Selective Robust Image Encryption for Social Networks | SpringerLink

Selective Robust Image Encryption for Social Networks Ahmet Emir Dirik1,2 and Nasir Memon2,3 1

Electronic Engineering Dept., Faculty of Engineering and Architecture, Uludag University, Bursa, 16059, Turkey 2 Center for Interdisciplinary Studies in Security and Privacy, NYU, Abu Dhabi, UAE 3 Computer Science & Engineering Dept., Polytechnic Institute of NYU, USA

Abstract. Current image encryption schemes do not achieve robustness to lossy compression and downsizing without sacrificing image quality. In this paper, we introduce a selective robust image encryption scheme for online social networks that provides a trade-off between robustness and security. With the selective encryption property, users have an option to share a blurred version of the original image and only a specific group of people having the right key can access the full content. Our method is based on the fact that image resizing and compression can be modeled as a low pass process which alters and/or removes high frequency components of image pixels. We achieve robustness to compression and downsizing by adjusting and shuffling DCT coefficients. Experimental results show that the proposed method can be applied in real world applications and social networks with satisfactory image quality. Keywords: image encryption, robustness, privacy, social networks.

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Introduction

With the advent of image sharing tools and online social networks, privacy is becoming an increasing concern. When a user shares an image, she may reveal sensitive information related to her private life. Besides, some shared images may contain offensive, embarrassing, or inappropriate content. Consider a scenario where John, a member of a social network would like to share an image. However, due to some privacy concerns, he wants to make the full content available only to a specific group of people. One solution is to design a client-to-client security mechanism for image distribution for Facebook and similar Web 2.0 services [7], [12]. Similar to file encryption, when a sender uploads an image to an online social network, the shared image can be encrypted using a secret key. This key can be associated with the users’ social network account and can be shared only with the accounts which are authorized by the sender. A serious challenge for such a scheme is that social network providers typically re-compress the image or modify the dimension and quality of uploaded images. A. Dziech and A. Czy˙zewski (Eds.): MCSS 2013, CCIS 368, pp. 71–81, 2013. c Springer-Verlag Berlin Heidelberg 2013 

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In this case, classic encryption schemes [1], [4], [5] fail due to unpredictable image modifications performed by the social network provider. Therefore, the decryption of the original image becomes impossible at the receiving end. For the above reasons, current social networks such as Facebook, Twitter, etc. may need image encryption systems which are robust to image transformations such as downsizing, cropping, and, compression. It is also desired that image decryption should yield acceptable image quality at the receiver/viewer end, even after the lossy manipulations performed by the social networking provider. Current robust image encryption schemes do not preserve acceptable image quality without sacrificing the level of protection of the encrypted content [11], [13], [15]. Design of a robust image encryption system [2], [6], [12] has to consider the trade off between security and robustness to image transformations. There have been a couple of approaches that have ben proposed for robust image encryption. One approach is to shuffle large image blocks like a jig saw puzzle [12]. This may not provide reasonable security, since attacks against the encryption scheme would be straightforward and even without such attacks, the image content may still be conceivable after encryption. Another approach is to shuffle DCT coefficients of the original image which have same spatial frequencies based on a secret key [9]. This approach requires the receiver to reshuffle DCT frequencies using the shared key and constructs the original image. Although this method is robust to JPEG compression, it has problems with image resizing and may yield low image quality after decryption [9]. In this paper, we introduce a robust image encryption scheme for online social networks taking into account the design challenges introduced above and discuss different trade-offs between robustness and security. Our method is based on the fact that image resizing and compression can be modeled as low pass processing which alters and/or removes high frequency components of image pixels. A robust image encryption scheme has to handle such deformations at high spatial frequencies after decryption. This fact limits pixel-wise shuffling along the image because it spreads out the image spectrum without considering any lossy image processing after encryption. Hence we propose an approach similar to [9] that employs shuffling of low and mid spatial frequency components in the transform domain, by utilizing DCT coefficients. The drawback of DCT shuffling is that it causes distortions in the image and yields dark or white noisy blocks since some pixel values can exceed the maximum value (255) after shuffling. Thus, the actual pixel values cannot be restored even without lossy operations after encryption. We propose below an approach that provides alleviates this problem and provides better distortion-security tradeoff.

2

Encryption Method

In order to get better distortion after decompression, the main idea of the proposed method is to shuffle DCT coefficients having equal frequencies. DCT coefficients are shuffled independently based on a secret key. The resulting content can then be decrypted if the correct decryption key is provided.

Selective Robust Image Encryption for Social Networks

(a) PSNR 39.06 dB

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(b) PSNR 50.85 dB

Fig. 1. Effect of DCT scaling on decrypted image quality. (a) and (b) are decrypted images of Fig. 2.b and Fig. 2.c. DCT scaling and decreasing the contrast as in Fig. 2.c improves the PSNR significantly. Notice the distorted small blocks on the wall in (a).

