Protecting Biometric Templates with Image Watermarking Techniques

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1 Protectig Biometric Templates with Image Watermarkig Techiques Nikos Komios ad Tassos Dimitriou Athes Iformatio Techology, GR Peaia Attiki, Greece Abstract. Biometric templates are subject to modificatios for idetity fraud especially whe they are stored i databases. I this paper, a ew approach to protectig biometric templates with image watermarkig techiques is proposed. The ovelty of this approach is that we have combied lattice ad block-wise image watermarkig techiques to maitai image quality alog with cryptographic techiques to embed figerprit templates ito facial images ad vice-versa. Thus, protectig them from beig modified. Keywords: Biometric templates, figerprits, face pictures, autheticatio, watermarkig. 1 Itroductio Visual based biometric systems use feature extractio algorithms to extract the discrimiat iformatio that is ivariat to as may variatios embedded i the raw data (e.g. scalig, traslatio, rotatio) as possible. Template-based methods crop a particular subimage (the template) from the origial sesory image, ad extract features from the template by applyig global-level processig, without a priori kowledge of the object s structural properties. Compared to geometric feature extractio algorithms, image template approaches eed to locate far fewer poits to obtai a correct template. For example, i the probabilistic decisio-based eural etwork (PDBNN) face recogitio syste oly two poits (left ad right eyes) eed to be located to extract a facial recogitio template. A early compariso betwee the two types of feature extractio methods was made by Bruelli ad Poggio [1]. They foud the template approach to be faster ad able to geerate more accurate recogitio results tha the geometric approach. I practice, there is ot always a perfect match betwee templates ad idividuals. Oe speaks of a false positive if the biometric recogitio system says 'yes', but the aswer should be 'o'. A false egative works the other way roud: the system says 'o', where it should be a 'yes'. Oe of the mai challeges with biometric systems is to miimise the rates of both false positives ad of false egatives. I theory oe is iclied to keep the false positives low, but i practical situatios it ofte works the other way roud: people that operate these systems dislike false egatives, because they slow dow the process ad result i extra work ad people complaiig. S.-W. Lee ad S.Z. Li (Eds.): ICB 2007, LNCS 4642, pp , Spriger-Verlag Berli Heidelberg 2007

2 Protectig Biometric Templates with Image Watermarkig Techiques 115 A potetial protectio approach of biometric templates is feasible with image watermarkig techiques i visual-based biometric systems. Watermarkig techiques attempt to protect the copyrights of ay digital medium by embeddig a uique patter or message withi the origial iformatio. The embeddig method ivolves the use of a umber of differet autheticatio, ecryptio ad hash algorithms ad protocols to achieve the validity ad copy protectio of the particular message. Oe of the most importat requiremets of watermarkig is the perceptual trasparecy betwee the origial work ad the watermarked. Especially for images that objective metrics are widely used [6]. The watermark message may have a higher or lower level of perceptibility, meaig that there is a greater or lesser likelihood that a give observer will perceive the differece. I this paper, we apply watermarkig techiques to biometric templates to overcome serious cases of idetity theft. I particular, we embed a perso s figerprit template ito his facial image with the form of a cryptographic ecoder that utilizes ecryptio algorithms, hash fuctios ad digital sigatures. Oce the facial image has bee watermarked, it ca be stored i public databases without riskig a idetity modificatio or fabricatio. Followig this itroductio, the paper is orgaized as follows. Sectio 2 presets curret work that combie watermarkig ad biometrics techiques. Sectio 3 discusses the requiremets for efficiet watermarkig ad briefly describes lattice ad block-wise embeddig methods ad how these ca be used alog with cryptographic techiques to protect biometric templates. Sectio 4 evaluates the performace ad the efficiecy of the two embeddig methods, through simulatio tests. Sectio 5 cocludes with remarks ad commets o the ope issues i watermarkig ad biometric templates. 