Abstract:
An object of the present invention is to provide a simple apparatus for and a simple method of embedding and extracting digital information with little clue to a third party as to embedded digital information with less effort, and the embedded information is securely reconstructed thereby. 
     To embed digital information, a band division portion receives a digital image signal, and then divides the same into ten frequency band signals through discrete wavelet transform so as to compute wavelet coefficients. A mapping portion maps inherent digital information to a pseudo-random number string. An information embedding portion embeds the mapped pseudo-random number string in a string structured by every or some of the computed wavelet coefficients in MRR (signals exclusive of an LL 3  signal). A band synthesis portion synthesizes the embedded LL 3  digital image signal. To extract the digital information, the band division portion divides a digital image signal to which information has been embedded into a plurality of frequency bands, and then computes wavelet coefficients therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for and a method of embedding and extracting digital information, and a medium having a program for carrying out the method recorded thereon, more particularly to, for copyright protection, an apparatus for and a method of embedding digital data such as copyright information in an image signal and extracting the same, and a medium having a program for carrying out the method recorded thereon. 
     2. Description of the Background Art 
     In recent years, developments in digital technology accelerate digitalization of multimedia data including audio, image and video. The digitalized multimedia data has been getting popular through high-speed transmission in volume. Such multimedia data, however, is easy to duplicate and hence any unauthorized person is free to duplicate copyrighted digital images for secondary utilization. 
     To get around such problem, an electron (digital) watermark technique is applied. The digital watermarking is a technique for embedding digital information in image data in an insensible form for human being. With such digital watermark technique, a copyright holder can claim that his/her copyright is illegally used by extracting embedded information as a proof. 
     The conventional digital watermark technique includes a method disclosed in Japanese Patent Laying-Open No.9-191394 (hereinafter, referred to as first document). The first document proposes a method of embedding, according to normal or random distribution, an embedding value in a key component for image quality after spectrum decomposition is done on data. More specifically, in the first document, embedment is done with an equation (1) and extraction with an equation (2), where Wi is a pseudo-random number string (embedding value), fi is a frequency coefficient of before-watermarking data, fi′ is a frequency coefficient of after-watermarking data, and α is a scaling parameter. 
     
       
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     Although the method advantageously renders the embedding value difficult to eliminate, it does not store positional information on the frequency coefficient into which the embedding value is inserted. To extract the embedding value, the method accordingly requires before-embedding original data. 
     Differently, Japanese Patent Laying-Open No.10-308867 (hereinafter, second document) discloses a method which does not require original data. In the second document, embedment is done with an equation (3) and extraction with (4), where avg(fi) is a partial average of the frequency coefficient fi of before-watermarking data. 
     
       
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     The equation (4) having no fi on the right part thereof indicates that there is no more need for the original data. However, the method is still required to perform DCT (Discrete Cosine Transform) and compute the reciprocal of the partial average of data. In this manner, applying such method to data in volume like image data results in great computation effort. 
     For betterment, another method is disclosed in Japanese Patent Laying-Open No.10-145757 (hereinafter, third document). 
     In the third document, embedment is done with an equation (5) and extraction with an equation (6), where |fi| is an absolute value of the frequency coefficient fi of before-watermarking data. 
     
       
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     For the purpose of reducing the effort great in the second document, the pseudo-random number string (embedding value) is first divided into several units, and then each unit of numbers is subjected to watermarking. When each unit is 8 by 8 in size, this method can be carried out by utilizing the procedure of MPEG, which is a manner of encoding moving images. In this manner, computational complexity accordingly gets lower, but the method is still required to compute the reciprocal of the partial average of data and great in computation effort. 
     The method in the third document is relevant to MPEG. Described below are methods of embedding (inserting) and extracting an embedding value disclosed in the second document. 
     First, the method of embedding is described by referring to FIG.  10 . An embedding signal first goes through an error correction encoder  81 , secondly through a spread spectrum modulator  82 , and lastly through a first spectrum transformer  83  and reaches a spectrum shaping device  84  as a first input. On the other hand, before-watermarking data is provided to a second spectrum transformer  85 . An output of the second spectrum transformer  85  is partially averaged temporally or spatially in a partial averaging device  86  before provided to the spectrum shaping device  84  as a second input, and is also provided to a delaying device  87 . An output of the spectrum shaping device  84  is added to that of the delaying device  87  in an adder  88 . An output of the adder  88  is inversely-transformed in an inverse transformer  89 , and thus data is watermarked. 
