Buyerseller watermarking protocol is defined as the practice of imperceptible altering a digital content to embed a message using watermarking in the encryption domain. This protocol is acknowledged as one kind of copyright protection techniques in electronic commerce. Buyerseller watermarking protocol is fundamentally based on publickey cryptosystem that is operating using the algebraic property of an integer. However, in general usage, digital contents which are handled in watermarking scheme mostly exist as real numbers in frequency domain through DCT, DFT, DWT, etc. Therefore, in order to use the watermarking scheme in a cryptographic protocol, digital contents that exist as real number must be transformed into integer type through preprocessing beforehand. In this paper, we presented a new watermarking scheme in an encrypted domain in an image that is based on the blockDCT framework and homomorphic encryption method for buyerseller watermarking protocol. We applied integralprocessing in order to modify the decimal layer. And we designed a directionadaptive watermarking scheme by analyzing distribution property of the frequency coefficients in a block using JND threshold. From the experimental results, the proposed scheme was confirmed to have a good robustness and invisibility.
1. INTRODUCTION
Advancement of computing and the networking technology, which enables highspeed calculation and ability to connect each person via internet have been growing very rapidly over the past decade. Like a twosided coins, while this advancement provided availability for users to perform various operations like composing, editing, transforming, duplicating, and fast distribution of numerous multimedia contents. It also permits malicious users to do unauthorized duplication and redistribution easily. Therefore, this technology is considered bring detrimental effect upon the protection of intellectual property right.
There are various kinds of techniques to solve these problems. Among them digital watermarking techniques are considered one of the most promising solutions. The idea of digital watermarking techniques is to embed tiny signals or message in certain digital contents in order to keep ownership information. So, at discovery of a pirate copy sometime later, the owner can assert the lawful ownership of the original digital contents through the detecting process of watermarks
[1

3]
.
In several company which utilized watermarking techniques, it has been studied cryptographic protocols that can offer a reliable business transaction process to all of the parties in electronic commerce, and buyerseller watermarking protocol is one of those protocols. Buyerseller watermarking protocol combines a cryptographic protocol with a digital watermarking scheme. This method will provide clarification of copyright responsibility at discovery of a pirate copy by some illegal users.
In the previous study, symmetric protocols were proposed. But, unfortunately, they were designed for both a buyer and a seller can access watermarked contents. Accordingly, when it is found an illegal replica, they have the customer's right problem which is not able to clarify responsibility issue
[4]
. Therefore, asymmetric protocols which were proposed after that time, is designed as a buyer can be only able to access watermarked contents while doing the transaction. In this way, they solved the problem of clarifying the responsible person at discovery of a pirate copy in symmetric protocols
[5

