Web-oriented nonblind image watermarking procedure

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Web-oriented nonblind image watermarking procedure
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  Web-oriented nonblind image watermarking procedure Franco Frattolillo University of SannioDepartment of EngineeringResearch Centre on Software TechnologyCorso Garibaldi, 10782100 Benevento, ItalyE-mail: frattolillo@unisannio.it Abstract.  The advances in multimedia and networking technolo- gies have created opportunities for Internet pirates, who can copy digital contents and illegally distribute them, thus violating the legal rights of content owners. In such a situation, digital watermarking has gained popularity as a main technology to implement the copy- right protection of multimedia digital contents distributed on the In- ternet. We present a novel “nonblind” watermarking procedure for JPEG images based on the use of protected extensible markup lan- guage (XML) documents. The procedure enables the copyright owner to insert a distinct watermark code identifying the buyer into the distributed images. Furthermore, to increase the security and robustness levels of the procedure, the watermark is repeatedly em- bedded into an image in the discrete cosine transform (DCT) do- main at different frequencies and by exploiting both block classifica- tion techniques and perceptual analysis. The embedded watermark is then extracted from an image according to the information con- tained in a protected XML document that is associated to the image.  © 2006 SPIE and IS&T.   DOI: 10.1117/1.2234479  1 Introduction Digital watermarking is considered one of the main securitytechniques that can be used for discouraging the unautho-rized trading of multimedia digital contents on the Internet.It is exploited to implement copyright protection systemsbased on embedding the perceptually invisible digital sig-natures of the copyright holders in the contents to be pro-tected, thus enabling the copyright holders to prove theownership of the protected contents by extracting the in-serted signatures. 1–3 The development of watermarking procedures involvesseveral design trade-offs. In particular, watermarking pro-cedures should 1,4 :1. provide a good degree of robustness against themost common, nonmalevolent content manipula-tions, including digital-to-analog conversion anddigital format conversion2. survive intentional attacks3. be imperceptible and convey as much informationas possible4. comprise watermark embedding and retrieval pro-cesses characterized by low complexity, so as tobe effectively exploitable in a web context, whichis commonly considered their typical utilizationscenarioHowever, all of these requirements and the resulting designconstraints appear to be partly contradicting. For example,“blind” watermarking procedures 3 can be considered wellsuited to be adopted in a web context, since they exploitinsertion schemes that do not need the srcinal, nonwater-marked multimedia digital contents to retrieve the hiddeninformation. As a consequence, such procedures do notforce the distinct web entities involved in “identificationand arbitration” protocols 5–7 to exchange unprotected,large-size multimedia digital contents through the insecurecommunication channels usually characterizing theInternet. 6 Furthermore, blind watermarking procedures areusually based on fingerprinting techniques that enable thecopyright owner to insert specific “anticollusion codes”able to identify the buyer within any copy of content that isdistributed. 8,9 The main aim is to make it possible to deter-mine if a user is illegally in possession of a content as wellas who has initially bought and then illegally shared it via,for example, peer-to-peer network applications. 1 However,“nonblind” watermarking procedures are typically consid-ered more robust then blind ones. 7,9 Unfortunately, differ-ently from blind ones, nonblind watermarking proceduresrequire the srcinal, unprotected multimedia digital con-tents to be able to run the watermark extraction algorithmson the corresponding protected or pirated copies. Therefore,the adoption of such watermarking procedures in a webcontext results in being very difficult.This paper presents a web-oriented, nonblind water-marking procedure for JPEG images based on theuse of protected extensible markup language   XML  documents. 