2014 22nd International Conference on Pattern Recognition Anti-Forensics of JPEG Detectors via Adaptive Quantization Table Replacement Chao Chen† , Haodong Li† , Weiqi Luo, Rui Yang‡ and Jiwu Huang§ School of Software, Sun Yat-Sen University, Guangzhou, China, 510006 of Information Science and Technology, Sun Yat-Sen University, Guangzhou, China, 510006 ‡ School of Information Management, Sun Yat-Sen University, Guangzhou, China, 510006 § College of Information Engineering, Shenzhen University, Shenzhen, China, 518060 † School Abstract—Due to the popularity of JPEG compression standard, JPEG images have been widely used in various applications. Nowadays, detection of JPEG forgeries becomes an important issue in digital image forensics, and lots of related works have been reported. However, most existing works mainly rely on a pretrained classifier according to the quantization table shown in the file header of the suspicious JPEG image, and they assume that such a table is authentic. This assumption leaves a potential flaw for those wise forgers to confuse or even invalidate the current JPEG forensic detectors. Based on our analysis and experiments, we found that the generalization ability of most current JPEG forensic detectors is not very good. If the quantization table changes, their performances would decrease significantly. Based on this observation, we propose a universal anti-forensic scheme via replacing the quantization table adaptively. The extensive experimental results evaluated on 10,000 natural images have shown the effectiveness of the proposed scheme for confusing four typical JPEG forensic works. I. random perturbation on the DCT coefficients. In [12], Luo et al. showed that nonaligned double JPEG compression usually occurs in JPEG forgeries, and proposed a detection method based on blocking artifact characteristics matrix. Recently, Bianchi and Piva [13] proposed an improved method based on the integer periodicity maps. Like a cat-and-mouse game, a wise attacker may perform some anti-forensic operations to remove those tampering traces and fool the forensic detectors. In order to find out the weaknesses of current forensic methods and further improve their robustness, researches have began to study the anti-forensic methods from the perspective of forgers. For instance, Gloe et al. [14] proposed two methods to trick the resampling detector and the sensor pattern noise based detector respectively. Stamm and Liu [15] introduced an anti-forensic scheme for removing detectable artifacts left by lossy image compression. Sutthiwan and Shi [16] proposed a method to deceive double JPEG compression detection by destroying JPEG grid structure via resizing images, and so on. I NTRODUCTION With the help of various powerful digital image editing software, it is increasingly easy to modify image contents without leaving any obvious visual artifacts. If those sophisticated tools were abused, it would inevitably lead to some potential serious moral, ethical and legal consequences. Therefore, digital image forensics [1], [2] becomes an important issue and it faces great challenges. So far, most anti-forensic methods aim at attacking a specific forensic detector, which means that they are difficult to be extended for confusing other forensic detectors. In this paper, we try to proposed a universal anti-forensic scheme for those quantization table based JPEG detectors. Based on our analysis and experiments, we found that the existing JPEG forensic detectors usually assume that the quantization table in the JPEG file header is authentic, and their performances are highly dependent on this table. If a test JPEG image is fed to a mismatched JPEG detector for a different quantization table, the detection performances will drop significantly. Therefore, we propose a simple yet very effective anti-forensic method against this common flaw in JPEG forensic methods via just replacing the quantization table of the file header. In the proposed strategy, we try to obtain a proper quantization table by adjusting the visual quality of the resulting JPEG image adaptively. The experimental results evaluated on 10,000 natural image with four typical double JPEG compression detectors have shown the effectiveness of the proposed antiforensic scheme. As the most popular image compression standard, JPEG