     Next, the method of extracting is described by referring to FIG. 11. A spectrum normalization unit  91  receives the watermarked data and then subjects the data to spectrum normalization so as to put the data back to a state before watermarking. Then, the normalized signal is analyzed in correlators  92 A to  92 Z. The correlators  92 A to  92 Z each detects a specific pseudo-random number sequence in the signal, if any, correlates the normalized signal to the pseudo-random number sequence, and then provides an output indicating a degree of the correlation therebetween to a judgment circuit  93 . The judgement circuit  93  sequentially selects an output being maximum as a most-probable current symbol among current symbols received from the correlators. Further, a sequence of the selected maximum likelihood current symbol is provided to the error corrector  94  so as to correct any error in the judgement made in the judgement circuit  93 . In this manner, the embedding signal is extracted as an output of the error corrector  94 . 
     To eliminate the need for original data, the method in the foregoing results in another need for a partial average to embed an embedding signal and for spectrum shaping by using the partial average. Further, to extract the embedding signal, there is still another need for spectrum normalization to put the embedding signal back to a state before watermarking. In a practical manner, the reciprocal of the partial average is computed. Still further, the method requires a plurality of correlators for extraction, and is required to correct errors at the end. Accordingly, the method bears a problem of great computation effort for extraction. 
     SUMMARY OF THE INVENTION 
     Therefore, in view of the above problems, an object of the present invention is, with the help of simple spectrum transformation for embedding and an inner product computed for extraction, to provide an apparatus for and a method of embedding and extracting digital information, in a simplified manner with less effort, with little clue to a third party as to the embedded digital information, and a medium on which a program for carrying out the method is recorded. 
     The present invention has the following features to achieve the object above. 
     A first aspect of the present invention is directed to a digital information embedding/extracting apparatus of a type embedding inherent digital information in a digital image signal, the apparatus comprising: 
     a band division portion for dividing the digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band division; 
     a mapping portion for mapping the inherent digital information to a pseudo-random number string; 
     an information embedding portion for embedding the pseudo-random number string in a string structured by the coefficients in every or some of the divided frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR); and 
     a band synthesis portion for reconstructing the digital image signal in which the pseudo-random number string has been embedded by using the MRR and the MRA to which information embedding processing is subjected. 
     As described above, in the first aspect, inherent digital information can be embedded without causing image degradation or necessitating positional information thereof. 
     A second aspect of the present invention is directed to a digital information embedding/extracting apparatus of a type extracting inherent digital information embedded in a digital image signal, the apparatus comprising: 
     a band division portion ( 11 ) for receiving a reconstructed digital image signal obtained by dividing said digital image signal through discrete wavelet transform or sub-band division and by embedding a pseudo-random number string in a string structured by coefficients in every or some of a plurality of frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR), and then dividing the reconstructed digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band transform; 
     a correlation value computation portion for computing an inner product of the string structured by the coefficients in the MRR among the divided frequency bands and a predetermined pseudo-random number assumed to be embedded; 
     a pseudo-random number string determination portion for determining the pseudo-random number string embedded in the digital image signal according to the value computed by the correlation value computation portion; and 
     an information extraction portion for extracting the inherent digital information mapped to the determined pseudo-random number string. 
     As described above, in the second aspect, embedded inherent digital information can be extracted in a simplified structure. 
     According to third and fourth aspects of the present invention, in the first and second aspects, respectively, 
     the pseudo-random number string is structured by numbers selected from normally-distributed average values of “0” and distribution values of “1”. 
     As described above, in the third and fourth aspects, a pseudo-random number string can be easily generated in an arbitrary length, whereby an embedding apparatus can be in a simplified structure. Further, since numbers in a string are totaled to be 0 if the string is long enough, a correlation peak value can be specified by computing an inner product. In this manner, an extracting apparatus in a simplified structure can be realized. 
     According to fifth and sixth aspects of the present invention, in the second and fourth aspects, respectively, 
     when the value computed by the correlation value computation portion is larger than a predetermined threshold value, the pseudo-random number string determination portion determines that pseudo-random number string which embedded in the string structured by the coefficients in the MRR is positively identified as being the pseudo-random number string. 
     As described above, in the fifth and sixth aspects, determination is made only by comparing an output value of the correlation value computation portion with a predetermined threshold value. Therefore, an extracting apparatus can be in a simplified structure. 
     According to seventh and eighth aspects of the present invention, in the first and third aspects, respectively, 
     on dividing the digital image signal into the coefficients in the plurality of frequency bands, the band division portion divides the digital image signal into a plurality of hierarchies, and 
     the information embedding portion embeds the pseudo-random number string in a string structured by every or some of the coefficients in a second or higher hierarchies in the MRR among the divided frequency bands. 
