Off-Line Authentication Using Watermarks Hyejoung Yoo1 , Kwangsoo Lee2 , Sangjin Lee2 , and Jongin Lim2 1
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sparxcom corporation, Anam Dong 5ga, Sungbuk Gu, Seoul, Korea
[email protected] Center for Information Security Technologies(CIST), Korea University, Anam Dong, Sungbuk Gu, Seoul, Korea {kslee,sangjin,jilim}@cist.korea.ac.kr
Abstract. In this paper, we propose a new method for the secure and practical watermarking system. This method is designed to use printed images in the off-line authentication, where one determines whether a printed image has been altered intentionally since the first printing time, perhaps by a malicious party. This system is based on the fragile watermarking technologies to detect the transforms such as PS(printing and scanning) distortions, slight rotations and croppings, etc. With the human visual system, this watermarking system can detect the watermark in a image with random noises and can be applied for the practical use. Keywords: off-line authentication, e-commerce, PS distortion, PSP distortion.
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Introduction
Digital watermarking is a technology to mark a discriminating information into digital images for the purposes of the ownership verification or authentication. This technology is becoming important in the electronic commerce because the digital data can be splitted on the developing network systems such as the World Wide Web. There are two types of watermarking systems. One is the robust watermarking system that is designed to resist attacks that attempt to remove or destroy the mark. Such attacks include lossy compression, filtering, and geometric scaling. The other is the fragile watermarking system that is designed to detect slight changes to the watermarked image with high probability. The fragile watermarking system is mainly used for the image authentication because it ensures the genuineness of images with high probability. In the off-line authentication, the watermark extracted on the scanning process determines the genuineness of the printed image of a digital image downloaded from network systems. The off-line authentication is important due to the usage of digital images on the internet and in electronic commerce such as on-line ticketing. There are some problems in the off-line authentication of the printed image with the information of the genuineness. On the process that the digital image is printed and the printed image is scanned, there is possibility that the image is K. Kim (Ed.): ICICS 2001, LNCS 2288, pp. 200–213, 2002. c Springer-Verlag Berlin Heidelberg 2002
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modified by random noises. We call this the PS(Printing and Scanning) distortion. For the correctness of the off-line authentication, the PS distortion must be removed. In this paper, we design the fragile watermarking system for the off-line authentication that resists some problems such as the PS distortion. The principal property in the scheme is the correctness and validity of the off-line authentication. With the human visual system, this watermarking system can detect the watermark in the image with random noises. Although invisibility is less important in the off-line authentication, our system has a regulation factor so that one can adjust the visibility of the mark in compliance with the various kinds of applications. If we decrease the value of the regulation factor, we can embed the watermark invisibly into the digital image and authenticate its printed image more securely. This paper is organized as follows. Section 2 is a brief review of the general flow of watermarking technologies. In section 3 we consider the basic properties of the watermark used in the offline authentication. In section 4 we present new watermarking method for off-line authentication. Section 5 shows the security of the method. We suppose various attack scenarios and explain them by the experimental results that our method is secure against these attacks. Conclusion and suggestion for the further research are offered in section 6.
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General Flow of Watermarking Technologies
Digital watermarking is a method to insert a digital information into an image so that the information can be extracted for the purposes of authorship or ownership verification. This technologies have been proposed as solutions for fundamental problems in digital communication. Early watermarking methods are no more than incrementing or decrementing an image component to encode binary ’1’ or ’0’, respectively[4]. Tirkel[6] and van Schyndel[7] applied the properties of m-sequences to produce oblivious watermarks resistant to filtering and cropping. It is reasonably robust to cryptographic attacks. Tirkel and Osborne[6] were the first to apply spread spectrum techniques to digital image watermarking. Spread spectrum method is cryptographically secure and is capable of achieving error free transmission of the watermarked image at the limits given by the maximum channel capacity[8]. A fragile watermark is a mark that is readily altered or destroyed when the host image is modified through a linear or non-linear transformation[9]. Since digital images are easy to modify, a secure authentication system is useful in proving that no tampering has occurred. The sensitivity of fragile watermarks to modification leads to their use in image authentication. That is, it may be of interest for parties to verify that an image has not been edited, damaged, or altered since it was marked. Image authentication systems can be adapted in law, commerce, defense, and journalism, etc. Schneider and Chang[10] presented a methodology for designing content based digital signatures which could be used to authenticate digital images.
