Abstract:
Provided are an image file distribution apparatus, an image file recovery apparatus, an image file distribution method, an image file recovery method, an image file distribution program, an image file recovery program, and a recording medium storing the program which can prevent a relatively large increase in the amount of data of an image file even when an (k, n) secret sharing scheme with high security is used. For example, distributed tag information is obtained from tag information of the image file by a (k, n)-threshold secret sharing scheme. For example, distributed image data is obtained from image data by a (k, L, n)-threshold ramp secret sharing scheme. For example, the distributed tag information and the distributed image data are combined to obtain combined data. Since the amount of data in the tag information is small, the use of the (k, n) secret sharing scheme does not cause a large increase in the amount of data. Since the (k, L, n)-threshold ramp secret sharing scheme does not cause a large increase in the amount of data, an increase in the total amount of data in the image data is relatively small.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of PCT International Application No. PCT/JP2016/057909 filed on Mar. 14, 2016, which claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2015-058907 filed Mar. 23, 2015. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The present invention relates to an image file distribution apparatus, an image file recovery apparatus, an image file distribution method, an image file recovery method, an image file distribution program, an image file recovery program, and a recording medium storing the program. 
       2. Description of the Related Art 
       [0003]    When a very large amount of data is stored in storage device, the cost of the storage device increases. Therefore, the cost of storing data can be reduced by a cloud storage server that is provided on the Internet. However, it is pointed out that the cloud storage server has a security problem. In particular, when a cloud is used, the task is to secure data. For example, the following have been proposed in order to improve security: a technique that distributes a plurality of partial data items to a plurality of data centers and stores the partial data items in the data centers, using a secret sharing scheme (JP2013-020313A); and an apparatus that backs up data (JP2007-102672A 2). 
         [0004]    There are various types of secret sharing schemes. A (k, n)-threshold secret sharing scheme (A. Shamir. “How to Share a Secret”, Comm. Assoc. Comput. Mach., Vol. 22, no. 11, pp. 612-613 (Nov. 1979)) distributes secret information into n distributed data items, collects k (k is equal to or greater than 2 and equal to or less than n) distributed data items among the n distributed data items, and recovers the k distributed data items. A (k, n)-threshold secret sharing scheme is a perfect secret sharing scheme and has information-theoretic security. Therefore, even when k−1 distributed data items less than a threshold value are acquired, a clue about secret information does not leak. However, in the (k, n)-threshold secret sharing scheme, the size of the divided data is equal to the size of the original secret data. Therefore, the (k, n)-threshold secret sharing scheme has the problem that the total amount of divided. data is n times more than the amount of original secret data and the amount of data increases. In order to solve the problem of the (k, n)-threshold secret sharing scheme, a (k, L, n)-threshold ramp secret sharing scheme (Hirosuke Yamamoto, “(k, L, n)-Threshold Secret Sharing System”, Transactions of the Institute of Electronics and Communication Engineers of Japan, vol. 168-A, No. 9, pp. 945-952, 1985) that can reduce the amount of divided data has been proposed. The (k, L, n)-threshold ramp secret sharing scheme (Hirosuke Yamamoto, “On Secret Sharing Systems Using (k L n) Threshold. Scheme”, Transactions of the institute of Electronics and Communication Engineers of Japan, vol. 168-A, No. 9, pp. 945-952, 1985) compresses the size of distributed data to 1/L instead of reducing security (information-theoretic security up to k-L) a little. The (k, L, n)-threshold ramp secret sharing scheme is the same as the (k, n)-threshold secret sharing scheme in that it collects k or more divided data items among n divided data items and recovers the original secret data. 
       SUMMARY OF THE INVENTION 
       [0005]    In contrast, in a case in which the number of collected divided data items is equal to or less than (k−L) (1≦L≦k), it is difficult to completely recover and estimate secret data from the divided data. However, in a case in which the number of collected divided data items is less than k and is greater than (k-L), the information of secret data is partially obtained. As a result, the (k, L, n)-threshold ramp secret sharing scheme has the problem that security is lower than that in the (k, n)-threshold secret sharing scheme. 
         [0006]    An object of the invention is to prevent a relatively large increase in the amount of data even when a (k, n)-threshold secret sharing scheme with high security is used. 