10,11 The procedure enables the copyright ownerto insert a distinct code identifying the buyer within eachcopy of the distributed images. Furthermore, to increase theprocedure security and robustness levels, the watermark isrepeatedly embedded into an image in the discrete cosinedomain   DCT   domain at different frequencies and by ex-ploiting both block classification techniques and perceptualanalysis. The embedded watermark is then extracted froman image according to the information contained in a pro-tected XML document that is associated to the image. Thus,the usual security and robustness levels characterizing thenonblind watermarking schemes can be achieved withoutrequiring unprotected, large-size images to be securely ex- Paper 05170R received Sep. 23, 2005; revised manuscript received Jan. 9,2006; accepted for publication Jan. 17, 2006; published online Aug. 23,2006. 1017-9909/2006/15  3   /033011/14/$22.00 © 2006 SPIE and IS&T. Journal of Electronic Imaging 15(3), 033011 (Jul–Sep 2006) Journal of Electronic Imaging Jul–Sep 2006/Vol. 15(3) 033011-1  changed and stored on the Internet whenever the watermarkextraction has to be performed. In fact, keeping XML docu-ments protected or securely exchanging them in a web con-text results nowadays in being rather easy due to the secu-rity technologies developed in the field of XML and “webservices.” 12–15 Moreover, using the XML technology makesit also easier to automate the document access in a webcontext, since XML is a standard technology well sup-ported by the Java world, and standard document parsers,such as Simple API for XML   SAX   and Document ObjectModel   DOM   parsers, are freely available.The paper is organized as the follows. Section 2 presentsthe proposed watermarking procedure. Section 3 briefly de-scribes the XML documents associated to the protected im-ages. Section 4 describes the watermark extraction process.Section 5 reports on some experimental results. Section 6reports conclusion remarks. 2 Watermarking Procedure The proposed watermarking procedure, whose scheme isshown in Fig. 1, makes it possible to insert into a JPEGimage a binary code which can be exploited to unambigu-ously identify a user. The code, which is represented by asequence      0,1   and whose length is denoted as  n   , canbe repeatedly embedded into the image in the DCT domainat different frequencies, denoted as    1 ,    2 ,...,    f  . In fact,each frequency    i  identifies an entry in each 8  8 DCTblock of a JPEG image, and so it can range 16 from 1 to8 2 =64. Furthermore, to increase the security and robustnesslevels of the procedure, the watermark insertion is assumedto be carried out at low, middle, and high frequencies cho-sen on the basis of the image to be watermarked.In principle, all the DCT coefficients at the differentfrequencies    i , with  i =1,...,  f  , could be modified by avalue representing a watermark information. However, inthe proposed procedure, the “perceptual capacity” of thecoefficients belonging to the luminance DCT blocks is pre-liminarily estimated by exploiting both block classificationtechniques and perceptual analysis. In fact, the block clas-sification techniques 16–18 are applied to indicate the bestDCT coefficients that can be altered without reducing thevisual quality. They classify each luminance DCT blockwith respect to its energy distribution by using four classi-fication masks. The possible types of classified blocks are“flat,” “diagonal edge,” “horizontal edge,” “vertical edge,”and “textured block.” The result of this procedure is a firstselection of DCT coefficients whose modification has aminimal or no impact to the perceptual quality of the im-age.The perceptual analysis is then applied to calculate the“just noticeable difference”   JND   values for the DCTcoefficients. 19–21 Such values are the thresholds beyondwhich any changes to the respective coefficient will mostlikely be visible. Therefore, let  X  b m      denote the coeffi-cient at the frequency     in the DCT block  b m , and letJND b m      denote the JND value calculated for the  X  b m     coefficient. We can calculate JND b m      asJND b m       max  C  b m     ,  C  b m      E  b m     g  ,   1  where  C  b m      represents the perceptual threshold of thecontrast masking and is expressed as C  b m      = max   t  b m     ,   X  b m     h t  b m     1− h  ,   2  where  E  b m      is the entropy value calculated over the eightneighbors of the  X  b m      coefficient, 19,20 and can be approxi-mated by the following expression: Fig. 1  Scheme of the proposed watermarking procedure. Frattolillo: Web-oriented nonblind image watermarking procedure Journal of Electronic Imaging Jul–Sep 2006/Vol. 15(3) 033011-2    E  b m       X  b m      −  u b m     q     .   