Abstract:
A communication terminal that can adjust which section of a one-time pad cipher key is used and achieve cipher communication when there is a possibility that the one-time pad cipher keys are not completely matched between communication terminals. A cipher key transfer device acquires a one-time pad cipher key from a key sharing system, divides the acquired one-time pad cipher key with a predetermined number of bits, and transfers the same to a mobile communication terminal after converting the same into one-time pad cipher key cartridges. Along with the partner&#39;s terminal, the mobile communication terminal negotiates which one-time pad cipher key cartridge will be used to perform cipher communication, decides the one-time pad cipher key cartridge to be used, and begins cipher communication.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to cipher communication technique using a one-time pad (OTP: One-Time Pad) cipher. 
       BACKGROUND ART 
       [0002]    Among a plurality of communication terminals, a key of cipher algorithm is shared to encrypt communication contents between the communication terminals, thereby preventing eavesdropping of the communication contents. 
         [0003]    At this time, when a block cipher algorithm is used as a cipher algorithm, plaintext data is divided into units (usually having a fixed length) called blocks, and encrypting process using a cipher key is repeatedly done for each block. 
         [0004]    Further, when a stream cipher algorithm is used as the cipher algorithm, a pseudo random number called key stream is generated from the cipher key, and encrypting process of plaintext data using the key stream is repeatedly done for each bit. 
         [0005]    In either case, the length of the cipher key shared between the communication terminals is 128 bits or 256 bits, and so on. Namely, it is a system to encrypt the communication contents based on the cipher key being shorter than the plaintext data. In these systems, a range of possible values of the cipher key is 128 to 256 powers of 2 types of combinations, and thus it is considered impossible to do brute-force attack for decrypting using all the combinations of the cipher keys if the current computer technology is employed. 
         [0006]    On the other hand, there is another cryptosystem called OTP, in which a random number having the same length as the plaintext data is prepared and the random number is used as a key for one-time encryption. 
         [0007]    In the OTP cipher system, the range of possible values of the cipher key is a huge space which is the same as the plaintext data, and thus it is provable that the decrypting is impossible even if computer technology is tremendously progressed. Varnam&#39;s Cipher is one type of the OTP cipher system, in which an exclusive logical sum (XOR) of the plaintext data and the cipher key is used as cipher text. 
         [0008]    In order to encrypt the communication contents using the OTP cipher system, it is necessary to previously share the OTP cipher key being longer than or equal to the plaintext data between the communication terminals. The quantum key distribution (QKD: Quantum Key Distribution) technique is considered to be an effective technique to share the OTP cipher key. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP2001-7800A 
         Patent Literature 2: JP2001-86110A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0011]    In the communication system using the OTP cipher system, it is necessary to adjust which part of the previously shared long OTP cipher key should be used for encryption between the communication terminals trying to carry out cipher communication using the OTP cipher. 
         [0012]    Conventionally, it is a premise that the communication terminal in the communication system using the OTP cipher system is a fixed terminal which is always connected to the key sharing system using the quantum key distribution technique and so on. Therefore, the OTP cipher keys are assumed to be completely matched between the communication terminals, and the OTP cipher keys can be simply used sequentially from the initial bit for the encryption. 
         [0013]    However, when the communication terminal is, for example a mobile communication terminal, the communication terminal is connected to the key sharing system irregularly, and thus the OTP cipher keys may not be completely matched between the mobile communication terminals. Further, another case can be considered, in which the connection of the mobile communication terminal may be released during the OTP cipher key is transferred to the mobile communication terminal, and a part of the OTP cipher key may be failed to be transferred. Because of these reasons, the OTP cipher keys may not be completely matched between the mobile communication terminals. 
         [0014]    The present invention aims to implement the cipher communication by adjusting which part of the OTP cipher key should be used when the OTP cipher keys may not be completely matched between the communication terminals. 
       Solution to Problem 
       [0015]    According to the present invention, a communication terminal includes: 
         [0016]    a cipher key block storage part which stores a cipher key for one-time pad cipher as a plurality of cipher key blocks generated by dividing the cipher key with a predetermined number of bits; 
         [0017]    an identifying information transmission part which transmits first identifying information that specifies the plurality of cipher key blocks stored by the cipher key block storage part to a partner&#39;s terminal of communication; 
         [0018]    an identifying information reception part which receives from the communication partner second identifying information that specifies only cipher key blocks included in cipher key blocks retained by the partner&#39;s terminal among the plurality of cipher key blocks specified by the first identifying information transmitted by the identifying information transmission part; and 
         [0000]    a cipher communication part which performs cipher communication with the partner&#39;s terminal by a one-time pad cipher using the cipher key blocks specified by the second identifying information received by the identifying information reception part. 
       Advantageous Effects of Invention 
       [0019]    In the communication terminal according to the present invention, the cipher key for the OTP cipher is stored as a cipher key block generated by dividing the key with a predetermined number of bits, after confirming the cipher key block which is shared with the communication partner prior to the cipher communication, the cipher key block to be used for the cipher communication is decided. By this operation, it is possible to adjust which part of the OTP cipher key should be used. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a configurational diagram showing a communication system  1  according to the first embodiment. 
