Multi-block Dependency Based Watermarking
Scheme for Binary-Text Image Authentication
Fan Chen, Yao Qin, Hongjie He
*
Sichuan Key Lab of Signal and Information Processing,Southwest Jiaotong University, Chengdu,Sichuan, China
E-mail: fchen@swjtu.edu.cn, hjhe@swjtu.edu.cn,
Abstract—Toimprove the ability against different
counterfeiting attacks, a watermarking algorithm is proposed
for binary-textimage (BTI) authentication. To protect the
uniform regions in BTI, the watermark information of a
fixed-size blockisgenerated according to the content of it and
divided into three parts. One part is embedded in the flippable
pixels of itself, and the other two parts are respectively
embedded in the flippable pixels of other two blocks in BTI,
which are randomly chosen based on the secret key. This
strategy can not only introduce the block-wise dependence, but
also make it possible for the authentication watermark of a
uniform block to be embedded in the BTI. In the tamper
detection stage, a multi-block based statistic detection method is
designed to verify the validity of an image block.Simulation
results show that the proposed algorithm can achieve a good
imperceptibility and have an ability resisting the maliciously
attacks such as collage attack, delete tampering, replace
tampering etc.
I. INTRODUCTION
With the proliferation of computer and network technology
intheoffice automation system,a paperless officehas become
an irresistible trend.A paperless office, which is achieved by
converting documents and other papers into digital form, can
not only make the customers enjoy the most expeditious,
efficient and safest service, but alsogenerate cost savings. On
the other hand, the digital data can easily be illegally
manipulated without any trace using the powerful editing
tools. Authentication to detect tampering and forgery is thus
of primary concern especially for the important documents
such as contracts, notices, certificates, vouchersetc. As a
result, how to protect the authenticity and integrity of digital
data content becomes one of the key problems to be settled in
theoffice automation system. Digital watermarking
technology has been proposed as an effective way toaddress
the above issue [1].
To give attention to documentspecialtyand file-size, the
important documents such as certificates and voucherhave has
been digitized and stored in binaryimage, which is called as
"binary-textimage (BTI)" since the document includes many
importanttext information. A watermarking algorithm for BTI
authenticationis a very challenging task due to the fact that
there are only two colors, black and white, in BTI.A
watermarking algorithm for BTI authentication should take
into imperceptibility and security account.
To maintain a good imperceptibility, the watermark data
are hidden in the "flippable pixels" in BTI. Many methods
[2-5] were proposed to judge the flippability of a pixel.
Specifically, the flippability of a center pixel in a block is
measured by changingof smoothness, connectivity,
boundaries or pixel spread deviation (PSD).Obviously, the
more the flippable pixels are, the more the watermark
capacity is. On the other side, tamper detection performance
is one of important evaluation indexes for image
authentication watermarking scheme. At the early stage of the
study, the BTI authentication watermarking scheme cannot
locate the tamper regions [2]since thewatermark payload is
poor. To achieve tamper localization, some BTI
authentication watermarking methods were proposed based
on image block [3, 4].However, the tamper detection rate of a
block is only 50% due to the fact that the watermark payload
is 1 bit per block. Moreover, these algorithms cannot verify
the authenticity of homogeneous blocksbecause no watermark
datais embedded in it. To protect the homogeneous regions in
which flappable pixels is lack, Zhang [6]adopted a label
symbol to distinguish the homogeneity of each block.
However, this method needs transmit lots of additional
information.Yang [7]generated the watermark information of
each block by encrypting the neighborhood blocks, which
could decrease the accuracy of tamper localization in terms
of a higher probability of false rejection (PFR). To hide
watermark bits of uniform regions, Li [8] proposed a
hierarchical structure based authentication watermarking, in
which an image was divided into five multi-level hierarchies
and each hierarchy was respectively watermarked. As a
uniform block is tampered, the higher-level block including it
can detect and localizethis tampering. As a result, this
scheme can protect the uniform regions and localize the
modifications in BTI by multi-level authentication. However,
the lower-levelprobability of false acceptance (PFA) and
higher-level PFRare high.On the other hand, Kwon [9]
proposed that the size of each block is decided by the number
of flippable wavelet coefficients in a block. Unfixed-size
blocks can prevent the PFA caused by the lack of flappable
pixels. However, the tamper localization performance would
be decreased with the increase of the uniform-region
size.More important, the existing BTI authentication
watermarking schemes are vulnerable to the collage attack
[10] because of block-wise independence.
To address the above problems, this paper proposes a BTI
authentication watermarking algorithm based on multi-block
dependency.The authentication watermark of
afixed-sizeblock is divided into three parts. One part is
embedded in itself, and the other two parts areembedded in
the other two blocks randomly chosen based on the secret key.
This strategy can not only introduce the block-wise