A Novel DCT-based Approach for Secure Color Image Watermarking

Transcription

1 A Novel DCT-based Aroach for Secure Color Image Watermarkng Narges Ahmd Amrkabr Unversty of Technology Reza Safabakhsh Amrkabr Unversty of Technology Abstract In ths aer, focusng on vsually meanngful color mage atermarks, e construct a ne dgtal atermarkng scheme based on the Dscrete Cosne transformaton. The roosed method uses the senstvty of human eyes to adatvely embed a atermark n a color mage. In addton, to revent tamerng or unauthorzed access, a ne atermark ermutaton functon s roosed, hch causes a structural nose over the extracted atermark. Also, e have roosed a rocedure to elmnate ths nose to decrease false ostves and false negatves n the extracted atermark. The exermental results sho that embeddng the color atermark adated to the orgnal mage roduces the most mercetble and the most robust atermarked mage under geometrc and valumetrc attacks.. Introducton The rad evoluton of the Internet technology makes the transmsson of dgtal multmeda content easer. So, dgtal atermarkng technques are roosed for coyrght rotecton or onersh dentfcaton of dgtal meda. A dgtal atermarkng technque must meet the follong three requrements: Imercetblty: Accordng to the Human Vsual System (HVS) arameters, the atermark embedded n the dgtal mage should be ercetually nvsble to human eyes. Robustness: The embedded atermark must be dffcult (hoefully mossble) to detect or delete, and any attack should result n severe degradaton n fdelty. Securty: The securty of the atermarkng system reles on the use of secret keys knon exclusvely by the coyrght oner. All the arameters usng n embeddng rocess can be a art of secret key. These keys are requred n the extracton rocess. In general, there s a tradeoff beteen the atermark embeddng strength (the atermark robustness) and qualty (the atermark nvsblty). Increased robustness requres a stronger embeddng, hch n turn ncreases the vsual degradaton of the mages. The roosed atermarkng scheme adots a color mage as the atermark so human eyes can easly verfy the extracton of ths vsually meanngful atermark. In general, a color mage can rovde more ercetual nformaton [].e. suffcent evdence aganst any llegal coyrght nvason. In sectons and 3, e ll descrbe the roosed embeddng and extracton rocesses. Secton 5 gves the exermental results. We have evaluated robustness and nvsblty of the roosed scheme and comared them to those of Tsa's [], s [,5], Frdrch's [3, 6] and 's [4] methods under varous attacks such as JPEG lossy comresson, medan flterng and crong.. Embeddng Process As shon n Fg., the roosed rocess embeds an M M color atermark mage W n an N N orgnal mage X usng secret keys K for better securty. All the relatons used n the embeddng rocess are nvertble. The follong aragrahs descrbe the model brefly. Y X W Secret keys (K) IDCT RGB YIQ Yˆ X Y Permutaton Yˆ DCT Embeddng aa DCT Fgure. The roosed embeddng rocess Intally, the orgnal mage X s converted to the NTSC color sace. One of the man advantages of the NTSC color sace s that the grayscale nformaton s searated from color data; as a result, the same sgnal can be used for both color and black-and-hte mages. Then the orgnal mage lumnance comonent Y s dvded nto non-overlang 8 8-xels blocks, and each block s ndeendently transformed to frequency doman reresentaton through D-DCT. W Frequency selecton Ŵ Ŷ mf Adataton Unt HVS Proceedngs of the Internatonal Conference on Informaton Technology: Codng and Comutng (ITCC 4)

2 A beneft of such a scheme s that f the mages are stored as comressed JPEG bt streams, the atermark can be nserted drectly nto the artally decomressed bt stream...mddle -frequency coeffcents selecton DCT concentrates the sgnal energy n the lo-frequency bands. So, embeddng the atermark n loer frequences roduces more robust atermarked mages th loer qualty. Hence, to meet both the nvsblty and robustness requrements, mddle-frequency coeffcents of the transformed blocks are selected n a zgzag order to embed the dgtal atermark n. The total number of selected coeffcents n each block, denoted as t, s determned by Eq. () []. t = ( 8M / N ) The frst coeffcent s selected randomly usng a secret key K as the seed of a seudo-random number generator (PRNG). All the selected coeffcents comose