CN101562681A - Method for generating digital photo containing watermark based on state parameter - Google Patents

Abstract

A method for generating watermarked digital photos based on state parameters: when a digital camera takes a photo, the image signal is processed by a central processing unit to obtain digital image data, and the central processing unit reads the focal length, aperture, exposure time, and shooting time when the photo is taken. Time and sensitivity are used as state parameters. Generate a camera key using the camera ID and the key setup function. The state parameter and the camera key are used as plaintext, and the authentication key of the digital photo is generated by hash function. Use the authentication key to generate the watermarked digital image data of the image data, and then synthesize it and the header file EXIF of the photo into a photo output file and save it in the external memory of the camera. The authentication key of the watermarked digital photo generated by this method has good confidentiality, which can effectively prevent the "collage attack" and effectively resist the "remake attack". At the same time, neither the participation of the digital camera nor the transmission of the key is required during the authentication, which realizes the openness and high efficiency of the digital photo authentication.

Description

A Method of Generating Watermarked Digital Photos Based on State Parameters

Technical field

The invention relates to a method for generating a watermarked digital photo, which is used for conveniently, timely and openly detecting and verifying the authenticity, integrity and originality of the digital photo.

Background technique

Today, with the increasing development of computer technology, network communication technology and digital imaging technology, digital cameras have entered thousands of households, and digital photos taken by digital cameras have become our main source of information acquisition and exchange and an important carrier of information dissemination. At the same time, the digital storage of images and the emergence of various image processing software make the editing, modification and synthesis of digital photos very simple. On the one hand, digital image processing technology can improve the display quality of images, enrich our life and facilitate our work. On the other hand, if tampering and forging digital photos are used in the fields of news media, court evidence and scientific development, it will have a serious negative impact on the integrity of the society, the credibility of the government and the authenticity of science. Falsifying the existence of digital photos puts digital photos at risk of losing trust. For example, in recent incidents such as the "Plaza Pigeon" and "Tibetan Antelope" that have attracted widespread attention, the photo provider used image processing tools to tamper with, collage, and synthesize multiple digital photos to generate "pseudo-photos" that became award-winning news photos. After multi-party evidence collection and identification, the photo providers of these events have publicly admitted that there are elements of modification in their award-winning news photos. Although the awards for these works were withdrawn, many resources (financial, material, time, etc.) were spent to verify their authenticity and integrity. This makes it inevitable that people will doubt the authenticity of news digital photos, and how many "lurking" fake photos in news photos have not been discovered, making the credibility of digital photos increasingly become a serious problem of social integrity. Therefore, the authenticity, integrity and originality of digital photos have been authenticated. One of the effective ways to solve this problem is to implement the watermark embedding module built in the digital camera and the corresponding authentication system.

Recent background art related to digital cameras and authentication watermarking can be found in the following documents:

[1].P.Blythe, J.Fridrich.Secure Digital Camera, Digital Forensic Research Workshop, Baltimore, August 11-13, 2004

[2]. Tian Xin, Liu Man, Yang Xianghui A digital camera with watermark embedding function, application number 2005100034309.X, public date 2007.7

[3]. Yu Miao, He Hongjie, Zhang Jiashu, A digital authentication watermarking scheme for JPEG images with superior localization and security, Sci China Ser F-Info Sci, 2007.50(3): 491-509.

[4]. X. Zhang, and S. Wang. Statistical fragile watermarking capable of locating individual tampered pixels. IEEE Signal processing Letter, October 2007, 14(10): 727-731

[5]. Hongjie He, Jiashu Zhang, Fan Chen. Adjacent-block Based Statistical Detection Method for Self-Embedding Watermarking Techniques, Signal processing, doi: 10.1016/j.sigpro.2009.02.009

The above document [1] discloses a digital camera that provides a function of embedding watermark information into the digital photo while the digital photo is being taken. The "watermark information" is used to detect the authenticity, integrity and Originality can also be used to identify that a digital photo was taken by a certain digital camera and a certain person. In order to avoid counterfeiting digital photos with watermarks by computer, the authentication key of the digital photos must be kept secret. This is because if someone obtains the authentication key of the watermarked digital photo, he can pass the authentication while tampering with the digital photo on the computer and re-embedding the watermark [2]. For this reason, Tian Xin et al. [2] of Huaqi Information proposed to use the "encapsulation" technology to build the camera key into the digital camera, so as to improve the confidentiality of the watermarked digital photo authentication key. At the same time, research on secure authentication watermarking algorithm is always one of the key technologies to solve this problem. In recent years, researchers have conducted in-depth research and analysis on digital image authentication watermarking algorithms from the aspects of security [3], positioning accuracy [4], and tamper recovery performance [5].

