A Lossless Watermarking Scheme for Halftone Image Authentication

Transcription

1 IJCSNS Internatonal Journal of Computer Scence and Network Securty, VOL.6 No.2B, February A Lossless Watermarkng Scheme for Halftone Image Authentcaton Jeng-Shyang Pan, Hao Luo, and Zhe-Mng Lu, Department of Electronc Engneerng, Natonal Kaohsung Unversty of Appled Scences, Kaohsung, Tawan Harbn Insttute of Technology Shenzhen Graduate School, Shenzhen, Chna Unversty of Freburg, Freburg, Germany Summary Nowadays, halftone mages appear routnely n books, magaznes, prnter outputs, and fax documents. It s desrable to embed data n halftone mages for copyrght protecton and content authentcaton. Ths paper proposes a novel watermarkng scheme for halftone mage authentcaton, explotng mage hash as a fragle watermark. After pxel blockng and orderng, a lookup table s constructed accordng to blocks frequency of occurrence. Watermark embeddng s to dsplace the orgnal blocks wth the correspondng smlar blocks n the lookup table, and n the reverse process watermark s extracted. Some extra blocks are randomly selected wth a secret key for the lookup table embeddng, and the orgnal data of these blocks are also nserted nto the mage. In mage authentcaton, the lookup table s reconstructed frst wth the secret key, and then a smple table-look-up procedure s employed to extract the watermark hash, fnally we only need to compare the watermark hash wth the hash of recovered mage: f they are equal, the orgnal mage suffers no alteraton; otherwse t s changed. As a lossless technque, the orgnal mage can be perfectly recovered by performng the reverse process of the watermark embeddng f the watermarked mage s ntact. It s necessary to keep the content of orgnal host mage unchanged n some specfc applcatons, where content accuracy of the host mage must be guaranteed, e.g. mltary maps, medcal mages, great works of art, etc. As a fragle watermarkng, even one pxel togglng can be detected. Because of the small quantty of watermark, low qualty dstorton s ntroduced to the halftone mage. Experment results demonstrate the effectveness of the scheme. Key words: Halftone mage, Lossless watermarkng, Hash sequence, Image authentcaton 1. Introducton Dgtal halftonng s a technque to transform mult-tone mages nto two-tone mages, e.g. 8-bt grayscale mages nto 1-bt bnary mages. The halftone mages can resemble the orgnal mages when vewng from a dstance by the low-pass flterng n the human vsual system. Most popular halftonng methods can be dvded nto three categores: ordered dtherng [1], error dffuson [2], and drect bnary search [3]. Among these, error dffuson acheves a preferable tradeoff between good vsual qualty and reasonable computatonal complexty. Wth halftone mages wdely used, content authentcaton, changes localzaton and copyrght protecton for ths knd of mages are recevng an ncreasng nterest among researchers wth dgtal watermarkng technques. Many watermarkng technques are proposed for mult-tone mages, and those for halftone mages are developed n recent years. In contrast, qute a small number of authentcaton watermarkng approaches are avalable for halftone mages. In the lterature, most methods cannot perfectly restore the orgnal mage n watermark extracton or detecton. Exstng watermarkng usually ntroduces rreversble degradaton to the host medum. Although the degradaton s slght, t cannot satsfy the requrement of some specfc stuatons, where content accuracy of the host mage must be guaranteed, e.g., mltary maps, medcal mages, great works of art, etc. Therefore, t s qute necessary to develop a lossless watermarkng method for authentcatng halftone mages. However, tll the present tme, there has been lttle attenton pad to the lossless watermarkng technques for halftone mages. Ths paper proposes a watermarkng scheme for halftone mages combnng the above two characterstcs, namely, lossless and for authentcaton purpose. In mage authentcaton, we only need to compare two hash sequences: the extracted watermark hash and the hash of the restored mage. If the two sequences are equal, the halftone mage suffers no alteraton; otherwse t s changed ntentonally or unntentonally. The rest of the paper s organzed as follows. Secton 2 revews the prevous work on halftone mage watermarkng, n partcular for secure authentcaton. Secton 3 extensvely descrbes the proposed scheme ncludng watermark embeddng and extracton, and meanwhle the authentcaton process. In Secton 4, expermental results are presented for the demonstraton of ts effectveness. Secton 5 concludes the paper. 2. Prevous work Manuscrpt revsed January 2006.