           [0021]      FIG. 2  is a block diagram showing function of a cipher key transfer device  102  according to the first embodiment. 
           [0022]      FIG. 3  is a diagram showing a format of an OTP cipher key cartridge  112  and a cartridging/encrypting process in the cipher key transfer device  102  according to the first embodiment. 
           [0023]      FIG. 4  is a block diagram showing function of a mobile communication terminal  103  according to the first embodiment. 
           [0024]      FIG. 5  is a diagram showing a format of the OTP cipher key cartridge  112  and an opening process of the mobile communication terminal  103  according to the first embodiment. 
           [0025]      FIG. 6  is a sequence diagram when the mobile communication terminals  103  start cipher communication. 
           [0026]      FIG. 7  is a sequence diagram while the mobile communication terminals  103  are implementing cipher communication. 
           [0027]      FIG. 8  is a sequence diagram when the mobile communication terminals  103  finish cipher communication. 
           [0028]      FIG. 9  explains discarding of the OTP cipher key cartridge  112 . 
           [0029]      FIG. 10  is a flowchart showing handling of the OTP cipher key cartridge  112  in the mobile communication terminal  103 . 
           [0030]      FIG. 11  shows a screen display of the mobile communication terminal  103 . 
           [0031]      FIG. 12  shows an example of hardware configuration of the mobile communication terminal  103 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    Hereinafter, embodiments of the present invention will be explained with reference to the drawings. 
         [0033]    In the following description, the processing device is a CPU  911  to be described later, and the like. Anon-volatile memory is a magnetic disk device  920  to be described later, and the like. Further, a volatile memory is a RAM  914  to be described later, and the like. 
       Embodiment 1 
       [0034]      FIG. 1  is a configurational diagram showing communication system  1  according to the first embodiment. 
         [0035]    The communication system  1  includes a key sharing system  101 , a plurality of cipher key transfer devices  102 , a plurality of mobile communication terminals  103 , and a network  104 . 
         [0036]    The key sharing system  101  provides means to share an OTP cipher key  111  between hubs. Here, the key sharing system  101  is assumed to be a system using a quantum cipher key distribution technique; however, the system also can employ another method. 
         [0037]    The key sharing system  101  includes a key sharing device  105  at each hub, and the key sharing devices  105  are connected by an optical fiber link  106 . Then, between the key sharing devices  105 , an OTP cipher key  111  having, for example, 20,000 bits per second is shared via the optical fiber link  106 . 
         [0038]    The cipher key transfer device  102  transfers the OTP cipher key  111  shared by the key sharing system  101  to a mobile communication terminal  103 . 
         [0039]    The cipher key transfer device  102  is provided at each hub and is connected to the key sharing device  105  provided at that hub. The cipher key transfer device  102  acquires the OTP cipher key  111  from the key sharing device  105  connected thereto. Then, the cipher key transfer device  102  divides the acquired OTP cipher key  111 , encrypts using a device key  113  to convert to a format of an OPT cipher key cartridge  112 , and transfers to the mobile communication terminal  103 . 
         [0040]    The mobile communication terminal  103  performs cipher communication with another mobile communication terminal  103  via a network  104  using the OTP cipher key cartridge  112  transferred from the cipher key transfer device  102 . 
         [0041]    For example, the mobile communication terminal  103  encrypts call data (voice data) using the OTP cipher key cartridge  112 , thereby communicating with another communication terminal  103 . 
         [0042]    The network  104  is used for a communication passage between the mobile communication terminals  103 . 
         [0043]    The OTP cipher key  111  is a cipher key for OTP shared between hubs in the key sharing system  101 ; the OTP cipher key  111  is, for example a true random number. Since 20,000 bits per second of the OTP cipher key  111  are shared between the key sharing devices  105  as discussed above, the OTP cipher key  111  becomes data with an extremely large bit string. 
         [0044]    The OTP cipher key cartridge  112  is a cipher key for OTP generated by dividing the OTP cipher key  111  with amount of bits required for cipher communication of a certain amount of communication data, and encrypting using the device key  113 . 
         [0045]    For example, the OTP cipher key cartridge  112  is, in case of the communication with encrypting the call data, a cipher key for OTP encryption which can encrypt the call data of a predetermined time length (10 minutes, for example). In this case, if a bit rate of the call data is 8000 bps (bit per second), the OTP cipher key cartridge  112  is 8000 bps×600 seconds (10 minutes)×2=9,600,000 bits. Here, the final duplation in the above equation is because the call is bi-directional communication. 
         [0046]    The device key  113  is a key previously shared between the cipher key transfer device  102  and the mobile communication terminal  103 . The device key  113  is used for encrypting the OTP cipher key cartridge  112 . 
         [0047]    Next, function of the cipher key transfer device  102  according to the first embodiment will be explained. 
         [0048]      FIG. 2  is a block diagram showing function of the cipher key transfer device  102  according to the first embodiment. 
