Data Hiding and Image Authentication for Color-Palette Images

Transcription

1 Data Hdng and Image Authentcaton for Color-Palette Images Chh-Yang Yn ( 殷志揚 ) and Wen-Hsang Tsa ( 蔡文祥 ) Department of Computer & Informaton Scence Natonal Chao Tung Unversty 00 Ta Hsueh Rd., Hsnchu, Tawan 300, R.O.C. Tel: Emal:whtsa@cs.nctu.edu.tw Abstract In ths paper, a novel method for smultaneously embeddng bnary data and fragle watermark sgnal wthn dgtal color-palette mages s proposed. The color-palette of an mage s frst re-sorted from dark-to-brght, whch makes the palette colors n the unque order even when the mage s re-saved by other applcatons. Moreover a data hdng method for replacng the palette ndex s desgned. The even-odd relaton between two pxels s utlzed for hdng any form of bnary data. Fnally, a novel method that embeds fragle watermark sgnals wthn color-palette mages s also proposed n ths study. The fragle watermark sgnals are embedded n the central pxel of every 3 3 block. Expermental results show the feasblty and practcablty of the proposed approaches. (KEYWORD: Data Hdng, Image Authentcaton, Fragle Watermark, Color-Palette Image). Introducton Wth the rapd growth of dgtal technques, the lfe-style of human bengs, the socal structures, and the contents of cvlzaton, have changed hugely. It s just started, and s unstoppable and unavodable. In the transton from the old age to the dgtal world, the dgtzaton of a cvlzaton s asset has become a very mportant ssue. Dgtzaton does provde many advantages. Frst, dgtal data could be preserved permanently wthout any degradaton. And data dstrbuton va the Internet s easy and fast. The management of dgtal data s easy, too, whch facltates people to quckly fnd what they want. But dgtzaton, on the other hand, does reveal some new problems, too. Generally speakng, t s dffcult to determne the fdelty or ntegrty of dgtal vsual data because the development of dgtal processng technologes has made producton of forgery an easy job. The problem orgnates from the ntrnsc features of dgtal nformaton: () makng copes s easy and cheap; () each copy s exactly the same as the orgnal one; (3) dstrbuton va the Internet s easy and fast. The ease of copyng and edtng facltates unauthorzed use, msappropraton, and msrepresentaton, whch have brought un-estmated ntellectual property losses. Thus, there s a great nterest n developng technologes that help to protect the ntegrty of a dgtal medum and the copyrght of ts owner. Many technques have been developed for resolvng the problem. Data hdng n mages s one of the technques that embed nformaton nsde an mage, called the cover mage n ths study, to provde data securty. The goal of data hdng s to embed a secret message nto a cover mage wthout makng perceptual changes to the cover mage under human observaton. An authorzed user can extract the secret message from the processed cover mage, called the stego-mage n ths study. An unauthorzed user cannot even sense the exstence of the secret message n the stego-mage, and message protecton s hence acheved. Many dfferent mage data hdng methods have been proposed durng the last few years