Instead of shuffling all 64 DCT coefficients, only middle and high DCT frequencies can be shuffled during encryption. As a result, selective encryption [10] can be achieved. In this case, robustness of the encryption method to lossy processes and distortions also increases since DCT 0 frequency contains most of the content of the image and it is not affected from lossy operations significantly. However, with this encryption scheme, people may opt using selective or full encryption methods based on their security and privacy concerns. One of the drawbacks of equal frequency shuffling is that encrypted 8 by 8 spatial blocks may have unexpected dark or light tones. This problem can be handled by limiting the power of DCT coefficients during encryption. In this paper we choose an empirically determined scaling factor to limit the power of DCT coefficients. However the power of DCT frequencies can be adjusted adaptively for less encryption-decryption distortion. An example of such encryption distortion is visible in Fig. 1.a. It is seen in the figure that there are some noisy blocks on the wall in the image on the left. The original image of Fig. 1 and its encrypted and compressed versions are depicted in Fig. 2. In Fig. 2 two different encrypted forms are presented. The encrypted image on the left side has been created by DCT shuffling. On the other hand the encrypted image on the right side has been created by DCT scaling during shuffling. This means that all DCT coefficients are scaled down with a specific factor α before the encryption. Here, α has been selected as 2. This scaling has an effect of limiting DCT coefficient powers and narrowing the range of the image histogram as it is seen in Fig. 2.e. Dark and high tones in the histogram in Fig. 2.d does not appear in Fig. 2.e. In the decryption step, during reshuffling, all DCT coefficients are upscaled with the same factor α used in the encryption step. This step restores the actual DCT powers and corrects the range of the decrypted image histogram. As a result, better image quality in terms of PSNR is achieved. For instance, the decrypted image in Fig. 1.b has been encrypted

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(a) Original image

(b) Encryption result with no (c) Encryption with DCT coDCT coefficient scaling. efficient scaling. Contrast narrowed. 14000

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Fig. 2. Encrypted and JPEG compressed versions of the original image in (a) with their corresponding histograms. In (b) the original image has been encrypted with DCT shuffling. In (c) DCT coefficients has been scaled for better image quality. See Fig. 1.

with DCT scaling factor 2 and has 50.85 dB PSNR. On the other hand, the same image encrypted without DCT scaling has 39.06 dB after the decryption (Fig. 1.a). The security of the proposed scheme is based on the strength of the key using shuffling DCT frequencies. In image encryption literature, chaos based methods [3],[8],[14] are widely used due to the desired properties of chaotic sequences such as they are unpredictable, non-periodic, and have good avalanche effect. Thus utilizing chaotic sequences would be one of the alternative choices for DCT shuffling. In this paper instead of discussing chaos based properties we will particularly focus on the robustness of the proposed encryption method.

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Fig. 3. Restaurant image : [436x684] pixels

(a) Full encryption result

(b) Decryption after JPEG compression (Q80) Fig. 4. Full encryption with JPEG compression (Q80)

An example of full encryption with 64 DCT frequencies and a decryption result after JPEG compression with quality 80 are given in Fig. 3 and 4. In Fig. 3 an image to be encrypted is shown. In Fig. 4, both encrypted and decrypted versions of the image in Fig 3 are provided. An example of the selective encryption method is given in Fig. 5. In this figure both selectively encrypted image and its decrypted form after JPEG compression with quality factor Q80 are provided. It is seen from the figure that selective encryption preserves main content but not reveal

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(a) Selective encryption result

(b) Decryption after JPEG compression (Q80) Fig. 5. Selective encryption (63 DCT coefficients) with JPEG compression Q80

the details of the original image. All details can only be accessed if a right key is provided. Besides selective encryption are not affected from external distortions significantly as shown in Fig. 5. Since equal DCT frequencies are shuffled during encryption instead of shuffling all DCT coefficients, the proposed scheme gains robustness not only for JPEG compression but also downsizing. To test this property, we took the image shown in Fig. 6 and encrypted using 63 DCT coefficients except DCT 0 frequency. Then, the encrypted image was compressed with JPEG quality Q75 and downsized with a factor 0.90. The encrypted image and its distorted and decrypted version are shown in Fig. 7. It is seen from the figure that even after compression and downsizing, introduced method yields a satisfactory image quality after the decryption.