7]
. However, these protocols also have another problem. Once the seller discovers a pirated copy, it is possible for her to transplant the watermark embedded in the illegal copy into another copy of a digital content, provided both copies are sold to the same buyer. This problem is called unbinding problem. ChinLaung Lei et al.
[8]
proposed a watermarking protocol that solves unbinding problem by designed buyerseller watermarking protocol based on anonymous certificates and one time publickey cryptosystem for seller not to use the watermark embedded in the illegal copy, in the event of discovery.
In watermarking techniques for digital images as one of the representative digital contents, it is desirable to embed watermarks in the frequency components based on DFT, DCT, DWT and etc. for robustness, invisibility and capacity purpose. In the buyerseller watermarking protocol, publickey cryptosystem is operated in the basis of the algebraic property of an integer, but generally, images which are handled in most watermarking schemes exist as real number in frequency domain. These data type differences produce some problems in combining watermarking techniques with cryptographic protocols directly. But, previous researchers are mostly had not considered on how to implement watermarking scheme in an encrypted domain.
Therefore, in this paper, the author presented a study of a new watermarking scheme in an encrypted domain in an image that is based on the blockDCT framework and homomorphic encryption method for buyerseller watermarking protocol. Our study started with investigation about how to divide frequency coefficients exists as real number into integer and decimal layer in order to implement watermarking scheme in a publickey cryptosystem. We applied integralprocessing in order to modify the decimal layer. Also, for robustness and invisibility requirements in watermarking scheme, we designed a directionadaptive watermarking scheme based on locally edgeproperties of each block in an image by analyzing distribution property of the frequency coefficients in a block using JND threshold.
2. RELATED WORKS
Previous research in cryptographic protocols utilized watermarking scheme to embed the information into the image. Unfortunately, there is an issue with data type compatibility between the publickey cryptosystem and the targeted object of operation (frequency domain) in those protocols which use the algebraic property of an integer and real number respectively. Therefore, it is difficult to embed the information with frequency coefficients in encrypted domain. But almost none of previous researchers were considered to apply watermarking scheme in the protocols.
For the implementation of watermarking scheme in watermarking protocol, Kuribayashi and H. Tanaka
[9]
proposed a new scheme that combines ownership information, a kind of binary bit stream, with frequency coefficients transformed integer type through requantization method in encrypted domain. In this scheme, frequency coefficients transformation by integralprocessing is employed which resulting an easy watermarking application scheme in cryptographic protocol. But, due to the larger reconstruction of quantization table based on the original image, this scheme is vulnerable to JPEG compression attack by the original quantization table, which usually processed in the size of 8×8 pixels. Moreover, the watermarks are easily removed by JPEG compression and reverse of even and odd number, because after presetting a quantization coefficient to the nearest even number, a requantized coefficient is added to '1' when the embedded bit is '1', otherwise it is leaved intact.
Fig. 1
shows requantization method.
Requantization procedure.
And Lee et al.
[10
,
11]
proposed another watermarking scheme for 3D vector contents in the Anonymous buyer seller watermarking protocol. Basically, this method works in the memon’s
[7]
buyer seller watermarking protocol. But in this watermarking scheme, to guarantee buyer’s anonymity in the purchase process, Lei’s watermarking generation sub protocol is applied. So this scheme can overcome piracy tracing, customer's right, and unbinding problems.
Other related method which has an important role in the proposed method is homomorphic encryption based on bilinear pairing on elliptic curve. It is a form of encryption where one can perform a specific algebraic operation on the plaintext by performing an algebraic operation on the ciphertext. In 2007, Goh
[12]
proposed a homomorhic encryption using bilinear map on elliptic curve. This cryptographic security scheme has been proven by its ability to solve the difficult subgroup decision problem on elliptic curve. It is similar to Paillier and OkamotoUchiyama cryptosystem. In this algorithm, key generation, encryption, decryption and homomorphic properties are elaborated as follows.

Key generation

Encryption

Decryption

Homomorphic properties
And many buyersellers watermarking protocols have proposed and the literature is rich in the relevant area
[13