2 Related Work Curret research efforts i combiig watermarkig techiques ad visual-based biometric systems follow a hierarchical approach, with the most explored area beig that of biometrics. Watermarkig techiques o the other had have bee explored less i cojuctio with biometrics templates. Despite the fact that several attempts of combiig watermarkig techiques ad biometric systems have already bee proposed. I Lucilla et al. [5], a techique for the autheticatio of ID cardholders is preseted, which combies dyamic sigature verificatio with hologram watermarks. Two biometric features are already itegral parts of ID cards for maual verificatio: the user s face ad sigature. This techique embeds dyamic features of the cardholder s sigature ito the persoal data prited o the ID card, thereby facilitatig automated user autheticatio based o the embedded iformatio. Ay modificatio of the image ca also be detected ad will further disallow the biometric verificatio of the forger. Jai ad Uludag [2] worked with hidig figerprit miutiae i images. For this purpose, they cosidered two applicatio scearios: A set of figerprit miutiae is trasferred as the watermark of a arbitrary image ad a face image is watermarked with figerprit miutiae. I the first sceario, the figerprit miutiae are trasferred via a o-secure chael hidde i a arbitrary image. Before beig embedded ito

3 116 N. Komios ad T. Dimitriou the host image, the figerprit miutiae are ecrypted, which further icreases the security of the data. The produced image is set through the isecure commuicatio chael. I the ed, the image is received ad the figerprit miutiae are extracted, decrypted ad ready for ay further processig. I the secod sceario, a face sca is watermarked with figerprit miutiae data ad the result is ecoded i a smart card. For the autheticatio of a user, the image is retrieved from the smart card, the figerprit miutiae are extracted from it ad they are compared to the miutiae obtaied from the user olie. The user is autheticated based o the two figerprit miutiae data sets ad the face image. Jai et al. [3] have preseted a figerprit image watermarkig method that ca embed facial iformatio ito host figerprit images. The cosidered applicatio sceario i this case is as follows: The figerprit image of a perso is watermarked with face iformatio of the same perso ad stored o a smart card. At a access cotrol site, the figerprit of the user is sesed ad compared with the oe stored o the smart card. After the figerprit matchig has successfully bee completed, the facial iformatio ca be extracted from the figerprit image o the smart card ad ca be used for further autheticatio purposes. 3 Combiig Image Watermarkig Techiques with Visual Based Biometric Systems Visual-based biometric systems use feature extractio techiques to collect uique facial patters ad create biometric templates. However, biometric templates are subject to fraud especially i passport cloig ad illegal immigratio. Image watermarkig techiques alog with cryptographic primitives ca be used to verify the autheticity of a perso ad also detect ay modificatio to biometric templates whe these are securely stored. Biometric templates of a figerprit ad a face sca ca be hashed ad ecrypted with cryptographic algorithms ad the embedded ito a image. For example, with the use of hash fuctios ad ecryptio methods, the ower of a facial image ca embed his/her template. The recipiet ca extract it by decryptig it ad therefore ca verify that the received image was the oe iteded by the seder. Ecryptig ad hashig watermarked iformatio ca guaratee the autheticatio of the ower ad the image itself sice the purpose of watermarks is two-fold: (i) they ca be used to determie owership, ad (ii) they ca be used to detect tamperig. There are two ecessary features that all watermarks must possess [7]. First, all watermarks should be detectable. I order to determie owership, it is imperative that oe be able to recover the watermark. Secod, watermarks must be robust to various types of processig of the sigal (i.e. croppig, filterig, traslatio, compressio, etc.). If the watermark is ot robust, it serves little purpose, as owership will be lost upo processig. Aother importat requiremet for watermarks is the perceptual trasparecy betwee the origial work ad the watermarked; ad for images objective metrics are widely used. The watermarked message may have a higher or lower level of perceptibility, meaig that there is a greater or lesser likelihood that a give observer will perceive the differece. The ideal is to be as imperceptible as possible ad it is required to develop models that are