However, the following problems still exist in the prior art: (1) The digital camera that takes the digital photo must participate in the authentication process of the digital photo, which is not feasible in some practical applications. For example, news reports sent back by journalists stationed abroad can only be released after passing the certification. When the digital camera that took the picture comes back to participate in the anti-counterfeiting certification, the news will become "old news". (2) Using the same digital camera, many watermarked digital photos can be obtained, and their authentication keys are all the same, which not only increases the risk of camera key leakage, but also provides a basis for the implementation of Fridrich et al. (J. Fridrich, M The "collage attack" proposed by Goljan and N.Memon.Cryptanalysis of the Yeung-Mintzer Fragile Watermarking Technique Electronic Imaging, April2002, 11(4):262-274) provides conditions. That is to say, the "fake photos" obtained by collaging and synthesizing multiple digital photos taken by the same digital camera may pass the authentication, and preventing such attacks will increase the difficulty and complexity of the authentication watermark algorithm design. (3) The most important thing is that the camera owner uses one or more digital photos, edits, modifies, synthesizes them with image processing software, prints out the synthesized photos, and then remakes the printed photos. The "fake photo" can still pass the authentication of the authentication watermark (this attack is called "remake attack" for short). At the same time, by tampering with the digital photo header file EXIF including the focal length, aperture, exposure time, shooting time, sensitivity and other state parameters, the attacker can destroy the obvious evidence of using the basic common sense of optical imaging to determine the "remake photo", so that the The authentication watermarking algorithm cannot provide strong evidence to prove that the "remake photo" is forged, that is, the existing authentication watermarking algorithm is difficult to effectively resist the "remake attack".

Contents of the invention

The purpose of the present invention is to provide a method for generating watermarked digital photos based on state parameters. The authentication key of the watermarked digital photos generated by the method has good confidentiality, which can effectively prevent "collage attack" and effectively resist "collage attack". Remake attack". At the same time, neither the participation of the digital camera nor the transmission of the key is required during the authentication, which realizes the openness and high efficiency of the digital photo authentication.

The present invention solves its technical problem, and the adopted technical solution is: the generation method of the watermarked digital photo based on state parameters, comprising the following steps:

(1) When a digital camera takes a picture, the signal acquired by the lens is processed by the central processor to obtain digital image data D, and at the same time the central processor reads the focal length, aperture, exposure time, shooting time, and sensitivity of the digital camera when the picture is taken as State parameter S;

(2), according to the "camera identity number C" preserved in the digital camera read-only memory, utilize the key setting function F(k) to generate the camera key K _L , where k is a key;

(3), using the state parameter S of step (1) and the camera key K _L of step (2) as plaintext, utilize the hash function H() to generate the authentication key K=H(K _L , S of the digital photo );

(4), with the authentication key K as the watermark embedding key, utilize the authentication watermark embedding algorithm to embed the authentication watermark in the image data D in step (1), generate digital image data X containing the watermark;

(5) Synthesize the digital image data X containing the watermark and the photo header file EXIF including the state parameter S and the camera ID number C into a photo output file and save it in the external memory of the digital camera.

Compared with prior art, the beneficial effect of the present invention is:

1. The present invention utilizes a hash function to generate the "authentication key" of the digital photo by using the state parameters of the digital camera such as focal length, aperture, exposure time, shooting time, and sensitivity as part of the hash function plaintext when the digital photo is taken. Since the state parameters of different digital photos are very unlikely to be exactly the same, according to the nature of the hash function, even if the digital photos taken by the same digital camera, the possibility of having the same authentication key is almost non-existent, thereby eliminating the need to implement "spelling". Post attack" condition. Therefore, the watermarking algorithm used in the present invention does not need to have the ability to resist "collage attack", thereby reducing the requirements for the design of the authentication watermarking algorithm, and the complexity of the watermarking algorithm is low.