         [0049]    The cipher key transfer device  102  includes a main memory device  201 , an auxiliary memory device  202 , a device key management part  203 , a cipher key acquisition part  204 , a cipher key cartridge generation part  205  (a cipher key block generation part), a block encrypting part  206 , a cipher key cartridge transfer part  207 , an interface part  208 , and a wired communication part  209 . 
         [0050]    The main memory device  201  is a volatile memory which is capable of retaining data only while electric power is supplied to the cipher key transfer device  102 . The auxiliary memory device  202  is a non-volatile memory which is capable of retaining data regardless whether electric power is supplied to the cipher key transfer device  102  or not. The device key management part  203  manages the device key  113  which is the key previously shared with the mobile communication terminal  103  by a tamper resistant device and so on. 
         [0051]    The interface part  208  is an interface to connect to the key sharing system  101 . For example, the cipher key transfer device  102  is always connected to the key sharing system  101  via the interface part  208 . The wired communication part  209  is an interface to connect to the mobile communication terminal  103 . For example, the cipher key transfer device  102  is connected to the mobile communication terminal  103  via the wired communication part  209  irregularly. 
         [0052]    The other functions will be described in detail in the explanation of the operation of the cipher key transfer device  102 . 
         [0053]    Next, the operation of the cipher key transfer device  102  will be explained. 
         [0054]    First, the operation at the time of acquiring the OTP cipher key  111  will be explained. 
         [0055]    The cipher key acquisition part  204  acquires the OTP cipher key  111  which has been shared between the key sharing devices  105  of the key sharing system  101  from the key sharing device  105  connected via the interface part  208  at every prescribed time period. The acquired OTP cipher key  111  is temporarily stored in the main memory device  201 . 
         [0056]    Next, the cipher key cartridge generation part  205  divides the OTP cipher key  111  using a processing device with amount of bits required for the cipher communication of a certain amount of communication data. Then, the cipher key cartridge generation part  205  makes the block encrypting part  206  encrypt each of the divided OTP cipher key  111  using the device key  113  managed by the device key management part  203 . By the above operation, the cipher key cartridge generation part  205  generates a plurality of OTP cipher key cartridges  112 . The generated OTP cipher key cartridge  112  is stored in the auxiliary memory device  202 . After generating the OTP cipher key cartridge  112 , the cipher key cartridge generation part  205  erases the OTP cipher key  111  from the main memory device  201 . 
         [0057]    Next, a detail of the process to convert the OTP cipher key  111  to the OTP cipher key cartridge  112  will be explained. 
         [0058]      FIG. 3  is a diagram showing a format of the OTP cipher key cartridge  112  and a cartridging/encrypting process in the cipher key transfer device  102  according to the first embodiment. 
         [0059]    An OTP cipher key block  301  is a block generated by dividing the OTP cipher key  111  with amount of bits required for the cipher communication of a certain amount of communication data. A device key ID  302  is an identifier to uniquely identify the device key  113 . An encrypting parameter  303  is an algorithm parameter (for example, specification of the encrypting mode or IV (Initialization Vector) value) specified when the encryption is done using the block cipher algorithm. 
         [0060]    An encrypted OTP cipher key block  311  is a cipher text which has been encrypted by the block encrypting part  206  using the block cipher algorithm when one of the OTP cipher key blocks  301  is assumed as plain text, the device key  113  as the cipher key, and the encrypting parameter  303  as an algorithm parameter. An OTP cipher key cartridge ID  312  is an identifier to uniquely identify the OTP cipher key cartridge  112 . A terminal ID (# 1 )  313  and a terminal ID (# 2 )  314  are identifiers to identify two mobile communication terminals which perform the cipher communication using the OTP cipher key cartridge  112 . 
         [0061]    The cipher key cartridge generation part  205  makes the block encrypting part  206  encrypt each OTP cipher key block  301  by the block cipher algorithm using the device key  113  and the encrypting parameter  303 . By the above operation, the encrypted OTP cipher key block  311  is generated. 
         [0062]    Then, the cipher key cartridge generation part  205  combines the OTP cipher key cartridge ID  312 , the terminal ID (# 1 )  313 , the terminal ID (# 2 )  314 , the device key ID  302 , the encrypting parameter  303  with the encrypted OTP cipher key block  311 , thereby forming one OTP cipher key cartridge  112 . 
         [0063]    Next, the operation will be explained when the OTP cipher key cartridge  112  is transferred. 
         [0064]    The cipher key cartridge transfer part  207  detects connection of the mobile communication terminal  103  via the wired communication part  209 . Then, the cipher key cartridge transfer part  207  transfers the OTP cipher key cartridges  112  stored in the auxiliary memory device  202  to the mobile communication terminal  103  via the wired communication part  209 . After confirming the normal transfer, the cipher key cartridge transfer part  207  erases the OTP cipher key cartridges  112  from the auxiliary memory device  202 . 
         [0065]    Here, between the key sharing devices  105 , the same OTP cipher key  111  is always shared. Further, since the cipher key transfer device  102  is always connected to the key sharing device  105 , the same OTP cipher key  111  is shared between the cipher key transfer devices  102  in principle. 