         [0007]    According to a first aspect of the invention, there is provided an image file distribution apparatus comprising: a (k, n)-threshold secret sharing device {(k, n)-threshold secret sharing means} for distributing tag information recorded in a tag information recording region of an image file into a plurality of first distributed tag information items, using a (k, n)-threshold secret sharing scheme; a (k, L, n)-threshold ramp secret sharing device {(k, L, n)-threshold ramp secret sharing means} for distributing image data recorded in an image data recording region of the image tile into a plurality of second distributed image data items, using a (k, L, n)-threshold ramp secret sharing scheme; a distributed tag information transmission device (distributed tag information transmission means) for transmitting each of the plurality of first distributed tag information items distributed by the (k, n)-threshold secret sharing device to different storage servers, and a distributed image data transmission device (distributed image data transmission means) for transmitting each of the plurality of second distributed image data items distributed by the (k, L, n)-threshold ramp secret sharing device to different storage servers. 
         [0008]    The first aspect of the invention also provides an image file distribution method. That is, this method comprises: allowing a (k, n)-threshold secret sharing device to distribute tag information recorded in a tag information recording region of an image file into a plurality of first distributed tag information items, using a (k, n)-threshold secret sharing scheme; allowing a (k, L, n)-threshold ramp secret sharing device to distribute image data recorded in an image data recording region of the image file into a plurality of second distributed image data items, using a (k, L, n)-threshold ramp secret sharing scheme; allowing distributed tag information transmission to transmit each of the plurality of first distributed tag information items distributed by the (k, n)-threshold secret sharing device to different storage servers; and allowing a distributed image data transmission device to transmit each of the plurality of second distributed image data items distributed by the (k, L, n)-threshold ramp secret sharing device to different storage servers. In addition, the first aspect of the invention provides a computer readable program that controls a computer of an image file distribution apparatus and a (non-transitory) recording medium that stores the program. 
         [0009]    The image file distribution apparatus may further comprise a combination device (combination means) for combining each of the distributed tag information items distributed by the (k, n)-threshold secret sharing device with each of the distributed image data items distributed by the (k, L, n)-threshold ramp secret sharing device to generate combined data. In this case, the distributed tag information transmission device and the distributed image data transmission device transmit the combined data generated by the combination device to different storage servers. 
         [0010]    The number of first distributed tag information items distributed by the (k, n)-threshold secret sharing device may be different from the number of second distributed image data items distributed by the (k, L, n)-threshold ramp secret sharing device. 
         [0011]    For example, the distributed image data transmission device transmits the plurality of second distributed image data items after the distributed tag information transmission device transmits the plurality of first distributed tag information items. 
         [0012]    The image file distribution apparatus may further comprise: a determination device (determination device) for determining whether a rate of utilization of a communication line which is used to transmit the plurality of second distributed image data items by the distributed image data transmission device is less than a threshold value. In this case, as the determination device determines that the rate of utilization of the communication line is less than the threshold value, the distributed image data transmission device transmits the plurality of second distributed image data items. 
         [0013]    The image file distribution apparatus may further comprise a distributed tag information storage device (distributed tag information storage means) for storing at least one of the plurality of first distributed tag information items distributed by the (k, n)-threshold secret sharing device. In this case, the distributed tag information transmission device transmits each of the distributed tag information items other than the distributed tag information item stored in the distributed tag information storage device among the plurality of first distributed tag information items to different storage servers. 
         [0014]    For example, the image file is a DICOM tile. 
         [0015]    The storage server to which the distributed tag information is transmitted by the distributed tag information transmission device may be different from the storage server to which the distributed image data is transmitted by the distributed image data transmission device. 
         [0016]    According to a second aspect of the invention, there is provided an image file recovery apparatus comprising: a distributed tag information reading device for reading a plurality of first distributed tag information items which have been obtained by distributing tag information recorded in a tag information recording region of an image file, using a (k, n)-threshold secret sharing scheme, and have been stored in different storage servers; a distributed image data reading device for reading a plurality of second distributed image data items which have been obtained by distributing image data recorded in an image data recording region of the image file, using a (k, L, n)-threshold ramp secret sharing scheme, and have been stored in different storage servers; a distributed tag information recovery device (distributed tag information recovery means) for recovering the plurality of first distributed tag information items read by the distributed tag information reading device, using the (k, n)-threshold secret sharing scheme; and a distributed image data recovery device (distributed image data recovery means) for recovering the plurality of second distributed image data items read by the distributed image data reading device, using the (k, L, n)-threshold ramp secret sharing scheme. 
