An Efficient Watermarking Technique for the Protection of Fingerprint Images
© D. Kirovski 2008
Received: 12 February 2008
Accepted: 11 September 2008
Published: 18 November 2008
This paper describes an efficient watermarking technique for use to protect fingerprint images. The rationale is to embed the watermarks into the ridges area of the fingerprint images so that the technique is inherently robust, yields imperceptible watermarks, and resists well against cropping and/or segmentation attacks. The proposed technique improves the performance of optimum multibit watermark decoding, based on the maximum likelihood scheme and the statistical properties of the host data. The technique has been applied successfully on the well-known transform domains: discrete cosine transform (DCT) and discrete wavelet transform (DWT). The statistical properties of the coefficients from the two transforms are modeled by a generalized Gaussian model, widely adopted in the literature. The results obtained are very attractive and clearly show significant improvements when compared to the conventional technique, which operates on the whole image. Also, the results suggest that the segmentation (cropping) attack does not affect the performance of the proposed technique, which also provides more robustness against other common attacks.
Depending on the embedding domain, existing algorithms for image watermarking usually operate either in the spatial domain [6, 7] or in a transform domain such as the discrete cosine transform (DCT) [8, 9] and the discrete wavelet transform (DWT) [10, 11]. However, most research works have been proposed in the transform domain because of its energy compaction property which suggests that the distortions introduced by the watermarks into the transform coefficients will spread over all the pixels in the spatial domain so as the changes introduced in these pixels values are visually less significant. Also, depending on the embedding rule used, the watermarks are often embedded using either an additive or a multiplicative rule. Additive rule has been broadly used in the literature due to its simplicity [8, 9, 12]. On the other hand, multiplicative rule is more efficient because it is image dependent and exploits the characteristics of the human visual system (HVS) in a better way [13–16].
Researchers in watermarking domain have focused their works on two fundamental issues: watermark detection and watermark decoding (extraction). In the latter, usually referred to as multibit watermarking, a full decoding is carried out to extract the hidden message, which can be an ownership identifiers, transaction dates, a serial numbers, and so forth. Such a watermarking can be found in fingerprinting, steganography, and the protection of intellectual property rights. In multibit watermarking, errors may occur when extracting the hidden message. Error probability can be used as a measure of the watermarking system performance.
In the literature, optimum decoders have been proposed and are based on a statistical modeling of the host data. Hernandez et al. propose a structure of optimum decoder for additive watermarks embedded within the DCT coefficients, modeled by a generalized Gaussian distribution (GGD). The problem of optimum decoding for multiplicative multibit watermarking has been addressed in [17–19]. In , the authors propose a new optimum decoder of watermarks embedded in the DFT coefficients modeled using a Weibull distribution, while Song in  proposes a general statistical procedure based on the total efficient score vector for both GGD and Weibull distribution. In , a new optimum decoder based on GGD has been proposed for extracting watermarks embedded within DWT coefficients.
Embedding watermarks into the ridges area increases its robustness because an attacker is interested in that area only (i.e., segmentation or cropping attack is usually performed to extract the ridges area from the background). Consequently, a part/portion of the watermark which is embedded within the background area can be removed, thus affecting the robustness of the watermark. Furthermore, to remove a watermark embedded in the ridges area, an attacker needs to apply strong attacks (such as additive noise and filtering) on that area, resulting in severe degradations of the quality of the image, thus, making it useless.
The human eye is less sensitive to noise and changes in the texture regions; this makes sense to select the ridges area for watermark embedding and ensures imperceptibility of the embedded watermarks.
The proposed technique starts by first extracting the ridges area using the segmentation technique proposed by Wu et al. , which has been modified to generate adaptive thresholds instead of fixed ones. The output of the segmentation results in a binary mask called segmentation mask. This mask is then partitioned into nonoverlapping blocks, where only the blocks belonging to the ridges area are used to carry the watermark. This is represented by another binary mask called watermarking mask. The proposed technique has been introduced to increase the performance of the optimum watermark decoder, whose structure is theoretically based on a maximum-likelihood (ML) estimation scheme. For the sake of illustration, the process of watermarking is applied in both the DCT and the DWT domains, where the transform coefficients in each domain are statistically modeled using a GGD that has been shown, in the literature, to be the most accurate statistical model. The results obtained in this work clearly demonstrate the performance improvements achieved by the proposed technique. Also, the segmentation process, which can be thought of as an attack for fingerprint images, is shown to have no influence on the overall performance of the optimum decoder.
The paper is organized as follows. Section 2 describes the technique used to extract the region of interest. A brief description of watermark generation and the embedding process for both the DCT and the DWT domains is given in Section 3. Then, in Section 4, the multibit watermark decoding (extraction) issue is addressed. The influence of attacks on the overall performance of the optimum decoder is assessed through experimentation whose results and analysis are reported in Section 5. Finally, conclusions are drawn in Section 6.