         [0066]    However, the mobile communication terminal  103  is connected to the cipher key transfer device  102  irregularly and acquires the OTP cipher key cartridge  112  from the cipher key transfer device  102  at the timing of connection. Therefore, the OTP cipher key cartridges  112  retained are sometimes different between the mobile communication terminals  103 . 
         [0067]    Further, for example, during the OTP cipher key cartridge  112  is transferred from the cipher key transfer device  102  to the mobile communication terminal  103 , a cable which connects the cipher key transfer device  102  and the mobile communication terminal  103  may be pulled out, and the connection may be released between the cipher key transfer device  102  and the mobile communication terminal  103 . At this time, the transfer of some OTP cipher key cartridges  112  is failed, and those OTP cipher key cartridges  112  may not be transferred to the mobile communication terminal  103 . This may cause a problem that the retained OTP cipher key cartridges  112  are different between the mobile communication terminals  103 . 
         [0068]    Next, the function of the mobile communication terminal  103  according to the first embodiment will be explained. 
         [0069]      FIG. 4  is a block diagram showing function of the mobile communication terminal  103  according to the first embodiment. 
         [0070]    The mobile communication terminal  103  includes a main memory device  401 , an auxiliary memory device  402  (a cipher key block storage part), a device key management part  403 , a cipher key cartridge reception part  404 , a block decrypting part  405 , an identifying information transmission part  406 , an identifying information reception part  407 , a cipher communication part  408 , an OPT encrypting/decrypting part  409 , a cipher key block erasing part  410 , a remaining amount notification part  411 , a liquid crystal display screen  412 , a vibrator  413 , a speaker  414 , a microphone  415 , a wireless communication part  416 , and a wired communication part  417 . 
         [0071]    The main memory device  401  is a volatile memory which is capable of retaining data only while electric power is supplied to the mobile communication terminal  103 . The auxiliary memory device  402  is a non-volatile memory which is capable of retaining data regardless whether electric power is supplied to the mobile communication terminal  103  or not. The device key management part  403  manages the device key  113  which is the key previously shared with the cipher key transfer device  102 . 
         [0072]    The liquid crystal display screen  412  is a display device to output text information or graphic information. The vibrator  413  is a device to generate vibration. The speaker  414  is a device to output voice. The microphone  415  is a device to input voice. 
         [0073]    The wireless communication part  416  is an interface to communicate with another mobile communication terminal  103  via the network  104 . The wired communication part  417  is an interface to connect to the cipher key transfer device  102 . 
         [0074]    The other functions will be explained in detail in the explanation of the operation of the mobile communication terminal  103 . 
         [0075]    Next, the operation of the mobile communication terminal  103  will be explained. 
         [0076]    First, the operation of the OTP cipher key cartridge  112  at the time of supplement. 
         [0077]    The cipher key cartridge reception part  404  receives the OTP cipher key cartridge  112  transferred by the cipher key transfer device  102  via the wired communication part  417  and stores in the auxiliary memory device  402 . 
         [0078]    Next, the operation of cipher communication using the OTP cipher key cartridge  112  will be explained. 
         [0079]    First, the cipher communication part  408 , for encrypting the communication contents, extracts the OTP cipher key block  301  from the OTP cipher key cartridge  112  stored in the auxiliary memory device  402  and stores in the main memory device  401 . After extracting the OTP cipher key block  301 , the cipher key block erasing part  410  erases the OTP cipher key cartridge  112  from the auxiliary memory device  402 . 
         [0080]    Then, cipher communication part  408  carries out cipher communication with another mobile communication terminal  103  via the wireless communication part  416  using each bit of the OTP cipher key block  301  stored in the main memory device  401  sequentially from the initial bit. After the cipher communication is finished, the cipher key block erasing part  410  erases the OTP cipher key block  301  from the main memory device  401 . 
         [0081]    Next, a detail of an extracting process from the OTP cipher key cartridge  112  to the OTP cipher key block  301  (an opening process of the OTP cipher key cartridge  112 ) will be explained. 
         [0082]      FIG. 5  is a diagram showing a format of the OTP cipher key cartridge  112  and an opening process in the mobile communication terminal  103  according to the first embodiment. 
         [0083]    Here, in  FIG. 5 , the OTP cipher key cartridge  112  and its configurational components are the same as ones in  FIG. 3 . 
         [0084]    The cipher communication part  408  confirms that the terminal ID (# 1 )  313  and the terminal ID (# 2 )  314  included in the OTP cipher key cartridge  112  are the terminal IDs of the self terminal and the mobile communication terminal  103  of the communication partner. Further, the cipher communication part  408  confirms that the device key ID  302  managed by the device key management part  403  matches the device key ID  302  included in the OTP cipher key cartridge  112 . 
         [0085]    When the above conditions are satisfied, the cipher communication part  408  makes the block decrypting part  405  decrypt the encrypted OTP cipher key block  311  included in the OTP cipher key cartridge  112 . At this time, the block decrypting part  405  decrypts the encrypted OTP cipher key block  311  by the block cipher algorithm using the device key  113  managed by the device key management part  403  and the encrypting parameter  303  included in the OTP cipher key cartridge  112 . By the above operation, the OTP cipher key block  301  is extracted. 