     According to ninth to twelfth aspects of the present invention, in the first, third, seventh and eighth aspect, respectively, 
     among the MRR, the information embedding portion embeds the pseudo-random number string in a string structured by every or some of coefficients in a region high in a horizontal frequency component and low in a vertical frequency component (HL region), and by every or some of coefficients in a region low in the horizontal frequency component and high in the vertical frequency component (LH region). 
     According to thirteenth to sixteenth aspects of the present invention, in the first, third, seventh and eighth aspects, respectively, 
     among the MRR, the information embedding portion embeds the pseudo-random number string in a string structured by every or some of coefficients in either an HL region or an LH region. 
     As described above, in the seventh to sixteenth aspects, embedded digital information can be retained even if the information is subjected to processing of cutting high frequency regions such as encoding. 
     A seventeenth aspect of the present invention is directed to a method of embedding/extracting digital information of a type embedding inherent digital information in a digital image signal, the method comprising the steps of: 
     dividing the digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band division; 
     mapping the inherent digital information to a pseudo-random number string; 
     embedding the pseudo-random number string in a string structured by the coefficients in every or some of the divided frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR); and 
     reconstructing the digital image signal in which the pseudo-random number string has been embedded by using the MRR and the MRA to which information embedding processing is subjected. 
     As described above, in the seventeenth aspect, inherent digital information can be embedded without causing image degradation or necessitating positional information thereof. 
     An eighteenth aspect of the present invention is directed to a method of embedding/extracting digital information of a type extracting inherent digital information embedded in a digital image signal, the method comprising the steps of: 
     receiving a reconstructed digital image signal obtained by dividing said digital image signal through discrete wavelet transform or sub-band division and by embedding a pseudo-random number string in a string structured by coefficients in every or some of a plurality of frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR), and then dividing the reconstructed digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band transform; 
     computing an inner product of the string structured by the coefficients in the MRR among the divided frequency bands and a predetermined pseudo-random number assumed to be embedded; 
     determining the pseudo-random number string embedded in the digital image signal according to the computed inner product; and 
     generating the inherent digital information mapped to the determined pseudo-random number string. 
     As described above, in the eighteenth aspect, embedded inherent digital information can be extracted in a simplified structure. 
     According to nineteenth and twentieth aspects, in the seventeenth and eighteenth aspects, respectively, 
     the pseudo-random number string is structured by numbers selected from normally-distributed average values of “0” and distribution values of “1”. 
     As described above, in the nineteenth and twentieth aspects, a pseudo-random number string can be easily generated in an arbitrary length, whereby an embedding apparatus can be in a simplified structure. Further, since numbers in a string are totaled to be 0 if the string is long enough, a correlation peak value can be specified by computing an inner product. In this manner, an extracting apparatus in a simplified structure can be realized. 
     According to twenty-first and twenty-second aspects of the present invention, in the eighteenth and twentieth aspects, respectively, 
     when the computed inner product is larger than a predetermined value, in the pseudo-random number string determination step, pseudo-random number string which embedded in the string structured by the coefficients in the MRR is positively determined as being the pseudo-random number string. 
     As described above, in the twenty-first and twenty-second aspects, determination is made only by comparing an inner product obtained through computation with a predetermined threshold value. Therefore, an extracting apparatus in a simplified structure can be realized. 
     According to twenty-third and twenty-fourth aspects of the present invention, in the seventeenth and nineteenth aspect, respectively, 
     in the band division step, the digital image signal is divided into a plurality of hierarchies when being divided into coefficients in a plurality of frequency bands, and 
     in the pseudo-random number string embedding step, the pseudo-random number string is embedded in a string structured by every or some of the coefficients in a second or higher hierarchies in the MRR among the divided frequency bands. 
     According to twenty-fifth to twenty-eighth aspects of the present invention, in the seventeenth, nineteenth, twenty-third and twenty-fourth aspects, respectively, 
     in the pseudo-random number string embedding step, among the MRR, the pseudo-random number string is embedded in a string structured by every or some of coefficients in an HL region, and by every or some of coefficients in an LH region. 
     According to twenty-ninth to thirty-second aspects of the present invention, in the seventeenth, nineteenth, twenty-third and twenty-fourth aspects, respectively, 
     in the pseudo-random number string embedding step, among the MRR, the pseudo-random number string is embedded in a string structured by every or some of coefficients in either a region high in a horizontal frequency component and low in a vertical frequency component (HL region) or a region low in the horizontal frequency component and high in the vertical frequency component (LH region). 
     As described above, in the twenty-third to thirty-second aspects, respectively, embedded digital information can be retained even if the information is subjected to processing of cutting high frequency regions such as encoding. 