2. For the duplicated photos, if the parameters in the header file in the digital photo are modified, the state parameters read by the authentication system are inconsistent with the state parameters entered as plaintext when generating the authentication key, which will cause the generated authentication key to be different from the one when the watermark was embedded. If the authentication keys used are different, the digital photos reprinted will not be able to pass the authentication; The focal length in the distant view photo, the difference is very obvious), it is easy to judge whether the digital photo is a remake of the digital photo. Therefore, the present invention can effectively resist remake attacks.

3. Use the "key setting function F()" to establish a hidden correspondence between the identity number of the digital camera (that is, the unique identifier of the digital camera) and the camera key. On the one hand, it realizes the confidentiality and security of the camera key. Concealment; on the other hand, according to the public digital camera ID number during authentication, use the "key setting function F()" in the authentication system to generate the corresponding camera key. At the same time, the state parameters are also publicly stored in the header file of the digital photo; therefore, neither key delivery is required for authentication, nor does the digital camera participate in providing the camera key. The authentication system can detect the authenticity, integrity and originality of digital photos according to the digital photos themselves, thus realizing an open, convenient and efficient authentication process for digital photos.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a logic block diagram of generating a watermarked digital photo according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a key setting function F(k) according to an embodiment of the present invention.

FIG. 3 is a logical block diagram of an authentication system according to an embodiment of the present invention.

Fig. 4 is a key setting function F(k) in an embodiment of the present invention, and a distribution diagram of camera keys corresponding to three cameras with different ID numbers and key k. In the figure, the symbols "□", "○" and "*" respectively represent the camera keys K _L of the three cameras with ID numbers C1, C2, and C3 when the key k is different.

Example

Fig. 1 shows, a kind of embodiment of the present invention is: a kind of generation method based on the watermark digital photo of state parameter, comprises the following steps:

1. When a digital camera takes a photo, the signal acquired by the lens is processed by the central processor to obtain digital image data D, and the central processor reads the focal length, aperture, exposure time, shooting time, and sensitivity of the digital camera when the photo is taken as the status parameter S.

2. According to the "camera ID number C" stored in the digital camera's read-only memory, use the key setting function F(k) to generate the camera key K _L , where k is the key.

In this embodiment, the key k of the "key setting function F(k)" is jointly set by the authentication system and the camera manufacturer, and is encrypted and packaged inside the digital camera together with the key distribution function, and can only be obtained by the "hash function H ()" call, any damage to the encapsulation will cause the key setting function F(k) to be unavailable.

The "key setting function F(k)" is used to establish the concealed corresponding relationship between the digital camera identity number C (the unique identifier of the digital camera) and the camera key K _L , that is, K _L =F(k, C). In the present invention, the key setting function can adopt any safe non-linear mapping function. In the present embodiment, the key setting function adopts the cascade-chaotic map as shown in Figure 2, and the chaotic map f adopts the existing literature (S.Lian, J.Sun, J.Wang, and Z.Wang A chaotic stream cipher and the usage in video protection, Chaos, Solitons and Fractals 34 (2007) 851-859), a discrete piecewise linear chaotic map implemented on the integer domain (fixed point),

表示不大于a的最大整数，

表示

个离散状态， $0<p<2^{n_0-1}$ 是控制参数，n₀是计算机的字长。考虑到现有计算机和安全性需要，后面的测试和使用中取n₀＝32。公式(1)表示根据控制参数p和状态z_i，能确定性地生成下一个状态z_i+1，初始状态z₀和控制参数p是产生该随机序列的密钥。本实施例将相机身份号C以四个字符为单位分段(如果C中的字符个数不能被4整除，则在C的末尾补“0”)，然后，以每段4个字符ASCII码的链接为混沌映射f的控制参数p，密钥k作为第一个混沌映射的初始状态z₀，第一个混沌映射的输出作为第二个混沌映射的初始状态z₀，第二个混沌映射的输出作为第三个混沌映射的初始状态z₀，以此类推，最后一个混沌映射的输出作为相机密钥K_L。in

Indicates the largest integer not greater than a,

express

a discrete state, $0<p<2^{{\mathrm{no}}_0-1}$ Is the control parameter, n ₀ is the word length of the computer. Considering the existing computer and security requirements, n ₀ =32 is used in the following tests and uses. Formula (1) indicates that the next state z _i+1 can be deterministically generated according to the control parameter p and the state z _i , and the initial state z ₀ and the control parameter p are the keys for generating the random sequence. In this embodiment, the camera ID number C is segmented in units of four characters (if the number of characters in C cannot be divisible by 4, "0" is added at the end of C), and then, the ASCII code of each segment is 4 characters The link is the control parameter p of the chaotic map f, the key k is used as the initial state z ₀ of the first chaotic map, the output of the first chaotic map is used as the initial state z ₀ of the second chaotic map, and the second chaotic map The output of the third chaotic map is used as the initial state z ₀ , and so on, the output of the last chaotic map is used as the camera key K _L .