         [0086]    Next, a method for adjusting the OTP cipher key cartridge  112  used for encrypting between the mobile communication terminals  103  and a method for erasing the used OTP cipher key cartridge  112  will be explained. 
         [0087]    First, at the start of the cipher communication, negotiation for deciding the OTP cipher key cartridge  112  to be used between the mobile communication terminals  103  will be explained. 
         [0088]      FIG. 6  is a sequence diagram when the mobile communication terminals  103  start cipher communication. 
         [0089]    Here, at the start of the communication, the mobile communication terminal  103  of the transmission side has the OTP cipher key cartridges  112  with IDs= 101  to  300 . Further, the mobile communication terminal  103  of the reception side has the OTP cipher key cartridges  112  with IDs= 201  to  400 . 
         [0090]    The identifying information transmission part  406  of the mobile communication terminal  103  of the transmission side transmits a proposal of cipher communication (Proposed IDs= 101  to  300 ) (an example of the first identifying information) showing that the mobile communication terminal  103  has the OTP cipher key cartridges  112  with IDs= 101  to  300  to the mobile communication terminal  103  of the reception side (S 501 ). 
         [0091]    The identifying information reception part  407  of the mobile communication terminal  103  of the reception side receives the proposal of cipher communication (Proposed IDs= 101  to  300 ). Then, the cipher communication part  408  compares using the processing device the IDs (= 101  to  300 ) shown by the received proposal of cipher communication with the IDs ( 201  to  400 ) of the OTP cipher key cartridge  112  retained by the mobile communication terminal  103  of the reception side. By this operation, the cipher communication part  408  identifies IDs of the OTP cipher key cartridges  112  shared by both mobile communication terminals  103  of the transmission side and the reception side. Here, both mobile communication terminals  103  share the OTP cipher key cartridges  112  with IDs= 201  to  300 . Then, the identifying information transmission part  406  of the mobile communication terminal  103  at the reception side transmits an acceptance of starting cipher communication (Accept IDs= 201 - 300 ) (an example of the second identifying information) showing that both mobile communication terminals  103  share the OTP cipher key cartridges  112  with IDs= 201  to  300  to the mobile communication terminal  103  of the transmission side (S 502 ). 
         [0092]    The identifying information reception part  407  of the mobile communication terminal  103  at the transmission side receives the acceptance of starting cipher communication (Accept IDs= 201 - 300 ). By the above operation, both mobile communication terminals  103  know IDs of the OTP cipher key cartridges  112  shared by both mobile communication terminals  103 . 
         [0093]    Then, cipher communication parts  408  of both of the mobile communication terminal  103  decide the OTP cipher key cartridges  112  to be used from IDs of the OTP cipher key cartridges  112  shared by both mobile communication terminals  103  by a method which has been previously shared using the processing device. For example, the cipher communication parts  408  select ID having the least value. Here, ID= 201  is selected. Then, the cipher communication parts  408  of both mobile communication terminals  103  carry out the opening process (refer to  FIG. 5 ) of the OTP cipher key cartridge  112  with ID= 201  (S 511 ). 
         [0094]    Thereafter, the cipher communication parts  408  of both mobile communication terminals  103  make the OTP encrypting/decrypting parts  409  encrypt the communication data using the OTP cipher key block  301  of the OTP cipher key cartridge  112  with ID= 201  sequentially from the front bit. Then, the cipher communication parts  408  of both mobile communication terminals  103  carry out cipher communication by transmitting/receiving the encrypted communication data (S 503 ). Here, the communication data is, for example, call data input from the microphone  415 . 
         [0095]    Next, the operation will be explained when the cipher communication continues exceeding the communication data amount of one piece of the OTP cipher key cartridge  112  and the OTP cipher key cartridge  112  has been exhausted. 
         [0096]      FIG. 7  is a sequence diagram while the mobile communication terminals  103  are implementing cipher communication. 
         [0097]    Here, both mobile communication terminals  103  have the OTP cipher key cartridges  112  with IDs= 201  to  300 . 
         [0098]    In  FIG. 7 , the opening process (S 511 ) of the OTP cipher key cartridge  112  with ID= 201  and the cipher communication (S 503 ) using the OTP cipher key cartridge  112  with ID= 201  are the same as the sequence shown in  FIG. 6 . 
         [0099]    After the communication of the communication data amount of one cartridge, the cipher key block erasing parts  410  of both mobile communication terminals  103  perform a discarding process (S 521 ) of the OTP cipher key cartridge  112  with ID= 201 . Further, the cipher communication parts  408  of both mobile communication terminals  103  select ID having the least value (here, ID= 202 ) out of the remaining OTP cipher key cartridges  112 , and the opening process of the selected OTP cipher key cartridge  112  is carried out (S 512 ). Thereafter, the cipher communication parts  408  of both mobile communication terminals  103  make the OTP encrypting/decrypting parts  409  encrypt the communication data using the OTP cipher key block  301  of the OTP cipher key cartridge  112  with ID= 202  sequentially from the front bit. Then, the cipher communication parts  408  of both mobile communication terminals  103  carry out the cipher communication by transmitting/receiving the encrypted communication data (S 504 ). 