has been widely used in various applications due to its high compression efficiency and good image quality fidelity. Most image forgeries are now stored as the JPEG format, and thus lots of related forensic works have been proposed in the past few years, such as identifying JPEG compression traces from bitmap images and further estimating the compression parameters previously used [3], [4], exposing those doubly JPEG compressed images [5], [6], locating the tampered regions in JPEG composite images [7], [8], and so on. Since double JPEG compression is a necessary operation after various tampering operations, double JPEG compression detection attracts considerable attention. In [9], Chen and Shi proposed a method for detecting double JPEG compression with different quantization table via modeling the relations among the magnitude of DCT (discrete cosine transform) coefficients. In [10], Li et al. extracted a 180-D feature set from the first digit of DCT coefficients in the individual AC mode for distinguishing between singly and doubly JPEG compressed images. In [11], Huang et al. proposed a scheme to detect double JPEG compression with the same quantization table by employing a 1051-4651/14 $31.00 © 2014 IEEE DOI 10.1109/ICPR.2014.126 The rest of this paper is arranged as follows. Section II points out a common flaw in existing JPEG forensic detectors, Section III describes the proposed anti-forensic method, Section IV demonstrates the experimental results and discussions for four typical JPEG detectors. Finally, the concluding remarks are drawn in Section V. 672 Training set with QA Test image with QA Training set with QB Fig. 1. II. Feature extraction Detector A Appropriate decision Detector B Inappropriate decision Feature extraction Feature extraction JPEG forensic detector selection based on the quantization table of the test JPEG image TABLE I. T HE C OMMON F LAW IN JPEG F ORENSIC M ETHODS TPR QF2 Detector trained with QF2 = 70 Detector trained with QF2 = 80 In the JPEG lossy compression standard [17], the input image is firstly divided into 8 × 8 non-overlapping blocks, and then each individual block B in the spatial domain is transformed to DCT frequency domain and obtain the DCT coefficients (denoted as D), which are then quantized by a 8×8 matrix called quantization table Q, the quantized coefficients (denoted as Dq ) are finally entropy encoded as bit streams. The JPEG decompression works inversely: entropy decode the bit streams to obtain the Dq , do the de-quantization to obtain D, and then perform the inverse DCT (IDCT) on D to obtain the 8 × 8 block in the spatial domain, finally round and truncate the pixel values into the range of {0, 1, . . . , 255}. The decompression operations can be formulated as follows: FOR THE MATCHED / MISMATCHED DETECTORS Method [9] Method [10] Method [9] Method [10] 50 49.30 16.67 47.04 7.92 60 34.92 7.30 44.40 38.52 70 95.70∗ 99.17∗ 60.72 27.64 80 82.94 62.84 97.52∗ 99.28∗ with QF2 = 50, 60, 70, 80, which are then fed to the pretrained detectors and obtain the true positive rate (TPR)1 . The experimental results are shown in Table I. It is clearly observed that when the test doubly compressed images are detected by a matched detector, all the TPR exceed 95% as highlighted with asterisks. However, when a mismatched detector is selected, for example, using a detector trained with QF2 = 70 to detect doubly compressed images with QF2 = 80, the TPR will significantly drop. Therefore, we can conclude that JPEG forensic detectors [9], [10] are highly dependent on the quantization table as described in previous analysis. B = [IDCT(D)] = [IDCT(Dq · Q)] Please note that the decompressed block B may not be exactly the same as the input block B due to the errors introduced by JPEG compression and decompression [4]. Please note that the quantization table Q is stored in the JPEG file header, it is right after the hexadecimal flag “FFDB”. In existing JPEG forensic methods, the detector selection is mainly based on this file header information, and always assume it is authentic. For a wise attacker, however, he or she can easily modify the file header information with the help of some text editors, such as Notepad++, so that a mismatched detector is selected and thus it would lead to poorer detection performances of the existing JPEG detectors. This