A Quantization based Robust Image Watermarking

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					    A Quantization based Robust Image Watermarking Algorithm in Wavelet
                                  Domain
                                         Vidyasagar M. Potdar, Elizabeth Chang
                  School of Information Systems, Curtin University of Technology, Perth, Western Australia
                              e-mail: vidyasagar.potdar, elizabeth.chang@cbs.curtin.edu.au



   Abstract — Watermarking which belong to the field of                    •     Section VII Discussion will be given of the ex-
information hiding has seen a lot of research interest recently.                 isting literature.
Watermarking is used in B2B and B2C e-commerce markets                     •     Section VII will continue our algorithm proposal.
for a variety of reasons including security, content protection,
                                                                           •     Section VIII will show our results.
copyright     management,      trust   management,      content
authentication, tamper detection and privacy. Recently many                •     Section IX concludes.
watermarking techniques have been proposed to support these
applications but one major issue with most of the                              II. DIGITAL WATERMARKING TECHNIQUE
watermarking techniques it that these techniques fail in the
presence of severe attacks. This has been a major threat to           The process of embedding a watermark in a multimedia
content providers in the e-commerce scenario because if the        object 1 is termed as watermarking. A Watermark can be
digital content is dramatically changed then it would be           considered as a kind of a signature, which reveals the owner
difficult to prove the existence of a watermark in it and          of the multimedia object. Content providers want to embed
consequently its ownership. To tackle this security threat         watermarks in their multimedia objects (digital content) for
towards ownership issues we propose a quantization based           several reasons like copyright protection, content authenti-
watermarking algorithm which offers incredible performance
                                                                   cation, tamper detection etc. A watermarking algorithm
in presence of malicious attacks which try to remove ownership
information. The proposed scheme is backed up with excellent       embeds a visible or invisible watermark in a given multi-
results.                                                           media object. The embedding process is guided by use of a
                                                                   secret key, which decides the locations within the multimedia
  Index Terms— Watermark Detection, Watermarking, DWT,             object (image) where the watermark would be embedded.
Quantization                                                       Once the watermark is embedded it can experience several
                                                                   attacks because the multimedia object can be digitally proc-
                      I. INTRODUCTION                              essed. The attacks can be unintentional (in the case of im-
                                                                   ages, low pass filtering or gamma correction or compression)
   Watermarking is a method of hiding proprietary informa-         or intentional (like cropping). Hence, the watermark has to
tion in digital media like photographs, digital music, or          be very robust against all these possible attacks. When the
digital video. The ease with which digital content can be          owner wants to check the watermarks in the possibly at-
exchanged over the Internet has created copyright in-              tacked and distorted multimedia object, s/he relies on the
fringement issues. Copyrighted material can be easily ex-          secret key that was used to embed the watermark. Using the
changed over peer-to-peer networks, and this has caused            secret key, the embedded watermark sequence can be ex-
major concerns for those content providers who produce             tracted. This extracted watermark may or may not resemble
these digital contents. In order to protect the interest of the    the original watermark, because the object might have been
content providers these digital contents can be watermarked.       attacked.
In this paper we provide a survey of the latest techniques that
are employed to watermark images. Image watermarking
techniques can be applied to digital videos as well. How-
ever, this paper will limited to the context of the image do-
main.
   The paper is organized into the following sections:

       •    Section II - We describe the process of digital
            watermarking.
       •    Section III - We discuss the main requirements of
            image watermarking.
       •    Section IV - Information on some main applica-
            tions of watermarking will follow.
       •    Section V - Will discuss different kinds of wa-
            termarks that could be embedded.
       •    Section VI introduces wavelet transform.
                                                                      1
                                                                        Multimedia object refers to images, video and audio clips or any digital
                                                                   content that can be used for the purpose of information hiding.
   Hence, to validate the existence of a watermark, either the                 E. Unambiguity
original object is used to compare and ascertain the water-
                                                                                The extracted watermark must identify the ownership
mark signal (non-blind watermarking), or a correlation
                                                                             unambiguously, which means the errors in an extracted wa-
measure is used to detect the strength of the watermark signal
                                                                             termark must be as low as possible. Craver et al. (1997)
from the extracted watermark (blind watermarking). In
                                                                             argued that an attacker can watermark a watermarked image
correlation based detection, the original watermark sequence
                                                                             and hence claim ownership, is called a counterfeiting attack
is compared with the extracted watermark sequence, and a
                                                                             [3]. So a watermarking algorithm should be able to resist
statistical correlation test is used to determine the existence
                                                                             such attacks.
of the watermark.
                                                                                        IV. WATERMARKING APPLICATIONS
       III. REQUIREMENTS OF DIGITAL WATERMARKING
                                                                                Before beginning the discussion on watermarking algo-
     There are three main requirements of digital                            rithms we discuss the main B2B and B2C watermarking
watermarking. They are transparency, robustness and                          applications.
capacity.                                                                      A. Copyright Protection