         [0100]    Next, when the cipher communication ends, discarding of the OTP cipher key cartridges  112  of the mobile communication terminals  103  will be explained. 
         [0101]      FIG. 8  is a sequence diagram when the mobile communication terminals  103  finish cipher communication. 
         [0102]    Here, both mobile communication terminals  103  have the OTP cipher key cartridges  112  with IDs= 203  to  300 , and the cipher communication is being carried out using the OTP cipher key cartridge  112  with ID= 203  (S 505 ). 
         [0103]    The cipher communication part  408  of one of the mobile communication terminals  103  transmits a proposal of finishing the cipher communication to the other mobile communication terminal  103  (S 506 ). Then, the cipher communication part  408  of the other mobile communication terminal  103  transmits back an acceptance of finishing the cipher communication (S 507 ). Thereafter, the cipher key block erasing parts  410  of both mobile communication terminals  103  carry out a discarding process of the OTP cipher key cartridge  112  with ID= 203  (S 523 ). 
         [0104]      FIG. 9  explains discarding of the OTP cipher key cartridge  112 . Here,  FIG. 9  shows an example when one OTP cipher key cartridge  112  is a cipher key for the OTP cipher being sufficient to encrypt call data of 10 minutes and the mobile communication terminals  103  carry out call of 24 minutes. 
         [0105]    First, the OTP cipher key cartridge  112  with ID= 201  is used for the call of the initial 10 minutes, and is erased when the initial 10-minute call is finished. Next, the OTP cipher key cartridge  112  with ID= 202  is used for the call of the 11 th -20 th  minutes, and is erased when the 11 th -20 th  call is finished. Then, the OTP cipher key cartridge  112  with ID= 203  is used for the call of the 21 st -24 th  minutes, and is erased when the call is finished. 
         [0106]    That is, although some part of the OTP cipher key cartridge  112  with ID= 203  remains unused at the end of the call, that part is erased without being used. Namely, the OTP cipher key cartridge  112  being sufficient for 30-minute call is used by 24-minute call. 
         [0107]    Hereinafter, handling of the OTP cipher key cartridge  112  will be explained by focusing on one mobile communication terminal  103 . 
         [0108]      FIG. 10  is a flowchart showing handling of the OTP cipher key cartridge  112  in the mobile communication terminal  103 . 
         [0109]    First, the identifying information transmission part  406  and the identifying information reception part  407  of the mobile communication terminal  103  carry out negotiation for starting communication with the mobile communication terminal  103  of the communication partner. By this operation, the value of the OTP cipher key cartridge ID  312  of the OTP cipher key cartridge  112  which is used for the first time communication is decided, and the decided value is set to the variable X and stored in the main memory device  401  (S 601 ). 
         [0110]    The cipher communication part  408  extracts the OTP cipher key block  301  from the OTP cipher key cartridge  112  with ID=X in the auxiliary memory device  402 , and develops and stores the OTP cipher key block  301  in the main memory device  401  (S 602 ). Further, the cipher key block erasing part  410  erases the OTP cipher key cartridge  112  with ID=X from the auxiliary memory device  402  (S 603 ). 
         [0111]    Subsequently, the cipher communication part  408  carries out the cipher communication using the extracted OTP cipher key block  301  (S 604 ). 
         [0112]    When the cipher communication is finished (YES at S 605 ), the cipher key block erasing part  410  erases the OTP cipher key block  301  from the main memory device  401  (S 609 ). 
         [0113]    On the other hand, if the cipher communication is continued, and the OTP cipher key block  301  remains (NO at both S 605  and S 606 ), the cipher communication part  408  returns the process back to (S 604 ), and carries out the cipher communication using the OTP cipher key block  301  which is being used. 
         [0114]    Further, if the cipher communication is continued, and the OTP cipher key block  301  is exhausted (NO at S 605  and YES at S 606 ), the cipher key block erasing part  410  erases the OTP cipher key block  301  from the main memory device  401  (S 607 ). Subsequently, the cipher communication part  408  sets the value of the OTP cipher key cartridge ID  312  of the OTP cipher key cartridge  112  which will be used for the next cipher communication to the variable X, and returns the process back to (S 602 ). Then, the cipher communication part  408  extracts the OTP cipher key block  301  from the new OTP cipher key cartridge  112 . 
         [0115]    Hereinafter, a method for notifying the user of the remaining amount of the OTP cipher key cartridge  112  of the mobile communication terminal  103  will be explained. 
         [0116]      FIG. 11  shows a screen display of the mobile communication terminal  103 . Here,  FIG. 11  shows an example when the verbal communication is done between the mobile communication terminals  103 . 
         [0117]    The remaining amount notification part  411  displays information  701  showing cipher communication is being implemented on the liquid crystal display screen  412  when the cipher communication is being carried out. Further, the remaining amount notification part  411  displays information  702  showing capable time length of cipher communication on the liquid crystal display screen  412  if all of the remaining part of the OTP cipher key cartridge  112  is used. Here, the capable time length of cipher communication can be calculated by dividing the number of remaining bits of the OTP cipher key cartridge  112  by twice of the bit rate of the communication. Further, the remaining amount notification part  411  displays information  703  showing total remaining amount of the OTP cipher key cartridge  112  on the liquid crystal display screen  412 . Further, the remaining amount notification part  411  displays information  704  showing remaining amount of cipher key of the OTP cipher key cartridge  112  which is currently used on the liquid crystal display screen  412 . 
