JP2004328191A - Electronic watermark embedding method, and electronic watermark detecting and falsification discriminating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electronic watermark embedding method and an electronic watermark detecting and falsification discriminating method for detecting the presence / absence of data falsification in the reliability certification of digital image data. <P>SOLUTION: Statistic information related to an image signal obtained from a first area is embedded to the first area as first electronic watermark information and information equivalent to the first electronic watermark information in a different expression form from that of the first electronic watermark information is embedded to a second area different from the first area as second electronic watermark information. In the case of detecting falsification, two embedded information items are detected and compared, and when the values of the two information items are coincident, it is discriminated that no falsification exists in the image signal in the first area and when the values of the two information items are dissident, it is discriminated that falsification exists in the image signal in the first area. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a digital watermark embedding method and a digital watermark detection / falsification determination method for detecting the presence / absence of data tampering in the reliability certification of digital image data.
[0002]
[Prior art]
In recent years, with the spread of digital technology, many digital contents have been distributed. Package media such as CDs (compact disks), MDs (mini disks) and DVDs (digital versatile disks) are typical examples.
[0003]
On the other hand, digital moving images and still images have been distributed on networks other than package media. At the same time, data has been tampered with by third parties during transmission because of the digital form and the use of network paths.
[0004]
The following method is generally used as a method for detecting tampering of such digital data. First, a hash value of digital data is calculated, and the calculation result is stored as separate data. The “hash value” is a result calculated by what is called a “hash function”. The “hash function” is a function h_K for which it is difficult to obtain x such that h_K (x) = y when a function value y is given.
[0005]
When a hash function is used for falsification detection, x has an arbitrary length and y has a fixed length. At this time, h_K changes according to the secret key K known only to the sender and the receiver. When K and x are given, h_K (x) needs to be designed so that it can be easily calculated. There is. Then, the hash value is attached to the data and sent to the recipient. To be absolutely secure, data and hash values may be encrypted before transmission.
[0006]
The receiver calculates a hash value using the same secret key as the sender, and verifies whether the hash value is equal to the transmitted hash value. If they are equal, it is determined that there is no tampering, and if they are not equal, it is determined that there is tampering.
[0007]
The above means is generally called "message authentication", and the calculation result of h_K is called "message authenticator".
[0008]
[Non-patent document 1]
Tatsuaki Okamoto and Hiroshi Yamamoto, "Series / Mathematics of Information Science, Modern Encryption", Sangyo Tosho Co., Ltd., June 30, 1997, P178-P180, 11.5 ElGamal 11.6 DSA signature (ISBN4-7828-5353) X C3355)
The signature in this document corresponds to the message authenticator described above.
[0009]
[Problems to be solved by the invention]
In the falsification detection using the message authenticator, it is necessary to attach another information called “message authenticator” separately from the image to be subjected to the falsification detection. However, when distributing an image over a network, the image can be reproduced if only the image signal itself exists, and it is inconvenient to distribute the image along with the accompanying information. Even if the information of the "message authenticator" is written in the header of the image signal, if the re-encoding is performed, the data is lost.
[0010]
The present invention has been made in view of the above points, and an object of the present invention is to provide a digital watermark embedding method and a digital watermark detection / falsification determination method that can determine whether an image has been tampered with an image alone.
[0011]
[Means for Solving the Problems]
Therefore, in order to solve the above problems, the present invention provides the following digital watermark embedding method and digital watermark detection / falsification determination method.
(1) embedding statistical information relating to an image signal obtained from a first area set in an input image signal in the first area as first digital watermark information;
In a second area different from the first area set in the image signal, information equivalent to the first digital watermark information having a different expression format from the first digital watermark information is stored in a second area. Embedding as digital watermark information;
A digital watermark embedding method, comprising:
(2) A digital watermark detection / falsification determination method for detecting two types of digital watermark information from an input image signal and determining whether the image signal is falsified,
In the image signal, statistical information on an image signal obtained from the first area is embedded as first digital watermark information in a first area, and the second area different from the first area has the statistical information. The first digital watermark information is a signal in which information equivalent to the first digital watermark information having a different expression format is embedded as second digital watermark information,
Detecting third digital watermark information corresponding to the first digital watermark information from the first area;
Detecting fourth digital watermark information corresponding to the second digital watermark information from the second area;
The value of the third digital watermark information is compared with the value of the fourth digital watermark information. If the two values match, it is determined that the first area of the image signal has not been tampered with. And when the two values do not match, determining that the first area of the image signal has been tampered with;
A digital watermark detection / falsification determination method characterized by having:
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, embedding of alteration detection data will be described. FIG. 1 is a flowchart showing one embodiment of a digital watermark embedding method according to the present invention. The method of the embodiment is executed by a program that causes a computer to execute the steps shown in FIG. This program may be read from a recording medium and taken into the computer, or may be transmitted via a communication network and taken into the computer.