2 148 IJCSNS Internatonal Journal of Computer Scence and Network Securty, VOL.6 No.2B, February 2006 Dfferent from grayscale or color mages, there are manly three challenges to embed data n halftone mages. The frst one s less nformaton redundancy for each pxel value has only one bt. Consequently many watermarkng approaches cannot be drectly transplanted to halftone mages. Another challenge s vsual qualty degradaton. To nsert data n halftone mages, change of the pxel value s ether from black to whte or from whte to black. Usually, human vsual system s senstve to the abrupt change, e.g. the whte cross and the black cross. The thrd challenge s lower embeddng capacty. Hgh capacty s one of the key factors to evaluate the performance of watermarkng technques. Actually, for halftone mages, ths challenge s closely related to the former two challenges. It s expected that a large quantty of data cannot be embedded nto halftone mages consderng vsual qualty degradaton, for less nformaton redundancy can be explored. Avalable data hdng methods for halftone mages can be dvded nto three classes: (1) Pxel-based: ths knd of methods s to change the values of ndvdual pxels, usually randomly selected [4] [5]. (2) Block-based: these methods dvde the host mage nto blocks and modfy characterstc of some blocks [6] [7]. (3) Hybrd-based: they nsert data by combnng the characterstcs of pxel-based and block-based [8]. Km and Aff ntroduce an authentcaton watermark AWST (authentcaton watermarkng by self togglng) for halftone mages n [9]. It conssts of followng steps: choosng a set of pseudo-random pxels n the mage, clearng them, computng the message authentcaton code (MAC) or the dgtal sgnature (DS) of the random-pxels-cleared mage, and nsertng the resultng code nto the selected random pxels. One dsadvantage of the AWST s t cannot obtan the orgnal mage n watermark extracton and mage authentcaton when the host mage s not changed, because t clears some pxels randomly selected and never can be recovered. However, our scheme overcomes ths problem: the host mage can be perfectly restored as long as t suffers no alteraton, otherwse even a sngle pxel change can be detected. The scheme can only smply detect the change wthout spatally locatng them. Fg. 1 Block dagram of watermark embeddng. 3.1 Watermark embeddng The block dagram of watermark embeddng s as shown n Fg. 1, and each step s detaled descrbed below. Watermark generaton Our scheme explots mage hash as a fragle watermark. Image hashng s known as the problem of mappng an mage to a short bnary strng. Image hash functon has the propertes that perceptually dentcal mages have the same hash value wth hgh probablty, whle perceptually dfferent mages have ndependent hash values. In addton, the hash functon s secure, so that an attacker cannot predct the hash value of a known mage. Image hashng s one-way, collson-free and relatvely easy to compute for any gven mage. Hence, the watermark can be vewed as adaptve for ts senstvty to change of the mage. Our scheme generates the watermark hash W H of the orgnal mage usng hash functons. Suppose W H s an L-bt 0-1 sequence, thus we need L blocks to embed data for each block can be nserted nto 1-bt data. Image blockng The scheme starts wth dvdng the halftone mage nto dsont pxel blocks. Suppose the sze of the halftone mage I and a block B are M N and m n respectvely, and I s segmented nto b blocks B 1, B 2, B b. Snce each pxel s ether black or whte, an m n block has totally 2 m n dfferent patterns. Some patterns never occur n I, whle some occur many tmes. 3. Proposed scheme Exstng watermarkng technques [10] can be classfed nto