is the main idea of the proposed anti-forensics method. Based on above analysis, it is observed that the quantization table Q plays a very important role in both JPEG compression and decompression, since it would affect the quality of the resulting JPEG images. Usually, the smaller quantization steps we use in Q, the better quality of the resulting JPEG image we obtain. What is more, the quantization table Q would significantly affect the statistic properties of resulting DCT coefficients. Therefore, the common way for JPEG forensic schemes is that: for a questionable JPEG image with a quantization table, they need to train a corresponding classifier by some controlled JPEG images with the same quantization table. Since the statistic properties of DCT coefficients are quite different for different quantization tables, it is expected that the generalization ability of the pre-trained classifier may not be very good, meaning that if a JPEG image with quantization table QA is fed to a detector B for quantization table QB , the detector would probably give an inappropriate decision, as illustrated in Fig. 1. III. T HE P ROPOSED A NTI -F ORENSIC M ETHOD In this section, we propose an anti-forensic method for fooling JPEG forensic detectors via adaptive quantization table replacement. As mentioned in Section II, the quantization table will affect both image quality and the detection performance of JPEG detectors. Generally, the more quantization steps we change, the poorer quality of image we obtain, however, the better we confuse the JPEG detectors. There may be a good trade off between the two things. Therefore, we should modify the quantization table carefully. To verify the above discussions, we conducted the following experiments. Firstly, we selected two forensic detectors [9], [10] for double JPEG compression detection, and trained two detectors for each method with quality factors QF2 = 70 and QF2 = 80 using 2,500 singly and the corresponding doubly JPEG compressed images with QF2 . In all doubly compressed images, the primary quality factor QF1 is randomly selected in the range of {50, 51, . . . 90} (excluding QF2 ). In the same way, we created other test doubly JPEG compressed images In order to preserve the image quality, we do not change the quantization steps for low frequency DCT components 1 True positive rate (TPR) means that double compressed images are predicted correctly, while true negative rate (TNR) means that singly compressed images are predicted correctly. Please note that the anti-forensic method aims to reduce the TPR, while the TNR is unchanged. 673 (a) Original JPEG (QF = 70) (b) The whole quantization table is replaced with QF = 50, 60, 80 and 90 respectively. The corresponding PSNRs compared with Fig. 2(a) are 19.68dB, 23.68dB, 20.73dB, and 15.78dB. (c) Preserving the first 5 quantization steps in the Zigzag order when replacing with QF = 50, 60, 80 and 90 respectively. The corresponding PSNR compared with Fig. 2(a) are 30.76dB, 36.54dB, 36.76dB, and 30.67dB. Fig. 2. Illustrations of decompressed JPEG images after replacing the whole and partial quantization tables (i.e. the first few steps in the Zigzag order). The reason is that most of natural image information is concentrated in a few low frequency DCT components. If those steps are modified, the decompressed images would change a lot, and some abnormal artifacts would also occur. As illustrated in Fig. 2, the quantization table of the original JPEG image with QF = 70 (Fig. 2(a)) is totally replaced with other four tables with QF = 50, 60, 80 and 90, respectively 2 . It is observed that the images become either too bright or too dark, since the quantization steps in low frequency components especially the DC component become larger and smaller, respectively. On the other hand, if only the quantization steps for higher frequency components are changed, the resulting images will look like the original ones, just as shown in Fig. 2(c). In this case, the corresponding PSNR between Fig. 2(c) and Fig. 2(a) are all higher than those between Fig. 2(b) and Fig. 2(a). three following steps. 