     A. Transparency or Fidelity                                               Watermarking can be used to protecting redistribution of
                                                                             copyrighted material over the untrusted network like Internet
  The digital watermark should not affect the quality of the                 or peer-to-peer (P2P) networks. Content aware networks
original image after it is watermarked. Cox et al. (2002)                    (p2p) could incorporate watermarking technologies to report
defines transparency or fidelity as ‘perceptual similarity                   or filter out copyrighted material from such networks.
between the original and the watermarked versions of the
cover work’ [1]. Watermarking should not introduce visible                     B. Content Archiving
distortions because if such distortions are introduced it re-                   Watermarking can be used to insert a digital object iden-
duces the commercial value of the image.                                     tifier or a serial number to help archive digital contents like
     B. Robustness                                                           images, audio or video. It can also be used for classifying
                                                                             and organizing digital contents. Normally digital contents
   Cox et al. (2002) defines robustness as the ‘ability to detect            are identified by their file names; however, this is a very
the watermark after common signal processing operations’                     fragile technique, as file names can be easily changed.
[1]. Watermarks could be removed intentionally or unin-                      Therefore, embedding the object identifier within the object
tentionally by simple image processing operations like con-                  itself reduces the possibility of tampering, and hence it can
trast or brightness enhancement, gamma correction etc.                       be effectively used in archiving systems.
Hence watermarks should be robust against a variety of such
attacks. Stirmark2 classifies attacks into four basic catego-                  C. Meta-data Insertion
ries, attacks that try to remove watermarks totally, attacks                   Meta-data refers to data that describes data. Images can be
that try to remove the synchronization between the embedder                  labelled with its content and can be used in search engines.
and the detector, cryptographic attacks and protocol attacks.                Audio files, for example, can carry the lyrics or the name of a
     C. Capacity or Data Payload                                             singer. Journalists could use photographs of an incident to
                                                                             insert the cover story of the respective news. Medical X-rays
   Cox et al. (2002) define capacity or data payload as ‘the                 could store patient records.
number of bits a watermark encodes within a unit of time or
work’ [1]. This property describes how much data should be                     D. Broadcast Monitoring
embedded as a watermark to successfully detect during ex-                      Broadcast Monitoring refers to the technique of
traction. Watermark should be able to carry enough infor-                    cross-verifying whether the content that was supposed to be
mation to represent the uniqueness of the image. Different                   broadcasted (on TV or Radio) has really been broadcasted or
applications have different payload requirements [1].                        not. Watermarking can also be used for broadcast monitor-
     D. Security                                                             ing. This has major application in commercial advertisement
                                                                             broadcasting where the advertiser wants to monitor whether
  According to Kerckhoff’s principle the security of a                       their advertisement was actually broadcasted at the right time
cryptosystem depends on the secrecy of the key and not on                    and for right duration.
the cryptographic algorithm. Same rule applies to water-
marking algorithms, i.e. the watermarking algorithms must                      E. Tamper Detection
be public but watermark embedding should be based on a                          Digital content can be detected for tampering by embed-
secret key [2].                                                              ding fragile watermarks. If the fragile watermark is de-
                                                                             stroyed or degraded, it indicates the presence of tampering
                                                                             and hence the digital content cannot be trusted. Tamper de-
2                                                                            tection is very important for some applications that involve
    Stirmark is a benchmark to test robustness of watermarking algorithms.
                                                                             highly sensitive data like satellite imagery or medical im-
http://www.petitcolas.net/fabien/watermarking/stirmark                       agery. Tamper detection is also useful in a court of law
where digital images could be used as a forensic tool to prove   number of matching pixel values in compared images and
whether the image has been tampered with.                         D denotes the number of different pixel values in compared
                                                                 images.