3. Using the state parameter S in step 1 and the camera key K _L in step 2 as plaintext, use the hash function H() to generate the authentication key K=H(K _L , S) of the digital photo. "Hash function H ()" in the present embodiment adopts existing literature (Wang Xiaomin, Zhang Wenfang, Zhang Jiashu. Based on the chaotic Hash function design of nonlinear digital filter, Computer Aided Design and Graphics Journal, 2006, 18 (6) : 870-875) proposed a chaotic hash function based on nonlinear filters.

4. Use the authentication key K as the watermark embedding key, use the authentication watermark embedding algorithm to embed the authentication watermark in the image data D in step (1), and generate the watermarked digital image data X. The "watermark embedding algorithm" in the present invention can use any existing authentication watermark algorithm, such as recoverable authentication watermark algorithm, positioning type authentication watermark algorithm and so on.

5. Synthesize the digital image data X containing the watermark and the photo header file EXIF including the state parameter S and the camera ID number C into a photo output file Y={X, S, C} and save it in the external memory of the digital camera.

Correspondingly, the authentication work of the present invention can be completed by a specific authentication software system in a general-purpose computer, or can be completed by a specific detection device equipped with a corresponding authentication system. The input of the authentication system is the tested digital photo file Y ^* = {X ^* , S ^* , C ^* }, and the output is the authentication result. The encapsulation of the digital camera and the authentication system itself can guarantee the confidentiality of the key distribution function F(k) sex. Figure 3 shows that the authentication process of the authentication system for the digital photo to be tested Y ^* ={X ^* , S ^* , C ^* } includes the following steps:

1. According to the digital photo to be tested Y ^* = {X ^* , S ^* , C ^* }, the camera model C ^* , state parameter S ^* and watermarked image data X ^* of the digital photo to be tested are respectively obtained through "data extraction";

2. Use the "key setting function F(k)" to generate the corresponding camera key K _L ^* according to the camera identity number C ^* ;

3. Utilize the "hash function H()" to generate the authentication key K ^* of the tested digital photo with the state parameter S ^* and the camera key _KL ^* as plaintext;

4. Using the authentication key K ^* , through the corresponding "watermark extraction and authentication" module, the authentication result of the tested digital photo is given. The information provided by the authentication result may include: tamper location, tamper degree, tamper recovery results, etc., which mainly depend on the performance of the authentication watermark algorithm used in the system.

It can be seen from the above authentication process that only the digital photo to be tested is required for authentication, thereby realizing an open and simple authentication process for the digital photo. The security of the algorithm lies in the secrecy of the key distribution function F(k) and the hard forgery of the authentication key.

Performance analysis and computer simulation test of the present invention:

1. Performance Analysis

In the present invention, the focal length, aperture, exposure time, shooting time, and sensitivity of the digital camera are used as the state parameter S and the camera key K _L when the digital photo is taken, and the authentication key K of the digital photo is generated by using a hash function. The above-mentioned method for generating a digital photo authentication key has the following advantages:

(1) Public authentication is achieved: use the key setting function F(k) to assign the camera key K _L to the digital camera, combined with the state parameter S and the camera identity number C publicly stored in the EXIF header file of the digital photo, there is no need for "non- Symmetric encryption" technology can realize the public authentication of digital photos;

(2) Increased confidentiality of the authentication key: On the one hand, the authentication key of the digital photo is unknown to anyone, which increases the confidentiality of the authentication key. On the other hand, since the state parameter S of the digital photo is involved in the generation of the authentication key, the authentication keys of the watermarked digital photos obtained by the same digital camera are different from each other, eliminating the conditions for implementing the "collage attack";

(3) It has the ability to resist "remake attack": for the watermarked digital photo Y={X, S, C}, the attacker can modify the digital photo data X through image editing software, and then use the same digital camera and personal information " Generate a watermarked digital photo Y ^* ＝{X ^* , S ^* , C} from a tampered digital photo. According to the principle of optical imaging, the state parameters S and S ^* of scene imaging and photo reproduction are very different. In order not to leave obvious forged evidence, the attacker needs to forge a state parameter S′ similar to S so that the equation H(F(k,C),S ^* )=H(F(k,C),S′ ) is established, so that the fake digital photo Y'={X ^* , S', C} can pass the authentication. From the nature of the hash function, it can be seen that finding any plaintext m≠m' makes H(m)=H(m') computationally infeasible. Therefore, the present invention can effectively prevent digital photo providers from using digital cameras to forge watermarks digital photo.