         [0017]    The second aspect of the invention also provides an image file recovery method. That is, this method comprises: allowing a distributed tag information reading device to read a plurality of first distributed tag information items which have been obtained by distributing tag information recorded in a tag information recording region of an image file, using a (k, n)-threshold secret sharing scheme, and have been stored in different storage servers; allowing a distributed image data reading device to read a plurality of second distributed image data items which have been obtained by distributing image data recorded in an image data recording region of the image tile, using a (k, L, n)-threshold ramp secret sharing scheme, and have been stored in different storage servers; allowing a distributed tag information recovery device to recover the plurality of first distributed tag information items read by the distributed tag information reading device, using the (k, n)-threshold secret sharing scheme; and allowing a distributed image data recovery device to recover the plurality of second distributed image data items read by the distributed image data reading device, using the (k, L, n)-threshold ramp secret sharing scheme. In addition, the second aspect of the invention provides a program that can be read by a computer of an image file recovery apparatus and a (non-transitory) recording medium that stores the program. 
         [0018]    The image file recovery apparatus may further comprise an image data reading stop device (image data reading stop means) for stopping the reading of the plurality of second distributed image data items by the distributed image data reading device. 
         [0019]    The image file recovery apparatus may further comprise an image file generation device (image file generation means) for generating the image file from the distributed tag information recovered by the distributed tag information recovery device and the distributed image data recovered by the distributed image data recovery device. 
         [0020]    In this case, the image file is, for example, a DICOM file. 
         [0021]    According to the first aspect of the invention, the tag information of the image file is distributed into a plurality of first distributed tag information items by the (k, n)-threshold secret sharing scheme. The plurality of first distributed tag information items are transmitted to different storage servers and are stored in the storage servers. In addition, the image data of the image file is distributed into a plurality of second image data items by the (k, L, n)-threshold ramp secret sharing scheme. The plurality of second image data items are transmitted to different storage servers and are stored in the storage servers. The security of the (k, n)-threshold secret sharing scheme is higher than that of the (k, L, n) threshold ramp sharing scheme. The amount of data in the (k, n)-threshold secret sharing scheme is more than that in the (k. L, n)-threshold ramp secret sharing scheme. In contrast, the security of the (k, L, n)-threshold ramp secret sharing scheme is lower than that of the (k, n)-threshold secret sharing scheme. The amount of data after distribution in the (k, L, n)-threshold ramp secret sharing scheme is less than that in the (k, n)-threshold secret sharing scheme. According to the invention, the image data that originally has a large size is distributed by the (k, L, n)-threshold ramp secret sharing scheme in which the amount of data after distribution is small. Therefore, it is possible to suppress a relatively large increase in the amount of data. Since tag information is more important than image data, the tag information is distributed by the (k, n) sharing scheme with high security such that data does not leak. Since the original amount of data in the tag information is small, the tag information is distributed by the (k, n) sharing scheme. In this case, even when the amount of data is large, the rate of increase in the amount of data is less than that in a case in which image data is distributed by the (k, n) sharing scheme. Therefore, it is possible to guarantee the security of important tag information while suppressing an increase in the amount of data after distribution. 
         [0022]    According to the second aspect of the invention, the distributed tag information and the distributed image data distributed by the first aspect of the invention are read. The read. distributed tag information and the read distributed image data are recovered. The tag information and the image data before distribution are obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a block diagram illustrating the electric configuration of an image file distribution/recovery apparatus. 
           [0024]      FIG. 2  illustrates the file structure of a DICOM file. 
           [0025]      FIG. 3  illustrates an aspect in which an image file is distributed. 
           [0026]      FIG. 4  is a table storing the address of a recording destination of combined data. 
           [0027]      FIG. 5  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0028]      FIG. 6  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0029]      FIG. 7  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0030]      FIG. 8  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0031]      FIG. 9  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0032]      FIG. 10  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
           [0033]      FIG. 11  illustrates an aspect in which the distributed combined data is recovered to the image file. 
           [0034]      FIG. 12  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  illustrates an embodiment of the invention and is a block diagram mainly illustrating the electric configuration of an image file distribution/recovery apparatus  10 .  FIG. 2  illustrates the file structure of a digital imaging and communication in medicine (DICOM) file  40 .  FIG. 3  illustrates an aspect in Which the DICOM file  40  is distributed by the image file distribution/recovery apparatus. 
         [0036]    The image file distribution/recovers apparatus  10  (an image file distribution apparatus and an image file recovery apparatus) distributes an image file using a secret sharing scheme. The image file distribution/recovery apparatus IC) according to this embodiment can perform distribution, using a (k, n)-threshold secret sharing scheme and a (k, L, n)-threshold ramp secret sharing scheme. The (k, n)-threshold secret sharing scheme distributes secret information into n distributed data items, collects k (k is equal to or greater than 2 and equal to or less than n) distributed data items among then distributed data items, and recovers the k distributed data items. The (k, n)-threshold secret sharing scheme is a perfect secret sharing scheme and has information-theoretic security. Therefore, even when k−1 distributed data items less than a threshold value are acquired, a clue about secret information does not leak. The (k, L, n)-threshold ramp secret sharing scheme compresses the size of distributed data to while reducing security (information-theoretic security up to k−L) a little. In these sharing schemes, n is a distribution number and k is a threshold value. 