2 [][5][6][8][3][4] and most of them can be seen as substtuton systems. Such methods try to substtute redundant parts of the mage wth the secret message. The man dsadvantage s the relatve weakness aganst modfcatons. Recently, the development of new robust watermarkng technques led to advances n the constructon of robust and secure mage data hdng systems. Image authentcaton, on the contrary, s a technque for verfyng the ntegrty and fdelty of an mage. The uses of fragle watermarks and sem-fragle watermarks are two of the technques developed for mage authentcaton. A fragle watermark s a knd of watermark that s desgned to be easly destroyed f the watermarked mage s manpulated n the slghtest manner. Image authentcaton can be acheved by nspectng whether the embedded sgnal s destroyed. But n some applcatons, on the other hand, nformaton preservng operatons (such as JPEG lossy compresson) should be consdered as legal operatons. A sem-fragle watermark, whch tells the dfference between an nformaton preservng operaton and an nformaton alterng one (such as feature replacement), s hence desgned to pont out real tamperng and gnore legal operatons n the authentcaton process. In the recent years, proposed systems that are used for verfyng the authentcty of a dgtal mage may be categorzed nto two types, the sgnature system [4] and the fragle watermark system. In a sgnature system, a dgest of the data to be authentcated s obtaned by the use of cryptographc hash functons. The recpent verfes the sgnature by examnng the dgest of the data and usng a verfcaton algorthm to determne f the data are authentc. A dsadvantage of the sgnature system s that the addtonal sgnature must be stored and transmtted separately from the protected mage. A fragle watermark system, on the contrary, embeds the authentcaton nformaton nsde the mage and provdes the ablty to localze the altered areas wthn the mage. Fragle watermark systems can be classfed nto two types, spatal-doman fragle watermarkng system [][0][][] or transformed-doman watermarkng one [3][7][9]. In ths study, we propose a novel method for smultaneously embeddng bnary data and fragle watermark sgnal wthn dgtal color-palette mages.. Characterstcs of Color-palette Images and a Palette-sortng Algorthm In order to hde nformaton wthn a color-palette mage, some characterstcs of them must be dentfed:. every color-palette mage contans at most 56 colors;. a color palette s used for storng 56 colors used n the mage; 3. every pxel can be thought as an ndex number, whch s a reference to the color palette, and so every pxel needs one byte of storage. In ths study, the proposed method s closely dependent on the pxel s ndex number wthn the color palette. But almost every mage processng software has a dfferent method to store the palette data. Even when we just re-save the same mage by dfferent software, the palette data may be dfferent from the orgnal one. As a result, we wll frst sort the color palette before the hdng process to make t n the dark-to-brght order. After sortng, we may reference the same palette data. Ths wll facltate the subsequent hdng process. Let p = R, G, B ) be the -th color wthn ( the color palette, where palette colors, we frst calculate To sort the µ ( p ) = ω R + ω G + ω B () for every color n the palette, where ω, ω, and ω are weghtng values that make color such as 3 (40,30,0) to be dfferent from (30,40,0), and ω ω ω. Then we sort 3 µ ( p ) n a 3

3 descendng order to obtan a sorted palette. Fg. (a) shows a color palette before sortng and Fg. (b) shows the sorted result of Fg. (a). µ ( β x, β y ) = ( Rx Ry ) + ( Gx Gy ) + ( Bx By ), () Ⅰ Ⅰ Ⅱ Ⅳ Ⅱ Ⅳ Ⅲ Ⅲ Fgure. A 3 3 block. (a) (b) Fgure. The color palette before and after sortng. (a) The color palette before sortng. (b) The color palette after sortng. 3. Data Hdng Scheme by Palette Index Replacement In ths secton, the proposed method for embeddng and extractng bnary data nformaton wll be descrbed. An even-odd relatonshp between two pxels s utlzed to embed or extract a bnary bt. The embeddng process wll be descrbed n Secton 3.. And the extractng process wll be descrbed n Secton 3.. Some expermental results wll be shown n Secton Proposed Data Hdng Method Let C be a cover mage of sze M N. Let S be the annotaton that wll be hdden nto C, whch has L characters n length. And let β be the sorted palette, wth β ( 0 ι 55 ) beng a color ι nsde β that has ndex ι. Before the embeddng process, S must frst be converted nto a bnary form S = s s s8 sl * 8 sl*8 accordng to the ASCII codes of the characters n S. As Fg. shows, we hde the bnary data nto the four marked area (Ⅰ, Ⅱ, Ⅲ, and Ⅳ) of every 3x3 block n C, and leave the central pxel unchanged, whch wll be used for hdng fragle nformaton as descrbed n Secton 4. colors be Let the Eucldean dstance between the two where 0 x, y 55.The algorthm for embeddng a bt of annotaton, say as follows. s, can be expressed brefly Step : Select a two-pxel block (Ⅰ,Ⅱ,Ⅲ or Ⅳ) Step: Step3: frst from a 3x3 block. Name the two nvolved pxels as α and α. By vewng every pxel as an ndex number n the color-palette mage, get the color ndces ι and ι of the two selected pxels α and α from the sorted palette β, respectvely. Use the followng rule to judge whether ι and ι are even or odd numbers: true f ι mod = 0; seven ( ι) = (3) false f ι mod =. Step4: In the case that s = and seven ι ) seven( ), modfy one of ( ι the pxels by another color n β to make the two ndces ether both even or both odd n a qualty preservng fashon. Let and be two canddate βr β r colors that wll be used to replace α or α, respectvely. Choose the color to β r be the one among β that has mnmal Eucldean dstance to α and satsfy the condton. On the other hand, choose the canddate color β r seven( ι ) seven( r ) to be the one among β that has mnmal Eucldean dstance to α and satsfy the condton that