18]
. Since the first introduction of the concept, several alternative design solutions have following problems;
Block diagram of the proposed watermarking scheme.
Region segmentation in a block.
In the
Table 1
, the feature of several buyerseller watermarking protocols are explained.
Feature of buyerseller watermarking protocols
Feature of buyerseller watermarking protocols
3. PROPOSED WATERMARKING SCHEME
In buyerseller watermarking protocol, generally the protocol consists of 3party nodes which include the buyer, the seller, and reliable certification authority. Also according to procedure of the protocol, it is divided into three subprotocols. They are watermark generation protocol, watermarking protocol and dispute resolution protocol. We focused our research on watermarking protocol to apply watermarking scheme in encrypted domain.
Firstly, the buyer creates two keys, encryption key
pk_{B}
and decryption key
sk_{B}
. And the buyer encrypts embedded watermark
w
with encryption function
E_{pkB}
( ). Secondly, the seller the seller receves encrypted watermark
E_{pkB}
(
w
) and
pk_{B}
from the buyer and encrypts contents for sale,
contents
, with
E_{pkB}
( ). So,
E_{pkB}
(
contents
) is formed. Then, the seller combines
E_{pkB}
(
w
) with
E_{pkB}
(
contents
) through the homomorphic operation and sends
E_{pkB}
(
w
) ⊕
E_{pkB}
(
contents
) to the buyer where “⊕” a linear operator. At this time, due to homomorphic operation on publickey cryptosystem,
E_{pkB}
(
w
) ⊕
E_{pkB}
(
contents
) =
E_{pkB}
(
w
⊕
contents
). Fanally, the seller sends the encryption contents
E_{pkB}
(
w
⊕
contents
) to the buyer and the watermarked contents can be obtained with decryption function
D_{skB}
( ).
 3.1 Embedding scheme
In image watermarking, the existing techniques can be classified into two broad categories based on their working domain, they are spatialdomain and frequencydomain techniques. Generally, it is known that frequencydomain watermarking schemes are stronger than spatialdomain one about requirements of the watermarking techniques i.e. robustness, invisibility, etc. Therefore, on a blockDCT framework, our proposed scheme is to embed watermarks into suitable frequency coefficients using JND threshold and is designed with consideration of blind detection. The embedding process is as follows:
1)
Adaptive decision of the watermarked blocks
: First, we get frequency coefficients using block DCT, the size of block is 8×8, and then each block is separated into vertical, horizontal and diagonal regions as shown in
Fig. 4
.
Compression quality q vs. BER (Bit Error Rate).
For the analysis of the distribution characteristic of AC components, JND (just noticeable difference) is calculated considering frequency, luminance and contrast sensitivity based on HVS (human visual system). We apply Watson’s method
[10]
due to its well performance. But, for the improvement of calculating speed, we apply S. Suthaharan’s method
[11]
to obtain frequency sensitivity. Next, using partitioned regions and JND, embedded blocks are chosen adaptively according to the following rules.
Where
J_{n}
the nth block in an image is,
k_{p,q,n}
is frequency coefficient in each block
J_{n}
. And
Th
. is threshold. When
J_{n}
is chosen for the watermark embedding, the watermarked image coefficient
is determined using equation (6).
2) Dividing DCT coefficients into integer and decimal layers
: Considering watermark insertion based on publickey cryptosystem, in this step, we divide frequency coefficients which appear as real number into integer and decimal layers with the decimal layer is modified integer type through integralprocessing as shown in equation (8).
Where
is interger part on
is Decimal part and
is the integer type one about
l
is the length of significant figures under a decimal point.
3) Watermark insertion
: Through the previously mentioned step, the obtained layers are combined with encrypted water marks using (9)~(17).
If watermark = 1 and
=
k
_{1,0,n}
or
k
_{0,1n}
, then
else if watermark = 1 and
=
k
_{1,1n}
, then
Where
are watermarked and become
is quantization table.
 3.2 Extracting scheme
The extraction process is simple. Because watermarked contents that are delivered to the buyer after the watermark embedding process are decrypted. So, watermark extraction process is performed simply without encryption and decryption key.
Where
is extracted watermark and
Q_{p,q}
is quantization table. We alter AC
_{1}
, AC
_{2}
and AC
_{4}
that have same quantization step size and design. The difference between AC
_{1}
and AC
_{2}
is more than or equal to
Q_{p,q}
/2 in the watermark embedding process. So, using equation (18)~(22), watermarks can be extracted simply.
4. EXPERIMENTAL RESULTS
In these experiments, we use five MATLAB standard images that have different frequency characteristics "Airplane", "Barbara", “Co”, Girl”, and "Gold". They have 256 level gray level with the size of 512×512. For the embedded information we use a combination of the English alphabets and Arabian figures which have been converted into the bit stream using 8 bit extended ASCII codes. In all experiments, the number of characters of embedded information was 35, and the length of the converted bit stream was 280. At the same time, we scrambled each bit for the watermarking security. We set the threshold value (Th.=5) and the quality factor (q=10)considering robustness of watermarks against JPEG compression in the following simulations. To evaluate the proposed watermarking scheme, we estimated the invisibility parameter using PSNR (peak signaltonoise ratio) between original images and watermarked one and evaluate robustness against JPEG compression by measuring number of error bits in watermarks as shown in
