CN108810555A - Thumbnail image method for secret protection based on compression of images with elastic resolution ratio - Google Patents

Abstract

A thumbnail image privacy protection method based on image compression and elastic resolution; by dividing the original image into blocks, and subdividing each block into sub-blocks, according to the thumbnail resolution requirements, within each block The sub-blocks are randomly scrambled to a certain degree; combined with JPEG compression and cryptography technology, the DC and AC coefficients in the 8×8 blocks that undergo DCT transformation are encrypted to varying degrees, and the thumbnails of the public part and the private part are obtained. Data is fully encrypted; the thumbnail image of the public part in the present invention can be used for picture preview in network applications, and at the same time provide a certain degree of privacy protection. Using image compression technology to reduce the overall storage overhead of encrypted images, especially the public part, can greatly save bandwidth. The invention also provides thumbnail privacy protection with elastic resolution, the security of the image is affected by the resolution, the lower the resolution is controlled, the higher the security of the image.

Description

Thumbnail image privacy protection method based on image compression and elastic resolution

【Technical field】

The invention belongs to the technical fields of digital image encryption, digital image privacy protection and information security, and in particular relates to a thumbnail image privacy protection method based on image compression and elastic resolution.

【Background technique】

Compared with sound and text information, digital image overcomes the difficulty of large storage space and can convey more information because of its vivid and vivid content, thus becoming one of the most important ways of disseminating information [1]. With the rapid development of Internet applications, various cloud storage devices have begun to be used as digital image management tools. It can easily share large files asynchronously without point-to-point transmission. Even if the mobile device is lost or damaged, it can also be stored from the cloud. It can be accessed and restored on the Internet, and the cost is low, which saves a lot of costs for users to transfer and manage data. Therefore, nearly one billion new photos are uploaded to cloud applications every week.

However, while cloud storage brings convenience, it also poses a threat to user privacy, because uploading any meaningful image to a cloud application is equivalent to making a backup on an untrusted third-party server, so user privacy is very difficult. It may be leaked. The reasons for this hidden danger are: first, the service provider will leak the user's photo to the partner due to some commercial interests; second, the user's account may be attacked by the attacker, which will make the user's photo The photos fell into the hands of the attacker, resulting in the leakage of the user's face information and life information.

The solution to this problem is to implement "end-to-end" encryption, that is, the sending end needs to encrypt the image before it is uploaded to the server, so that the image obtained by the server end is an encrypted image, and no authorized third party Without the key, the ciphertext image cannot be restored to the original image, which greatly improves the security of the image content from being stolen, and plays a very good role in protecting digital images involving personal privacy. Technologies in this area include image encryption technology based on matrix transformation pixel replacement, image encryption technology based on secret division and sharing, and image encryption technology based on modern cryptosystem [2].

However, although the traditional classic image encryption algorithm has solved the problem of image privacy protection well, it is very difficult to retrieve the image due to the incompatible format or the extreme confusion of the encrypted image, and the user cannot preview the image online. References [3] and [4] solved the problem of format incompatibility, but still did not solve the problem of image preview. In [5], Wright et al. proposed a thumbnail image privacy protection algorithm at the conference on information hiding and multimedia security. (TPE), choose to provide a low-resolution JPEG thumbnail for each image, which provides a compatible image format and hides the details of the image, which can protect the user's privacy to a certain extent and can be previewed online .

However, whether it is the traditional image encryption algorithm or the latest thumbnail image privacy protection algorithm, the spatial correlation of the image is changed, which greatly increases the storage cost of the encrypted image file, and the image reconstruction of the TPE algorithm has obvious block effects. , the visual quality cannot reach the effect accepted by the human eye. The thumbnail privacy-preserving algorithm proposed in reference [6] has improved image security, file size and image reconstruction quality, but the overall effect is still unsatisfactory.

references

[1]Gonzalez, Rafael C., and Richard E. Woods."Digital image processing."(2012).

[2] Li Changgang, Han Zhengzhi, and Zhang Haoran. "A Review of Image Encryption Technology." Computer Research and Development 39.10 (2002): 1317-1324.

[3]Tierney, Matt, et al."Cryptagram: Photo privacy for online socialmedia."Proceedings of the first ACM conference on Online social networks.ACM,2013.