  F. Digital Fingerprinting
  Digital Fingerprinting is a technique used to detect the                     VI. WAVELET PRELIMINARIES
owner of the digital content. Fingerprints are unique to the
                                                                    In the last few years wavelet transform has been widely
owner of the digital content. Hence, a single digital object
                                                                 studied in signal processing in general and image compres-
can have different fingerprints, because they belong to dif-
                                                                 sion in particular. In some applications wavelet based wa-
ferent users
                                                                 termarking schemes outperforms DCT based approaches.
                                                                 One such scheme is proposed here [4]. Before discussing the
    V. WATERMARKS AND WATERMARK DETECTION
                                                                 literature we point out the advantages of the DWT transform
  Basically there are three main types of watermarks that can    and its characteristics.
be embedded within an image.
                                                                   A. Characteristics of DWT
  A. Pseudo-Random Gaussian Sequence                             1) The wavelet transform decomposes the image into three
  A Gaussian sequence watermark is a sequence of numbers            spatial directions i.e. horizontal, vertical and diagonal.
comprising 1 and -1, which has an equal number of 1’s and           Hence wavelets reflect the anisotropic properties of
-1’s. It is termed as a watermark with zero mean and one            HVS more precisely [19].
variation. Such watermarks are used for objective detection      2) Wavelet Transform is computationally efficient and can
using a correlation measure.                                        be implemented by using simple filter convolution.
                                                                 3) Magnitude of DWT coefficients is larger in the lowest
  B. Binary Image or Grey Scale Image Watermarks                    bands (LL) at each level of decomposition and is smaller
                                                                    for other bands (HH, LH, HL) [20].
   Some watermarking algorithms embed meaningful data in
                                                                 4) The larger the magnitude of the wavelet coefficient the
the form of a logo image instead of a pseudo-random gaus-
                                                                    more significant it is.
sian sequence. Such watermarks are termed as binary image
                                                                 5) Watermark detection at lower resolutions is computa-
watermarks or grey scale watermarks. Such watermarks are
                                                                    tionally effective because at every successive resolution
used for subjective detection. Based on the type of water-
                                                                    level there are few frequency bands involved.
mark embedded, an appropriate decoder has to be designed
                                                                 6) High resolution sub-bands helps to easily locate edge
to detect the presence of watermark.
                                                                    and textures patterns in an image.
Watermark Detection                                                B. Advantages of DWT over DCT

   If it’s a pseudo random gaussian sequence then hypothesis     1) Wavelet transform understands the HVS more closely
testing is done to detect the presence of watermark. Suppose        than the DCT.
W is the original watermark bit sequence and W’ is the ex-       2) Wavelet coded image is a multi-resolution description of
tracted watermark bit sequence then we can calculate bit            image. Hence an image can be shown at different levels
error rate (BER) to detect the presence of watermark. If the        of resolution and can be sequentially processed from
BER is zero it indicates the presence of watermark however          low resolution to high resolution.
if it is one it indicates absence of watermark. BER is cal-      3) Visual artefacts introduced by wavelet coded images are
culated as follows. Suppose D is the retrieved signal and N is      less evident compared to DCT because wavelet trans-
the number of bits in watermark then:                               form doesn’t decompose the image into blocks for
                                                                    processing. At high compression ratios blocking arte-
             ⎧
             ⎪1 if Wi ≠ Wi ⎪
           D=⎨
                          '
                             ⎫
                                 BER (W , W ' ) =
                                                    ∑D              facts are noticeable in DCT; however, in wavelet coded
                            '⎬
                                                                    images it’s much clearer.
             ⎪0 if Wi = Wi ⎪
             ⎩               ⎭                      N            4) DFT and DCT are full frame transform and hence any
                                                                    change in the transform coefficients affects the entire
  Normalized Correlation Coefficient can also be used to            image except if DCT is implemented using a block
detect the presence of watermark.                                   based approach. However DWT has spatial frequency