3

Experimental Results

As stated in the introduction, encrypted images can be distorted with uncontrolled operations such as lossy compression, downsizing just after being shared. Even if the encrypted image would lose information, we would like to attain reasonable image quality after decryption. To evaluate the applicability of the proposed method we conducted an experimental setup involving 100 images

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Fig. 6. Boy image : [436x684] pixels

(a) Selective encryption result

(b) Decryption after downsizing (0.90) and JPEG compression (Q75) Fig. 7. Selective encryption with JPEG compression (Q75) and downsizing (0.90)

taken from the Greenspun’s image database1 . First, we tested the robustness property so all images were compressed with JPEG with quality factors ranging from Q100 to Q50. Second, we applied JPEG compression (Q75) and downsizing together to the encrypted images. Here, downsizing ratios are selected from 0.9 to 0.5. In all of the experiments, DCT scaling ratio α is fixed to 2. The result of the first experiment is provided as box plot in Fig. 8. Here, 100 images were encrypted and compressed with different JPEG quality factors. The 1

http://philip.greenspun.com

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Fig. 8. Robustness to different JPEG compression levels in terms of PSNR

Table 1. Average PSNR [dB] values after JPEG compression and decryption for full and selective encryption

Full (64 coef) Selective (63 coef) Selective (62 coef)

Q100 43.3786 43.6511 44.1216

Q80 36.7855 36.8365 36.9211

Q60 35.3165 35.3490 35.4014

PSNR was measured for each encrypted image after compression and decryption steps. The PSNR statistics of each individual experiment are depicted with different boxes. In each box, the central red line shows the median of the data, and the edges of the box shape are 25th and 75th percentiles. Dotted lines show the extreme data points. From the figure it is seen that reducing the JPEG quality factor for encrypted images result in degradation of the quality, as well. However, the median of PSNR for Q50 is close to 35 dB which still provides a reasonable image quality. The average PSNR values for Q100, Q80, and Q60 are also provided in Table 1. In this experiment we also evaluated the robustness of selective encryption in which DCT 0 and/or near DCT 0 frequencies were not shuffled. This makes the appearance of the encrypted image not fully random revealing the rough content of the original image. Thus, 100 images were also encrypted shuffling 63 and 62 DCT frequencies and compressed with Q100, Q80, and Q60 quality factors. For 63 coefficient case, DCT 0 frequency was not touched during encryption. For 62 coefficient case, DCT 0 and DCT 1 frequencies were not shuffled. The mean of the PSNR values of compressed and decrypted images

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Fig. 9. Robustness to downsizing and compressing (Q75) in terms of PSNR Table 2. Average PSNR [dB] values after JPEG compression (Q75), downsizing and decryption for full and selective encryption

Full (64 coef) Selective (63 coef) Selective (62 coef)

0.9 31.6953 33.8296 34.0918

0.8 31.3111 33.6610 33.9536

0.6 30.6843 33.4120 33.7904

are provided in Table 1 for two selective encryption cases. It is intuitive that average PSNR values are better for selective cases than the full encryption case where all DCT frequencies are shuffled. This is because, for the selective case, the main content is visible and not shuffled during the encryption. Thus, in selective case, the content of the original image is not affected much from the compression and downsizing, resulting a better looking decrypted image in terms of PSNR. In the second experiment, the robustness of the encryption method to both JPEG compression and downsizing is evaluated. First, images were encrypted. Then they were compressed with JPEG quality factor 75 and reduced their dimensions with downsizing factor ranging from 0.9 to 0.5. PSNR statistics of decrypted images for different downsizing factors are given in Fig. 9. It is seen from the figure that the median PSNR values are between 30dB and 32dB for different downsizing factors. This shows the efficacy of the proposed method under lossy compression and different downsizing operations. A more detailed analysis of full and selective encryption cases are provided in Table 2. Similar to the results in the first experiment, it is seen from the table that the average PSNR becomes better for selective encryption cases. However shuffling

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less coefficients, reveal more information about the original content and reduces the security. The trade-off between quality and security can easily be seen from both Tables 1 and 2.

4

Conclusion

In this paper we introduced a robust selective image encryption which survives from lossy compression and downsizing. Selective encryption makes it possible to reveal some portions of the content after encryption without sacrificing the robustness property. This property could be applied in social networking applications, providing a way for the users who may want to share an original image only with an authenticated group of people. In this case the blurry version of the original image is available for unauthenticated members and the original version will only be accessed when a right decryption key is provided. Nevertheless, selective encryption should be used carefully considering the information revealed by the encrypted images. Experimental and statistical analysis suggest that the proposed method provides satisfactory results even if the encrypted image is compressed with low JPEG quality factors and downsized afterwards. Considering both quality and security issues, introduced method could be a good option to employ in social networking applications compared to jig-saw like encryption schemes. Moreover the proposed method can be used in any encryption system or application where robustness to any external distortions and/or lossy processes are required. Compared to the similar robust encryption schemes, proposed method provides better image quality in terms of PSNR. Acknowledgement. This work was partially supported by the Center for Interdisciplinary Studies in Security and Privacy, NYU, Abu Dhabi.

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