     A thirty-third aspect of the present invention is directed to a recording medium on which a program to be run in a computer device is recorded, the program being for realizing in the computer device an operational environment comprising the steps of: 
     dividing a digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band division; 
     mapping inherent digital information to a pseudo-random number string; 
     embedding the pseudo-random number string in a string structured by coefficients in every or some of the divided frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR); and 
     reconstructing the digital image signal in which the pseudo-random number string has been embedded by using the MRR and the MRA to which information embedding processing is subjected. 
     A thirty-fourth aspect of the present invention is directed to a recording medium on which a program to be run in a computer device is recorded, the program being for realizing an operational environment in the computer device comprising the steps of: 
     receiving a reconstructed digital image signal obtained by dividing said digital image signal through discrete wavelet transform or sub-band division and by embedding a pseudo-random number string in a string structured by coefficients in every or some of a plurality of frequency bands exclusive of a lowest frequency band referred to as MRA (hereinafter, referred to as MRR), and then dividing the reconstructed digital image signal into coefficients in a plurality of frequency bands through discrete wavelet transform or sub-band transform; 
     computing an inner product of the string structured by the coefficients in the MRR among the divided frequency bands and a predetermined pseudo-random number assumed to be embedded; 
     determining the pseudo-random number string embedded in the digital image signal according to the computed inner product; and 
     generating the inherent digital information mapped to the determined pseudo-random number string. 
     According to thirty-fifth and thirty-sixth aspects of the present invention, in the thirty-third and thirty-fourth aspects, respectively, 
     the pseudo-random number string is structured by numbers selected from normally-distributed average values of “0” and distribution values of “1”. 
     According to thirty-seventh and thirty-eighth aspect, in the thirty-fourth and thirty-sixth aspect, 
     when the computed inner product is larger than a predetermined value, in the pseudo-random number string determination step, pseudo-random number string which embedded in the string structured by the coefficients in the MRR is positively determined as being the pseudo-random number string. 
     According to thirty-ninth and fortieth aspect, in the thirty-third and thirty-fifth aspects, 
     in the band division step, the digital image signal is divided into a plurality of hierarchies when being divided into coefficients in a plurality of frequency bands, and 
     in the pseudo-random number string embedding step, the pseudo-random number string is embedded in a string structured by every or some of the coefficients in a second or higher hierarchies in the MRR among the divided frequency bands. 
     According to forty-first to forty-fourth aspects, in the thirty-third, thirty-fifth, thirty-ninth and fortieth aspects, respectively, 
     in the pseudo-random number string embedding step, among the MRR, the pseudo-random number string is embedded in a string structured by every or some of coefficients in an HL region, and by every or some of coefficients in an LH region. 
     According to a forty-fifth to a forty-eighth aspects, in the thirty-third, thirty-fifth, thirty-ninth and fortieth aspects, respectively, 
     in the pseudo-random number string embedding step, among the MRR, the pseudo-random number string is embedded in a string structured by every or some of coefficients in either an HL region or an LH region. 
     As described above, the thirty-third to forty-eighth aspects are directed to a recording medium on which a program for carrying out the method of embedding and extracting digital information in the seventeenth to thirty-second aspects is recorded. The recording medium is to provide the method of embedding and extracting digital information in the seventeenth to thirty-second aspects to any existing apparatus as a software. 
    
    
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of a digital information embedding apparatus  1   a  according to a first embodiment of the present invention; 
     FIG. 2 is a block diagram exemplarily showing the detailed structure of a band division portion  11  in FIG. 1; 
     FIG. 3 is a block diagram exemplarily showing the detailed structure of a first band dividing filter  100  in FIG. 2; 
     FIG. 4 is a diagram illustrating, in a two-dimensional frequency region, signals subjected to discrete wavelet transform by the band division portion  11  in FIG. 1; 
     FIG. 5 is a block diagram exemplarily showing the detailed structure of an information embedding portion  13  in FIG. 1; 
     FIG. 6 is a block diagram exemplarily showing the detailed structure of a band synthesis portion  14  in FIG. 1; 
     FIG. 7 is a block diagram exemplarily showing the detailed structure of a first band synthesis filter  400  in FIG. 6; 
     FIG. 8 is a block diagram showing the structure of a digital information extracting apparatus  1   b  according to a second embodiment of the present invention; 
     FIG. 9 is a diagram showing the structure of a system in which digital information embedding/extracting program is operated; 
     FIG. 10 is a block diagram showing the structure of a conventional apparatus for embedding an embedding value; and 
     FIG. 11 is a block diagram showing the structure of a conventional apparatus for extracting an embedding value. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 is a block diagram showing the structure of a digital information embedding apparatus according to a first embodiment of the present invention. In FIG. 1, the digital information embedding apparatus  1   a  is provided with a band division portion  11 , a mapping portion  12 , an information embedding portion  13 , and a band synthesis portion  14 . Hereinafter, it is stepwise described how the digital information embedding apparatus  1   a  is operated by further referring to FIGS. 2 to  7 . 