[4] Ra, Moo-Ryong, Ramesh Govindan, and Antonio Ortega."P3: TowardPrivacy-Preserving Photo Sharing."Nsdi.2013.

[5] Wright, Charles V., Wu-chi Feng, and Feng Liu. "Thumbnail-preserving encryption for JPEG." Proceedings of the 3rd ACM Workshop on Information Hiding and Multimedia Security. ACM, 2015.

[6] Marohn, Byron, et al."Approximate Thumbnail Preserving Encryption."Proceedings of the 2017 on Multimedia Privacy and Security. ACM, 2017.

【Content of invention】

The purpose of the invention is to solve the following key technical problems:

First, an image encryption method that preserves the original image format is proposed to solve the problem of incompatibility of encrypted image formats.

Second, a thumbnail image privacy protection method is proposed. The encrypted image can not only protect the user's privacy, but also preview online through the thumbnail.

Thirdly, an image encryption method based on image compression is proposed, so that the encrypted image can save file storage space and transmission bandwidth as much as possible.

Fourth, a thumbnail image encryption method with flexible resolution is proposed, and the thumbnail resolution of the image can be controlled by the user.

In order to solve the above problems, the present invention reads the format of the original image and stores it in the format of the original image after image encryption processing, so that the usability of the encrypted image is stronger.

In order to solve the above problems, the present invention proposes to create a low-resolution thumbnail of the original image. The detail part of the image is discarded in the thumbnail, only the rough outline of the image is kept, and it is difficult to restore the approximate original image from the thumbnail. This method not only protects the private information in the image, but also enables online preview through encrypted images.

In order to solve the above problems, the present invention uses image compression technology to control the additional storage overhead caused by image encryption, and through reasonable image quantization, image redundancy is removed to reduce the size of the original file as much as possible. And combined with the characteristics of image compression technology, the encrypted image is divided into public part and private part. The public part is a small thumbnail, and only the public part is loaded when previewing online through the thumbnail, which can greatly save bandwidth.

In order to solve the above problems, the present invention provides a thumbnail image encryption technology with elastic resolution, and the exposure degree of image details is controlled by the user himself. At the same time, the security of the image not only depends on the security of the algorithm, but also depends on the size of the thumbnail resolution. The higher the resolution of the image, the less the details of the image are exposed, and the higher the security of the image.

In order to solve the above problems, the present invention also provides a prototype system for thumbnail encrypted picture storage. It supports flexible selection of picture encryption level and corresponding resolution level for picture uploading, and it will be decrypted into high-definition images at the terminal when downloading.

Technical scheme of the present invention:

A thumbnail image privacy protection method based on image compression and elastic resolution, the method includes two parts: a thumbnail encryption method and a thumbnail decryption method;

1. Thumbnail encryption method

1. Divide the image into blocks in the spatial domain: first read the image information in the bitmap image file, assuming that the image size of the image is N×N, divide the original image into blocks according to n×n, n<N, The same below;

1.1. Divide each n×n block twice, and the size of each sub-block is b×b, b<n, the same below;

1.2. Randomly scramble several sub-blocks in each n×n block. Considering the nature of JPEG compression, in order to obtain better image restoration quality, n mod8≡0, b mod8≡0, and b involved in scrambling The number of ×b sub-blocks is determined by the sub-key k ₀ to flexibly control the resolution of the thumbnail image, and the random positions of the sub-blocks participating in the scrambling are generated by a pseudo-random number generator;

Assuming that each N×M block contains m b×b sub-blocks, then the degree of scrambling of the image is divided into 5 levels: when level=0, the image is not scrambled or compressed, and the original image is completely preserved; level= 1, m/4 sub-blocks participate in scrambling; level=2, m/2 sub-blocks participate in scrambling; level=3, 3m/4 sub-blocks participate in scrambling; level=4, m sub-blocks participate in scrambling ;level will be used as the "seed" of the pseudo-random number generator to determine the random scrambling position of the sub-block;

2. Using the JPEG compression method to transform the image into the frequency domain through the forward discrete cosine transform (DCT), the DCT transformation of the image should follow the following rules:

2.1. Subtract 128 from each pixel data of the image, so that the range of pixel values falls into the range between -128 and 127, which can greatly reduce the probability that the decimal number is 3 digits after DCT transformation, which is convenient when using VIL encoding. Reduce the number of bits encoded;