         [0037]    The (k, n)-threshold secret sharing scheme includes, for example, any type of scheme, such as a secret sharing scheme, a threshold sharing scheme, a secret division scheme, a. threshold value division scheme, or a threshold secret division scheme as long as it distributes secret information into n distributed data items, collects k (k is equal to or greater than 2 and equal to or less than n) distributed data items among the n distributed data items, and recovers the k distributed data items. Similarly, the (k, L, n)-threshold ramp secret sharing scheme includes any type of scheme as long as it compresses the size of distributed data to 1/L while reducing security (information-theoretic security up to k-L) a little. 
         [0038]    The image file distribution/recovery apparatus  10  can communicate with p personal computers l to p through a network and can communicate with m storage servers  31  to  3   m  through the network. 
         [0039]    The overall operation of the image file distribution/recovery apparatus  10  is controlled by a control device  11 . 
         [0040]    The image file distribution/recovery apparatus  10  includes a compact disc read only memory (CD-ROM) drive  23 . When a compact disc read only memory (CD-ROM)  24  (recording medium) storing a program, which will be described below, is inserted into the image file distribution/recovery apparatus  10 , the program stored in the CD-ROM  24  is read and installed in the image file distribution/recovery apparatus  10 . The program is not limited to the recording medium, such as the CD-ROM  24 , and may be stored in other recording media, such as a memory card, or may be downloaded and installed in the image file distribution/recovery apparatus  10  through the network. 
         [0041]    The image file distribution/recovery apparatus  10  includes an operation device  12  that is operated by a user. Commands that are input to the operation device  12  are transmitted to the control device  11 . In addition, a memory  13  that stores, for example, data is connected to the control device  11 . 
         [0042]    In this embodiment, the DICOM file  40  in which, for example, roentgen image data of a patient is stored as secret information is distributed. However, the invention is not limited to the DICOM file  40  and other files may be distributed. 
         [0043]    Referring to  FIG. 2 , the DICOM file  40  includes a tag information recording region  41  and an image data recording region  42 . 
         [0044]    Medical image data, such as roentgen image data, is recorded in the image data recording region  42 . 
         [0045]    Tag information (management information) about a patient corresponding to an image indicated by the image data recorded in the image data, recording region  42 , such as a patient name or a patient ID, is recorded in the tag information recording region  41 . 
         [0046]    Referring to  FIG. 1  and  FIG. 3 , the DICOM file  40  is transmitted from any one of the personal computers l to n and is input to a data input/output device  14  of the image file distribution recovery apparatus  10  through the network. The DICOM file  40  is transmitted to a primary storage memory  15  and is then stored in the primary storage memory  15 . The control device  11  divides the DICOM file  40  into tag information  50  and image data  60 . Since data indicating the amount of data in the tag information  50  is recorded in the tag information recording region  41 , the tag information  50  includes the data indicating the amount of data. The image data.  60  is recorded after the tag information  50 . In this way, it is possible to divide the DICOM file  40  into the tag information  50  and the image data  60 . 
         [0047]    The divided tag information  50  is input to a first distributed data generation device  16 . The first distributed data generation device  16  distributes the tag information  50  into a plurality of (n) first distributed tag information items, using the (k, n)-threshold secret sharing scheme. In this embodiment, the first distributed data generation device  16  {a (k, n)-threshold secret sharing device} distributes the tag information  50  into three distributed tag information items  51 ,  52 , and  53  (the number of distributed tag information items may not be three). The distributed tag information items  51 .  52 , and  53  are transmitted to a distributed/recovered data storage memory  18  and is then temporarily stored in the distributed/recovered data storage memory  18 . 
         [0048]    The divided image data  60  is input to a second distributed data generation device  17 . The second distributed data generation device  17  distributes the image data  60  into a plurality of (n) second image data items, using the (k, L, n)-threshold ramp secret sharing scheme. In this embodiment, the second distributed data generation device  17  {a (k, L, n)-threshold ramp sharing device} distributes the image data  60  into three image data items  61 ,  62 , and  63  (the number of image data items may not be three). The distributed image data items  61 ,  62 , and  63  are transmitted to the distributed/recovered data storage memory  18  and are then temporarily stored in the distributed/recovered data storage memory  18 . 