1) Generate a series of candidate quantization tables Qc , whose first N steps in Zigzag order are identical to those in target table Qt , while the (N + 1)-th to the last steps are replaced by quantization tables with quality factors QFc , where QFc are the neighbors of QFt with an interval r and a step 1: QFc = {QFt − r, · · · , QFt + r} ∩ {1, 2, · · · , 100} Based on the above analysis, we propose an adaptive antiforensic strategy to replace quantization table as illustrated in Fig. 3. For a given image I and a target quantization table Qt with a quality factor QFt , the proposed method includes the Compress the given image I with each Qc respectively, and then we replace the quantization table stored in the resulting JPEG file header with the targeted table Qt . Calculate the PSNR of the resulting images compared with their corresponding versions before quantization table replacement. If there are some images with PSNR higher than a preset value P dB, we then choose the one with the smallest PSNR as the output image; otherwise, the one with the largest PSNR is selected. 2 In our experiments, we employ quality factors QF ∈ {1, 2, · · · , 100} to create different test quantization tables. However, our method can be extended to arbitrarily customized quantization tables. Please note that there are three parameters, i.e. N , r and P , in the proposed method. By carefully selecting these parameters, we can obtain a JPEG image with the target 2) 3) 674 JPEG images with candidate tables Input image I J1 Jn PSNR(Ji,Ji,Qt) Replace with target table Qt Candidate tables Target table Qt …… J1,Qt …… Selection based on PSNR Output JPEG image with target table Qt Jn,Qt JPEG images after replacing with target table Qt Fig. 3. Illustration of the proposed method TABLE II. TPR & TNR WITH THE DETECTOR [9] (%) BEFORE AND AFTER ANTI - FORENSIC OPERATIONS TABLE III. TPR & TNR WITH THE DETECTOR [10] AFTER ANTI - FORENSIC OPERATIONS (%) BEFORE AND QF2 No Attack N =4 TPR N =5 N =6 50 81.68 ↓11.44 ↓10.78 ↓ 3.74 60 87.14 ↓ 2.98 ↓ 3.12 ↓ 3.80 70 95.94 ↓10.72 ↓ 4.90 ↓ 2.64 80 98.34 ↓18.24 ↓15.54 ↓14.00 90 100 ↓20.72 ↓16.62 ↓18.96 QF2 No Attack N =4 TPR N =5 N =6 50 88.62 ↓34.04 ↓30.66 ↓20.04 60 94.80 ↓29.50 ↓20.96 ↓19.70 70 97.94 ↓22.26 ↓10.96 ↓ 6.38 80 98.20 ↓36.92 ↓24.28 ↓22.70 90 98.98 ↓33.16 ↓25.78 ↓32.22 TNR 94.14 95.02 96.98 98.68 100 TNR 92.16 95.76 98.33 99.49 99.99 quantization table and have a good tradeoff between the image quality and the performance of anti-forensic. We must note that the proposed method just changes the JPEG file header, all original quantized DCT coefficients Dq for each 8×8 block are well preserved, which means that if the original quantization table Qc is given (or as the side information), we can recover the original JPEG image without any quality loss. TPR degrades, the better performances we achieve. Please note that the algorithm parameters are set as r = 20, P = 35, and N = 4, 5, 6. Besides, we also show the corresponding TNR in all the following experiments. A. Anti-forensics of Double JPEG Compression with the Different Quantization Tables To show the visual quality of the proposed method, we illustrate two examples for fooling double JPEG compression detection with the parameters P = 35, r = 20, N = 5 in Fig. 4. The given singly compressed images and their quantization tables are shown in Fig. 4(a), while the target quantization tables Qt are shown in Fig. 4(c). For fooling double JPEG detection, we adaptively select the most suitable quantization table Qc for each image according to the proposed method, and then obtain the doubly compressed images with Qc as shown in Fig. 4(b). Finally, the quantization table Qc are replaced with Qt , the resulting JPEG images are shown in Fig. 4(c). In 4(c), the highlighted quantization steps in Qt are different from those in Qc . Although most quantization steps have been changed, there are no obvious artifacts, and the PSNR of the two images in Fig. 4(c) compared with Fig. 4(b) are both higher than 35dB. IV. In this experiment, we generated doubly compressed images (i.e. positive instances) by successively compressing the original uncompressed images with primary quality factor QF1 and secondary quality factor QF2 , where QF1 = QF2 . Thus the negative instances were those singly compressed images with QF2 . For making positive instances with anti-forensic operation, we firstly compressed the original uncompressed images with QF1 , and then performed the anti-forensic operations as described in Section III for a target QF2 . In our experiments, QF1 