                NC (W , W ' ) =
                                  ∑W W '                            locality which means if signal is embedded it will affect
                                                                    the image locally [21]. Hence a wavelet transform
                                ∑Wi 2 ∑W 'i2                        provides both frequency and spatial description for an
                                                                    image.
   To detect the presence of a logo watermark it can be            C. DWT Decomposition
achieved by doing subjective comparison by visual inspec-
tion, whereas an objective analysis could be made by cal-           Wavelet transforms use wavelet filters to transform the
culating similarity, a measure which would be outlined as        image in the wavelet domain. There are many available fil-
                       S                                         ters; however, the most commonly used filters for water-
follows:   S R=                       where S denotes the        marking are Haar Wavelet Filter, Daubechies Orthogonal
                   ( S + D)                                      Filters and Daubechies Bi-Orthogonal Filters. Each of these
filters decomposes the image into several frequencies. Single   nique. The authors argue that if the watermark is embedded
level decomposition gives four frequency representations of     in the low frequency components it is robust against low pass
the images. These four representations are called the LL, LH,   filtering, lossy compression and geometric distortions on the
HL, HH sub-bands as shown in Fig.2.                             other hand if the watermark is embedded in high frequency
                                                                components it’s robust against contrast and brightness ad-
                                                                justment, gamma correction, histogram equalization and
                                                                cropping and vice-versa. Thus to achieve overall robustness
                                                                against a large number of attacks the authors propose to
                LL1                  HL1                        embed multiple watermarks in low frequency and high fre-
                                                                quency bands of DWT [14].
                                                                   Kundur and Hatzinakos (2004) present image fusion wa-
                                                                termarking technique. They use salient features of the image
                                                                to embed the watermark. They use a saliency measure to
                LH1                  HH1                        identify the watermark strength and later embed the water-
                                                                mark additively. Normalized correlation is used to evaluate
                                                                the robustness of the extracted watermark. Later the authors
                                                                propose another technique termed as FuseMark [15] which
                                                                includes minimum variance fusion for watermark extraction.
     Fig.2 Single level Decomposition using DWT                 Here they propose to use a watermark image whose size is a
                                                                factor of the host by 2xy.
                   VII. RELATED WORK                               Tao and Eskicioglu (2004) present an optimal wavelet
                                                                based watermarking technique. They embed binary logo
   Wavelet based watermarking techniques are gaining at-        watermark in all the four bands. But they embed the wa-
tention because of the upcoming JPEG2000 standard. This         termarks with variable scaling factor in different bands. The
standard used the Discrete Wavelet Transform to compress        scaling factor is high for the LL sub band but for the other
the image. In this section we discuss wavelet based water-      three bands its lower. The quality of the extracted watermark
marking algorithms. We classify these algorithms based on       is determined by Similarity Ratio measurement for objective
their decoder requirements as Blind Detection or Non-blind      calculation [16].
Detection. As mentioned earlier blind detection doesn’t            Ganic and Eskicioglu (2005) inspired by Raval and Rege
require the original image for detecting the watermarks;        (2003) propose a multiple watermarking technique based on
however, non-blind detection requires the original image.       DWT and Singular Value Decomposition (SVD). They ar-
After initial discussion on these algorithms we focus on the    gue that the watermark embedded by Raval and Rege (2003)
main quantization based watermarking algorithms which use       scheme is visible in some parts of the image especially in the
a logo as a watermark.                                          low frequency areas, which reduces the commercial value of
                                                                the image. Hence they generalize their technique by using all
  A. DWT based Non-Blind Watermark Detection                    the four sub bands and embedding the watermark in SVD
                                                                domain. The core technique is to decompose an image into
   This technique requires the original image for detecting
                                                                four sub bands and then applying SVD to each band. The
the watermark. Most of the techniques surveyed in this sec-
                                                                watermark is actually embedded by modifying the singular
tion use a smaller image as a watermark and hence cannot
                                                                values from SVD [18]. Fig. 3 gives complete technical de-
use correlation based detectors for detecting the watermark;
                                                                tails of the parameters used in these algorithms.
as a result they rely on the original image for informed de-
                                                                   All these algorithms discussed here require the original
tection. The size of the watermark image (normally a logo) is
                                                                image for detecting the presence of watermark which is not
smaller compared to the host image.
                                                                feasible in all scenarios. Hence we now discuss some blind
   Xia et al. (1997) present a wavelet based non-blind wa-
                                                                watermarking algorithms which embed an image logo as a
termarking technique for still images where watermarks are
                                                                watermark.
added to all bands except the approximation band [12]. A
multi-resolution based approach with binary watermarks is         B. DWT based Blind Watermark Detection
presented by Hsu and Wu [13]. Here both the watermark
logo as well as the host image is decomposed into sub bands        Voyatzis and Pitas (1999) who presented the ‘toral auto-
and later embedded. Watermark is subjectively detected by       morphism’ concept provide a technique to embed binary
visual inspection however an objective detection is em-         logo as a watermark, which can be detected using visual
ployed by using normalized correlation.                         models as well as by statistical means. So in case the image
   . Lu et al. (2001) present another robust watermarking       is degraded too much and the logo is not visible, it can be
technique based on image fusion [17]. They embed a grey-        detected statistically using correlation [9]. Watermark em-
scale and binary watermark which is modulated using the         bedding is based on a chaotic (mixing) system. Original
‘toral automorphism’ described in [9]. Watermark is em-         image is not required for watermark detection. However the
bedded additively. The novelty of this technique lies in the    watermark is embedded in spatial domain by modifying the
use of secret image instead of host image for watermark ex-     pixel or luminance values. A similar approach is presented
traction and use of image dependent and image independent       for the wavelet domain [10], where the authors propose a
permutations to de-correlate the watermark logos [17]. Ra-      watermarking algorithm based on chaotic encryption.
val and Rege (2003) present a multiple watermarking tech-
   Some recent work in quantization based watermarking is                              called the approximate band and it contains most of the en-
more interesting where a logo is embedded as a watermark                               ergy. In the algorithm we decompose the image into four
and the detection is blind. Tsai et al. (2000) present a scalar                        levels and embed the watermark in HL, LH, HH sub-bands.
quantization based watermarking algorithm. They embed                                  Here we assume the size of the watermark logo is in multiple
the watermark in the middle and low frequency components                               of the sub-band size. We propose a quantization based wa-
of the wavelet sub-bands. The algorithm shows robustness                               termarking algorithm. We incorporate implicit visual
against JPEG compression only. However, it’s not robust                                masking by embedding the watermark in the LH, HL and HH
against other geometric and image processing attacks is not                            sub-bands.
discussed.
   Chen et al. (2003) present another quantization based                                 A. Embedding Algorithm
watermarking algorithm which improves on the Tsai algo-                                   Suppose the original image is an 8 bit grey scale image of
rithm by incorporating variable quantization, resistance                               size S x S and watermark is a binary image of size S/16 x
against a wide range of attacks like blurring, noising, sharp-                         S/16.
ening, scaling, cropping and compression. They employ the
concept of digital signature and time stamps to overcome                                 Step 1: Wavelet Transform
counterfeit attacks. Technical details of these algorithms are                         The original image I is decomposed by one level to obtain
presented in Table 1.                                                                  LL, LH, HL and HH sub-bands.