3  In Eq.   1   g  is assumed equal to 0.5. In Eq.   2   h  is assumedequal to 0.7, and  t  b m      is equal to  t       X  b m  1   /   X   1  ,where  X   1   is a dc coefficient corresponding to the meanluminance of the display, whereas  X  b m  1   is the dc coeffi-cient of the block  b m . In fact,  t       can be approximated bythe value  q      /2, where  q      represents the coefficient of the quantization matrix corresponding 20 to the frequency    .Finally, in Eq.   3   u b m      is equal to round    X  b m      /  q     .Once block classification and perceptual analysis haveidentified the DCT coefficients that could usefully host wa-termark information without reducing the image visualquality, the “choice rule” block takes charge of selectingwhich of these coefficients have to be actually modified bythe insertion procedure at each of the  f   frequencies chosenon the basis of the image to be watermarked   see Fig. 1  . Infact, the insertion procedure performed at each frequency   i , with  i =1,...,  f  , assumes that each bit of the user se-quence     is inserted into a given image by altering a pair of selected DCT coefficients associated to that frequency andcharacterized by similar values. More precisely, let  b l    i  indicate the entry at the frequency    i  in the DCT block  l ,where  l  may vary from 1 to  t  JPEG , which denotes the maxi-mum number of DCT blocks contained in a JPEG imagewith given characteristics. Let      i   denote the set of theDCT entries  b l    i   whose associated values have been iden-tified by the block classification and perceptual analysis asvalid candidates at the frequency    i , with  i =1,...,  f  , to hostthe bits belonging to the user sequence    . Let     i   denotethe sequences of the pairs of DCT entries   b m    i  , b n    i  whose values are actually selected to host the bits of theuser sequence     at the frequency    i , with  i =1,...,  f   and  m and  n  being two indices in the range from 1 to  t  JPEG . The“choice rule” states that a pair   b m    i  , b n    i   is allowed tobelong to      i   only if   X  b m    i   X  b n    i  , " i =1,...,  f   andwith  b m    i   and  b n    i   belonging to     i  . This can also besynthesized by the following expression: Fig. 2  Insertion procedure. Frattolillo: Web-oriented nonblind image watermarking procedure Journal of Electronic Imaging Jul–Sep 2006/Vol. 15(3) 033011-3   Consequently, if      is  n    bits long, the process that selectsthe DCT coefficients at the frequency    i  must choose atleast 2 n    coefficients. Therefore, if the insertion frequen-cies are  f  , the total number of DCT coefficients to be se-lected is 2 n    f  . Finally, the cardinality of       i   is equal to n   .The bits of the user sequence     are inserted into animage as symbols obtained by applying the “encoding func-tion”  E  , which must be defined within the watermarkingprocedure. In fact, E  defines an encoding rule by which thebits 0 and 1 are translated to the symbols belonging to thealphabet composed by   ր , ց  , respectively called the upsymbol and the down symbol. Thus, a user sequence      0,1   is translated to a corresponding sequence of sym-bols      ր , ց   depending on the function  E  . For ex-ample, the user sequence   01101...   is translated to thesequence of symbols   րցցրց ...  , if the function E  associates the up symbol to 0 and the down symbol to 1.Let     be a user sequence, and let     be the correspondingsequence of symbols obtained by applying an E   function.Let      j   denote the  j ’th symbol of     , and let    1 ,    2 ,...,    f  be the insertion frequencies. Let  W  b m    i   denote the water-marked DCT coefficient at the frequency    i  in the block b m    i  , and let    i   j   denote the  j ’th pair in the set      i  .The insertion procedure, whose scheme is exemplified inFig. 