three categores: robust, fragle and sem-fragle. Among them, fragle watermarks are easly corrupted by mage processng operatons, and thus often used for checkng mage ntegrty and authentcaton. Authentcaton watermark s a hdden data nserted nto an mage that can be appled to detect any unauthorzed change of the mage. Our scheme s a block-based method. Fg. 2 An example of an orgnal block and ts smlar blocks. Block classfcaton and orderng Ths step ams to select approprate blocks to embed data. To all blocks n I, we count ther occurrence respectvely, and then rank the frequency of occurrence. Sorted n descendng order, suppose t 1, t 2, t ( 2 m n ) denotes the occurrence tmes

3 IJCSNS Internatonal Journal of Computer Scence and Network Securty, VOL.6 No.2B, February of block pattern p 1, p 2, p, respectvely. It s clear that Formula (1) s satsfed. t = b (1) l l= 1 Lookup table constructon A lookup table T s made up of orgnal blocks O 1, O 2, O u, smlar blocks S 1, S 2, S u, and frequency of occurrence of O 1, O 2, O u. S (1 u) s the correspondng smlar block of O. Smlar blocks that never occur n the mage are used to dsplace orgnal blocks for watermark embeddng. In the context, two blocks wth the same sze are consdered smlar f the Hammng dstance between them s 1, namely, only one pxel value s dfferent. Therefore each block has 2 m n smlar blocks. As an example a 4 4 block and ts 16 smlar blocks are shown n Fg. 2. Accordng to the frequency of occurrence, we select k block patterns wth hghest k occurrence t 1, t 2, t k as a canddate block set. To a block n the set, f all of ts smlar ones occur at least once n I, then the block s dscarded from the canddate set. If one or more smlar blocks do not occur, we randomly select one as ts smlar block. All canddate blocks are nvestgated lke ths, and orgnal blocks and correspondng smlar blocks are recorded n T. Besdes ths, we need to record the frequency of occurrence for computng embeddng capacty. So far a lookup table s constructed. It s essental that each smlar block n T must be dfferent from any other blocks. More detals of the lookup table constructon can be seen n [11]. Fg. 3 An example of a lookup table. As an example shown n Fg. 3, the lookup table s constructed on the halftone Lena mage, wth the sze of the Lena and a block and 4 4 pxels, respectvely. Only 5 blocks wth hghest frequency of occurrence and ther smlar blocks are gven. Watermark embeddng To embed data, we compare the block B (1 b) wth O 1, O 2, O u one by one. If B s the same as O, 1-bt data can be nserted wth the rule: If 0 s to be embedded, we do not change B, f 1 s to be embedded, we dsplace B wth S. The operaton as shown n Formula (2) s repeated for all blocks B 1, B 2, B b. B = O f WH = 0 B = S f WH = 1 Overhead nformaton embeddng In [11], the lookup table needs extra storage space for watermark embeddng and extracton. Snce dfferent mage has dfferent lookup table, no unversal table s sutable for all mages. Besdes, the lookup table s also need to be protected. Our scheme overcomes the dsadvantage successfully by embeddng the lookup table n the host mage as overhead nformaton. The overhead nformaton s embedded as follows. () Use a pseudo-random number generator wth a secret key K to choose a set of non-repeatng blocks G n I. Note these blocks must not concde wth the watermarked blocks. () Embed G n I. An m n block of G s rearranged to a 0-1 strng and embedded as the same way of watermark embeddng. The process s repeated for all blocks n G. Snce a block dsplacement can embed one bt, Formula (3) must be satsfed. In Formula (3), 2umn s the total bts of the lookup table T. k l= 1 (2) tl WH+ 2umn (1 k ) (3) () Dsplace blocks n G wth blocks n T drectly. 