was randomly selected from 50 to 90 with a step 1, and QF2 was in the range from 50 to 90 with step 10. For a given QF2 , we randomly selected 5,000 positive instances and the corresponding negative instances to train two SVM classifiers [20] using the feature sets [9] and [10], respectively. Finally, the remaining 5,000 positive instances and their corresponding anti-forensically modified counterparts were fed to the resulting classifiers for calculating the TPR. E XPERIMENTAL R ESULTS The experimental results are shown in Table II and Table III. It is observed that the proposed method can reduce the TPR in all the cases. For example, when N = 4, the average decrements are 12.82% and 31.17%, respectively. It is also observed that the performances tend to decrease when with increasing the N . Because most of natural image information is concentrated in lower DCT frequency components, when N is larger (i.e. the first N quantization steps remain the same), it is expected that more statistical properties of test JPEG image would be well preserved after anti-forensic operations, and thus the pre-trained classifier would still work. We also note that the performances are dependent on the QF2 . For instance in Table III, the results for QF2 = 70 are relatively poorer than others. In our experiments, 10,000 uncompressed gray-scale images of size 512×512 are downloaded from BOSSBase image database [18], and the MATLAB JPEG Toolbox [19] is used for JPEG compression and decompression. Three different kinds of forensic methods for JPEG images are considered, including double JPEG compression detection with different quantization tables [9], [10], double JPEG compression detection with the same quantization table [11], and nonaligned double JPEG compression detection [13]. Since the antiforensic operations aim to confuse those positive instances, we will compare the TPR of each forensic detector before and after performing quantization table replacement. The more 675 10 7 6 10 14 24 31 37 6 4 4 6 10 16 20 24 5 5 6 8 10 23 24 22 7 7 6 5 6 10 16 23 28 22 8 8 10 14 24 34 41 34 6 7 9 12 20 35 32 25 8 10 13 17 31 52 48 37 7 9 15 22 27 44 41 31 11 13 22 34 41 65 62 46 10 14 22 26 32 42 45 37 14 21 33 38 49 62 68 55 20 26 31 35 41 48 48 40 29 38 47 52 62 73 72 61 29 37 38 39 45 40 41 40 43 55 57 59 67 60 62 59 8 11 16 35 36 33 (a) Singly JPEG compressed images and their quantization tables (QF =80 and 70, respectively) 3 4 7 11 14 17 6 4 3 5 8 13 16 20 7 7 4 5 7 16 17 15 5 5 4 6 8 19 19 18 8 4 4 7 11 16 19 16 6 4 5 8 13 18 22 18 4 5 6 8 14 24 22 17 4 5 7 9 16 28 26 20 5 6 10 16 19 31 29 22 6 7 12 18 22 35 33 25 10 7 7 10 15 18 23 29 32 26 8 11 18 20 26 33 36 29 14 18 22 24 29 34 34 28 16 20 25 28 33 39 38 32 20 26 27 27 31 28 29 28 23 29 30 31 36 32 33 32 (b) JPEG re-compressed above JPEG images with the selected quantization tables Qc (the corresponding target tables are shown in Fig. 4(c)) 6 10 14 24 31 37 6 4 4 6 10 16 20 24 8 11 16 35 36 33 5 5 6 8 10 23 24 22 8 10 14 24 34 41 34 6 5 6 10 16 23 28 22 8 10 13 17 31 52 48 37 6 7 9 12 20 35 32 25 11 13 22 34 41 65 62 46 7 9 15 22 27 44 41 37 10 7 7 7 8 14 21 33 38 49 62 68 55 10 14 22 26 32 42 45 37 29 38 47 52 62 73 72 61 20 26 31 35 41 48 48 40 43 55 57 59 67 60 62 59 29 37 38 39 45 40 41 40 (c) Resulting JPEG images after replacing with the target tables Qt (QF =70 and 80, respectively) Fig. 4. Two image examples generated by the proposed method with parameters N = 5, r = 20, P = 35. TABLE IV. The reasons are complicated due to the distributions of DCT coefficients for QF = 70 and its relation with its adjacent QF , the generalization ability of the pre-trained classifier. We will further analyze such a problem in our future work. TPR B. Anti-forensics of Double JPEG Compression with the Same Quantization Table In this experiment, we try to evaluate the anti-forensic performances on attacking the detector proposed in [11], which aims at exposing double JPEG compression with the same quantization table. Thus, we created the singly compressed images (i.e. negative instances) and the doubly compressed images (i.e. positive instances) by compressing the original uncompressed ones with the same QF once and twice respectively, where QF was in the range from 70 to 90 with a step 5. The anti-forensic modified images (i.e. positive instance with