4 seven ι ) seven( ). And then ( r modfy one of the pxels by followng condton: f f µ ( α, β ) > µ ( α, β ) r µ ( α, β ) < µ ( α, β ) r r r Otherwse, n the case that seven ( ι ) = seven( ι), just leave the two pxels unchanged. seven ( ι ) = seven( ι), modfy one of canddate colors β r and r seven( ι ) seven( ι), just leave the ι replaceα by β, r replaceα by β. s = r and (4) Step5: In another case that s = 0 and the pxels to make one of the ndex be even and the other be odd. Perform smlar operatons n Step 4 to obtan two β from β and use them to replace α or α accordng to the condton lsted n Eq. (4). Otherwse, f s and = 0 two pxels unchanged. In the proposed method, the even-odd relatonshp of the two chosen pxels s utlzed to embed a bnary bt of annotaton nformaton. The ndces ι and are tuned up to become both even or both odd to represent the fact that a s embedded. On the contrary, a 0 s embedded by adjustng the ndces of the two chosen pxels to become dfferent n the even-odd relaton, that s, for one ndex to become even and the other to become s odd. There s a good chance that the orgnal ndces just ft these two constrants. In ths knd of stuaton, what we have to do s just to leave the two pxels unchanged. On the other hand, when the ndces of the two pxels do not meet the constrants, a color wth ts ndex satsfyng to the constrants descrbed n Step 4 and Step 5 must be found to replace the correspondng pxel. In order to preserve the qualty of the stego-mage, the color wth the mnmal Eucldean dstance to the correspondng pxel s the best canddate for replacement. 3. Data Extracton Process In some conventonal methods, besdes the embeddng result, an extracton process needs ether a lookup table or the source mage to obtan the secret message. Ths s nconvenent for the recever to get the secret message because the user has to keep the source mage or a table. In our proposed method, only the stego-mage s needed to extract the annotaton message. Because the annotaton embedded n the mage can be any knd of descrpton wth an unfxed length, n our mplementaton the length of the annotaton s frst embedded before the annotaton content s treated. As a result, the length nformaton must frst be extracted n the extracton process so that we can know how many blocks should be examned before endng the process. Let E be the stego-mage to be processed, β be the sorted palette of E, and be the -th extracted bt of the annotaton data. The process of extractng a bt from the stego-mage s descrbed below: Step: Choose a two-pxel block (see Fgure ) from a 3 3 block of E. And name the two nvolved pxels α and α. Step: Get the ndces and ι ι of α and α, respectvely, from β accordng to Step3: the frst ther RGB values. The annotaton s can be determned by the followng rule: f ι and ι are both even s = or both odd; (5) 0 otherwse. In the entre extracton process, we frst extract l bts of the data that compose a number that s the value of the length of the annotaton embedded n E. Then we can know how many two-pxel blocks we should examne and after repeatng the above-mentoned process by s l 8 tmes, we can get the bnary form of the extracted annotaton S = s s s8 s l *8 s l*8. Accordng to the ASCII codes, S s then converted nto character form that fnally results n the embedded annotaton.