     First, by referring to FIGS. 2 to  4 , it is described how a signal is subjected to discrete wevelet transform in the band division portion  11 . After the transform, a band thereof is hierarchically divided into three. FIG. 2 is a block diagram exemplarily showing the detailed structure of the band division portion  11  in FIG.  1 . In FIG. 2, the band division portion  11  is provided with first to third band dividing filters  100 ,  200  and  300 , all of which are equal in structure. By going through each of the first to third band dividing filters  100 ,  200  and  300 , an image signal is divided into four frequency bands, and wavelet coefficients are then determined for every frequency band. Also, as to coefficients, sub-band division will do. 
     The band division portion  11  first receives a digital image signal  71  in the first band dividing filter  100 . Therein, the digital image signal  71  is divided into four signals varied in bands, i.e., an LL 1  signal, LH 1  signal, HL 1  signal, and HH 1  signal (hereinafter, referred collectively to as first hierarchical signal) on the basis of parameters of horizontal and vertical frequency components. The second band dividing filter  200  receives only the LL 1  signal in the lowest band, and then again divides the signal into four signals varied in bands, i.e., an LL 2  signal, LH 2  signal, HL 2  signal, and HH 2  signal (hereinafter, referred collectively to as second hierarchical signal). Then, the third band dividing filter  300  receives only the LL 2  signal in the lowest band, and then again divides the signal into four signals varied in bands, i.e., an LL 3  signal, LH 3  signal, HL 3  signal, and HH 3  signal (hereinafter, referred collectively to as third hierarchical signal). 
     FIG. 3 is a block diagram exemplarily showing the detailed structure of the first band dividing filter  100  in FIG.  2 . In FIG. 3, the first band dividing filter  100  is provided with first to third two-band division portions  101  to  103 . These first to third two-band division portions  101  to  103  are each provided with one-dimensional low-pass filters (LPF)  111  to  113 , one-dimensional high-pass filters (HPF)  121  to  123 , and down-samplers  131  to  133  and  141  to  143  for decimating the signal at a ratio of 2:1. 
     The first two-band division portion  101  receives the digital image signal  71 , filters any horizontal component thereof through both the LPF  111  and HPF  121 , and then outputs two signals. Thereafter, the first two-band division portion  101  decimates the filtered signals at a rate of 2:1, respectively, by using the downsampler  131  and  141 , and then outputs the signals to the next stage. The second two-band division portion  102  receives the signal from the downsampler  131 , and then filters any vertical component thereof through both the LPF  112  and HPF  122 . Thereafter, the second two-band division portion  102  decimates the filtered signals at a rate of 2:1, respectively, by using the downsamplers  132  and  142 , and then outputs two signals as LL 1  and LH 1 . The third two-band division portion  103  receives the signal from the downsampler  141 , and then filters any vertical frequency component thereof through both the LPF  113  and HPF  123 . Then, the third two-band division portion  103  decimates the signals at a rate of 2:1, respectively, by using the downsamplers  133  and  143 , and then outputs two signals as HL 1  and HH 1 . 
     In this manner, the first band dividing filter  100  outputs four signals, i.e., the LL 1  signal low in both horizontal and vertical components, the LH 1  signal low in horizontal but high in vertical, the HL 1  signal high in horizontal but low in vertical, and the HH 1  signal high in both. The four signals are, in other words, wavelet coefficients. The second and third band dividing filters  200  and  300  treat any incoming signal in a similar manner to the above. 
     After going through the first to third band dividing filters  100 ,  200  and  300 , the digital image signal  71  is divided into  10  band signals, i.e., LL 3 , LH 3 , HL 3 , HH 3 , LH 2 , HL 2 , HH 2 , LH 1 , HL 1 , and HH 1 . 
     FIG. 4 is a diagram illustrating these ten band signals in a two-dimensional frequency region. In FIG. 4, the vertical axis represents a vertical frequency component, which increases as is directed downward, and the horizontal axis represents a horizontal frequency component, which increases as is directed rightward. Each area in FIG. 4 is data serving as one image, and a ratio of area sizes is equivalent to that of the number of data in the band signals. In detail, in a case where the number of data in the LL 3 , LH 3 , HL 3 , and HH 3  being the third hierarchical signals is “1”, the number of data in the LH 2 , HL 2  and HH 2  being the second hierarchical signals is “4”, and the number of data in the LH 1 , HL 1  and HH 1  being the first hierarchical signals is “16”. 