                                                                                          Step 2: Block Mean Intensity Calculation
                                                            Water mark
                                Filter Used




                                                                           Watermark
                                                                           Embedded
                                              Decompo-
                                               Level of
             Name




                                                sition




                                                               type




                                                                                       For each sub-band except the LL sub-band, starting at the top
                                                                                       left corner divide the sub-band into non-overlapping blocks
        DWT Based Non-Blind Watermarking Algorithms
                                                                                       of 8x8 and calculate their mean intensity values of the
        Ganic and
                                                            Grey         High and      wavelet coefficients. Then find the minimum and maximum
                               Haar              1         Scale         Low pass
      Eskicioglu 2005
                                                           Image          bands        mean intensity values and use that to construct a quantization
         Tao and
                               Haar              2
                                                           Binary
                                                                         All Bands     table. Divide the mean intensity values into N equal sets. N is
      Eskicioglu 2004                                       Logo
        Kundur and         Daubechies
                                                            Grey
                                                                         High ass
                                                                                       a positive constant that can be chosen to control the trade-off
                                                 3         Scale
      Hatzinakos 2004      10 pt wavelet
                                                           Image
                                                                          bands        between robustness and transparency. For example if the
                                                                         2nd Level     range of mean intensity values is [20, 350] and the quanti-
      Raval and Rege        Not Speci-                     Binary
                                                 2                        LL and
           2003                fied                        Logo
                                                                         HH bands      zation step size being 40 we can generate a quantization table
       Hsu and Wu          Daubechies                      Binary           Not
          1998               tap-6
                                               Multi
                                                            Logo         Specified
                                                                                       like this and assign binary values to it. For example:
                                                            Grey
      Chae and Man-                                                        Not
                               Haar              1         Scale                          0-    41-    81-   121-   161-   201-   241-    281-   321-
       junath 1998                                                       Specified
                                                           Image                          40    80     120   160    200    240    280     320    360
        Kundur and                                         Binary        Low Pass         1     0      1     0      1      0      1       0      1
                            Not Spec.            4
      Hatzinakos 1997                                       Logo          Bands
           DWT Based Blind Watermarking Algorithms
      Chen et al. 2003      Not Spec.            4
                                                           Binary
                                                                            LL           Step 3: Watermark Permutation
                                                            Logo
                                                           Binary                        Before embedding the watermark Wo , the watermark
      Tsai et al. 2000      Not Spec.            3                          LL
                                                            Logo
                                                           Binary         Visually     should be permuted to W p by a seed S and a pseudo random
          Lu et al.                            Not        and Grey        Signifi-
                            Not Spec.
           1999                               Spec.         scale
                                                           Image
                                                                           cant
                                                                           Coef.
                                                                                       permutation function f (.) . The seed is secret.
                                                           Binary
        Kundur and
                           Daubechies          Not         Water-        High pass
        Hatzinakos
          1998
                           10 pt wavelet      Spec.         mark           bands          Step 4: Quantifying the DWT Coefficients
                                                            {-1,1}
                                                                                          After obtaining the quantization table Q , we can quantify
           Table 1: List of DWT based Blind and                                        the DWT coefficients by the following method. We use one
           Non-Blind Watermarking Algorithms                                           block of 8x8 to embed one watermark bit. Hence we can
                                                                                       embed S/16 x S/16 bits in an image of size S x S. The em-
   The main issues with these quantization based algorithms                            bedding begins by finding the nearest reconstruction point
are that it only tackles a subset of attacks. For example                              based on the quantization step size. Suppose the binary
Tsai’s algorithm is only robust against JPEG compression                               watermark bit is 1 and the mean intensity of the first block is
however Chen’s algorithm is doesn’t tackle geometric at-                               158. Based on this value we compare with the quantization
tacks like rotation or image processing attacks like gamma                             table and find that the nearest reconstruction point for 1 is in
correction. Hence we propose a new algorithm which is ro-                              the range 161-200 hence the mean intensity of that block
bust against Cropping, Gamma Correction, JPEG Compres-                                 should be uniformly scaled so that the new mean is centred
sion, JPEG2000, Resizing, Rotation (90, 180 degrees), Salt                             on 180 which is the median value for the range 160-200. The
and Pepper.                                                                            selected block is shown in the table below.

