usable for scientific and educational purposes, where the experiments
involve many time-consuming and often not clearly
determined investigations. Figures below provide sample
flows of processing. Figure 3 shows a simple process of embedding
after which cover image is distorted three times. Then
it is stored in a Display module and a LSB Decoder tries to
read the hidden message. In the end report is created from
whole process.
Fig. 3. Example flow of experiment representing an attack on chosen
steganographic method
Rys. 3. Przykładowy przebieg eksperymentu odpowiadającemu
atakowi na wybraną metodę staganograficzną
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