2.2. Combined with the nature of JPEG compression, DCT transformation is applied to each 8×8 image block to obtain 1 DC coefficient and 63 AC coefficients. At the same time, the DC coefficient is called a low-frequency coefficient, and the AC coefficient is called a high-frequency coefficient;

`3. Quantize the DCT transformation result in step 2, that is, divide each 8×8 block of the image by the default quantization table in JPEG compression, and round the result to an integer; thus most of the high-frequency coefficients in the DCT coefficients become is "0", some coefficients that are not set to "0" will also lose some precision; the purpose of quantization is to filter the high-frequency coefficients in the image, and the main information of the image is concentrated in the low-frequency part of the image, which can play a role The effect of compression will not have a big impact on the main information of the image.

4. Perform a zigzag scan on each 8×8 DCT coefficient quantized in the third step, so that a 1×64 one-dimensional sequence will be obtained for the DCT coefficient of the 8×8 block;

4.1. Separate the high-frequency coefficient AC from the low-frequency coefficient DC, store the high-frequency coefficient as a private part, and replace the position of the high-frequency coefficient with "0";

Step 4.2. Perform zero-run encoding and Huffman encoding on the separated high-frequency coefficients, further compress the sequence encoding length of the data, and obtain binary sequences w ₀ , w ₁ ,..., w _i ;

The 4.3rd, carry out stream cipher encryption to the high-frequency coefficient through the 4.2 step;

The specific method of stream cipher encryption is: use subkey k ₁ to generate binary key stream r ₀ , r ₁ ,..., r _i through the stream cipher generator, and press the binary w _i sequence in step 4.2 The following formula obtains the encrypted w _i 'sequence:

w _i ′ will be saved as a private part;

Fifth, the coefficients after the high-frequency coefficients are set to "0" are izigzag scanned and dequantized. The result of dequantization discards the details of the image and only retains the rough features of the image;

The 6th, carry out inverse discrete cosine transform (IDCT) transformation to the space domain through the DCT coefficient processed in the 5th step;

Step 7. Subtract 128 from all pixel values in step 2.1, and add 128 here to restore the pixel value to the range of 0-255, thus obtaining a thumbnail image of the corresponding scrambling level;

Step 8. Encrypt the thumbnail in step 7 again, each block size is 8×8;

8.1. At this time, the pixel value in each block of the thumbnail is the average value of the block, so it is necessary to randomly scramble the lower six bits of the pixel in each block, so that the pixel value of the block is not an accurate average value, but close to the mean;

Section 8.2, in order to ensure the compression effect of the image, use the same subkey k ₂ for 64 pixels in the same block to scramble the lower six positions of the pixel value;

Although the sub-blocks in each block are scrambled to a certain extent in the first step, it brings a certain degree of difficulty for the attacker to brute-force the image. However, in steps 3-4.1, the DCT transformation is performed on each 8×8 block, and all ACs are set to “0”. Due to the characteristics of DCT transformation, after the image undergoes IDCT transformation, each 8×8 block The value of the pixel in is the average value of the block, thus accurately exposing the average value of each 8×8 block after inter-block scrambling. Therefore, in step 8.2, the subkey k ₂ is used as the "seed" of the random number generator, and the lower six bits of each pixel value in each 8×8 block are randomly scrambled; Correlation, easy to compress, the subkeys in each block are the same, which not only hides the average value of the block to a certain extent, but also makes each block have a certain degree of similarity with the original image.

2. Thumbnail decryption method

Step 9. Divide the encrypted thumbnail image into 8×8 blocks, and use the subkey k ₂ to restore and decrypt the lower six bits of the pixel value in each block;

10th, decrypt the high-frequency coefficient AC separated in the 4th step according to the following formula according to the subkey _k1 ;

Step 11. Perform DCT transformation and quantization on the thumbnail image as in steps 2-3;

In step 11.1, the high-frequency coefficients in the DCT coefficients are replaced by the high-frequency coefficients decrypted in step 10;

Section 11.2, perform inverse quantization and IDCT transformation on the thumbnail image to obtain a slightly higher resolution thumbnail image;

Twelfth, use the subkey k ₀ to restore the scrambled block to the position of the original image, and restore the high-definition original image.