4 Protectig Biometric Templates with Image Watermarkig Techiques 117 used to compare two differet versios of the works ad evaluate ay alteratios. Evaluatig the perceptibility of the watermarks ca be doe with distortio metrics. These distortio metrics do ot exploit the properties of the huma visual system but they provide reliable results. Also, there is a objective criterio that relies o the sesitivity of the eye ad is called Watso perceptual distace. It is also kow as just oticeable differeces ad cosists of a sesitivity fuctio, two maskig compoets based o lumiace, cotrast maskig, ad a poolig compoet. Table 1 gives the metrics that are used more ofte. Table 1. Quality Measuremets Sigal to Noise Ratio (SNR) Peak Sigal to Noise Ratio (PSNR) Image Fidelity (IF) 2 ~ 2 SNR Im, / I I Normalized Cross Correlatio (NC) 2 ~ PSNR MN max I 2 m, / I I ~ 2 I I Mea Square MSE I 2 Error (MSE) m, I Correlatio Quality (CQ) 2 IF 1 / Im, Watso Distace (WD) 1 ~ MN ~ 2 NC I I / Im, ~ CQ I I / I Dwat 1 / 4 4 0, w, i, j, k c c d i, j k There are plety of image watermarkig techiques available i the literature but we have combied lattice ad semi-fragile, or block-wise, embeddig methods to take advatage of their uique features. Briefly, the lattice watermarkig system embeds oly oe bit per 256 pixels i a image. Each bit is ecoded usig the trellis code ad produces a sequece of four bits. The trellis codig is a covolutio code ad the umber of states is 2 3 =8 with possible outputs 2 4 =16. After the ecodig procedure, the bits eed to be embedded i 256 pixels which meas that each of the four bits is embedded i 256/4=64 pixels [6]. The block-wise method ivolves the basic properties of the JPEG compressio where DCT domai takes place. Four bits are embedded i the high-frequecy DCT of each 8x8 (64 pixels) block i the image ad ot i the low-frequecy i order to avoid ay visual differeces that would lead to uacceptably poor fidelity. By usig the block-wise method, the image ca host 16 times more iformatio tha lattice. Specifically 28 coefficiets are used which meas that each bit is embedded i seve coefficiets. The seve coefficiets that host oe bit are chose radomly accordig to a seed umber ad thus, each coefficiet is ivolved i oly oe bit [6]. By combig the two methods, we ca exploit their advatages particularly, i circumstaces where both the quality ad the ability to otice the corrupted blocks is essetial. I a image, the part that is likely to be illegally altered is watermarked with the block-wise method while the rest of the image is watermarked with the lattice method. I a facial image, for example, the areas of the eyes, mouth ad jaw ca be used to embed the figerprit template. The roud area of the face ca be used to embed additioal iformatio, such as the ame ad/or address of the perso show i the photo with the lattice method. If a adversary chages for example the color of

5 118 N. Komios ad T. Dimitriou the eyes or some characteristics of the face (e.g. addig a moustache) the combied algorithm is able to determiate the modified pixels. This is achieved by comparig the extracted message with the origial. The combiatio of the two embeddig methods is implemeted i a cryptographic ecoder-decoder. The authority that wishes to protect a face or figerprit photo, extracts the biometric template(s) of that perso ad alog with a short descriptio ad a uique feature of the image are iserted i a hash fuctio ad the result is ecrypted with a 1024-bit secret key. The sigature, together with the short ad the extracted uique descriptio, is embedded with the lattice method while the biometric template is embedded with the block-wise algorithm. As a uique descriptio we have used the sum of the pixel values of the four blocks i the corers. The Secure Hash Algorithm (SHA) ad the Rivest, Shamir, Aldema (RSA) [8] have bee used to hash ad sig the figerprit template, short ad uique descriptios. The desig of the ecoder is illustrated i Fig. 1a. Biometric Template Short Descriptio Origial Image Watermarked Image Biometric Template Short Descriptio Extracted Descriptio Extract Uique Descriptio Detect Extract Uique Descriptio SHA Hash Fuctio Sigature 1024-bit Secret Key RSA Ecryptio 1024-bit Public Key RSA Decryptio Biometric Template Short Descriptio Uique Descriptio SHA Hash Fuctio Close? Sigature Message Short Descriptio Uique Descriptio Embed =? Both Yes? Watermarked Image Valid \ Ivalid (a) (b) Fig. 1. Cryptographic Ecoder / Decoder From the watermarked versio of the image, at the decoder s side, the sigature, the short ad uique descriptios are extracted with the lattice method while the biometric template is extracted with the block-wise algorithm. The, the uique descriptio is compared with the extracted oe. Thus, the first step is to verify whether the uique descriptios match. I the case of the watermark beig copied ad embedded i aother image, the extracted descriptio will ot be the same. This is due to the fact that the pixel values of the image have bee slightly chaged to host the watermark, the extracted descriptio caot be exactly the same, but ca be very close. Therefore, some upper ad lower boudaries have bee determied for this step of verificatio.