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The goal of this method was to develop the way to be able to prove some form of authenticity, while still allowing desired forms of manipulation such as lossy compression. Lin and Chang[11] described an effective technique for image authentication which could prevent malicious manipulations but allowed JPEG lossy compression. The authentication signature was based on the invariance of the relationship between DCT coefficients at the same position in the separate blocks of an image. Lu, Mark Liao, and Sze[12] proposed a combined watermarking scheme which could simultaneously achieve the copyright protection and the content authentication by hiding watermarks only once. In this method, they proposed to quantize the selected wavelet coefficients into masking threshold units. Then, the watermark was embedded by modulating the quantization result to either a right or left masking threshold unit using the cocktail watermarking[13]. Wang and Knox[14] presented the watermarking method which was an invisible watermark based on a digital halftoning screen. When the image to be protected was halftoned in preparation for printing, the digital watermark, in the form of a logo-style image, was embedded into the correlation patterns of the halftoned image. Even if one of the main distribution methods of the digital data is a printed media, however, there are few watermarking methods that are robust to printing and scanning noises, i.e, PS distortion. We propose a new technique to be robust to PS distortion . This is the first proposal of the off-line authentication scheme using watermarks.
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Basic Properties
There are two kinds of attacks. One is an intentional attack such as the duplication and the other is an unintentional attack such as the compression and the PS distortion through the flow of the scheme. In case of an intentional attack, attackers may try to duplicate a printed document or a printed ticket to fool the system without triggering alarm to enter a theater or a stadium with the copy of that. Some noises are added to the watermarked image. D/A and A/D transform noises would make trouble to authenticate the data. And a slight rotation may happen in scanning processes of the image for the authentication of a ticket. In the following, we discuss the seven basic properties required for off-line authentication. 1. robustness to the PS distortions: In the off-line authentication scheme, since printing and scanning are indispensable procedure, the PS distortion is added essentially to the watermarked image. Therefore, the robustness to the PS distortion is one of the most important properties of the off-line authentication scheme. 2. robustness to slight rotations: When the image is scanned, a slight rotation may occur. Therefore, the watermark must be robust to slight rotations.
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3. robustness to the cropping: This is an essential property of all sorts of watermarking schemes. The watermark must be detected until the damage does not affect the value of the documents or the tickets. 4. robustness to folding and crumpling: Watermark is inserted in a document or in a ticket which the user carries at his pleasure. So there is a possibility that the watermarked image is folded or crumpled. 5. fragility to copying: The fragility to copy is the most important property for off-line authentication scheme such as on-line issuing of documents and on-line ticketing. If the scheme is not fragile to copy, it is not applicable to the real life and will be very useless. 6. fragility to the PSP distortions: Reprinting after scanning of the printed ticket is one way of copying. A ticket is easily copied in high quality by this method. The watermark is fragile to the PSP distortion. 7. propriety of the time requirement: In order to apply to the real life, the propriety of the time requirement is very important. If embedding and detecting processes are not efficient, the off-line authentication scheme cannot be realized. Especially, the detecting time of the on-line ticketing scheme is more important. The detecting time must be reasonble enough to enter a theater or a stadium without any delay.