It can be found that the final restored image is not the original image, because the lossy mode in the compression algorithm in the encryption algorithm of the present invention quantizes the transformed coefficients, in other words the algorithm of the present invention is lossy, But the final image recovery quality is acceptable.

Advantages and beneficial effects of the present invention:

1. The present invention provides a privacy protection scheme with flexible resolution, and in practical applications, the resolution of the image can be controlled by the user himself. At the same time, the elastic resolution also flexibly controls the security of the thumbnails. For images with low security requirements, a higher resolution can be used for online preview; for images with strong security requirements, low resolution can be used. Facilitate image privacy protection;

2. In addition, the public part and the private part of the image are stored separately, and neither party who saves these two parts can decrypt the image alone, which makes it more difficult for attackers to crack the encrypted image;

3. Compared with the previous thumbnail image privacy protection scheme, the present invention combines the JPEG image compression algorithm, which not only removes the detailed information in the image, but also reduces the storage cost of the encrypted image more efficiently, and because the present invention’s scheme The resolution in the image is flexible, so when the selected resolution is higher, the advantage of saving the storage cost of the encrypted image will be more obvious. Whether it is high resolution or low resolution, the size of the encrypted image is significantly smaller than the original image , which was not achieved by previous schemes;

4. In the algorithm of the present invention, in order to reduce the storage overhead, DCT transformation is used, and the coefficients are quantized, which makes the original image lose some information irretrievably. In other words, the solution of the present invention is lossy. However, under the premise of ensuring image security and compression efficiency, it provides acceptable image restoration quality.

The best embodiments of the present invention will be described in detail below with reference to the description of the drawings and specific implementation methods, and these and other advantages of the present invention will be more obvious.

【Description of drawings】

Fig. 1 has provided the example diagram for the image block of N * N size among the present invention;

Fig. 2 is according to the embodiment of the present invention, carries out the M200 flow chart of elastic resolution thumbnail encryption to image;

Fig. 3 shows the M300 flow chart of dividing blocks and scrambling sub-blocks in the present invention.

FIG. 4 is an example of random scrambling between sub-blocks when the scrambling level is 1-4 in 32×32 blocks according to an embodiment of the present invention;

Figure 5 shows the effect diagram of scrambling between image sub-blocks under different scrambling levels;

FIG. 6 shows a schematic diagram of setting the AC coefficients to 0 after DCT transformation, where the minimum unit block represents a pixel.

Fig. 7 is a specific example of setting the AC coefficient to 0 after DCT transformation after inter-block scrambling;

Fig. 8 has provided the schematic diagram of random scrambling of the lower six bits of the pixel value in the present invention;

Figure 9 shows the comparison between the encrypted image public part and the original image file size, and the data unit is KB;

Figure 10 shows the comparison between the encrypted image overall and the original image file size, and the data unit is KB;

Figure 11 lists the results of edge detection on thumbnails at different scrambling levels;

Fig. 12 is a flowchart of the thumbnail image decryption algorithm M1200;

Figure 13 uses the PSNR value to measure the reconstruction quality of the decrypted image, and the data unit is db;

Figure 14 shows the image reconstruction renderings of three different resolution images.

【Detailed ways】

The specific implementation manners of the present invention will be described below in conjunction with the accompanying drawings.

For the sake of convenience, Fig. 1 provides an example diagram of the block in the present invention, divides the image of N * N size into several n * n blocks, and carries out secondary division to each n * n block, is divided into b Example graph of ×b sub-blocks.

Example 1:

FIG. 2 shows a flow chart of an M200 embodiment of the algorithm encryption process of the present invention.

As shown in FIG. 2, the thumbnail encryption process starts at step S210.

S220 divides the image into blocks, divides an image with an image size of 256×256 into 64 32×32 blocks, divides each 32×32 block into 8×8 sub-blocks, and S230 divides the sub-blocks in each block Perform random scrambling. In this process, the degree of scrambling between sub-blocks is determined by the level of scrambling. The level is different, and the degree of scrambling between sub-blocks is different, which directly determines the resolution of the final image. The randomness of sub-blocks The scrambling position is determined by the subkey k ₀ (see M300 for the detailed inter-subblock scrambling process).