2. Ridges Area Detection and Extraction
A captured fingerprint image usually consists of two areas: the foreground and the background. The foreground or ridges area is the component that originates from the contact of a fingertip with the sensor. The noisy area at the borders of the image is called the background area. An extraction of the ridges area can be carried out by using a segmentation technique whose objective is to decide whether a part of the fingerprint image belongs to the foreground (which is of our interest) or belongs to the background. Several methods and techniques have been proposed in the literature for segmenting fingerprint images [21, 22]. However, in our case, the technique must be robust to common watermarking attacks in the sense that it also detects the same ridges area even if a fingerprint image is subjected to attacks such as compression, filtering, noise addition. Unfortunately, most of these techniques are not robust enough to resist image manipulations. In this work, we propose to use Harris corner point features to segment the fingerprint images. A Harris corner detector is based on a local autocorrelation function of a signal to measure the local changes of the signal with patches shifted by a small amount in different directions . It has been found in  that the strength of a Harris point in the foreground area is much higher than that in the background area. However, the authors proposed to use different thresholds, which are determined experimentally for each image. Also, they noticed that some noisy regions are likely to have a higher strength which cannot be eliminated even by using high threshold value and proposed to use a heuristic algorithm based on the corresponding Gabor response. In our case, we found that an adaptive threshold can be obtained by using Otsu thresholding method  which provides an excellent threshold for fingerprint images from different databases. When some morphological methods are applied to eliminate the noisy regions, excellent segmented images are obtained.
The output of the segmentation process yields a segmented image and/or a segmentation mask. Since Harris point features method is a pointwise method, the segmentation mask is a binary mask (i.e., 1 if the pixel is assigned to the foreground area and 0 otherwise) of the same size as the original image.
As can be seen from Table 2, all test images that form the three databases are robust to mean filtering attack and the technique can extract the same watermarking mask even for a filtering attack with a window size of . However, the test images from database DB2 are more sensitive to WSQ compression and AWGN attacks than the images from the other databases. Images from DB1 are very robust to WSQ compression and images from DB3 are less sensitive to AWGN.
3. Watermark Generation and Embedding
As mentioned previously, the DCT and DWT domains are used to embed the watermark. The DCT can be applied either to the entire image or blocks as in the JPEG standard  as well as the DWT. The watermarking algorithm considered in this work relies on the embedding of a spread spectrum watermark, which spreads the spectrum of the hidden signal over many frequencies making it difficult to detect . The embedding stage starts by decomposing the fingerprint image into blocks as described in the previous section (i.e., spatial blocks of pixels) and only the ridges area blocks are selected to carry the watermark. Thus, using a watermarking mask , if , then block is selected; otherwise, it remains unchanged.
Assuming that the watermark carries a hidden message M with information that can be used, for instance, to identify the intended recipient of the protected image; this message is mapped by an encoder into a binary sequence of bits (by denoting for bit and for bit 0).
Let be a pseudorandom sequence uniformly distributed in , generated using a pseudorandom sequence generator (PRSG) initialized by a secret key . This pseudorandom sequence is the spreading sequence of the system. Every bit from the sequence is then multiplied by a set from the sequence in order to generate an amplitude-modulated watermark, consisting of the spread of the bits .
3.1. DCT Domain
After selecting the blocks to be watermarked, a DCT transform is applied on blocks of pixels, as in the JPEG algorithm . Specifically, the application of the DCT on blocks leads to 64 coefficients which are zigzag scanned (i.e., arranged in decreasing order) to obtain one dimensional vector representing the entire set of the DCT coefficients to be watermarked (the DC component for each block is not used). In order to increase the security level, we propose to introduce some uncertainty about the selected coefficients altered by permuting the coefficients in using a key .
where and represent the set of the original coefficients and the associated watermarked coefficients belonging to the set , respectively. is a gain factor used to control the strength of the watermark by amplifying or attenuating the watermark effect on each DCT coefficient, so that the watermark energy is maximized while the alterations suffered by the image are kept invisible.
The hidden watermark can be retrieved if one knows (a) the entire procedure through which the watermark has been generated, (b) the secret key used to initialize the PRSG, and (c) the second key which is used to permute the coefficients. Thus, an attacker will not be able to extract the watermark without knowledge of the secrete keys and , even if the entire watermark generation and embedding process are known.