         [0049]    The distributed tag information items  51 ,  52 , and  53  and the distributed image data items  61 ,  62 , and  63  stored in the distributed/recovered data storage memory  18  are transmitted to a combination device  19  (a combination device). The combination device  19  combines the distributed tag information item  51  with the distributed image data item  61 , combines the distributed tag information item  52  with the distributed image data item  62 , and combines the distributed tag information item  53  with the distributed image data item  63  to generate combined data items  71 ,  72 , and  73 . 
         [0050]    Among the generated combined data items  71 , and  73 , the combined data item  71  is transmitted to the first storage server  31  by a data communication device  22  (a distributed tag information transmission device and a distributed image data transmission device), the combined data item  72  is transmitted to the second storage server  32  by the data communication device  22 , and the combined data item  73  is transmitted to the third storage server  33  by the data communication device  22 . As such, the combined data items  71 ,  72 , and  73  are transmitted to a plurality of different storage servers  31 ,  32 , and  33  and are stored in the storage servers  31 ,  32 , and  33 . 
         [0051]      FIG. 4  illustrates an example of a recording destination table indicating the recording destinations of the combined data items  71  to  73 . 
         [0052]    In the recording destination table, the addresses of the recording destinations of the combined data items  71  to  73  are stored for each DICOM file number (No.) and each DICOM file name. 
         [0053]    The address (of a recording region) of the storage server  31  which is the recording destination of the combined data item  71 , the address (of a recording region) of the storage server  32  which is the recording destination of the combined data item  72 , and the address (of a recording region) of the storage server  33  which is the recording destination of the combined data item  73  are stored as a first recording destination, a second recording destination, and a third recording destination in the recording destination table so as to correspond to the file name of the DICOM file  40 , respectively. 
         [0054]    The recording destination table is stored in the memory  13 . The recording destination table stored in the memory  13  is read to recognize the addresses of the recording destinations of the desired combined data items  71 ,  72 , and  73 , which makes it possible to read desired combined data from the storage servers  31 ,  32 , and  33 . 
         [0055]      FIG. 5  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus  10 . 
         [0056]    The DICOM file  40  transmitted from any one of the personal computers l to p is input from the data input/output device  14  (Step  81 ) and the control device  11  divides the DICOM file  40  into the tag information  50  and the image data  60  (Step  82 ). The first distributed data generation device  16  distributes the tag information  50  into the distributed tag information items  51 ,  52 , and  53  (Step  83 ). The second distributed data generation device  17  distributes the image data  60  into the distributed image data items  61 ,  62 , and  63  (Step  84 ). 
         [0057]    As described above, the combination device  19  combines the distributed tag information item  51  with the distributed image data item  61 , combines the distributed tag information item  52  with the distributed image data item  62 , and combines the distributed tag information item  53  with the distributed image data item  63  to generate the combined data items  71 ,  72 , and  73  (Step  85 ). The combined data item  71  is transmitted to the first storage server  31 , the combined data item  72  is transmitted to the second storage server  32 , and the combined data item  73  is transmitted to the third storage server  33  (Step  86 ). 
         [0058]    In the above-described embodiment, for example, the combined data items  71  obtained by combining the distributed tag information items  51  and the distributed image data items  61  are transmitted to different storage servers  31 . However, for example, the distributed tag information items  51  and the distributed image data items  61  may be transmitted to different storage servers  31 , without being combined with each other. For example, the data communication device  22  (a distributed image data transmission device) transmits the distributed tag information item  51  that is not combined to the storage server  31 , transmits the distributed tag information item  52  that is not combined to the storage server  32 , and transmits the distributed tag information items  53  that is not combined to the storage server  33 . The distributed image data item  61  may be transmitted to the storage server  31  or may be transmitted to a storage server other than the storage servers  31  to  33  in which the distributed tag information items  51  to  53  are stored. Similarly, the distributed image data item  62  may be transmitted to the storage server  32  or may be transmitted to a storage server other than the storage servers  31  to  33  in which the distributed tag information items  51  to  53  are stored. The distributed image data item  63  may be transmitted to the storage server  33  or may be transmitted to a storage server other than the storage servers  31  to  33  in which the distributed tag information items  51  to  53  are stored. 
         [0059]      FIG. 6  corresponds to  FIG. 3  and illustrates an aspect in which the image file distribution/recovery apparatus  10  distributes the DICOM file  40 . 
         [0060]    In the above-described embodiment, the number of distributed tag information items  51  to  53  (three distributed tag information items which are a plurality of first distributed tag information items) generated by the first distributed data generation device  16  is equal to the number of distributed image data items  61  to  63  (three distributed image data items which are a plurality of second distributed image data items) generated by the second distributed data generation device  17 . However, the numbers may be different from each other. 