Figure 4 shows a specific example of random scrambling between blocks. For a 32×32 image block, it is divided twice, and the size of each sub-block is 8×8, so the 32×32 block contains 16 sub-blocks. In each unit in 4, the left picture is the original block, and the right picture is the result of random scrambling of the sub-blocks in the left picture. The shaded blocks are randomly selected blocks that need to be scrambled. According to the definition in step 1.2 of the thumbnail encryption algorithm, when the scrambling level is 0, no scrambling is performed, when the scrambling level is 1, 4 blocks that need to be scrambled are randomly selected, ..., and so on, the scrambling level is 4, random scrambling is performed in all 16 sub-blocks, as shown in FIG. 4 .

Figure 5 shows the effect diagrams of scrambling between image sub-blocks under different scrambling levels in turn. It can be seen that the rough outline of the image can still be seen in the effect of local block scrambling. When level=0, the image remains the original image, and as the scrambling level increases, the image becomes more and more blurred, which lays the foundation for the elastic resolution thumbnail in the algorithm of the present invention.

S240 in Figure 2 performs DCT transformation on every 8×8 block of the image according to the method in JPEG compression, and quantizes it to obtain high-frequency coefficient DC and low-frequency coefficient AC, and retains DC and separates AC.

S250 in FIG. 2 sets the part of the original AC coefficients in S240 to 0, and performs inverse quantization and IDCT transformation to obtain a thumbnail image with lower resolution.

Figure 6 shows a schematic diagram of setting the AC coefficients to 0 after DCT transformation for each 8×8 block. The upper left corner of the left figure is the DC coefficient, and the rest are AC coefficients. The right figure shows the original AC coefficient after it is separated. position is set to 0.

Figure 7 is a schematic diagram of an image after performing DCT transformation on the scrambled image and setting AC to 0 according to the scrambling level of 3. DC concentrates the most important energy in the image, so when AC is separated, most of the information in the image are preserved, the details are less, so the resolution of the image will be lower.

S260 in Figure 2 uses zigzag to scan the DCT coefficients of S240 to obtain a one-dimensional block sequence, and separates the AC coefficients in it, and uses the subkey k ₁ to perform stream cipher encryption and save it as a private part.

S270 in Figure 2 re-divides the thumbnail image generated by S250 into blocks. The size of each block is 8×8, and the same seed key is used to scramble the lower six bits of the pixel value in each block. This part is used as the public Partially saved.

Figure 8 shows a schematic diagram of 256 gray-level pixel values scrambling the lower six positions. After scrambling, the pixel value is no longer the exact average value of the original image, but because the upper 2 bits have not changed, the pixel value is still close to on average.

The thumbnail encryption process ends at S280;

Thus, the encrypted image of the elastic resolution thumbnail of the public part and the encrypted sequence of the private part are obtained. In order to measure the cost of the storage space consumed after the image is encrypted, the present invention compares the storage space consumed by the ciphertext and the plaintext.

Figure 9 compares the file size (unit: KB) between the public part of the encrypted image and the plaintext under different scrambling levels. When level=0, it represents the original picture, and its file size is 39.7KB. When the scrambling level is above 1, the public part is about 0.5KB. Compared with the original image, the thumbnail part is greatly reduced, which will greatly save the storage overhead of the server in the public part, and at the same time save bandwidth to a greater extent and improve the performance of the server.

Figure 10 shows the file size comparison between the encrypted file overall (public part + private part) and the original image. It can be seen from the figure that the solution of the present invention not only has no additional overhead, but also reduces the storage space of the original image. A lot, which is a great improvement compared to the previous thumbnail image privacy protection algorithm.

Edge detection is a common method to attack privacy nowadays, because the boundary part of the image contains the most important information in the image. The present invention uses the most commonly used Canny operator edge detection method in the MATLAB toolbox to evaluate the security of the thumbnail image.

Figure 11 shows some feature points that can be detected using edge detection at different levels of the original image and level0-level4, where level0 and level1-level4 represent the results of edge detection on the original image and thumbnails with different resolutions, respectively. It can be seen that as the level of scrambling increases, the effect of image edge detection becomes worse and worse. For experts in the field of computer vision or signal processing, because they also collect a variety of data sets, they may be able to learn from these exposures. The feature points of the image can analyze some information, but if the image is not in any data set, it is impossible to discern what is in the image, or for a layman, it is impossible to obtain any information from the results in the image.