[0013]
A case where a digital image of 720 pixels horizontally by 480 pixels vertically as shown in FIG. 2 will be described. First, an area dividing step S1 shown in FIG. 1 is performed on this digital image. First, the area is divided into an area area 1 (720 horizontal pixels × 464 vertical pixels) where tampering detection is to be performed and another area area 2 (720 horizontal pixels × 16 vertical pixels).
[0014]
Then, as shown in FIG. 3, the image is further divided into areas 11 to 14 in a predetermined unit (360 pixels horizontally × 232 pixels vertically) with respect to area 1. Similarly, as shown in FIG. 4, the image is divided into areas 21 to 24 in a predetermined unit (128 horizontal pixels × 16 vertical pixels) for area 2 as well. Further, each area 21 to area 24 is divided into eight images by a predetermined unit (8 horizontal pixels × 16 vertical pixels). For example, the area 21 is divided into eight areas 211 to 218 as shown in FIG.
[0015]
Next, an average luminance value calculation step S2 of each divided area is performed. First, an average luminance value of the area 11 is obtained. .., Y83520 (0 ≦ Yi ≦ 255, 1 ≦ i ≦ 83520), and SUM1
SUM1 = Y1 + Y2 + Y3 +... + Y83520
(Hereinafter, the number of pixels of each divided region 83520 = 360 × 232 is set as N1)
Then, the average luminance value AVG1 is
AVG1 = SUM1 // N1
Is determined by (The operator // indicates rounding after the decimal after division.)
This average luminance value AVG1 becomes the first digital watermark information which is statistical information relating to an image signal obtained from the first area, as described later.
[0016]
Next, an addition / subtraction amount calculation step S3 for each divided area is performed. The addition / subtraction amount DELTA1 is obtained as follows.
[0017]
DELTA1 = | SUM1-AVG1 × N1 |
Next, a pixel operation step S4 for each divided area is performed according to the above DELTA1. First, it is calculated whether SUM1-AVG1 × N1 is positive or negative. If positive, the pixel value is subtracted by DELTA1 so that the sum SUM1 of the luminance values of each pixel becomes AVG1 × N1. Conversely, if negative, add.
[0018]
Here, the reason why the sum of the luminance values is added or subtracted by DELTA1 will be described below. When encoding is performed by MPEG or the like after embedding the digital watermark tampering detection data, basically, the average luminance value of each DCT block is stored, that is, AVG1 of area 11 is stored. However, some calculation errors appear due to image processing such as an integer operation and a pre / post filter performed in each calculation step of MPEG. Thereby, AVG1 before MPEG encoding and AVG1 calculated after encoding may have different values. In consideration of this and the fact that AVG1 is a rounding operation, DELTA1 is added or subtracted so that AVG1 becomes an integer.
[0019]
In addition, from the relationship between AVG1, SUM1, and N1 described above,
DELTA1 <N1
Holds. Therefore, the subtraction method is performed so as not to concentrate on a certain area and not to subtract a large value in order to prevent image quality deterioration. For example, as shown in FIG. 6, the luminance value is subtracted by one for every three pixels, and the same operation is continued until DELTA1 becomes zero. Alternatively, subtraction may be performed after a portion where the luminance change of the image is less noticeable is selected in advance by calculation.
[0020]
As a result, the average luminance value AVG1 is embedded in the area 11 as the first digital watermark information.
[0021]
Next, a digital watermark embedding step S5 is performed on the area 21 in the area 2 corresponding to the area 11. FIG. 7 is a flowchart showing the procedure of the digital watermark embedding step S5. First, step S51 is performed in which the average luminance value AVG1 of the area 11 is expressed in binary notation (b1, b2, b3, b4, b5, b6, b7, b8). b1 is the most significant bit, and b8 is the least significant bit. The average luminance value AVG1 expressed in binary is the second digital watermark information that is equivalent to the first digital watermark information having a different expression format from the first digital watermark information, as described later.
[0022]
Next, step S54 for calculating the average luminance value AVG2 of the area 211 shown in FIG. 5 is performed. As in AVG1, assuming that the sum of the luminance values of all the pixels in the area 211 is SUM2 and the total number of pixels is N2,
AVG2 = SUM2 // N2
Is determined by
[0023]
Next, step S56 or S57 of performing a pixel operation on the area 211 based on the value of the most significant bit b1 is performed. If the binary value b1 is 0, addition / subtraction is performed to make AVG2 an even number. The addition / subtraction amount DELTA2 is obtained as follows if AVG2 is an even number.
[0024]
DELTA2 = | SUM2-AVG2 × N2 |
If AVG2 is an odd number,
d1 = | SUM2- (AVG2 + 1) × N2 |
d2 = | SUM2- (AVG2-1) × N2 |
Is smaller than DELTA2.
[0025]
Here, the reason that the absolute value difference is selected to be smaller is to minimize the image quality deterioration due to the difference value DELTA2 being reflected in the addition and subtraction of the pixels in the pixel block in the following steps. . Then, as in the case of the area 11, the relationship between SUM2, AVG2, and N2 is checked, and the addition or subtraction of the luminance value by DELTA2 is performed without any image quality deterioration.