6 Protectig Biometric Templates with Image Watermarkig Techiques 119 The ext step is to decrypt the sigature usig the 1024-bit RSA public key ad retrieve the hash value. The biometric template, short ad uique descriptios that have bee extracted, is agai the iput to the hash fuctio. The obtaied hash value is the compared with the oe decrypted from the sigature. The third step of the decoder is to verify whether the decrypted hash value matches exactly with the oe calculated by the decoder. If both hash values ad uique descriptios are valid, the autheticatio process is successful. The whole desig of the decoder is preseted i Fig. 1b. 4 Experimetal Results I order to evaluate the performace ad the efficiecy of the embeddig methods, excessive tests have take place. A umber of cases have bee cosidered each with a differet variable parameter. A grayscale bitmap image with 300x300 (Fig. 2a) resolutio has bee used for the experimets. The differece betwee the origial ad the watermarked image was evaluated by the ideal values of the origial image that are preseted i Table 2. Table 2. Ideal Values of the test Image Quality Measuremets Ideal Values Quality Measuremets Ideal Values MSE 0 NC 1 SNR (db) 97 CQ PSNR (db) 104 Watso Distace 0 IF 100 Cases have also bee cosidered oce agai each with a differet variable parameter, idepedet of lattice ad block-wise methods i order to maitai image quality. Whe combied image watermarkig is performed, a small part of the image is watermarked with the block-wise method ad the rest is watermarked with the lattice method. We assume that the most vulerable to illegal modificatios, is the small part. The biometric template is embedded i the small part while the short ad extracted descriptios are embedded i the large part of the image. The experimetal tests of the quality measuremets were performed usig: oly lattice; oly block-wise; combied block-wise ad lattice. It was foud that the lattice method achieves better results tha the block-wise ad expected that the produced result values of the combied case would be i betwee the values of those produced by the two methods. Particularly, i the case of the lattice algorithm the maximum umber of the embedded bit ca be 351 (oe bit per 256 pixels). The formulas that are used to evaluate the differeces betwee two images are preseted i Table 1. The tests were executed usig a rage of values for the parameter i order to coclude what the best values are. The parameters are the embeddig stregth (β) ad the lattice spacig (α). The rage of the α value was from 0.35 to 5.33 ad the rage of β from 0.7 to 1.1. The icesemet steps for a was 0.02 ad for β 0.1.

7 120 N. Komios ad T. Dimitriou The measuremet values for the lattice method are very close to the ideal oes. More specifically, the directio towards zero is achieved usig low values of α i the case of MSE. If at the same time the value of β that is used is low, the MSE is decreased eve further. I the case of SNR ad PSNR, the result values are higher whe the parameters α ad β are low. The image fidelity (IF), which is defied as a percetage of how idetical the images are, the value of 100% is cosidered to be the optimum ad as ca be oticed from Table 3, the results are very close to this. Utilizig the NC ad the CQ quality measuremets, it is observed that their measuremets are closer to the ideal oes (Table 2), as the values of α ad β are decreased. The above observatios are also justified from the Watso measuremet which is based o lumiace, cotrast, ad poolig maskig. Table 3. Results From Lattice, Block-Wise ad Combied Embeddig Methods alpha(a)=1.53, beta(β)=0.8, alpha(a) =0.2 alpha(a)=0.93, beta(β)=1.0, alpha(a)=0.1 Lattice alpha(a), beta(β) Block- Wise alpha(a) Combied alpha(a),beta(β) alpha(a) MSE SNR PSNR IF NC CQ Watso- Distace Lattice alpha(a), beta(β) (a) Block-Wise alpha(a) Combied alpha(a),beta(β), alpha(a) MSE SNR PSNR IF NC CQ Watso-Distace (b) Therefore, it could