         [0061]    Referring to  FIG. 6 , the first distributed data generation device  16  distributes the tag information  50  into four distributed tag information items  51  to  54 . The second distributed data generation device  17  distributes the image data  60  into three distributed image data items  61  to  63 . As described above, three distributed tag information items  51  to  53  among the distributed tag information items  51  to  54  distributed by the first distributed data generation device  16  are combined with the distributed image data items  61  to  63  distributed by the second distributed data generation device  17  to generate combined data items  71  to  73 , respectively. As described above, the generated combined data items  71 ,  72 , and  73  are transmitted to the storage servers  31 ,  32 , and  33 , respectively. Among the four distributed tag information items  51  to  54 , the distributed tag information  54  that is not used to generate the combined data items  71  to  73  is transmitted to a storage server other than the storage servers  31  to  33  by the data communication device  22 . 
         [0062]    In the above-described embodiment, the number of distributed tag information items  51  to  54  generated by the first distributed data generation device  16  is greater than the number of distributed image data items  61  to  63  generated by the second distributed data generation device  17 . However, the number of distributed tag information items may be less than the number of distributed image data items. In this case, the distributed image data item that is not used to generate the combined data items  71  to  73  is transmitted to a storage server other than the storage servers  31  to  33  to which the combined data items  71  to  73  have been transmitted. 
         [0063]      FIGS. 7 and 8  are flowcharts illustrating another procedure of the image file distribution/recovery apparatus  10 . In the processes illustrated in  FIGS. 7 and 8 , the same processes as those illustrated in  FIG. 5  are denoted by the same reference numerals and the description thereof will not be repeated. 
         [0064]    As described above, three (a plurality of first) distributed tag information items  51  to  53  and three (a plurality of second) distributed image data items  61  to  63  are generated (Steps  81  to  84 ). The combined data items  71  to  73  are not generated and the data communication device  22  (a distributed tag information transmission device) transmits the three distributed tag information items  51 ,  52 , and  53  to the storage servers  31 ,  32 , and  33 , respectively (Step  87 ). 
         [0065]    When the distributed tag information items  51 ,  52 , and  53  are transmitted to the storage servers  31 ,  32 , and  33 , respectively, the control device  11  detects the rate of utilization of the network connecting the image file distribution/recovers apparatus  10  and the storage servers  31  to  3   n  {the network (communication line) used to transmit a plurality of second distributed image data items by the data communication device  22 } (Step  80 ). When the control device  11  (a determination device) determines that the detected rate of utilization is less than a threshold value (YES in Step  89 ), it is possible to rapidly transmit the distributed image data items  61  to  63  to the storage servers  31  to  3   n , respectively. The data communication device  22  (a distributed image data transmission device) transmits a plurality of second distributed image data items  61 ,  62 , and  63  to different storage servers among the storage servers  31  to  3   n  (Step  90 ). When the rate of utilization is equal to or greater than the threshold value (NO in Step  89 ), the transmission of the plurality of second distributed image data items  61 ,  62 , and  63  is temporarily stopped until the rate of utilization is less than the threshold value. When the rate of utilization is less than the threshold value, the plurality of second distributed image data items  61 .  62 , and  63  are transmitted. 
         [0066]    In the above-described embodiment, the rate of utilization of the network is detected and the distributed image data items  61  to  63  are transmitted to the storage servers  31  to  3   n  in a case in which the detected rate of utilization is less than the threshold value. However, even when the rate of utilization is not detected, the distributed tag information items  51  to  53  may be transmitted to different storage servers among the storage servers  31  to  3   n  and then the distributed image data items  61  to  63  may be transmitted to different storage servers among the storage servers  31  to  3   n.    
         [0067]      FIG. 9  is a flowchart illustrating still another procedure of the image file distribution/recovery apparatus  10 . In  FIG. 9 , the same processes as those illustrated in  FIG. 5  are denoted by the same reference numerals and the description thereof will not be repeated. 
         [0068]    As illustrated in  FIG. 6 , the distributed tag information items  51  to  54  are generated and the distributed image data items  61  to  63  are generated (Steps  81  to  84 ). The distributed tag information  54  is stored in the distributed/recovered data storage memory  18 , without being transmitted from the image file distribution/recovery apparatus  10  to any of the storage servers  31  to  3   n  (Step  101 ). As such, at least one of a plurality of first distributed tag information items is stored in the distributed/recovered data storage memory  18 . 