                         VIII. PROPOSED SCHEME                                            0-    41-    81-   121-   161-   201-   241-    281-   321-
                                                                                          40    80     120   160    200    240    280     320    360
                                                                                          1     0      1     0      1      0      1       0      1
  Discrete wavelet transform is a technique using which a
2D image can be transferred from spatial domain to fre-                                  The scaling can be done as follows; suppose the original
quency domain. After one level DWT an image I is de-                                   mean is M o and the final mean should be M f then the scal-
composed into four sub-bands LL, HL, LH, and HH. LL is
ing parameter can be calculated as M f / M o . We apply scal-               termark is recovered. Using the secret seed S and the in-
ing to individual wavelet coefficients within the 8x8 block.                verse permutation function f (.) we can recover the water-
                                                                            mark.
   Step 5: Inverse Quantization                                                A comparative study is shown in Table 2 which shows the
   We apply inverse wavelet transform to generate the wa-                   proposed algorithm and its comparison with the existing
termarked image. The secret keys used in this algorithm are                 work.
the seed S , the quantization table Q . These are termed as
verification keys.                                                                        IX. RESULTS AND DISCUSSION

                                                                               In order to prove the feasibility of our algorithm we con-
  Step 6: Digital Signature and Time Stamping
                                                                            ducted several experiments. The most classical example
  The keys are digitally signed. Suppose DS = SignKey ( S , Q)
                                                                            image of Lena was used for experiments. This image ( I ) is
Where Sign Key ( S , Q ) is the digital signature signed by the             shown in the following figure. The watermark logo ( W ) that
owner’s private key. This digital signature is now time                     is used in the experiments is shown next. The size of the
stamped by a trusted third party like CA e.g. Verisign. The                 original image is 512x512 pixel grey scale image whereas
CA then computes T = T Key ( D s ) where TKey denotes the                   the size of the watermark logo is 32x32 pixel.
time stamp by the CA’s private key. The time stamped digital
signature is then stored together with the verification keys
and is used for proving the ownership of the content.

 Proper-        Tsai, 2000    Barni,         Chen,          Proposed
 ties                         2001           2003           Algorithm
 Blindness      Yes           Yes            Yes            Yes
 Quantization   Constant      Variable       Variable       Variable
 Step Size
 Quantization   Scalar        Scalar         Scalar         Scalar
 Type
 HVS            No            Yes            Yes            Implicit
 Modeling
 Sub-bands      Middle-low    High           LL             HH, HL, LH                     (a)                         (b)
 Counterfeit    Not Dis-      Not Dis-       Timestamp      Timestamp
 Attack         cussed        cussed
 Visual         Yes           No             Yes            Yes                Fig. 2 (a) Image used for Watermarking
 Recognizable
 Pattern                                                                              (b) Binary Image used as a Watermark
 Robustness     JPEG          Compression,   Compression,   JPEG
 against        Compression   Cropping,      Cropping,      JPEG 2000
 Attacks                      Morphing       Blurring,      Cropping           We first decomposed the image I into four sub-bands and
                                             Noising,       Scaling
                                             Sharpening,    Salt and        embedded the watermark W in all the bands except LL. We
                                             Scaling        Pepper          then extracted the watermark without applying any attacks.
                                                            Gamma
                                                            Correction,     To our surprise the watermark extracted from diagonal
                                                            Rotation (90,
                                                            180 deg)        sub-band (HH) was not similar to the original. This is at-
                                                                            tributed to the rounding errors inherent with the D sub-band
   Table 2: Comparison of various proposed methods                          because of its small values. The extracted watermarks in
                   based on DWT                                             Table 3.
  B. Extraction Algorithm
The extraction algorithm begins by verifying the digital sig-
nature and the time stamp. If the timestamp and the signature
cannot be verified the algorithm doesn’t proceed to the next                         HL               LH              HH
step. If the verification is positive the extraction algorithm
                                                                                Table 3: Extracted watermarks without applying
begins and uses the quantization table Q and the secret seed
                                                                                                  any attacks
S to recover the watermark. The original image in not re-
quired during extraction.                                                     After embedding the watermark the output image is as
                                                                            shown in Fig 3.
   Step 1: Wavelet Transform
The original image I is decomposed by one level wavelet
transform to obtain LL, LH, HL and HH sub-bands.