3.2 Watermark extracton and mage authentcaton The block dagram of watermark extracton and mage authentcaton s as shown n Fg. 4. Lookup table reconstructon The watermarked mage I s segmented nto m n blocks B (0 b). Before watermark extracton, the lookup table must be reconstructed frst. We use the same key K to fnd out the blocks G, and thus T can be recovered by extractng G drectly. Watermark extracton Let W denotes the nformaton H extracted. Accordng to the lookup table T the watermark s extracted as shown n Formula (4). We compare the block B (1 b) wth blocks n T. If B s the same as O (1 u), 0 s extracted; f B s the same as S (1 u), 1 s extracted. W H = 0 f B = O W H = 1 f B = S (4)

4 150 IJCSNS Internatonal Journal of Computer Scence and Network Securty, VOL.6 No.2B, February 2006 The extracted nformaton W conssts of the extracted H watermark H 1 and the orgnal data of G. If 0 extracted, we do not change the block; f 1 extracted, we dsplace S wth B. After we recover G, the restored mage R s obtaned. Image authentcaton The same hash functon s used to compute the hash sequence H2 of R. Thus mage authentcaton s reduced to a task of comparng H1 and H2. On one hand, f the host mage I suffers no alteraton, H1 s equal to H2, otherwse dfferent. On the other hand, I suffers no alteraton f H1 s equal to H2. In a word, f H1 s dfferent from H2, I s changed, at least a sngle pxel. Fg. 4 Block dagram of watermark extracton and mage authentcaton. Fg. 6 Expermental results on halftone Baboon Image. (a) Orgnal Baboon, (b) Watermarked Baboon wthout alteraton, (c) Restored Baboon of (b), (d) Watermarked Baboon wth alteraton, (e) Restored Baboon of (d), (f) Orgnal watermark, (g) Watermark extracted from (b), (h) hash of (c), () Watermark extracted from (d), () hash of (e). 4. Expermental results Fg. 5 Expermental results on halftone Lena Image. (a) Orgnal Lena, (b) Watermarked Lena wthout alteraton, (c) Restored Lena of (b), (d) Watermarked Lena wth alteraton, (e) Restored Lena of (d), (f) Orgnal watermark, (g) Watermark extracted from (b), (h) hash of (c), () Watermark extracted from (d), () hash of (e). In the experment, halftone Lena mage and Baboon mage are selected to test the effectveness of the method. As shown n Fg. 5(a), the halftone Lena mage s dvded nto 4 4 blocks. The orgnal watermark,.e. the hash sequence of Lena, s computed by the MD5 hash functon. After translatng the strng nto 0-1 sequence, 128-bt dgest s obtaned. In authentcaton, we compare the watermark extracted from the watermarked mage, and the hash sequence computed from the restored mage. When the two

5 IJCSNS Internatonal Journal of Computer Scence and Network Securty, VOL.6 No.2B, February sequences are equal, as shown n Fg. 5(g) and Fg. 5(h), we can confrm the watermarked Lena suffers no alteraton. Both of them are equal to the orgnal watermark, as shown n Fg. 5(e). Whle f the watermarked Lena s tampered by a mark KUAS HIT (Fg. 5(d)), the two sequences are dfferent, as shown n Fg. 5() and Fg. 5(). Therefore, we can make a udgment by vrtue of the two sequences are equal or not: f equal, the mage suffers no alteraton; otherwse changed. The Baboon mage s also dvded nto 4 4 blocks. As shown n Fg. 6, expermental results also verfy effectveness of the scheme. 