Fig. 12 is a flow chart of the decryption method M1200 of a thumbnail image. _The method starts from S1210, and in step S1220, the lower six bits of all pixel values will be restored through the subkey k2. S1230 will use the subkey k ₁ to decrypt the private part of the data to obtain the decrypted AC sequence.

In step S1240, apply DCT transformation and quantization to the thumbnail image according to JPEG compression. In step S1250, replace the AC coefficient obtained in step S1240 with the AC coefficient decrypted in S1230, and perform inverse quantization and IDCT transformation to obtain a restored image.

S1260 performs an inverse scrambling operation on the image according to the subkey k ₀ to obtain a restored high-definition original image, and the thumbnail image decryption algorithm ends at S1270.

It should be noted that the final restored image is not the original image, because the lossy mode in JPEG compression is used in the encryption algorithm, and the coefficients after DCT transformation are quantized, and part of the information of the original image is lost. In other words It is said that the algorithm of the present invention is lossy, but the final image restoration quality is acceptable.

Figure 13 lists the restoration quality data of the image of the present invention. Studies have shown that when the PSNR value of the reconstructed image and the original image is above 28db, the image quality acceptable to human eyes can be achieved.

It can be seen from Figure 13 that the restoration quality of the image is within the acceptable range of the human eye, and the reconstruction quality of the image will hardly change due to the change of the size of the block and the level of scrambling, because the loss of the image It is only reflected in the process of quantization after DCT transformation. Although the present invention divides different blocks, the sub-blocks in each block are multiples of 8×8, and the pixel positions in each sub-block have not changed. Therefore, the image Neither the block size nor the degree of scrambling between sub-blocks is an influencing factor of image quality.

FIG. 14 shows a schematic diagram of reconstructed images of images with different resolutions when the scrambling level is level=4. The image resolutions of the first column to the third column are 256×256, 512×512 and 1024×1024 respectively. It is not difficult to find that when the resolution of the image is higher, no trace of image loss can be found from the perspective of the human eye. Therefore, the reconstruction quality effect of the algorithm of the present invention is very considerable.

Claims (1)

1. a kind of thumbnail image method for secret protection based on compression of images with elastic resolution ratio, this method includes that thumbnail adds Decryption method and thumbnail decryption method two parts；

First part, thumbnail encryption method

Image is eventually divided into common portion and private part by thumbnail image ciphering process, and common portion contracts for low resolution Sketch map, private part are complete encrypted binary sequence, and particular content is as follows：

1st, by image block in spatial domain：First read the image information in bitmap image file, it is assumed that the image of the image Size be N × N, to original image according to n × n carry out piecemeal, n < N, similarly hereinafter；

1.1st, secondary divided block carried out to the block of each n × n, the size of each sub-block is b × b, b < n, similarly hereinafter；

1.2nd, random scrambling is carried out to several sub-blocks in every n × n blocks, it is contemplated that the property of JPEG compression, in order to preferably Image Quality of recovery is obtained, 8 ≡ 0 of n mod, 8 ≡ 0 of b mod is taken to participate in the number of b × b sub-blocks of scramble by sub-key k₀ It determines, the resolution ratio of thumbnail image is flexibly controlled with this, and participate in the random site of the sub-block of scramble by pseudo random number Generator generates；

Assuming that including the sub-block of m b × b in the block of each N × N, then the scramble degree of image is divided into 5 grades：Level= When 0, scramble does not compress image yet, and artwork is fully retained；Level=1, m/4 sub-block participate in scramble；The son of level=2, m/2 Block participates in scramble；The sub-block of level=3,3m/4 participate in scramble；Level=4, m sub-blocks are involved in scramble；Level is by conduct " seed " of pseudorandom number generator determines the random scrambling position of sub-block；

2nd, it uses for reference JPEG compression method and image is converted into frequency domain by Forward Discrete Cosine Transform (DCT), here to image Following rule should be followed by carrying out dct transform：

2.1st, each pixel data of image being subtracted 128 so that the range of pixel value falls into the range between -128 to 127, Decimal number reduces the position of coding for 3 probability when being easy to use VIL codings after dct transform can be substantially reduced in this way Number；