2. Computer simulation test

1. Hard to forge the authentication key

The difficulty of forgery of the authentication key is illustrated below by testing the sensitivity of the hash function to plaintext. For the convenience of testing, we choose the initial plaintext sequence as: "South-west Jiaotong University 2008". The value of the ciphertext sequence ((the number string represented by hexadecimal notation, i.e. the authentication key) generated by the chaotic hash function based on the nonlinear filter adopted in this embodiment is as in the second line "original ciphertext" of Table 1 Shown. Then test the authentication keys obtained in the following six situations:

Case 1: The first capital letter S is changed to lowercase;

Case 2: Remove the connector in the word "South-west";

Case 3: add a space at the end of the original user key;

Case 4: add a % at the end of the original user key;

Case 5: delete the word 2008 at the end of the original user key;

Case 6: Change the word 'Jiaotong' to 'Jiao tong'.

Table 1 Values of two subkeys k ₁ and k ₂ under different conditions

Table 1 lists the sub-keys generated under the above six situations. For further tests on the characteristics of the hash function, see the literature (Wang Xiaomin, Zhang Wenfang, Zhang Jiashu. Chaotic Hash function design based on nonlinear digital filter, computer-aided design Journal of Graphics and Graphics, 2006, 18(6): 870-875). It can be seen from Table 1 that there are no two similar ciphertext sequences in the table, and they can be regarded as random sequences that are statistically independent of each other. Therefore, it is difficult for an attacker to forge a meaningful plaintext sequence that is equal to a specific ciphertext .

2. The confidentiality of the camera key K _L

The security of the key distribution function F(k) in the present invention lies in the secrecy of the key k. For a specific digital camera, its camera key not only depends on the camera identity number C, but also depends on the key k set jointly by the authentication system and the camera manufacturer. Under the condition that the key k is unknown, it is difficult to infer the camera key K _L corresponding to the camera according to the camera ID number C. In order to test the sensitivity of the camera key K _L to the key k, 1000 keys are randomly selected to test three digital cameras. The ID numbers of the three digital cameras are:

C1='32167Epson Photo PC 700/750Z'

C2 = '32166 Epson Photo PC 700/750Z'

C3='321676Kodak DC-290'

Figure 4 is the distribution diagram of three digital cameras and the camera keys corresponding to key k when the key k takes 1000 different values. The three cameras correspond to the camera key K _L for a particular key k. It can be seen from Figure 4 that for the above three digital cameras with different ID numbers (C1, C2, C3), ^the corresponding camera key K _L is ^{approximately} randomly distributed. Therefore, under the condition that the key k is unknown, even if the camera ID number is disclosed, it is difficult for an attacker to deduce or guess the camera key K _L corresponding to the digital camera.

Claims (1)

1, a kind of generation method of the moisture printing word photo based on state parameter comprises the steps:

When (1) digital camera is taken pictures, the signal that camera lens obtains is handled through central processing unit and is obtained DID D, and the focal length when central processing unit reads digital camera and takes this photo simultaneously, aperture, time for exposure, shooting time, photosensitivity are as state parameter S;

(2), according to " the camera ID C " that preserve in the digital camera read-only memory, utilize key that function F (k) is set and generate camera key K _L, wherein k is a key;

(3), with the state parameter S of step (1) and the camera key K of step (2) _LAs expressly, utilize hash function H () to generate the authenticate key K=H (K of this digital photos _L, S);

(4), with authenticate key K as the watermark embedded key, utilize authenticating water-mark to embed among the view data D of algorithm in step (1) and embed authenticating water-mark, generate moisture printing digital image data X;

(5) moisture printing digital image data X being synthesized the photo output file with the photo header file EXIF that comprises state parameter S and camera ID C is kept in the external memory of digital camera.

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