Abstract:
Provided are a data transmitting apparatus and a data receiving apparatus which use a Y-00 protocol and are capable of preventing an eavesdropper&#39;s decryption based on a transition pattern of a multi-level signal level. The data transmitting apparatus  101,103  of the present invention includes: a multi-level code generation section  111  for generating, by using predetermined key information  11 , a multi-level code sequence  12  in which a value changes so as to be approximately random numbers; and a multi-level signal modulator section  112,125  for generating a converted multi-level signal  23  in accordance with information shared with the receiving apparatus  201,203 , the multi-level code sequence  12  and information data  10 , modulating the converted multi-level signal  23  in a predetermined modulation method, and outputting a resultant modulated signal  14 . The converted multi-level signal  23  is a signal having a plurality of signal point allocations which are different from one another. The multi-level signal modulator section  112,125  switches the plurality of signal point allocations of the converted multi-level signal  23  in accordance with the information  21  shared with the receiving apparatus  201, 203.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an apparatus for performing cipher communication in order to avoid interception (such as eavesdropping) by a third party. More specifically, the present invention relates to a data transmitting apparatus and a data receiving apparatus for performing data communication through setting a specific encoding/decoding (modulation/demodulation) method between a legitimate transmitter and a legitimate receiver. 
         [0003]    2. Description of the Background Art 
         [0004]    Conventionally, in order to perform communication between specific parties, there has been generally adopted a structure for realizing cipher communication by sharing original information (herein after referred to as key information) between transmitting and receiving ends so as to mathematically perform an operation (encoding) and an inverse operation (decoding) of plain text which is information data to be transmitted between the transmitting and receiving ends. 
         [0005]    On the other hand, there have been suggested, in recent years, several encryption methods, which positively utilize physical phenomenon occurring on a transmission line. As one of the encryption methods, there is a method called Y-00 protocol for performing the cipher communication by utilizing a quantum noise generated in the transmission line. 
         [0006]      FIG. 11  is a diagram showing an exemplary configuration of a conventional transmitting/receiving apparatus using the Y-00 protocol disclosed in Japanese Laid-Open Patent Publication No. 2005-57313. Hereinafter, the configuration and an operation of the conventional transmitting/receiving apparatus disclosed in the Japanese Laid-Open Patent Publication No. 2005-57313 will be described. As shown  FIG. 11 , the conventional transmitting/receiving apparatus includes a transmitting section  901 , a receiving section  902 , and a transmission line  910 . The transmitting section  901  includes a first multi-level code generation section  911 , a multi-level processing section  912 , and a modulator section  913 . The receiving section  902  includes a demodulator section  915 , a second multi-level code generation section  914 , and a decision section  916 . The eavesdropping receiving section  903  is an apparatus used by an intercepting party, and is not including in the conventional transmitting/receiving apparatus. 
         [0007]    First, the transmitting section  901  and the receiving section  902  previously retain first key information  91  and second key information  96 , respectively, which are key information having contents identical to each other. Hereinafter, an operation of the transmitting section  901  will be described first. The first multi-level code generation section  911  generates, based on the first key information  91 , a multi-level code sequence  92 , which is a multi-level pseudo random number series having M digits of values from “0” to “M−1” (M is an integer of 2 or more), by using a pseudo random number generator. The multi-level processing section  912  generates, based on information data  90  and the multi-level code sequence  92  which are to be transmitted to the receiving section  902 , a multi-level signal  93  which is a intensity modified signal, by using a signal format described hereinbelow. 
         [0008]      FIG. 12  is a diagram showing a signal format used by the multi-level processing section  912 . As shown in  FIG. 12 , in the case where the number of the digits of the values constituting the multi-level code sequence  92  is M, signal intensity of the multi-level code sequence  92  is divided into 2M signal intensity levels (herein after, simply referred to as a level). These 2M levels are made into M pairs (herein after referred to as a modulation pair), and to one level of each of the M modulation pairs, a value “0” of the information data  90  is allocated, and to the other level, a value “1” of the information data  90  is allocated. Generally, the allocation is made such that levels corresponding to the value “0” of the information data  90  and levels corresponding to the value “1” of the information data  90  are evenly distributed over the whole of the 2M levels. In  FIG. 12 , “0” is allocated to a lower level of an even-numbered modulation pair, and “1” is allocated to a higher level of the same. On the other hand, with respect to an odd-numbered modulation pair, “1” is allocated to a lower level of the odd-numbered modulation pair, and “0” is allocated to a higher level of the same. Accordingly, the values “0” and “1” are alternatively allocated to each of the 2M levels. 
         [0009]    The multi-level processing section  912  selects a modulation pair corresponding to each of the values of the multi-level code sequence  92  having been inputted, then selects one level of the modulation pair, the level corresponding to the value of the information data  90 , and outputs a multi-level signal  93  having the selected level. The modulator section  913  converts the multi-level signal  93  outputted by the multi-level processing section  912  into a modulated signal  94  which is a light intensity modulated signal, and transmits the modulated signal  94  to the receiving section  902  via the transmission line  910 . (Note that, in the Japanese Laid-Open Patent Publication No. 2005-57313, the first multi-level code generation section  911  is described as a “transmitting pseudo random number generation section”, the multi-level processing section  912  as a “modulation method specification section” and a “laser modulation driving section”, the modulator section  913  as a “laser diode”, the demodulator section  915  as a “photo-detector”, the second multi-level code generation section  914  as a “receiving pseudo random number generation section”, and the decision section  916  as a “determination circuit”.) 