2.2nd, 1 DC coefficient and 63 are obtained to each 8 × 8 image block application dct transform in conjunction with the property of JPEG compression A AC coefficients, while DC coefficients are known as low frequency coefficient, AC coefficients are referred to as high frequency coefficient；

3rd, the dct transform result in the 2nd step is quantified, i.e., it will be default in the block divided by JPEG compression of image every 8 × 8 Quantify table, as a result rounds up and retain integer；Most of high frequency coefficient in this way in DCT coefficient becomes " 0 ", some are not set to It some can also be lost in precision for the coefficient of " 0 "；The purpose of quantization be filter image in high frequency coefficient, image it is main Information concentrates on the low frequency part of image, can play the role of compression in this way, and will not be generated to the main information of image big Influence；

4th, every 8 × 8 DCT coefficient after quantifying to the 3rd step carries out zigzag scannings, the in this way DCT coefficient for 8 × 8 pieces It will obtain 1 × 64 one-dimensional sequence；

4.1st, high frequency coefficient AC is detached with low frequency coefficient DC, high frequency coefficient is stored as private part, and by high frequency coefficient Position replace with " 0 "；

4.2nd, zero RLE and Huffman encoding, the sequential coding of further compressed data are carried out to the high frequency coefficient of separation Length obtains binary sequence w₀, w₁..., w_i；

4.3rd, stream cipher encrypting is carried out to the high frequency coefficient of the 4.2nd step；

The specific method of stream cipher encrypting is：Use sub-key k₁Binary key stream r is generated by stream cipher generator₀, r₁..., r_i, and to the binary system w in the 4.2nd step_iSequence obtains encrypted w as follows_i' sequence：

w_i' private part will be used as to preserve；

5th, izigzag scannings thus are carried out to the coefficient after high frequency coefficient reset and inverse quantization, the result of inverse quantization abandons The detail section of image only remains image rough features；

6th, reverse discrete cosine transform (IDCT) is carried out to the DCT coefficient by the processing of the 5th step and is converted into spatial domain；

7th, all pixel values are subtracted 128 in the 2.1st step, 128 will be added herein, pixel value is restored to 0-255 Range, just obtained the thumbnail of corresponding scramble grade in this way；

8th, the thumbnail in the 7th step is subjected to block encryption again, each block size is 8 × 8；

8.1st, each of thumbnail pixel value in the block is the average value of the block at this time, thus needs to each picture in the block The low six progress random scrambling of element so that the pixel value of the block is not accurate average value, but close to average value；

8.2nd, in order to ensure the compression effectiveness of image, same sub-key k is used for 64 pixels in same piece₂It carries out Low six position of pixel value is random；

Although having carried out a degree of scramble to each sub-block in the block in step 1, attacker's Brute Force picture strip is given Difficulty centainly is carried out.But in the 4.1st steps of 3-, dct transform is carried out to every 8 × 8 block, and by all AC resets, Due to the characteristic of dct transform so that after image carries out idct transform, the value of pixel is being averaged for the block in each 8 × 8 pieces Value just accurately exposes between block 8 × 8 pieces each of after scramble of average value in this way.Thus sub-key is taken in the 8.2nd step k₂As " seed " of random number generator, low six progress random scrambling of each pixel value in the block to each 8 × 8；For The correlation of pixel in maintainance block, convenient for compression, each sub-key in the block is identical, not only hides to a certain extent in this way The average value of block, and each block is made with artwork to have certain similarity；

Second part, thumbnail decryption method

9th, encrypted thumbnail image is subjected to piecemeal according to 8 × 8, and uses sub-key k₂To each pixel value in the block Carry out low six reduction decryption oprerations；

10th, according to sub-key k₁The high frequency coefficient AC detached in the 4th step is decrypted as follows；

11st, thumbnail is carried out such as the dct transform in the 3rd steps of 2- and quantization；

11.1st, the high frequency coefficient in DCT coefficient is replaced by the high frequency coefficient decrypted in the 10th step；

11.2nd, inverse quantization is carried out to thumbnail and idct transform obtains slightly high-resolution thumbnail；

12nd, sub-key k is used₀The block of scramble is restored to the position of original image, reverts to the original image of high definition.