a transformed M M mageŷ mf. Furthermore, by usng a seed value K, the oston of each coeffcent n the related block n Ŷ mf s selected... Permutaton of the atermark For gettng more securty and more robustness W must be shuffled before embeddng nto X []. Usng a random ermutaton ll add a nose to the extracted atermark (Fg. 6). Here, e roose a D ermutaton functon that generates a structural nose over the extracted atermark that can be modeled (Fg. 8). Consequently, e roose a ost-rocessng algorthm to elmnate ths nose. Ths algorthm ll be descrbed n secton 3. The roosed ermutaton functon s nsred from automorhsm. A one-arameter automorhsm s as follos [8]: = j k3 mod M 3 k + j It means that each xel at coordnates (, j) of the atermark mage s moved to (, j ) usng Eq. (). The number of transformatons erformed on the atermark mage (K 4 )and k 3 are ket as secret keys. In the mage ermutaton alcatons, ths functon s erodc th erod R. The erod deends only uon the arameters k 3 [, M-] and M. Fgure shos the erodc roerty of the ermutaton here arameters of Eq. () are k 3 = and M=8. It shos that for these arameters the erod s equal to 96. () () (c) Fgure. Permuted atermark after tmes th Eq. (), here a) =, b) =5, and c) =96.3. Adatng atermark to the orgnal mage For more securty and mercetblty, the atermark strength must deend on the ntensty values of the DCT coeffcents of the orgnal mage. In ths ay, the atermark sgnal ll be strengthened n large DCT values, and attenuated n small ones. Ths adataton rocedure s fulflled through the follong stes: ) Snce the number of blocks n both the atermark mage and the orgnal mages must be equal, the ermuted atermark s dvded nto non-overlang nxn blocks, here n = 8M / N. Then each block s DCT transformed ndvdually to obtan Ŵ. ) A block-based mean s calculated over Ŵ usng Eq. (3). ( M / n ) f ( ) ( ) ˆ = W = (3) ( M / n ) here reresents the block number and W ˆ ( ) s the th block n Ŵ, and f () s an nxn matrx used for the eghted averagng. The result ( Ŵ ) s an nxn matrx. 3) Usng Eq. (5), Each block n Ŵ s normalzed and then adated to Ŷ mf by multlyng t by Yˆ mf hch s the mean of all coeffcents n Ŷ mf. ( ) a = Yˆ mf (4).4. Inserton rocess A good vsual model should rovde the maxmum atermark strength and sze that can be nserted thout vsual dstortons. We roose Eq. (5) for embeddng the adated atermark n a Ŷ mf. ˆ ˆ aa ( ) = Ymf ( ) + α W a ( ), α (5) Here, reresents the current block number, and α s calculated va HVS arameters to meet nvsblty and robustness requrements. Consequently, a one-to-one relacement of each coeffcent n th the corresondng one n Ŷ s aa erformed to obtan Ŷ. Ths rocedure needs the secret Proceedngs of the Internatonal Conference on Informaton Technology: Codng and Comutng (ITCC 4)

3 keys K and K, and the zgzag attern to fnd the exact lace of each coeffcent. If the amltude of such atermarkng scheme ( α ) s ncreased to thart flterng attacks for a robustness, the atermark may become vsble. The just notceable dfference threshold (JND) α can be used to determne the maxmum amount of the atermark sgnal that can be tolerated at each regon n the mage thout affectng ts vsual qualty. In an mage-deendent adataton, α s calculated based on HVS roertes: frequency senstvty, lumnance senstvty and contrast maskng. We ll sho that α s ndeendent from the orgnal mage. Fnally, an 8 8 nverse-dct s aled to Yˆ to obtan NTSC format of the atermarked mage ( Y ). Consequently, th a color sace converson the atermarked mage ( X ll be obtaned. 3. Extracton Process The detecton rocess roosed n ths aer s dected n Fg. 3. The recovered atermark W ~ s extracted from the ossbly dstorted mage X ~ usng the orgnal magex and the secret keys. Secret keys (K) X ~ YIQ Y DCT Yˆ Frequency selecton Inverse W ~ Post -rocess Fgure 3. The roosed extracton rocess Intally, the attacked atermarked mage X ~ s converted to the NTSC color sace and then dvded nto non-overlang 8 8 blocks, and each block s DCT transformed ndeendently to obtan Y ˆ. After extractng t coeffcents from each block n Y ˆ, reverse adataton ll be aled to obtan W ~ W ~ IDCT ~ Inverse W Adataton ~ W aa W ˆ aa and W ~, resectvely. Next, the nverse DCT can be aled to each n n block n W ~ to obtan the satal-doman atermark mage W ~. Fnally, functon s erformed (R-K 4 ) tmes over W ~ to obtan the embedded atermark (W ~ ). A more relable detecton rocess has loer false ostves and false negatves n the extracted atermark. So, e roose a ost-rocessng rocedure to obtan a X hgher qualty atermark, esecally for human eyes. The roosed extracton scheme decreases both false negatves and false ostves. 