4. Concluson Ths paper presents a lossless watermarkng scheme for halftone mage authentcaton. The hash sequence of the mage s embedded as an adaptve fragle watermark. To udge weather the orgnal mage s changed or not, we only need to compare the extracted watermark and the hash sequence of the restored mage. If they are exactly the same, the mage suffers no alteraton; otherwse t s changed. As long as the watermarked mage s not unauthorzed changed, the orgnal mage can be perfected recovered. Besdes, no nformaton needs to be saved except a secret key. Acknowledgment The authors would lke to express ther thanks to Dr. Ban Yang for hs valuable advce. References [1] R. A. Ulchney, Dgtal Halftonng, Cambrdge, MA: MIT Press, [2] R. W. Floyd and L. Stenberg, An adaptve algorthm for spatal gray scale, Proc. SID 75 Dgest. Socety for nformaton Dsplay, pp , [3] D. Leberman and J. Allebach, Dgtal halftonng usng drect bnary search, Proc. of st IEEE Int. Conf. on Hgh technology, pp , September 1996 [4] M.S. Fu, and O.C. Au, Data Hdng by Smart Par Togglng for Halftone Images, IEEE Int Conf Acoustcs Speech and Sgnal Processng, vol. 4, pp , 2000 [5] M.S. Fu, and O.C. Au, Data Hdng Watermarkng for Halftone Images, IEEE Trans Image Processng, pp [6] Z. Baharav, and D. Shaked, Watermarkng of Dther Halftone Images, Hewlett-Packard Labs Tech Rep, HPL-98-32, [7] H. Z. Hel-Or, Watermarkng and Copyrght Labelng of Prnted Images, Journal of Electronc Imagng, pp [8] S.C. Pe, and J.M. Guo, Hybrd Pxel-Based Data Hdng and Block-Based Watermarkng for Error-Dffused Halftone Images, IEEE Trans Crcuts and Systems for Vdeo Technology, pp , 2003 [9] H.Y. Km, and A. Aff, Secure Authentcaton Watermarkng for Bnary Images, Proc Brazlan Symp on Computer Graphcs and Image Processng, pp , 2003 [10] J. S. Pan, H. C. Huang and L. C. Jan, Intellgent Watermarkng Technques, World Scentfc, [11] P. S. Lao, J. S. Pan, Y. H. Chen and B. Y. Lao, A Lossless Watermarkng Technque for Halftone Image, Internatonal Workshop on Intellgent Informaton Hdng and Multmeda Sgnal Processng, Melbourne, Australa, May 15, Jeng-Shyang Pan receved the B. S. degree n Electronc Engneerng from the Natonal Tawan Unversty of Scence and Technology, Tawan n 1986, the M. S. degree n Communcaton Engneerng from the Natonal Chao Tung Unversty, Tawan n 1988, and the Ph.D. degree n Electrcal Engneerng from the Unversty of Ednburgh, U.K. n Currently, he s a Professor n the Department of Electronc Engneerng, Natonal Kaohsung Unversty of Appled Scences, Tawan. Professor Pan has publshed more than 50 ournal papers and 120 conference papers. He onts the edtoral board for LNCS Transactons on Data Hdng and Multmeda Securty, Sprnger, Internatonal Journal of Knowledge-Based Intellgent Engneerng Systems, IOS Press, and Internatonal Journal of Hybrd Intellgent System, Advanced Knowledge Internatonal. He s the Co-Edtors-n-Chef for Internatonal Journal of Innovatve Computng, Informaton and Control. Hs current research nterests nclude data mnng, nformaton securty and mage processng. Hao Luo receved the B. S. degree and the M. S. degree from Harbn Insttute of Technology (HIT), Harbn, Chna n 2002 and 2004, respectvely. He s a Ph.D canddate n the School of Electronc Engneerng n HIT. Hs research nterests are manly n nformaton securty and mesh retreval. Zhe-Mng Lu receved the B.S., M.S. and Ph. D. degrees n Electrcal Engneerng from Harbn Insttute of Technology n 1995, 1997 and 2001, respectvely. He was the Alexander von Humboldt Research Fellow n Unversty of Freburg n Germany, from Oct., 2004 to Jan He has publshed more than 110 papers and four books. He has been program commttee members n several nternatonal conferences. He s now the Professor and Drector of the Vsual Informaton Analyss and Processng Research Center, Harbn Insttute of Technology Shenzhen Graduate School. Hs research nterests are mage processng, pattern recognton, nformaton hdng and vsual nformaton retreval.