be suggested that the optimum parameter values are those that give the best results. They could eve be the zero values. But at the decoder s side ot all the bits are extracted correctly. Specifically whe usig low values of α ad β, the decoder is ot able to get the correct embedded bits. I coclusio it ca be said that a trade-off betwee the quality results ad the decoder s result is ecessary i order to determie the optimum values. From the tests we cocluded that suggested values could be α 1.53 ad β = 0.8 (Table 3b). Similarly, i the case of the block-wise method, the tests were executed for the same image i order to be comparable with those of the lattice method. Oe major differece is the umber of bits that are embedded. Sice the method embeds four bits i every 64 pixels ad the image has pixels i total, the umber of bits ca be hosted i The size of the iformatio that ca be watermarked is sigificatly

8 Protectig Biometric Templates with Image Watermarkig Techiques 121 higher ad i fact is 16 times greater tha the size i the lattice method. Therefore, before eve executig the test, it is expected that the results will ot be as good. The iformatio i the block-wise method is much more, which meas that the alteratios i the image will produce worse values i the quality measuremets. The observatio of the results proves what is beig stated i the begiig. The values of the quality measuremets are ot as good i compariso with those of the lattice method sice the measuremet of the MSE is higher tha the ideal value, which is zero. The values of the SNR ad PSNR, which are widely used, show that as the value of the parameter alpha (a) is icreased, the result becomes worse. I the case of the IF, NC, ad CQ, the measuremets seem to be distat from the ideal values as alpha (a) takes higher values. The same coclusio ca be phrased for the perceptual distace give by the Watso model, where the results are worse as the value of alpha (a) is icreased. It seems that as the value of alpha is icreased, the watermarked image has poorer fidelity. So the optimum value of the parameter should perhaps possibly be a small oe e.g However, it seems that values below 0.05 do ot allow the decoder to get the right message. The chose value of alpha depeds o how sesitive the user wats the method to be i order to locate the corrupted bits ad mark the correspodig blocks. Higher values icrease the sesitivity but at the same time the quality of the image is reduced. So it is agai ecessary to make a trade-off betwee the results ad the sesitivity. A possible suggested value could be a 0.2 (Table 3b). Ideed the results were ot as good as those of the lattice method but they were better tha those of the block-wise method. I Table 3 some result values of the combiatio are give i order to compare them with those of the two methods whe they are applied idividually. Table 3 justifies that the combiatio produces quality measuremets betwee the two methods. Table 4 presets the maximum umber of bits that ca be hosted i the image usig the two embeddig methods ad a combiatio of them. Table 4. Maximum Number of Embedded Bits Lattice Block- Wise Combied Max Embedded Bits The last test was to verify that i case somebody modifies the block-wise part of the image, which is the biometric template, the decoder realizes the modificatio, iforms the authority that the autheticatio applicatio failed ad outputs a file with the modified blocks marked. The part that is likely to be illegally altered is the eyes or jaw ad the biometric template(s) of the facial ad/or figerprit images which are embedded with the block-wise method (Fig. 2b). I the watermarked versio the distace betwee the eyes was chaged ad this image was iserted i the decoder i order to verify its autheticity. The autheticatio process failed ad a marked image was produced (Fig. 2c). By observig the last image it is clear that the decoder has successfully located the modified blocks.