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The Scheme
In this section, we present the watermarking scheme for the off-line authentication which discriminates the genuineness of the printed image. To our knowledge, this is the first practical proposal about the authentication of the printed data. Here, we handle images which is used for the off-line authentication in practical application such as on-line issuing of documents and on-line ticketing. 4.1
Off-Line Authentication
Digital data can be manipulated and distributed easily by using computers. The easy manipulation of digital data causes a real threat for the data issuers. Issuers of on-line data have had no means to authenticate their printed data practically up to now. Lin and Chang[15] showed several techniques for extracting invariants from the original and rescanned image. But they did not consider the fragility to copy and to psp distortion. They proposed a hypothetical model of the pixel value distortions and the parameters of this model were approximated by experiments. There are some problems in their analysis of the geometirc distortions of RSC(rotation, scaling, and cropping) to apply to ps model of off-line authentication. First, the image is not rotated around the same center, that is, the central point of each rotation may be different in ps process. The rotations are not closed under the composite operation(R1 R2 = R), and, if the image is rotated more than once, the new kind of noise is added to that, not a general rotation error. Second, rotation and cropping are not commutative(RC = CR) unless
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the rotation degree equals zero. Scaling and cropping are also not commutative (SC = CS) unless the scaling factors are all equal. In order to construct the off-line authentication scheme, the printed materials which are issued via internet must preserve the contents in their integrity. Data integrity verification is equivalent to the functionality of the authentication. There are two types of watermarking schemes for authentication. The first type is sensitive even to the slightest change of the digital medium, whereas the other type notifies an authentication violation only when significant modifications of the visual content occur. The former is useful for channel authentication and the latter is useful for tracing the changes of the contents of the documents. In our case the traceability of the changes is much more important. One of the general properties of the watermarking schemes for the digital data authentication is perceptual transparency. An embedded watermark should not be visible under normal observation and should not interfere with the functionality of the image. In most cases, this refers to preserving the aesthetic qualities of an image and increasing the security. In our case, we see that authentication of the contents is more important than aesthetic qualities of the image. The invisibility is less important where the images in the documents or tickets are not for the commercial purpose. But the degree of visibility must be decided with prudence, since it affects the security of the scheme. Our method has the regulation factor of the visibility. As this regulation factor is decreased, the watermark becomes more invisible. 4.2
Embedding
Off-line authentication scheme contains PS steps. Printing and scanning add random noises to the data. These noises are resulted from the luminance and contrast variations, the blurring of adjacent pixels, and the geometric distortions. Therefore, even if we print/scan the same image by the same printer/scanner, the results are different. Thus, the watermark must be robust to random PS noises. At the same time, it should be fragile to the PSP distortion and the copy distortion. We model the variation of the gray values of pixels before and after the scanning process. The variance is usually larger on dark and bright pixels. Table 1 shows the repetitive experimental results about variation of the pixel values which are caused by scanning process. We describe the embedding approach which intensifies the regularity of the watermark and the irregularity of the random noises caused by scanning process. First, we must randomize the watermark information. This process prevents an attacker who knows the embedded watermark from re-embedding it into the scanned tickets and re-authenticating them. But we omit this procedure, since we embed the logo-style watermark in order to authenticate whether the printed data are issued by the determined issuer or not. The algorithm that we present below is guaranteed to produce the watermark which is pertinent to the off-line authentication scheme.
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Table 1. Scanners CCD error variance OV MV MV−OV ED (Original Value) (Error Mean Value) (Error Deviation) 250 253 +3 5 200 ∼ 210 206 +6 −3 ∼ 16 190 ∼ 200 193 +3 −6 ∼ 12 179 ∼ 180 169 −1 −10 ∼ 11 130 ∼ 140 129 −1 −10 ∼ 8 80 ∼ 90 84 +4 −3 ∼ 14 60 ∼ 70 69 +9 0 ∼ 19 50 ∼ 60 70 +20 13 ∼ 28 30 ∼ 40 48 +18 20 ∼ 28 0 ∼ 10 35 +35 30 ∼ 48