[0026]
On the other hand, when b1 is 1, the same operation is performed to make AVG2 an odd number. Next, an operation is performed on the area 212 according to the value of b2, and a similar operation is performed up to the area 218.
[0027]
As a result, the average luminance value AVG1 represented as a binary number in the area 21 is embedded as the second digital watermark information. (As a result, the bits b1 to b8 are embedded as digital watermark information in the areas 211 to 218, respectively, such that the bit b1 expressed in binary is the area 211 and the bit b2 is the area 212.)
The steps described above are the description of embedding the digital watermark in the area 11. If this operation is performed up to the area 14, the operation of embedding the data for falsification detection in the input image is completed.
[0028]
Next, falsification detection will be described. FIG. 8 is a flowchart showing one embodiment of the digital watermark detection / falsification determination method according to the present invention. The method of the embodiment is executed by a program that causes a computer to execute the steps shown in FIG. This program may be read from a recording medium and taken into the computer, or may be transmitted via a communication network and taken into the computer.
[0029]
First, the image area dividing step S7 is performed in the same procedure as when embedding. Next, step S8 of calculating the average luminance value of the area 11 is performed. This is the same as when embedding,
AVG1 = SUM1 // N1
Ask for. (The calculated AVG1 is the third digital watermark information corresponding to the first digital watermark information.)
Next, step S9 for detecting the digital watermark WM embedded in the area 21 is performed. First, as in the case of embedding the average value AVG2 of the area 211,
AVG2 = SUM2 // N2
Ask for. If AVG2 is even, b1 = 0, and if AVG2 is odd, b1 = 1. The same operation is performed up to the area 218. Then, the binary representation (b1, b2, b3, b4, b5, b6, b7, b8) is converted into a decimal number WM with b1 being the most significant bit and b8 being the least significant bit. (The calculated digital watermark WM in binary number is the fourth digital watermark information corresponding to the second digital watermark information.)
AVG1 = WM (1)
Is satisfied, it is determined that there is no tampering, and if not, it is determined that there is tampering in the area 11.
[0030]
Here, the expression (1) is used as a criterion for tampering because the average luminance value of the area 11 changes when the user replaces a part of the area 11 with another image. The reason for this is that the presence or absence of tampering can be determined by comparison with.
[0031]
The steps described above are the explanation of the digital watermark falsification detection for the area 11. If this operation is performed up to the area 14 or until it is determined that there is tampering, the operation of tampering detection for the input image is completed. Also, in FIG. 8, if it is determined that tampering has been performed even once, the user is informed of the tampering and the process is terminated. If there is, all of them can be transmitted to the user.
[0032]
Although the average luminance value is used as the first digital watermark information in the above-described embodiment, other statistical information such as an average color difference value, a total luminance value, a total color difference value, and the like may be used. Further, in the above-described embodiment, a binary expression (binary expression) is used as the expression format of the second digital watermark information, but a decimal expression, a hexadecimal expression (hex expression), or the like may be used.
[0033]
【The invention's effect】
As described above, according to the present invention, by embedding statistical information relating to an image signal equivalent to the first area in the image signal in the second area, the image signal can be obtained without the need for a message authenticator or additional information. By itself, it is possible to determine whether the image signal has been tampered with.
[Brief description of the drawings]
FIG. 1 is a flowchart showing one embodiment of a digital watermark embedding method according to the present invention.
FIG. 2 is a diagram illustrating an example of an image area dividing method.
FIG. 3 is a diagram illustrating an example of an image area dividing method.
FIG. 4 is a diagram illustrating an example of an image area dividing method.
FIG. 5 is a diagram showing an example of an image area dividing method.
FIG. 6 is a diagram illustrating an example of a luminance value subtraction method.
FIG. 7 is a detailed flowchart of step 5 in the embodiment shown in FIG. 1;
FIG. 8 is a flowchart illustrating an embodiment of a digital watermark detection / falsification determination method according to the present invention.

Claims (2)

Embedding statistical information relating to an image signal obtained from a first area set in an input image signal into the first area as first digital watermark information;
In a second area different from the first area set in the image signal, information equivalent to the first digital watermark information having a different expression format from the first digital watermark information is stored in a second area. Embedding as digital watermark information;
A digital watermark embedding method, comprising: An electronic watermark detection / falsification determination method for detecting two electronic watermark information from an input image signal and determining whether the image signal is falsified,
In the image signal, statistical information on an image signal obtained from the first area is embedded as first digital watermark information in a first area, and the second area different from the first area has the statistical information. The first digital watermark information is a signal in which information equivalent to the first digital watermark information having a different expression format is embedded as second digital watermark information,
Detecting third digital watermark information corresponding to the first digital watermark information from the first area;
Detecting fourth digital watermark information corresponding to the second digital watermark information from the second area;
The value of the third digital watermark information is compared with the value of the fourth digital watermark information. If the two values match, it is determined that the first area of the image signal has not been tampered with. And when the two values do not match, determining that the first area of the image signal has been tampered with;
A digital watermark detection / falsification determination method characterized by having:

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