4. Evaluaton Process By usng W ~ and the regstered atermark W, the arbtrator can verfy the onersh of the orgnal mage. The evaluaton of the atermarked mage nvsblty s calculated th Masked Peak Sgnal to Nose Rato (MPSNR) that s a ercetual qualty metrc that exlots the contrast senstvty and maskng henomena of the HVS and s based on a mult-channel model of the human satal vson [7]. Mc MPSNR ( vdb) = log( ) (6) MSE here MSE s the Mean Squared-Error beteen X and X. Values of X are beteen and Mc. The extracted atermark robustness s evaluated by Normalzed Correlaton () rato beteen W andw ~ [7]. In comarng to atermarkng systems, the system th hgher MPSNR value has better vsual qualty, and under an assumed attack, the system th hgher value s more robust [7]. 5. Exermental Results In our exerments, a RGB-color Mnature (Mornng Star mnature, anted by Mahmud Farshchan) th 5 5 xels, and a 56-color atermark (Mahmud Farshchan's hoto) th 8 8 xels are used. After erformng the roosed scheme, the MPSNR value of the atermarked mage (Fg. 4) s equal to 57.9vdB, hch ndcates a hgher qualty atermarked mage n terms of human vsual system. The value of the extracted atermark form an unattacked atermarked mage s equal to ;.e. no degradaton n the extracted atermark. In Tsa's method vsual qualty s 39.65dB and value s 9. Fgure 4. Watermarked mage, MPSNR=57.9 vdb Extracted atermark, = As shon n Fg. 5, for α <. the embeddng strength arameter α ncreases, the nvsblty decreases and the robustness ncreases. But for α >., hgher Proceedngs of the Internatonal Conference on Informaton Technology: Codng and Comutng (ITCC 4)

4 values for α results n less robust atermarked mage. On the other hand, based on the HVS, an MPSNR value hgher than 3 s sutable [7]. So, e selected α =. hch results n MPSNR=57.9vdB and = for an assumed attack such as 75% crong over the atermarked mage. The roosed system as also tested th dfferent orgnal mages and th ths secfc atermark. The results ere the same as above; so the vsual qualty of the roosed scheme s ndeendent of the orgnal mage. 5.. The ost rocessng unt advantages The roosed scheme has been tested usng a PRNG for atermark ermutaton, and an assumed attack such as mage crong to sho the benefts of ost-rocessng. Images shon n Fgs. 6 and 6 are a atermarked mage after 75% crong and addtve-nose attacks, resectvely. Fgs. 6(c) and 6(e) are the extracted atermarks, usng seudo-random atermark ermutaton thout ost-rocessng. Fgs. 6(d) and 6(f) are the extracted atermarks usng seudo-random atermark ermutaton and ost-rocessng. Accordng to the calculated ratos for each extracted atermark, t s obvous that erformng ost-rocessng over an extracted atermark mroves qualty of the oututs,.e. less false ostves and false negatves n the extracted atermark. As shon n Fg. 7, usng the embeddng rocess th seudo-random ermutaton, the vsual qualty of a ostrocessed atermark (Rost) s hgher than that of the atermark thout ost-rocessng (Rand) n the extracton algorthm. It shos that even usng seudorandom atermark, the robustness of the roosed scheme s better than Tsa's method. So, usng ost-rocessng mroves the vsual qualty of the extracted atermark. 5.. ermutaton advantages In Fg. 7, the robustness of a seudo-random ermutaton atermark (Rost) and a D ermutaton () are beng comared under varous mage crong ratos. Ths fgure shos that usng ermutaton mroves the atermark robustness and removng addtve nose after atermark extracton mroves the extracted atermark qualty. Fg. 8 shos the extracted atermark from a 75% croed atermarked mage (Fg. 6). Comarng Fg. 6(d) and Fg. 8, although the values are close to each other, the ercetual qualty mrovement s notceable Robustness aganst attacks Fgure 9 llustrates that the value ll only slghtly decrease even for the hghest JPEG lossy comresson ratos. Qualty Embeddng strength MPSNR Fgure 5. Embeddng strength ( α ) versus vsual qualty (MPSNR*.) and robustness () n the roosed embeddng rocess (c) (d) (e) (f) Fgure 6. Usng