   Step 2: Watermark Recovery
   For each sub-band except the LL sub-band, starting at the
top left corner we divide the sub-band into non-overlapping
blocks of 8x8 and calculate their mean intensity values of the
wavelet coefficients. These values are then compared with
the quantization table Q to generate the watermark bit. All
the watermark bits are thus generated and the permuted wa-
             Fig 3: Watermarked Lena Image                            Finally we added some salt and pepper noise to see the
                                                                   robustness against this kind of attack and it’s clearly visible
  A. Results                                                       that the watermark is extracted.
   After embedding the watermark we applied seven differ-             Since we incorporate digital signature and time stamping
ent attacks and recovered watermarks which were visually           technique the algorithm is also robust against counterfeit
recognizable. We attacked the watermarked image with the           attacks.
following attacks Cropping, Gamma Correction, JPEG                    The inputs required for the extraction algorithms are not
Compression, JPEG2000, Resizing, Rotation (90, 180 de-             many. We only require the quantization table and the secret
grees), Salt and Pepper. The results are shown in Table 4.         seed for permuting the watermark. If the quantization table
                                                                   is not available we can still manage if we have the quantiza-
    Attacks            HL             LH             HH            tion step size and its range. If the range is not available we
  JPEG 2000                                                        can use identify the range from the watermarked image but
                                                                   this results in reduced quality of the extracted watermark. If
                                                                   the quantization table is not available the extracted water-
                                                                   mark is again of a lower quality as shown in Table 4. Here
  JPEG
                                                                   we constructed the quantization table based on the water-
  Compres-
                                                                   marked image by gathering the range and using the quanti-
  sion
                                                                   zation step size.
                                                                      Our algorithm achieves good results by using smaller
  Gamma                                                            block size. We use a block size of 8x8 to quantize the coef-
  Correction                                                       ficients; however the scheme proposed by Meerwald [23]
                                                                   used block size of 16x16 coefficients or 256 coefficients. At
  Cropping                                                         the same time our algorithm offers more robustness to at-
                                                                   tacks compared to the previously published schemes which
                                                                   are listed in Table 2.
  Resizing                                                            Using this algorithm grey scale image could also be em-
                                                                   bedded as a watermark. We could just consider greyscale
                                                                   image as a set of multiple binary images. The most signifi-
  Rotation 90                                                      cant bits planes of the grey scale image could be embedded in
                                                                   this manner. A grey scale watermark has a greater prob-
                                                                   ability of survival because it preserves the contextual rela-
  Rotation 180                                                     tionship.

                                                                                              X. CONCLUSION
  Salt       and
  Pepper                                                              In this paper we presented a quantization based
                                                                   watermarking algorithm where we used an binary logo as a
             Table 4: Attack Characterization                      watermark. We embedded the logo in HH, HL and LH
                                                                   sub-bands of the first level wavelet decomposition. The
  B. Discussion and Analysis                                       proposed algorithm is shown to be robust against the
                                                                   following attacks Cropping, Gamma Correction, JPEG
   The proposed algorithm is shown to be robust against            Compression, JPEG2000, Resizing, Rotation (90, 180
seven major attacks.                                               degrees), Salt and Pepper.
   Robustness against gamma correction attack is one of the
main features of this algorithm. Although distortions exist
the watermark is still visible.
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