Table 2
and
Fig. 4
.
The number of watermarkallowable blocks, embedded bits, and PSNR in each image
The number of watermarkallowable blocks, embedded bits, and PSNR in each image
In JPEG compression simulations, error bits appeared when quality factor q is 50, but, because we inserted same watermarks several times over and set q = 10, it is possible to extract watermarks until quality factor q reach 10.
5. CONCLUSIONS
Watermarking scheme which is based on DCT and homomorphic encryption has some application difficulties in buyerseller watermarking protocol due to incompatibility of data type in publickey cryptosystem. We learned that Kuribayashi and H. Tanaka's scheme employed integralprocessing quantization to transform frequency coefficients which make it easier to apply watermarking scheme in cryptographic protocol but unfortunately, this method also showed some disadvantages in robustness issue due to its watermarks are easily removed by JPEG compression and reverse of even and odd number. To improve the robustness and invisibility of the previously available method, the author proposed a new watermark embedding algorithm into frequency coefficients considering edgeproperties of each block with JND threshold. Also, the author managed to perform embedding process by divided frequency coefficients into integer and decimal layer and modified decimal layer into integer type, damage of frequency information is less than Kuribayashi and H. Tanaka's scheme. Moreover, by considering the size of general contents and watermarks after encryption procedure, the amount of insertion bit is relatively small in proposed scheme.
Therefore, further research about techniques that can increase the amount of insertion information is needed in the future.
Original image Airplane (topleft), watermarked image(topright), watermark allowable block(bottomleft), difference image between original and watermarked one(bottomright).
Original image Barbara (topleft), watermarked image(topright), watermark allowable block(bottomleft), difference image between original and watermarked one(bottomright).
Original image Co (topleft), watermarked image (topright), watermark allowable block (bottomleft), difference image between original and watermarked one(bottomright).
Original image Girl (topleft), watermarked image (topright), watermark allowable block (bottomleft), difference image between original and watermarked one (bottomright).
Original image Gold (topleft), watermarked image (topright), watermark allowable block(bottomleft), difference image between original and watermarked one (bottomright).
BIO
TeakYoung Seong
received a B.S. and a M.S. degrees from Busan University of Foreign Studies in 2004 and 2006 respectively. He is doing a Ph. D. course in Interdisciplinary Program of Information Security at Pukyong National University. His research interests include multimedia signal and image processing, digital watermarking.
GiChang Kwon
received a B.S., a M.S., and Ph.D.degrees from Andong National University in 1985, Daegu Univsrsity in 1993, and Yeongnam University in 2000. Currently, he is a professor in Department of IT Cooperative System at Geongbuk Provincial College. His research interests include multimedia contents and image processing, digital contents and smart system.
SukHwan Lee
He received a B.S., a M.S., and a Ph. D. degrees in Electrical Engineering from Kyungpook National University, Korea in 1999, 2001, and 2004 respectively. He is currently an associate professor in Department of Information Security at Tongmyong University. His research interests include multimedia security, digital image processing, and computer graphics.
KwangSeok Moon
He received the B.S., and M.S., and Ph.D degrees in Electronics Engineering in Kyungpook National University, Korea in 1979, 1981, and 1989 respectively. He is currently a professor in department of Electronic engineering at Pukyong National University. His research interests include digital image processing, video watermarking, and multimedia communication.
KiRyong Kwon
He received the B.S., M.S., and Ph.D. degrees in electronics engineering from Kyungpook National University in 1986, 1990, and 1994 respectively. He worked at Hyundai Motor Company from 19861988 and at Pusan University of Foreign Language from 19962006. He is currently a professor in Department of IT Convertgence and Application Engineering at the Pukyong National University. He has researched University of Minnesota in USA on 20002002 with PostDoc. and Colorado State University on 20112012 with visiting professor. He is currently the General Affair Vice President of Korea Multimedia Society. His research interests are in the area of digital image processing, multimedia security and watermarking, bioinformatics, weather radar information processing.
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