anti-forensic operation) were generated by setting QF as the targeted quality factor and compressing them with the adaptively selected quantization tables twice. In order to obtain the best parameter mpnc of the algorithm [11], 2,500 singly compressed images and 2,500 doubly compressed ones were used to search the highest accuracy of the detector for each QF , and then we tested the remaining images and obtain the TPR (before and after anti-forensic operations) of the detector with the resulting best parameters. THE TNR(%) AND TPR(%) VARIOUS QF . OF DETECTOR IN [11] FOR QF No Attack N =4 N =5 N =6 70 80.00 ↓70.94 ↓70.07 ↓71.27 75 80.65 ↓70.79 ↓68.86 ↓71.12 80 89.20 ↓82.87 ↓82.74 ↓79.67 85 90.80 ↓76.41 ↓77.88 ↓76.68 90 93.93 ↓88.60 ↓89.40 ↓88.40 95 97.61 ↓96.68 ↓96.81 ↓96.88 TNR 54.07 56.08 83.81 86.12 90.67 96.45 decrements of TPR exceed 70% for all the listed cases. Please note that here the around 18% of the resulting positive JPEG images with the proposed method are lower than 35dB when QF = 95 based on our experiments, and the smallest one is around 30dB. However, we can also obtain the average PSNR over 35dB in this case. C. Anti-forensics of Nonaligned Double JPEG Compression In this experiment, we try to attack the nonaligned double JPEG compression detector proposed in [13]. In this case, those singly compressed images (i.e. negative instances) were created by compressing the original uncompressed ones with QF2 only once, where QF2 was in the range from 50 to 90 with a step 10. For generating the positive instances, we firstly compressed the uncompressed ones with QF1 , where QF1 was randomly selected from 50 to 90 with a step 1, and then we decompressed the JPEG images and cropped them with a random shift (i, j), where 1 ≤ i, j ≤ 8, and (i, j) = (8, 8). Finally we re-compressed the resulting images The experimental results are shown in Table IV, it is observed that the proposed method can completely fool the detector [11], most TPR drop to less than 10% and the average 676 TABLE V. THE TNR(%) AND TPR(%) OF DETECTOR IN DIFFERENT N AND QF2 . [13] R EFERENCES FOR [1] QF2 No Attack N =4 TPR N =5 N =6 50 20.34 ↓ 6.16 ↓ 4.42 ↓ 2.76 60 31.80 ↓ 9.50 ↓ 6.30 ↓ 4.18 70 51.26 ↓15.36 ↓11.04 ↓ 8.12 80 70.94 ↓17.96 ↓13.40 ↓11.18 90 90.84 ↓28.62 ↓20.96 ↓18.22 TNR 95.92 96.72 97.76 99.10 98.44 [2] [3] with QF2 . For creating those positive instances with antiforensic operation, we compressed each of the cropped ones with a quantization table selected by our proposed method as described in Section III, and then replaced it by QF2 . To determine the best decision threshold of the detector [13], we selected 5,000 singly compressed images and 5,000 nonaligned doubly compressed ones for each QF2 to conduct training stage. The TPR for different cases were obtained by testing the remaining 5,000 nonaligned double JPEG compressed images and their corresponding counterparts after anti-forensic operation. [4] [5] [6] [7] [8] The experimental results are shown in Table V. It is observed that when the quality factor QF2 are lower such as 50 and 60, the corresponding TPR before anti-forensic operation are relatively low with the detector [13]. After performing the proposed method, we can further decrease the TPR. When the QF2 is larger, the degree of decrements on TPR would increase. On average, The larger N is, the smaller the decrements we obtain. As shown in the Table V, the average decrements for N = 4, 5, 6 are 15.52%, 11.22%, and 8.89% respectively. The reasons are similar as described in subsection IV-A. V. [9] [10] [11] [12] C ONCLUSION [13] In this paper, we first point out the common flaw in the existing JPEG forensic methods: the detection performances of their pre-trained classifiers are highly dependent on the quantization table in the JPEG file header, which is assumed to be authentic, and then we propose a universal anti-forensic scheme against such JPEG detectors via replacing the quantization table according to the visual quality of the resulting JPEG image. The extensive experiments evaluated on 10,000 images have shown the effectiveness of the proposed anti-forensic scheme against four typical JPEG detectors, including double compression detectors for different and same quantization tables, and nonaligned double compression detector. 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