         [0118]    The remaining amount notification part  411  displays the information  701  to  704  on the liquid crystal display screen  412 , thereby notifying the user of the remaining amount of the OTP cipher key cartridge  112 . 
         [0119]    Further, when the capable time length of cipher communication becomes equal to or less than a certain value or when the remaining amount of the OTP cipher key cartridge  112  is decremented by one and so on, the remaining amount notification part  411  outputs vibration using the vibrator  413  or effect sound or audio guidance using the speaker  414 . 
         [0120]    By this operation, the remaining amount notification part  411  can notify the user of the decrease of the OTP cipher key cartridge  112  even if the user cannot look at the liquid crystal display screen  412 . 
         [0121]    As discussed above, in the communication system  1  according to the first embodiment, the OTP cipher key  111  shared by the hubs using the key sharing system  101  is converted to the OTP cipher key cartridge  112  and transferred to the mobile communication terminal  103 . Then, the mobile communication terminal  103  retrieves the OTP cipher key cartridge  112  to carry out cipher communication using the OTP cipher key cartridge  112  with another mobile communication terminal  103 . By the above operation, the cipher communication using the OTP cipher system is implemented between the mobile communication terminals  103 . 
         [0122]    In particular, in the communication system  1  according to the first embodiment, information of the OTP cipher key cartridge IDs  312  included in the OTP cipher key cartridges  112  are exchanged with each other at the start of the cipher communication. Therefore, even if the OTP cipher key cartridges  112  transferred to the mobile communication terminals  103  do not match completely, the cipher communication can be implemented by adjusting which of the OTP cipher key cartridges  112  should be used between the mobile communication terminals  103 . 
         [0123]    Here, when the key is shared by the quantum cipher key distribution technique, it is said that the distance between the key sharing devices  105  in the key sharing system  101  is limited to around 50 to 100 kilometers. Therefore, if the communication terminal is a fixed terminal, the cipher communication using the OTP cipher is limited to the communication terminals within a range of around 50 to 100 kilometers. However, in the communication system  1  according to the first embodiment, the cipher communication using the OTP cipher system can be implemented between the mobile communication terminals  103 , and thus the limitation of the distance is eliminated. 
         [0124]    Further, in the communication system  1  according to the first embodiment, the used OTP cipher key cartridge  112  is erased immediately when it becomes unnecessary. Therefore, it is possible to prevent decrypting the contents of the past cipher communication by removing the OTP cipher key cartridge  112  from the mobile communication terminal  103 . 
         [0125]    In particular, at the time when the OTP cipher key cartridge  112  is opened and developed in the main memory device  401 , the OTP cipher key cartridge  112  is erased from the auxiliary memory device  402 . The main memory device  401  is capable of retaining data only while electric power is supplied to the mobile communication terminal  103 ; the data cannot be retained when electric power is not supplied. Therefore, if the electric power to the mobile communication terminal  103  is turned off after transmitting the encrypted communication data and before erasing the OTP cipher key cartridge  112 , the OTP cipher key cartridge  112  developed is automatically erased from the main memory device  401 . Accordingly, even if the above case happens, it is possible to prevent decrypting the contents of the past cipher communication by removing the OTP cipher key from the mobile communication terminal  103 . 
         [0126]    Further, in the communication system  1  according to the first embodiment, various remaining amount and their variation are notified through the liquid crystal display screen  412 , the vibrator  413 , and the speaker  414  of the mobile communication terminal  103 . By this notification, the user can know the decrease of the OTP cipher key cartridges  112  and the remaining of the capable time length of cipher communication. 
         [0127]    The hardware configuration of the mobile communication terminal  103  of the above embodiment will be explained. 
         [0128]      FIG. 12  shows an example of hardware configuration of the mobile communication terminal  103 . 
         [0129]    As shown in  FIG. 12 , the mobile communication terminal  103  is provided with a CPU  911  (also referred to as a Central Processing Unit, central processing device, processing device, computation device, microprocessor, microcomputer, or processor) which executes programs. The CPU  911  is connected to a ROM  913 , a RAM  914  (an example of the main memory device  401 ), a liquid crystal display screen  412 , a keyboard  902  (K/B), a vibrator  413 , a speaker  414 , a microphone  415 , a wireless communication board  915  (an example of the wireless communication part  416 ), a wired communication board  916  (an example of the wired communication part  417 ), a magnetic disk device  920  (an example of the auxiliary memory device  402 ) via a bus  912 , and controls these hardware devices. In place of the magnetic disk device  920 , a storage device such as an optical disk device or memory card read/write device may be employed. The magnetic disk device  920  is connected via a predetermined fixed disk interface. 