seudo-random ermutaton n the embeddng rocess, 75% croed atermarked mage, nosy atermarked mage, (c) extracted atermark from a, =5, (d) c after ost-rocessng, =9, (e) extracted atermark from b, =, (f) e after ostrocessng, = Croed to N% of orgnal mage Tsa Rost Rand Fgure 7. Comarson of robustness beteen the roosed scheme (), Tsa's method, seudo-random ermutaton thout ost-rocessng (Rand) and seudorandom ermutaton th ost-rocessng (Rost) aganst an assumed attack (mage crong) Fgure 8. Extracted atermark usng ermutaton n the embeddng rocess thout ost-rocessng, =6, th ost-rocessng, = Proceedngs of the Internatonal Conference on Informaton Technology: Codng and Comutng (ITCC 4)

5 Fgure shos the robustness of the roosed scheme under varous medan-flter szes n comarson to that of other methods. Exerments sho that the atermark s successfully extracted from the atermarked mage even f the mage undergoes more than once blurrng attack. On the average, the roosed method s the most robust system under medan flter attack. Furthermore, the values do not decrease hen sze of the flter ncreases. As shon n Fg., the roosed scheme s the most robust method aganst mage crong attack. 6. Conclusons In ths aer, e roosed a ne block DCT-based dgtal atermarkng scheme, hch nserts an adated coloratermark n the mddle-frequency coeffcents of the orgnal color mage transform. The roosed adatve embeddng rocedure mroved robustness and nvsblty of the atermarked mage. Image-deendent adatatons not only take the advantage of HVS frequency senstvty, but also rely on adatng the atermark to local mage roertes n order to rovde maxmum erformance n terms of robustness, hle satsfyng the nvsblty constrant. For gettng hgher securty and hgher robustness, a D erodc ermutaton functon has been descrbed hch mroved robustness and ercetual qualty of the extracted atermark. Consequently, to decrease both false ostves and false negatves n the extra cted atermark, a ost-rocessng algorthm as roosed hch mroved the vsual qualty of the extracted atermark. The vsual qualty of the roosed scheme as ndeendent of the orgnal mage. The exermental results shoed that the roosed scheme as the most robust method, esecally for hgher lossy comresson ratos. Also, on the average, t as the most robust system under blurrng attacks. Indeed, the atermarked mage robustness dd not decrease hen blurrng ncreased due to larger flter sze. In addton, the roosed scheme as the most robust method under mage crong attacks. 7. References [] C-S Tsa, C-C Chang, T-S Chen, and M -H Chen, Dstrbuted multmeda databases: Technques and Alcatons, Natonal Chung Chang Unversty, and Natonal Tachung Insttute of Technology, Taan,. [] I.J., J. Klan, T. Leghton, and T. Shamoon, Secure sread sectrum atermarkng for multmeda, IEEE Trans. on Image Processng, Vol. 6, No., 997, [3] J. Frdrch, Combng lo-frequency and sread sectrum atermarkng, SPIE Symosum on Otcal Scence Engneerng and Instrumentaton, San Dego, USA, July 998. [4] E. and J. Zhao, Toards robust and hdden mage coyrght labelng, IEEE Internatonal Worksho on Non- Lnear Sgnal and Image Processng, Neos Marmaras, Greece, June 995, [5] I.J. and M.L. Mller, Robust dgtal atermarkng, Unted States Patent, atent number 67879, August,. [6] J. Frdrch, Image atermarkng for tamer detecton, IEEE Internatonal Conf. On Image Processng (ICIP 98), Vol., 998, [7] M. Kutter and F.A.P. Pettcolas, Far evaluaton methods for mage atermarkng systems, Journal of Electronc Imagng, Vol. 9, No. 4, October, [8] G. Voyatzs and I. Ptas, Chaotc mxng of dgtal mages and alcatons to atermarkng, Euroean Conf. on Multmeda Alcatons, Servces and Technques (ECMAST 96), Vol., 996, [9] R.B. Wolfgang, C.I. Podlchuk, and E.J. Del, Percetual atermarks for dgtal mages and vdeo, IEEE roc., Vol. 87, No. 7, July 999, JPEG comresson qualty Tsa Frdrch Proosed method Fgure 9. Comarson of robustness beteen the roosed scheme and other methods aganst JPEG lossy comresson attack NxN medan flter Tsa Frdrch Prosed method Fgure. Comarson of robustness beteen roosed scheme and other methods aganst blurrng attack Croed to N% of orgnal mage Tsa Frdrch Fgure. Comarson of robustness beteen roosed scheme and other methods aganst mage crong attack Proceedngs of the Internatonal Conference on Informaton Technology: Codng and Comutng (ITCC 4)