3.2. DWT Domain
Each block selected to carry the watermark is transformed using the DWT at a level , which produces (i) a low-resolution subband , (ii) high-resolution horizontal subbands , (iii) high-resolution vertical subbands and (iv) high-resolution diagonal subbands . A watermark should be embedded in the high-resolution subbands, where the human eye is less sensitive to noise and distortions [30, 31]. In this work, all coefficients of the high-resolution subbands are used to carry the watermark sequence and the set of coefficients to watermarked is defined as . The watermark is then embedded by following the same steps described above for the DCT domain.
4. Optimum Watermarking Decoder
5. Experimental Results
To gauge the effectiveness of our proposed technique, experiments were performed with test images from the databases FVC2004 (DB1, DB2, and DB3). In the DWT domain, the images were transformed using Daubechies9/7 wavelets  at the 3rd decomposition level and all coefficients of the high-resolution subbands ( , , and subbands of the levels ) were used to carry the watermark. Daubechies9/7 wavelets were used because they have been adopted by the FBI as part of the WSQ compression standard for fingerprint images . In all experiments, a blind watermark decoding is used so that the parameters and of each set are directly estimated from the DCT and the DWT coefficients of the watermarked images since the strength is chosen to be sufficiently small to not alter the visual quality of the original images. For the sake of fair comparison, the performance of the proposed technique is compared against the conventional technique using the same decoder. By conventional watermarking, it is meant a technique which operates on the whole transform coefficients as described in [10, 19]. The performance is assessed by the bit error rate (BER), that is, the average number bit errors. For the sake of illustration, only results related the test images shown in Figures 2, 3, and 4 are plotted because the results from other images are very similar.
Number of bits per image. Watermark embedded using the proposed technique.
As can be seen from Figures 5 and 6, the proposed technique outperforms the conventional one, even without applying any attack. Another point that should be raised is the influence of the size of sets on the performance of the decoder: the larger the set, the better the results. This is justified by the fact that a larger set provides more redundancy in the sense that each bit is carried by a higher number of coefficients. Furthermore, from the view point of implementation, a large set can be accurately modeled and the distribution of its coefficients is well approximated. However, in the case of the conventional technique operating on images from DB1 (i.e., image 98_2 and image 20_1, where the background is almost white), the BER is high and almost unchanged against an increase of the size of . One can explain this by the fact that, in general, since a white background and smooth areas produce large number of null coefficients in both the DCT and DWT domains and according to the multiplicative rule used, these null coefficients cannot carry significant portion of watermark, thereby making these coefficients not reliable for decoding.
Extensive experiments have also been conducted to gauge the performance of the proposed technique with respect to robustness in comparison with the conventional technique. Three sets of experiments have been carried out to measure the robustness of the watermark against the common attacks, namely, WSQ compression, mean filtering, and AWGN. In all these experiments, the value of the strength is chosen in such a way to obtain PSNR value 40 and the number of coefficients per bit is set to 500. Each attack has been applied several times by varying the attack strength and reporting the average value of BER over 100 different pseudorandom watermarks. Note that results related to one image from each database are plotted since results of other images are similar.
This paper proposes an efficient technique for use in fingerprint images watermarking. The rationale of the technique consists of embedding the watermark into the ridges area of the fingerprint images which constitutes the region of interest. The key features of the proposed technique are to (i) preserve the watermark from segmentation which can be considered as a special case of the cropping attack, (ii) increase the robustness of the watermark against known attacks such as filtering, noise, and compression, and (iii) allow to embed imperceptible watermarks by embedding in highly textured areas. The technique starts by first extracting the ridges area from fingerprint images using the segmentation technique proposed by Wu et al. , which has been modified to generate adaptive thresholds instead of fixed ones, thereby making it more practical. This leads to a binary mask referred to as the segmentation mask. In order to ensure that the full watermark is embedded into the ridges area, the segmentation mask is partitioned into blocks, represented by another binary mark called watermarking mask.
The proposed technique has been applied to the optimum multibit, multiplicative watermark decoding. The watermark is embedded in the well-known transform domains, namely, the DCT and the DWT. The optimum decoder is based on the ML scheme and the coefficients of the two domains are modeled by a generalized Gaussian distribution. It is worth mentioning that the number of bits that an image can carry is image dependent (i.e., it depends on the ridges area meaning that a larger area allows more bits to be embedded and vice versa). The results obtained clearly show the improvements introduced by the proposed technique even in the absence of attacks. Furthermore, as the segmentation technique removes the part of the watermark embedded within the background area, it affects the performance of the conventional optimum decoder. However, this attack has no effect on the proposed technique. Moreover, the proposed technique provides more robustness in the presence of attacks such as WSQ compression, mean filtering, and additive white noise.
Finally, it should be mentioned that the proposed technique can be easily applied to other biometric images such as face, hand, and iris, since this type of images has only one defined region of interest. Also, it can be used to some natural images whose region of interest can be defined and extracted.
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