     Next, it is described how the mapping portion  12  is operated. The mapping portion  12  generates a pseudo-random number string corresponding to inherent digital information. The pseudo-random number string is generated by randomly selecting numbers from a population constituted by normally-distributed average values of “0” and distribution values of “1”. It is preferable if the mapping portion  12  is set to select a pseudo-random number string unique to the inherent digital information. Herein, the mapping portion  12  stores a table showing the correspondence between the inherent digital information and the pseudo-random number string. In a case where the inherent digital information is information on a copyright holder including his/her name and the date and time of creation of works, the table shows the correspondence between such information and the pseudo-random number string. The table is structured not to include any identical pseudo-random number string. Accordingly, on receiving any inherent digital information, e.g., information on a copyright holder, the mapping portion  12  refers to the table to output a pseudo-random number string. 
     Next, by referring to FIG. 5, it is described how the information embedding portion  13  is operated. FIG. 5 is a block diagram exemplarily showing the detailed structure of the information embedding portion  13  in FIG.  1 . In FIG. 5, the information embedding portion  13  is provided with an absolute value computation portion  31 , a delaying device  32 , a multiplier  33 , and an adder  34 . 
     The information embedding portion  13  reads the wavelet coefficients of the LH 2  signal in FIG. 2 in a predetermined order from MRR of the signal divided in the band division portion  11 , and then provides the same to the absolute value computation portion  31  and the delaying device  32 . The absolute value computation portion  31  takes an absolute value of the received wavelet coefficients so as to output the same to the multiplier  33 . The delaying device  32  has the received wavelet coefficients delayed so as to output the same to the adder  34 . On the other hand, the pseudo-random number string Wi outputted from the mapping portion  12  is provided to the multiplier  33 . The multiplier  33  multiplies the output of the absolute value computation portion  31  by the pseudo-random number string Wi, and then further multiplies the resultant value by the scaling parameter α. The adder  34  receives both outputs of the multiplier  33  and the delaying device  32 , and then adds the outputs to output. In this example, the above-described processing can be expressed by an equation (7), where fi is the wavelet coefficient, |fi| is the absolute value of fi, Wi is the pseudo-random number string outputted from the mapping portion  12 , and fi′ is the wavelet coefficient subjected to embedment. Note that a herein is an integer smaller than 1. 
     
       
           f   i   ′+f   i   +αW   i   (7) 
       
     
     After the processing subjected to the wavelet coefficients of the LH 2  signal is completed, the information embedding portion  13  reads the wavelet coefficients of the LH 3  signal in a predetermined order, and then provides the same to the absolute value computation portion  31  and the delaying device  32 . Thereafter, the processing is carried out in a similar manner to the wavelet coefficients of the LH 2  signal. Note that, although the wavelet coefficients of the LH 2  signal are treated before those of the LH 3  signal in this example, the order may be inverted. Herein, the order in which the LH 2  and LH 3  signals are treated and the predetermined order for reading each wavelet coefficients thereof are both key information for extraction. The information is also used to extract inherent digital information. 
     Next, by referring to FIG. 6, it is described how the band synthesis portion  14  is operated. FIG. 6 is a block diagram exemplarily showing the detailed structure of the band synthesis portion  14  in FIG.  1 . In FIG. 6, the band synthesis portion  14  is provided with first to third band synthesis filters  400 ,  500  and  600 , all of which are equal in structure. These first to third band synthesis filters  400 ,  500  and  600  each receives four signals varied in frequency bands, and then synthesizes the signals to output as one signal. 
     The first band synthesis filter  400  receives the LL 3  signal, HL 3  and HH 3  signals, and the LH 3  signal in which the pseudo-random number string has been embedded, and then synthesizes these signals to generate the LL 2  signal. The second band synthesis filter  500  receives the synthesized LL 2  signal, the HL 2  signal and the HH 2  signal, and the LH 2  signal in which the pseudo-random number string has been embedded, and then synthesizes these signals to generate the LL 1  signal. Thereafter, the third band synthesis filter  600  receives the synthesized LL 1  signal, and the HL 1  signal, the HH 1  signal and the LH 1  signal, and then synthesizes these signals to reconstruct the digital image signal  72 . 
     FIG. 7 is a block diagram exemplarily showing the detailed structure of the first band synthesis filter  400  in FIG.  6 . In FIG. 7, the first band synthesis filter  400  is provided with first to third two-band synthesis portions  401  to  403 . These first to third two-band synthesis portions  401  to  403  are each provided with LPFs  411  to  413 , HPFs  421  to  423 , and upsamplers  431  to  433  and  441  to  443  for inserting zero to the signal at a ratio of 2:1, and adders  451  to  453 . 