5 3.3 Expermental Results In our experments, the mages Lena, Baboon, and Pepper wth sze 5 5, whch are shown n Fg. 3(a), (b), and (c), are used as the cover mages. And Fg. 3(d), (e), and (f) are the stego-mages after embeddng 0,000 characters. The PSNR values of the stego-mages are lsted n Table. The PSNR values are hgh, whch mean that only lttle perceptual effect s created durng the annotaton embeddng process. And the embedded annotaton can be extracted wthout any error, accordng to the expermental results. Table. The PSNR values after embeddng 0,000 characters. Lena Baboon Pepper PSNR Authentcaton Scheme by Nearest Palette Color Replacement Method In ths secton, a novel method for authentcatng the ntegrty and fdelty of the color-palette mage s proposed. Alternatons to the watermarked mage can be detected and localzed. And the verfcaton processes can be proceeded wthout referencng the orgnal mage. In Secton 3, the secret data s embedded n the surroundng eght pxels of a block and the central pxel s left unchanged. In the proposed method, the central pxel of every block s utlzed to embed the fragle watermark. The fragle watermark embeddng process wll be shown n Secton 4.. The authentcatng process wll be descrbed n Secton 4.. And several expermental results are shown n Secton Proposed Fragle Watermark Embeddng Method Let C be a cover mage of sze M N, β be the sorted palette of C, and β ( 0 55 β be the -th color n ). The cover mage C s frst dvded nto non-overlappng 3 3 mage blocks, lke the one shown below. Ⅰ Ⅰ Ⅱ Ⅳ Ⅱ Ⅳ Ⅲ Ⅲ We utlze the surroundng four two-pxel blocks (I, II, III and IV) to embed the secret data by usng the method descrbed n Secton 3. After embeddng, every block s central pxel s stll unchanged. Ths pxel s hence used for embeddng the fragle watermark. To embed the fragle watermark nto a 3 3 block B, the algorthm can be shown as the followng steps: Step : Calculate the mean value µ of B, whch can generally represent the color feature of a block. And the central pxel of the block s supposed to be close to µ n color. Snce µ s the mean color of a block, there s a good chance that t s not contaned n β. Step: Fnd a color β from β that s closest k to µ accordng to the Eucldean Step3: dstance of the RGB values, and k mod Q = 0 strength factor.. Here Q s a watermark Replace the color value of central pxel of the block by β. k The magntude of Q n Step s a tradeoff between the probablty to prove the tamperng and the qualty of the stego-mage. When Q s a small value, the qualty of stego-mage s relatvely hgher because the canddate colors could be more so that we can fnd a closer color for replacng the central pxel of every 3 3 block. But the opportunty of provng and localzng the tamperng wll became relatvely lower. On the contrary, f Q s large, the mage qualty wll become poorer but the probablty of detectng tamperng wll become hgher. The detaled data about the nfluence of Q and some dscussons wll be shown n Secton 4.3.

6 (a) (d) (b) (e) (c) (f) Fgure 3. The cover mages and stego-mage wth 0,000 characters embedded. (a) Cover mage Lena. (b) Cover mage Baboon. (c) Cover mage Pepper. (d) Stego-mage Lena wth 0,000 characters embedded. (e) Stego-mage Baboon wth 0,000 characters embedded. (f) Stego-mage Pepper wth 0,000 characters embedded.