         [0069]    The data communication device  22  transmits the distributed tag information items  51  to  53  other than the distributed tag information  54  stored in the distributed/recovered data storage memory  18  to different storage servers among the storage servers  31  to  3   n  (Step  102 ). The data communication device  22  transmits the distributed image data items  61  to  63  to different storage servers among the storage servers  31  to  3   n  (Step  103 ). 
         [0070]      FIGS. 10 to 12  are diagrams illustrating an example in which the distributed tag information items  51  to  53  and the distributed image data items  61  to  63  that are distributed as described above are recovered. 
         [0071]      FIG. 10  is a flowchart illustrating the procedure of the image file distribution/recovery apparatus  10 .  FIG. 11  illustrates an aspect in which the combined data items  71  and  73  (the distributed tag information items  51  to  53  and the distributed image data items  61  to  63 ) that are distributed as described above are recovered. 
         [0072]    The distributed combined data items  71  to  73  are stored in different storage servers  31  to  33 , respectively, such that the combined data item  71  is stored in the storage server  31 , the combined data item  72  is stored in the storage server  32 , and the combined data item  73  is stored in the storage server  33 . As illustrated in  FIG. 4 , addresses indicating the storage destinations of the combined data items  71 ,  72 , and  73  are stored in the memory  13 . Therefore, the addresses are read and commands to transmit the combined data items  71 ,  72 , and  73  are transmitted from the control device  11  of the image file distribution/recovery apparatus  10  to the storage servers  31 ,  32 , and  33 , respectively. 
         [0073]    When the storage servers  31 ,  32 , and  33  receive the transmission commands from the image file distribution/recovery apparatus  10 , the storage server  31  transmits the combined data item  71  to the image file distribution/recovery apparatus  10 , the storage server  32  transmits the combined data item  72  to the image file distribution/recovery apparatus  10 , and the storage server  33  transmits the combined data item  73  to the image file distribution/recovery apparatus  10 . 
         [0074]    The data communication device  22  of the image file distribution/recovery apparatus  10  receives the combined data items  71  to  73  transmitted from the storage servers  31  to  33 . That is, the data communication device  22  reads a plurality of combined data items  71  to  73  (Step  111 ). 
         [0075]    The control device  11  divides the combined data items  71  to  73  into a plurality of first distributed tag information items  51  to  53  and a plurality of second distributed image data items  61  to  63 , respectively (Step  112 ). The division positions of each of the distributed tag information items  51  to  53  and each of the distributed image data items  61  to  63  are known by storing the amount of data in each of the distributed tag information items  51  to  53  in the memory  13  at the time of distribution and by storing the start positions of the distributed image data items  61  to  63  in the memory  13 . Each of the divided distributed tag information items  51  to  53  and each of the divided distributed image data items  61  to  63  are temporarily stored in the distributed/recovered data storage memory  18 . 
         [0076]    The control device  11  reads the distributed tag information items  51  to  53  temporarily stored in the distributed/recovered data storage memory  18  and transmits the distributed tag information items  51  to  53  to a first distributed data recovery device  20 . That is, the control device  11  (a distributed tag information reading device) reads a plurality of divided first distributed tag information items  51  to  53  (Step  113 ). As such, the tag information recorded in the tag information recording region  41  of the DICOM file  40  is distributed into a plurality of first distributed tag information items  51  to  53  by the (k, n)-threshold secret sharing scheme, the plurality of first distributed tag information items  51  to  53  are stored in different storage servers  31  to  33 , and the control device  11  (a distributed tag information reading device) reads the plurality of first distributed tag information items  51  to  53 . 
         [0077]    The control device  11  reads the distributed image data items  61  to  63  temporarily stored in the distributed/recovered data storage memory  18  and transmits the distributed image data items  61  to  63  to a second distributed data recovery device  21 . That is, the control device  11  (a distributed image data reading device) reads a plurality of divided second distributed image data items  61  to  63  (Step  114 ). As such, the image data recorded in the image data recording region  42  of the DICOM file  40  is distributed into a plurality of second distributed image data items  61  to  63  by the (k, L, n)-threshold ramp secret sharing scheme, the plurality of second distributed image data items  61  to  63  are stored in different storage servers  31  to  33 , and the control device  11  (a distributed image data reading device) reads the plurality of second distributed image data items  61  to  63 . 
         [0078]    The first distributed data recovery device  20  recovers the distributed tag information items  51  to  53  (a plurality of first distributed tag information items) (Step  115 ). The first distributed data recovery device  20  (a distributed tag information recovery device) recovers the first distributed tag information read by the control device  11  (a distributed tag information reading device) using the (k, n)-threshold secret sharing scheme. The tag information  50  before distribution is obtained by the recovery. 