2, is synthetized by the following expressions: " i  = 1, ... ,  f  , "  j  = 1, ... , n        j   =  ր  Þ  W  b m    i   =  X  b m    i   − JND b m    i  W  b n    i   =  X  b n    i   + JND b n    i      j   =  ց  Þ  W  b m    i   =  X  b m    i   + JND b m    i  W  b n    i   =  X  b n    i   − JND b n    i  ,   b m    i  , b n    i   =   i   j  ,   i   j       i  which assume that the  j ’th symbol      j   derived from     isinserted in the DCT coefficients identified by the  j ’th pair   i   j  =  b m    i  , b n    i   belonging to      i  , " i =1,...,  f   and "  j =1,..., n   . In fact, since the “choice rule” imposes that  X  b m    i   X  b n    i   for each selected pair of DCT coefficients,the insertion process attempts to maximize the differenceexisting between the coefficients of the pair according tothe direction specified by the insertion symbol and by anamount that should not compromise the final visual qualityof an image. Therefore, the insertion process should be car-ried out according to the following main rules:1. The insertion frequencies should be evenly distrib-uted among the low, middle, and high frequencies,and should be chosen so that attacks characterizedby a filtering behavior on an image would end upreducing its final visual quality. This can beachieved by selecting frequencies characterized byhigh spectrum values, which, if filtered, can impairthe image.2. At each insertion frequency, the pairs of the se-lected DCT coefficients should belong to spatialregions that cannot be cropped without impairingthe image.Once the symbols of the sequence     have been insertedinto the image at the chosen frequencies, to increase thesecurity and robustness levels of the watermarking proce-dure against collusion and averaging attacks, 8,9 it is neces-sary to hide the modifications made to the DCT coefficientsof the image. In fact, note that both the set of the insertionfrequencies    i  and the sets      i  , with  i =1,...,  f  , are al-ways the same for all the copies of a given image to beprotected. Consequently, the DCT coefficients modified atthe different insertion frequencies remain the same for allthe copies of the image. Therefore, to prevent malicioususers from individuating the DCT coefficients modified bythe insertion process, the JND values modulated by a bi-nary pseudonoise sequence      −1,1   must be added to allthe unmodified DCT coefficients of a watermarked image.This addition is carried out by the following expression:   X  b k     i   =  X  b k     i   +   k    k  JND b k     i  ,  i    1, ... ,  f    or   i  = 1, ... ,  f    and  b k     i     b m    i  , b k     i     b n    i  ,  b m    i  , b n    i       i     , where 0    k   0.5 is a randomly varied amplitude factor.Finally, note that the proposed procedure directly acts onJPEG compressed images. To this end, as shown in Fig. 1,the incoming JPEG image is processed, and the sequence of Huffman codes, each representing one non-zero DCT coef-ficient of the image in the form of one   run, level   pair, isextracted and decoded. 16 Then, the DCT coefficients se-lected by the procedure are altered by inserting the water-mark information. After the insertion, the DCT coefficientsare Huffman reencoded in the form of    run, level   pairs,thus generating the watermarked version of the image.However, since the number of bits needed to encode a   run,level   pair is set by the JPEG standard and may be the sameeven for different   run, level   pairs, 16 the final size of theimages watermarked by exploiting the proposed procedureends up differing from the size of the corresponding srci-nal images by a small amount essentially determined by thenumber  f   of the chosen insertion frequencies. In fact, theconducted tests have determined increments in the size of the watermarked images about 4 to 9%, with  f   varying inthe range   3,15  . Frattolillo: Web-oriented nonblind image watermarking procedure Journal of Electronic Imaging Jul–Sep 2006/Vol. 15(3) 033011-4   3 XML Documents The characteristics of the proposed watermarking proce-dure make it necessary to save the insertion informationused to protect the copies of an image to make it possible tocarry out the watermark extraction. To this end, as reportedin Sec. 1, the proposed procedure assumes that such infor-mation is stored in XML documents. In particular, the XMLdocument associated to all the watermarked copies of animage, whose internal structure is out of the scope of thepaper and is not shown for the sake of brevity, has to in-clude:1. the insertion frequencies    1 ,    2 ,...,    f  2. the sets     i  , " i =1,...,  f  3. the encoding function E  Fig. 3  Geometric resynchronization of “Lena.” Frattolillo: Web-oriented nonblind image watermarking procedure Journal of Electronic Imaging Jul–Sep 2006/Vol. 15(3) 033011-5 
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