1. Decide the information rate of the watermark W to be embedded. 2. Find the parts Is of the original image I of which pixel values are contained in [b1 , b2 ], where the pixel values in [b1 , b2 ] have the small changing variances by the scanning process, so I(i, j) ∈ Is is more robust to PS distortions. In our experiments, we let b1 = 80 and b2 = 180. 3. Divide I by the size of W in order to disperse the watermark message so as to be robust to PS distortions. For example, let the size of I be M × N and the size of the logo be a × b, then the number of the division parts of I is l, where l = M/a × N/b. We denote these division parts by Di ,1 ≤ i ≤ l. In case of M/a ∈ / Z or N/b ∈ / Z, we append 0 to the remainder parts in order to equalize the sizes of the remainders with the size of W . Thus, size(Di ) = size(W ) for all i, 1 ≤ i ≤ l. 4. For all i, check that Di is contained the part of Is for some s. If more than half of Di is contained in Is , we say that Is contains Di and denote Di ⊂ Is . 5. Each division part Di is assigned to a different value of the regulation factor ri ; ri ≤ 0.05 if Di ⊂ Is f or some s ri > 0.05 if s such that Di ⊂ Is l and then calculate I ∗ = i=1 Ii∗ , where Ii∗ = Di + ri W (1 ≤ i ≤ l), where ∗ Ii (j, k) = Di (j, k) + ri W (j, k). Depending on the value of ri , the different amount of partial information of W is embedded in each division part. The higher the value of ri is, the more the partial information of W is embedded into Di . Here, all regualation factors can be assigned to the same, that is, ri = r for all i. The visibility of W is decided by the regulation factor. The more visible the mark is, the better detected it is. By this step the interference of the PS distortions decreases and the embedded watermark gets to be more invisible. 6. The watermarked image, Iem, is lined with I ∗ (j, k) depending on i. We show the logo-style watermark, the original image, and the watermarked image in Figure 1.
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(a)
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Fig. 1. (a) watermark: sparxcom co. logo (100 × 100); (b) original image: grandbleu (600 × 800); (c) watermarked image with r = 0.035, l = 48, b1 = 80, and b2 = 180
One of the practical applications of our watermarking scheme is on-line ticketing. On-line ticketing is a kind of e-commerce system all sorts of which users can purchase via internet. After the user chooses the ticket to buy, the watermark is embedded in the image which was contained in the chosen ticket. The user had no sooner paid for the chosen ticket than he received the ticket at home or in his office by using his printer. At this point, copy control technology about the digital data of the ticket is needed separately, because once a dishonest buyer of a ticket gets the ticket in digital form, he can reprint it as often as he wants. The user can enter the theater conveniently with the watermark embedded ticket which is printed in from 300dpi up. 4.3
Detecting
When a user goes to a theater or a stadium with the printed ticket, the staff of a theater or a stadium wants to know whether the ticket is genuine or not. If the ticket is forged, the staff rejects the entrance. With our watermarking scheme, the staff can judge the genuineness of the tickets using the exclusive use terminal(the scanner) within 2 seconds before the user is authorized to enter a theater or a stadium. The time required for this procedure is reasonable for the entrance without any delay. Detection of the watermark involves scanning the printed image and looking for the correlations of the scanned image with respect to the neighboring regions of the image. Detection is progressed as follows: 1. The printed document is scanned in from 300dpi up using the terminal (the scanner). The necessary part of the scanned is the image part. There is a possibility that the image is rotated slightly when the document is scanned.
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Fig. 2. (a) scanned ticket containing watermarked image with r = 0.070; (b) detected watermark (100 × 100)
2. We revise the rotation error to some degrees and cut the scanned image as the same size as the original. We denote this Iem∗ . Iem∗ is different with Iem because of PS distortions, geometric distortions, and cutting distortions, etc. Iem∗ = Iem + N, where N is a random noise. 3. Divide Iem∗ by the size of the embedded watermark. Since watermark is for authentication and exclusive use terminal s used by the watermark provider, it is right that the size of the embedded watermark makes public to the exclusive use terminal. 4. Look into the correlation of the division parts, and extract this correlation from each division part. 5. Extracted information from each division part lies one upon another. 6. Calculate the similarity of the embedded watermark and the extracted watermark. The embedded information was determined at a theater or a stadium, so it does not affect the security of the system that the original embedded watermark is inserted into the exclusive use terminal of the theater or the stadium. Figure 2 shows the scanned image and the detected watermark.
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Experimental Result
In this section, we show that our scheme satisfies the basic properties discussed in section 3. We experiment with popular devices in table 2. The results are as follows: 1. robustness to the PS distortion Figure 2 shows our scheme is robust to the PS distortion. 2. robustness to slight rotation Proposed detection algorithm corrects this rotation to some degrees and authenticates correctly.