     The first two-band synthesis portion  401  receives the LL 3  and LH 3  signals, and then converts the signals into signals twice in size (length) by using the upsamplers  431  and  441 , respectively. Then, any vertical component of the converted two signals is filtered through the LPF  411  and HPF  421 , respectively, and then the filtered two signals are added to output. The second two-band synthesis portion  402  receives the HL 3  and HH 3  signals, and then converts the two signals into signals twice in size (length) by using the upsamplers  432  and  442 . Thereafter, any vertical component of the converted two signals is filtered through the LPF  412  and HPF  422 , respectively, and then the filtered two signals are added to output. The third two-band synthesis portion  403  receives outputs from the adders  451  and  452 , and then respectively converts the outputs into two signals twice in size (length) by using the upsamplers  433  and  443 . Then, any horizontal component of the converted two signals is filtered through the LPF  413  and HPF  423 , and then the filtered signals are added to output. 
     Accordingly, the first band synthesis filter  400  outputs the LL 2  signal low in both horizontal and vertical components, which is the second hierarchical signal. Note that, the second and third band synthesis filters  500  and  600  treat any incoming signal similarly to the above. 
     In such manner, the band synthesis portion  14  reconstructs, before outputting, the ten frequency band signals (LL 3 , LH 1 , LH 2 , LH 3 , HL 1 , HL 2 , HL 3 , HH 1 , HH 2  and HH 3 ) to the digital image signal  72  to which embedment has already been done. 
     As is known from the above, according to the digital information embedding apparatus  1   a  of the first embodiment, a signal is hierarchically divided into three bands, and then each absolute value of the wavelet coefficients of the LH 2  and LH 3  signals, among MRR, is multiplied by a pseudo-random number string in an arbitrary order. Thereafter, the resultant values are respectively multiplied by a scaling parameter, and then the wavelet coefficients of the LH 2  and LH 3  signals are added thereto, respectively. Accordingly, the pseudo-random number string is embedded. In this manner, the embedded pseudo-random number string can be retained even if the data is subjected to encoding. It means that inherent digital information corresponding to the pseudo-random number string is retained, and a digital information embedding apparatus can thus be realized in a simple structure. 
     Discrete wavelet transform performed in the digital information embedding apparatus  1   a  of the first embodiment is not limited to three hierarchies. The signal can be divided into more hierarchies until the LL signal reaches a 1 by 1 element. Further, bands for embedding are not limited to the LH 2  and LH 3  signals, but may be arbitrarily selected among MRR or may be MRR in its entirety. If this is the case, the order in which the wavelet coefficients thereof are subjected to processing is arbitrary, but should be determined in advance. 
     Second Embodiment 
     FIG. 8 is a block diagram showing the structure of a digital information extracting apparatus according to a second embodiment of the present invention. The digital information extracting apparatus  1   b  of the second embodiment is for extracting the digital information embedded by the digital information embedding apparatus  1   a  of the first embodiment. 
     In FIG. 8, the digital information extracting apparatus  1   b  of the second embodiment is provided with the band division portion  11 , a correlation value computation portion  21 , a pseudo-random number string determination portion  22 , and an information generating portion  23 . The band division portion  11  of the digital information extracting apparatus  1   b  is structurally the same as the band division portion  11  of the digital information embedding apparatus  1   a , and is provided with the same reference numeral not to be described again. 
     The band division portion  11  receives a digital image signal  73 . The digital image signal  73  is the digital image signal  72  outputted from the band synthesis portion  14  of the digital information embedding apparatus  1   a , or a signal encoded or decompressed. The band division portion  11  subjects the received digital image signal  73  to discrete wavelet transform to divide the same into ten frequency band signals (LL 3 , LH 1 , LH 2 , LH 3 , HL 1 , HL 2 , HL 3 , HH 1 , HH 2  and HH 3 ), and computes wavelet coefficients for every signal. Herein, the band division portion  11  outputs, to the correlation value computation portion  21 , the wavelet coefficients of the LH 2  and LH 3  signals, among MRR, in the same order as the first embodiment, i.e., first of the LH 2  signal and then of the LH 3  signal. The wavelet coefficients thereof are in a string in the same order as the first embodiment. On the other hand, a pseudo-random number string assumed to be embedded in the digital image signal  73  (hereinafter, referred to as assumed pseudo-random number string) is provided to the correlation value computation portion  21 . The correlation value computation portion  21  computes an inner product of the string structured by the wavelet coefficients of the LH 2  and LH 3  signals and the assumed pseudo-random number string, and then divides the value by the length of the assumed pseudo-random number string. The resultant value can be expressed by an equation (8), where fi * is the wavelet coefficient string structured by the wavelet coefficients of the LH 2  and LH 3  signals (considered to be slightly different from fi′ depending on processing or due to tampering), and Vi is the assumed pseudo-random number string.              z   =       1   M            ∑     i   =   1     M            f   i   *          V   i                   (   8   )                                
     Herein, M denotes the length of the wavelet coefficient string fi* and the assumed pseudo-random number string Vi. In a case where the digital image signal  73  is 512 pixels by 512 pixels, the maximum value of M is “20480”. In the equation (8), when the assumed pseudo-random number string Vi is equal to the pseudo-random number string Wi in the equation (7) (Vi=Wi), the value of z will be larger than a case their not being equal (Vi≠Wi). 