7 4. Authentcaton Process The authentcaton process can proceed wthout referencng the orgnal mage n our proposed method. Let T be the mage that s suspcous of beng tampered. And let β be the sorted palette of T, and β be the -th color n β (0 55 ). To authentcate whether T has been tampered, we frst dvde T nto 3 3 non-overlappng blocks. For every block, we focus on the central pxel p and get the palette ndex k of p from β, where the RGB values of β k are equal to those of p. In the embeddng process descrbed n Secton 4., the central pxel of every block s replaced by the color whose ndex s a multple of Q. Therefore, k should be a multple of Q f ths block has not been tampered wth. That s, f k s a multple of Q, the block s judged as not beng tampered wth. On the other hand, the block s decded as beng tampered wth f k s not a multple of Q. For vsualzaton, we replace the block by ts nverse color f the authentcaton result ndcates the block has been altered. Ths wll help us to localze the tamperng area n the suspcous mage. 4.3 Expermental Results In our experments, we use three cover mages as shown n Fg. 4(a), (b), and (c) wth sze 5 5. And Fg. 4 (d), (e), (f) are the mages after embeddng a fragle watermark wth Q = 8. We almost cannot tell the dfference between the cover and the stego-mages. The PSNR values are shown n Table. Table. PSNR values of watermarked mages. Jet Lena Pepper PSNR Fgs. 5(a), (b), and (c) are the mages that have been tampered. And the authentcaton results are shown n Fgs. 5(d), (e), and (f). In Fg. 5(a), we tampered the stego-mage by rubbng the characters and the flag on the body of the arplane. In Fg. 5(b), we altered Lena s har color and paste some wg on her hat. In Fg. 5(c), we cropped two green peppers and pasted them onto dfferent parts of the stego-mage. The expermental results showed that the altered areas are accurately marked out. Fg. 6 are the embedded results of Lena by applyng dfferent Q values. When Q becomes larger, the qualty of the stego-mage goes down. In Fg. 6 (c) and Fg. 6 (d), we can fnd some false contourng effect on the grl s shoulder and cheek because there are only 4 or colors can be used for replacng the central pxel of every 3x3 block. The relaton between the sze of Q and the PSNR values of the stego-mage s shown n Table 3. Table 3. The PSNR values of stego-mage by applyng dfferent Q values. Q=6 Q=3 Q=64 Q=8 PSNR Dscussons In ths thess, a system that embeds annotaton data and a fragle watermark smultaneously wthn the color-palette mage has been proposed. The color-palette mage contans a palette wth all the colors used n that mage. In order to reference the palette n the same order, the color palette s frst reordered from dark to brght. In the embeddng process, the annotatons are embedded n the eght surroundng pxels of a 3 3 block. To embed a bt of data, two pxels are frst selected and the ndces of these two pxels are obtaned from the color palette. The ndces are tuned up to become both even or both odd f a s to be embedded. Otherwse, the ndces are tuned up to become dfferent n the even-odd relaton, that s, wth one even and the other odd. To embed the fragle watermark, the mean value of every 3 3 block s calculated. And the central pxel s replaced by a color that s closest to the mean value of the block. And the magntude of the ndex of ths color must be a multple tmes of the value of a predefned watermark strength factor.

8 (a) (d) (b) (e) (c) (f) Fgure 4. The cover mages and the fragle watermarked mages (a) Cover mage Jet. (b) Cover mage Lena. (c) Cover mage Pepper. (d) Watermarked mage Jet. (e) Watermarked mage Lena. (f) Watermarked mage Pepper.

9 (a) (d) (b) (e) (c) (f) Fgure 5. The suspcous mages and ther verfcaton results. (a) Tampered Jet. (b) Tampered Lena. (c) Tampered Pepper. (d) Verfcaton result of tampered Jet. (e) Verfcaton result of tampered Lena. (f) Verfcaton result of tampered Pepper.

10 (a) Q = 6 (b) Q = 3 (c) Q = 64 (d) Q = 8 Fgure 6. The watermarked mage wth dfferent Q values. (a) Watermarked mage wth Q = 6. (b) Watermarked mage wth Q = 3. (c) Watermarked mage wth Q = 64. (d) Watermarked mage wth Q = 8. References: [] D. C. Wu and W. H. Tsa, Embeddng of Any Type of Data n Images Based on a Human Vsual Model and Multple-Based Number Converson, accepted and to appear n Pattern Recognton Letters. [] D. C. Wu and W. H. Tsa, A Method for Creatng Perceptually Based Fragle Watermarks for Dgtal Image Verfcaton, submtted to IEEE Transactons on Multmeda. [3] D. Kundur and D. Hanznakos, Towards a Tell- Tale Watermarkng Technque for Tamper-Proofng, n Proc. IEEE Internatonal Conference on Image Processng, vol., pp , Chcago, Illnos, 998. [4] D. Stnson, Cryptography Theory and Practce, CRC Press, Boca Raton, 995. [5] E. H. Adelson, Dgtal sgnal encodng and decodng apparatus, U.S. Patent ,990. [6] H. Y. Chang, Data hdng and watermarkng n color mages by wavelet transforms, Master thess, Department of Computer and Informaton Scence, Natonal Chao Tung Unversty, Tawan, Republc of Chna, 999. [7] J. Frdrch, Image watermarkng for tamper detecton, n Proc. IEEE Internatonal Conference on Image Processng, vol. II, 998, pp

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