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Fig. 3-1. Off-line security (a1) scan of the rotated image after ImageCutter.exe (a2) detected watermark of (a1) (b1) scan of the crumpled image (b2) detected watermark of (b1)
3. robustness to cropping Our scheme, of course, is robust to cropping less than a half since the mark information is divided and inserted all over the image. 4. robustness to folding and crumpling We have folded a ticket(an image) into from two to eight leaves. Also, we have crumpled a ticket(an image) at random into the bargain. Experimental results show that our scheme is robust to folding and crumpling. 5. fragility to copy We do experiment the fragility to copy about the various brightness of the various duplicators. From these experimental results, we can conclude that our scheme is fragile to copy. We have found that the brighter the duplicator is, the better detected the watermark is . (c1) and (c2) in Figure 3 show the result of the copy using the most bright duplicator, SINDORICOH NT 4240
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Fig. 3-2. Off-line security (c1) scan of the copy (c2) detected watermark of (c1) (d1) scan of the psp distorted image (d2) detected watermark of (d1) Table 2. Experimental environment Computer Scanner Printer Duplicator Paper
Pentium III 600, Ram 128 FUJITSU M3292DC HP 2000c, HP-Laserjet 1100, SINDORICOH LP 3250, etc SINDORICOH NT 4140, ST 4520, XEROX X-230, Cannon NP 6650, CLC-900, LC-2300 general copying papers
6. fragility to the PSP distortion In our experiment, we have printed and scanned in 300dpi. We can conclude that the proposed scheme is fragile to psp distortions. 7. propriety of time requirement In our scheme, it takes 2 or 3 seconds for the watermark to be embedded and takes 1 or 2 seconds for the document or the ticket to be authenticated. We conclude that the required time is reasonable for realization of the off-line authentication scheme.
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Fig. 4-1. On-line security (a1) JPEG 95% (a2) detected watermark of (a1) (b1) high-pass filtering(−8/9) (b2) detected watermark of (b1)
We also experiment the robustness to the existing various attacks on the digital information of the watermarked image without D/A and A/D transformations separately. The results show in Table 3. Figure 3 and Figure 4 present the security of our scheme. In (c) and (d) of Figure 3, the watermarks are embedded with r = 0.090(high value) in order to show the powerful fragility to these attacks, but, in other experiments, the regulation factor is 0.040.
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Conclusion and Further Research
In this paper we have presented the off-line authentication scheme using watermarks. This scheme is applicable to on-line issuing of documents, on-line ticketing etc. Off-line authentication is a new application of the watermarking scheme. As we have mentioned above, our scheme is robust to the essential and various at-
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Fig. 4-2. On-line security (c1) rotation(2 degree) (c2) detected watermark of (c1) (d1) Jitter attack (d2) detected watermark of (d1)
tacks. These observations promise a significant performance improvements, but there is still much room for future work. As for the present study, the problems that it raises are as follows: 1. analyzing more accurate models of distortions Our analyzing of the embedding and the detecting processes are based on the augmentation of the regularity of the watermark and the irregularity of the various noises. A more accurate model must be considered in order to improve overall performance. 2. modeling the attacks more minutely This paper has presented the basic attack models for off-line authentication scheme. We must model the attacks more minutely and analyze the security against each attack. 3. analyzing potential performance improvement given side information at the detector Our watermarking algorithm assumes knowledge of the original and the size of the watermark at the detector. It increases the speed of the detection but is not ideal for the security. Without any ne-
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1. adding noise: gaussian(0.04, 0.09), salt and pepper(0.05, 0.09), and multiplicative noise. 2. rotation: The mark is detected in case that the rotation degree is less than 2. 3. Stirmark 3.1: The mark is detected in case that the rotation degree is less than 2. The mark is not detected after general linear geometric transformation. 4. Jitter attack: Copy odd rows to even rows after deletion of even rows.
cessities of these side information(full blind extraction), more sophisticated algorithms can be possibly developed. Therefore, we analyze the importance between the performance improvement given side information at the detector and the security without any necessity of that.
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