     An output of the correlation value computation portion  21  is provided to the pseudo-random number string determination portion  22 . The pseudo-random number string determination portion  22  compares a predetermined threshold value with the received output. The threshold value used therein is obtained by an equation (9) next below.                S   Z     =       α   pM            ∑     i   =   1     M                 f   i   *               (     p   ≧   2     )                   (   9   )                                
     In the equation (9), a is the scaling parameter applied in the first embodiment, and p is an integer equal to or larger than two. 
     When the output of the correlation value computation portion  21  is larger than the predetermined threshold value, the pseudo-random number string determination portion  22  notifies the information extraction portion  23  that the pseudo-random number string used in the correlation value computation portion  21  (i.e., the wavelet coefficient string structured by the wavelet coefficients of the LH 2  and LH 3  signals) is positive. In response thereto, with the pseudo-random number string notified as being positive, the information extraction portion  23  extracts inherent digital information mapped thereto. The mapping is done by using the same correspondence table as stored in the mapping portion  12  in the first embodiment. On the other hand, when the output of the correlation value computation portion  21  is smaller than the predetermined threshold value in the pseudo-random number string determination portion  22 , by using the assumed pseudo-random number string, the information extraction portion  23  extracts inherent digital information mapped thereto. Note that, when no assumed pseudo-random number string is determined to be positive enough, the pseudo-random number string determination portion  22  determines that the digital image signal has no pseudo-random number string embedded. 
     As is known from the above, according to the digital information extracting apparatus  1   b  of the second embodiment, an inner product of an embedded wavelet coefficient string in a predetermined frequency band and a pseudo-random number string assumed to be embedded by the digital information embedding apparatus  1   a  is first computed to obtain a correlation value. Thereafter, the correlation value is compared with a predetermined threshold value to determine whether or not the pseudo-random number string is positive, and then inherent digital information is extracted. In this manner, inherent digital information can be extracted through an easy operation. What is better, a third party may have little clue as to the inherent digital information if he/she has no information embedded in a predetermined frequency band. Such information includes, to be more specific, wavelet coefficients in use, an order in which the wavelet coefficients are structured in a string, and the length of a pseudo-random number string. 
     Note that, the digital information extracting apparatus lb of the second embodiment obtains a correlation value by dividing an inner product of a wavelet coefficient string in a frequency band and a pseudo-random number string by the length thereof. However, only the inner product is sufficient to obtain a correlation value. 
     Further, a signal used in the digital information embedding and extracting apparatuses of the first and second embodiments may be a digital image signal, specifically, may be a still image or a moving image. With a moving image signal structured by 30 frames per second, for example, digital information may be embedded in or extracted from every frame or every five frame, for example, in the aforementioned manner. 
     Typically, the processing executed respectively by the digital information embedding and extracting apparatuses of the first and second embodiments is realized as a computer program (hereinafter, referred to as digital information embedding/extracting program). FIG. 9 is a diagram showing the structure of a system in which the digital information embedding/extracting program is operated. In FIG. 9, a CPU  51  controls the program operation. The program or various types of data is stored in a main storage  52 . This digital information embedding/extracting program is stored in a recording medium  53 . The recording medium  53  may be in any type as long as the medium is readable/recordable, such as floppy disk or MO disk. Further, the recording medium  53  may be incorporated into a hard disk in advance, for example, and is not required to be portable. In the system shown in FIG. 9, the digital information embedding/extracting program is stored in the main storage  52 , and is operated under the control of the CPU  51 . Various types of provisional data required for the processing is kept in the main storage  52 . The table showing the correspondence between the inherent digital information and the pseudo-random number string used in the mapping portion  12  is stored in the recording medium  53 . Herein, the digital information embedding/extracting program and the correspondence table may be stored in any location as long as each location can be specified. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.