         [0130]    The magnetic disk device  920 , the ROM  913 , or the like stores an operating system  921  (OS), a window system  922 , programs  923 , and files  924 . The CPU  911 , the operating system  921 , and the window system  922  execute each program of the programs  923 . 
         [0131]    The programs  923  include software and programs that execute the functions described as the “cipher key cartridge reception part  404 ”, the “block decrypting part  405 ”, the “identifying information transmission part  406 ”, the “identifying information reception part  407 ”, the “cipher communication part  408 ”, the “OTP encrypting/decrypting part  409 ”, the “cipher key block erasing part  410 ”, the “remaining amount notification part  411 ” and the like in the above description; and other programs. The programs are read and executed by the CPU  911 . 
         [0132]    The files  924  store information, data, signal values, variable values, and parameters to be stored in the “OTP cipher key cartridge  112 ”, the “OTP cipher key block  301 ”, the “device key  113 ” and the like in the above explanation as the entries of a “database”. The “database” is stored in a recording medium such as a disk or memory. The information, data, signal values, variable values, and parameters stored in a storage medium such as a disk or memory are read out to a main memory or cache memory by the CPU  911  through a read/write circuit, and are used for the operations of the CPU  911  such as extraction, search, look-up, comparison, computation, calculation, process, output, print, and display. The information, data, signal values, variable values, and parameters are temporarily stored in the main memory, cache memory, or a buffer memory during the operations of the CPU  911  including extraction, search, look-up, comparison, computation, calculation, process, output, print, and display. 
         [0133]    Here, as with the mobile communication terminal  103 , the cipher key transfer device  102  is provided with a CPU  911  which executes programs. The CPU  911  is connected to a ROM  913 , a RAM  914  (an example of the main memory device  201 ), an LCD  901 , a keyboard  902  (K/B), a communication board  915 , a magnetic disk device  920  (an example of the auxiliary memory device  202 ) via a bus  912 , and controls these hardware devices. 
         [0134]    The magnetic disk device  920 , the ROM  913 , or the like stores an operating system  921  (OS), a window system  922 , programs  923 , and files  924 . The CPU  911 , the operating system  921 , and the window system  922  execute each program of the programs  923 . 
         [0135]    The programs  923  store software and programs that execute the functions described as the “cipher key acquisition part  204 ”, the “cipher key cartridge generation part  205 ”, the “block encrypting part  206 ”, the “cipher key cartridge transfer part  207 ”, and the like in the above explanation; and other programs. 
         [0136]    The files  924  store information, data, signal values, variable values, and parameters of the “OTP cipher key  111 ”, the “OTP cipher key cartridge  112 ”, the “device key  113 ”, and the like in the above explanation, as the entries of a “database”. 
         [0137]    Further, the arrows in the flowcharts in the above explanation mainly represent input/output of data and signals. The data and signal values are stored in the memory of the RAM  914 , the recording medium such as an optical disk, or in an IC chip. The data and signals are transmitted online via a transmission medium such as the bus  912 , signal lines, or cables; or electric waves. 
         [0138]    Further, a “part” in the above explanation may be a “circuit”, “device”, “equipment”, “means” or “function”; or a “step”, “procedure”, or “process”. A “device” or a “terminal” may be a “circuit”, “equipment”, “means”, or “function”; or a “step”, “procedure”, or “process”. Namely, a “part” may be realized as firmware stored in the ROM  913 . Alternatively, a “part” may be implemented by only software; by only hardware such as an element, a device, a substrate, or a wiring line; by a combination of software and hardware; or furthermore by a combination of software, hardware, and firmware. The firmware and software are stored, as programs, in the recording medium such as the ROM  913 . The program is read by the CPU  911  and executed by the CPU  911 . Namely, the program causes the computer or the like to function as a “part” described above. Alternatively, the program causes the computer or the like to execute the procedure and method of the “part” described above. 
       REFERENCE SIGNS LIST 
       [0139]      1 : communication system;  101 : key sharing system;  102 : cipher key transfer device;  103 : mobile communication terminal;  104 : network;  105 : key sharing device;  106 : optical fiber link;  111 : OTP cipher key;  112 : OTP cipher key cartridge;  113 : device key;  201 : main memory device;  202 : auxiliary memory device;  203 : device key management part;  204 : cipher key acquisition part;  205 : cipher key cartridge generation part;  206 : block encrypting part;  207 : cipher key cartridge transfer part;  208 : interface part;  209 : wired communication part;  301 : OTP cipher key block;  302 : device key ID;  303 : encrypting parameter;  311 : encrypted OTP cipher key block;  312 : OTP cipher key cartridge ID;  313 : terminal ID (# 1 );  314 : terminal ID (# 2 );  401 : main memory device;  402 : auxiliary memory device;  403 : device key management part;  404 : cipher key cartridge reception part;  405 : block decrypting part;  406 : identifying information transmission part;  407 : identifying information reception part:  408 : cipher communication part;  409 : OTP encrypting/decrypting part;  410 : cipher key block erasing part;  411 : remaining amount notification part;  412 : liquid crystal display screen;  413 : vibrator;  414 : speaker;  415 : microphone;  416 : wireless communication part; and  417 : wired communication part.