9 122 N. Komios ad T. Dimitriou (a) (b) (c) Fig. 2. Origial Image (a), Watermarked Image (b), Marked Image (c) Throughout the paper we have cosidered the case where a figerprit ad/or facial template is embedded ito a facial image. That does ot mea that a facial image caot be embedded with our method ito a figerprit image, which is illustrated i Fig. 3. Similar to Fig. 2, Fig. 3a is the origial image, Fig. 3b is the watermarked ad Fig. 3c is the marked image geerated by our testbed. (a) (b) (c) Fig. 3. Origial Image (a), Watermarked Image (b), Marked Image (c) The potetial dager with sesitive databases cotaiig biometric idetifiers, such as facial, figerprit images ad templates, is that they are likely to be attacked by hackers or crimials. Watermarkig the iformatio i these databases ca allow the itegrity of the cotets to be verified. Aother dager is that this critical data ca be attacked while it is beig trasmitted. For example, a third party could itercept this data ad maliciously alter the data before re-trasmittig it to its fial destiatio. The trasmissio problem is eve more critical i cellular ad wireless chaels. The chaels themselves are quite oisy ad ca degrade the sigal quality. Additioally, data trasmitted through wireless chaels is far from secure as they are omi-directioal, ad as such ca be eavesdropped with relative ease. The growth of the wireless market ad e-commerce applicatios for PDAs requires a robust cryptographic method for data security. There are compact solid state sesors already available i the market, which ca capture figerprits or faces for the purpose of idetity verificatio. These devices ca also be easily attached to PDAs ad other had-held cellular devices for use i idetificatio ad verificatio. Cosiderig all the oise ad distortio i cellular chaels, our combied watermarkig techique alog with the cryptographic ecoder/decoder will maily work i smudgig, compressio ad filterig. Our cryptographic ecoder/decoder will oly fail whe oise ad distortio is detected i the sesitive areas of the images that have bee embedded with the block-wise algorithm. If our watermarked image is trasferred i a oisy chael, the we eed to reduce the amout of iformatio iserted with the block-wise method to have a high rate of success.

10 Protectig Biometric Templates with Image Watermarkig Techiques Coclusio Watermarkig biometric data is of growig importace as more robust methods of verificatio ad autheticatio are beig used. Biometrics provides the ecessary uique characteristics but their validity must be esured. This ca be guarateed to a extet by watermarks. Ufortuately, they caot provide a foolproof solutio especially whe the trasmissio of data is ivolved. A receiver ca ot always determie whether or ot he has received the correct data without the seder givig access to critical iformatio (i.e., the watermark). I this paper we have preseted a cryptographic ecoder/decoder that digitally sigs biometric templates, which are embedded with combied lattice ad block-wise image watermarkig techiques ito a image. Combiig image watermarkig techiques with cryptographic primitives eables us to protect biometric templates that have bee geerated by a visual-based biometric system without ay distortio of the image. Sice biometric templates are essetial tools for autheticatig people, it is ecessary to protect them for possible alteratios ad fabricatios i cojuctio with their biometric image(s) whe these are stored i private/public databases. Image watermarkig techiques i cojuctio with cryptographic primitives provide a powerful tool to autheticate a image, its biometric template ad ay additioal iformatio that is cosidered importat accordig to a particular applicatio. I the passport-based sceario, for example, the photograph ad the private iformatio (i.e. ame/address) of a idividual ca be protected with the proposed approach. Our results showed that we ca combie watermarkig techiques to securely embed private iformatio i a biometric image without fadig it out. Refereces 1. Bruelli, R., Poggio, T.: Face recogitio: features versus templates. IEEE Tras. O Patter Aalysis ad Machie Itelligece 15, (1993) 2. Jai, A.K., Uludag, U.: Hidig Figerprit Miutiae i Images. I: Proc. Automatic Idetificatio Advaced Techologies (AutoID), New York, pp (March 14-15, 2002) 3. Jai, A.K., Uludag, U., Hsu, R.-L.: Hidig a face i a figerprit image. I: Proc. Iteratioal Coferece o Patter Recogitio (ICPR), Caada, (August 11-15, 2002) 4. Kug, S.Y., Mak, M.W., Li, S.H.: Biometric Autheticatio: A Machie Learig Approach. Pretice Hall Iformatio ad System Scieces Series (2005) 5. Lucilla, C.F., Astrid, M., Markus, F., Claus, V., Ralf, S., Edward, D.J.: Biometric Autheticatio for ID cards with hologram watermarks. I: Proc. Security ad Watermarkig of Multimedia Cotets SPIE 02, vol. 4675, pp (2002) 6. Peticolas, F., Aderso, R., Kuh, M.: Iformatio hidig a survey. IEEE Proceedigs 87(7), (1999) 7. Wog, P.H.W., Au, O.C., Yueg, Y.M.: Novel blid watermarkig techique for images. IEEE Tras. O Circuits ad Systems for Video Techology 13(8), (2003) 8. Hao, F., Aderso, R., Daugma, J.: Combiig Crypto with Biometrics Effectively. IEEE Trasactio o Computers 55(9), (2006)