         [0079]    The second distributed data recovery device  21  recovers the distributed image data items  61  to  63  (a plurality of second distributed image data items) (Step  116 ). The second distributed data recovery device  21  (a distributed image data recovery device) recovers the plurality of second distributed image data items  61  to  63  read by the control device  11  (a distributed image data reading device) using the (k, L, n)-threshold ramp secret sharing scheme. The image data  60  before distribution is obtained by the recovery. 
         [0080]    The control device  11  reads the recovered tag information  50  and the recovered image data  60 . The control device  11  (an image file generation device) combines the tag information  50  and the image data  60  to generate the DICOM file  40  (Step  117 ). The generated DICOM file  40  is transmitted to any one of the personal computers l to n and the image and the tag information are reproduced. 
         [0081]    In the above-described embodiment, the combined data items  71  to  73  stored in different storage servers  31  to  33  are read and recovered to the DICOM file  40 . In a case in which the distributed tag information items  51  to  53  and the distributed image data items  61  to  63  which are not combined with each other are stored in different storage servers  31  to  33 , the distributed tag information items  51  to  53  can be read by the data communication device  22  (a distributed tag information reading device) and the distributed image data items  61  to  63  can be read by the communication device  22  (a distributed image data reading device). Similarly, the distributed tag information items  51  to  53  and the distributed image data items  61  to  63  can be recovered to the image tile. In addition, in a case in which the distributed tag information items  51  to  53  are stored in different storage servers  31  to  33  and the distributed image data items  61  to  63  are distributed and stored in a storage server other than the storage servers  31  to  33 , similarly, the distributed tag information items  51  to  53  and the distributed image data items  61  to  63  can be recovered to the DICOM file  40 . 
         [0082]      FIG. 12  illustrates yet another procedure and is a flowchart illustrating the procedure of the image file distribution/recovery apparatus  10 . 
         [0083]    In the procedure illustrated in  FIG. 12 , after the distributed tag information items  51  to  53  are read, the distributed image data items  61  to  63  are read. 
         [0084]    Similarly to the above, the data communication device  22  reads a plurality of first distributed tag information items  51  to  53  stored in the storage servers  31  to  33  (Step  121 ). 
         [0085]    The user may recover only the distributed tag information items  51  to  53 . In a case in which it is considered that the distributed image data items  61  to  63  do not need to be recovered, the user inputs a read stop command to the image file distribution/recovery apparatus  10  through the operation device  12 . When the read stop command is input (YES in Step  122 ), the control device  11  (an image data reading stop device) controls the data communication device  22  such that the reading of the distributed image data items  61  to  63  is stopped. The first distributed data recovery device  20  recovers the read distributed tag information items  51  to  53  (Step  127 ). 
         [0086]    In a case in which it is considered that the distributed image data items  61  to  63  need to be recovered, the read stop command is not input to the image file distribution/recovery apparatus  10  (NO in Step  122 ). Similarly to the above, the data communication device  22  reads the distributed image data items  61  to  63  stored in the storage servers  31  to  33  (Step  123 ). The first distributed data recovery device  20  recovers the read distributed tag information items  51  to  53  to obtain the tag information  50  (Step  124 ). The second distributed data recovery device  21  recovers the read distributed image data items  61  to  63  to obtain the image data  60  (Step  125 ). The tag information  50  and the image data  60  are combined to obtain the DICOM file  40  (Step  126 ). 
         [0087]    As described above, in a case in which the tag information  50  is divided into a plurality of distributed tag information items  51  to  54 , the distributed tag information item  54  is stored in the distributed/recovered data storage memory  18 , and the distributed tag information items  51  to  53  are stored in the storage servers  31  to  33 , the tag information  50  and the image data  60  may be recovered by the same method as described above. 
         [0088]    In the image file distribution/recovery apparatus  10  illustrated in  FIG. 1 , an image file is distributed and recovered by hardware. Some or all of the distribution and recovery processes may be performed by software. In addition, the image file distribution/recovery apparatus  10  can perform both the distribution and recovery of an image file. However, an apparatus that performs only the distribution of an image file and an apparatus that performs only the recovery of an image file may be separately provided. 
         [0089]    In the above-described embodiment, the tag information recorded in the tag information recording region of the image file is distributed by the (k, n)-threshold secret sharing scheme. However, the name of the region in which the tag information is recorded is not limited to the tag information recording region and management information of the image file, such as header information recorded in a header recording region, may be distributed by the (k, n)-threshold secret sharing scheme.