         [0010]    Next, an operation of the receiving section  902  will be described. The demodulator section  915  converts the modulated signal  94  which is received via the transmission line  910  from a light signal to an electrical signal (herein after referred to as photo-electric conversion) and outputs a resultant signal as a multi-level signal  95 . The second multi-level code generation section  914  generates, based on the second key information  96 , a multi-level code sequence  97  which is a pseudo random number series constituted of multi levels, and is the same as the multi-level code sequence  92 . The decision section  916  determines, based on respective values of the multi-level code sequence  97  inputted by the second multi-level code generation section  914 , respective modulation pairs used for the multi-level signal  95 . The decision section  916  performs binary decision by using the determined modulation pairs and the multi-level signal  95  inputted by the demodulator section  915 , and then obtains information data  98  which is equivalent to the information data  90 . 
         [0011]      FIG. 13  is a diagram specifically illustrating an operation of the conventional transmitting/receiving apparatus. Hereinafter, with reference to  FIG. 13 , the operation of the conventional transmitting/receiving apparatus will be described in the case where the number of the digits of the values constituting the multi-level code sequence  92  is 4 (M=4). As shown in (a) and (b) of  FIG. 13 , an exemplary case will be described where the value of the information data  90  changes “0 1 1 1”, and the value of the multi-level code sequence  92  changes “0 3 2 1”. In this case, a level of the multi-level signal  93  of the transmitting section  901  changes “1 4 7 2” as shown in  FIG. 13(   c ). 
         [0012]    Specifically, at a time period t 1  shown in  FIG. 13(   c ), a 0th modulation pair corresponding to a value “0” of the multi-level code sequence  92  (a pair of level  1  and level  5 ) is selected. Thereafter, level  1  of the 0th modulation pair corresponding the value “0” of the information data  90  is selected, and the selected level  1  comes to a level of the multi-level signal  93  at the time period t 1 . In a similar manner, at a time period t 2 , a third modulation pair corresponding to a value “3” of the multi-level code sequence  92  (a pair of level  4  and level  8 ) is selected. Thereafter, level  4  of the third modulation pair corresponding to the value “1” of the information data  90  is selected, and the selected level  4  comes to a level of the multi-level signal  93  at t 2 . For a time period t 3  and a time period t 4  as well, a level of the multi-level signal  93  is selected in a similar manner. In this manner, at each of the time periods t 1  and t 3 , in which the value of the multi-level code sequence  92  is even-numbered, the lower level of the modulation pair corresponds to the value “0” of the information data, and the higher level of the modulation pair corresponds to the value “1” of the information data. On the other hand, at each of the time periods t 2  and t 4 , in which the value of the multi-level code sequence  92  is odd-numbered, the lower level of the modulation pair corresponds to value “1” of the information data, and the higher level of the modulation pair corresponds to the value “0” of the information data. 
         [0013]    The multi-level signal  95  inputted by the decision section  916  of the receiving section  902  is a signal which changes as shown in  FIG. 13(   e ), and includes noise, such as a shot noise, which is generated through the photo-electric conversion in the demodulation section  915 . The decision section  916  selects the respective modulation pairs corresponding to the respective values of the multi-level code sequence  97  (see  FIG. 13(   d )) which are equal to the values of the multi-level code sequence  92 , and sets an intermediate level of each of the modulation pairs as a decision level thereof, as shown in  FIG. 13(   e ). The decision section  916  then determines whether the multi-level signal  95  is higher or lower than the decision level. 
         [0014]    Specifically, at a time period t 1  shown in  FIG. 13(   e ), the decision section  916  selects the 0th modulation pair (the pair of level  1  and level  5 ) which corresponds to the value “0” of the multi-level code sequence  97 , and sets level  3 , which is an intermediate level of the 0th modulation pair, as the decision level. Since the multi-level signal  95  is generally distributed in lower levels than the decision level, the decision section  916  then determines that the multi-level signal  95  is lower than the decision level at t 1 . In a similar manner, at a time period t 2 , the decision section  916  selects the third modulation pair (a pair of level  4  and level  8 ) which corresponds to the value “3” of the multi-level code sequence  97 , and sets level  6 , which is an intermediate level of the third modulation pair, as the decision level. Since the multi-level signal  95  is generally distributed in lower levels than the decision level at t 2 , the decision section  916  then determines that the multi-level signal  95  is lower than the decision level at t 2 . At time periods t 3  and t 4  as well, decision is made in a similar manner, and accordingly, a result of the binary decision performed by the decision section  916  comes to “lower, lower, higher, lower”. 
         [0015]    Next, in the case where the values of the multi-level code sequence  97  are each an even number (in the case of each of the time periods t 1  and t 3 ), the decision section  916  determines that a lower level of the selected modulation pair is “0” and that a higher level thereof is “1”, and then outputs the determined values as the information data  98 . On the other hand, in the case the values of the multi-level code sequence  97  are each an odd number (in the case of time periods t 2  and t 4 ), the decision section  916  determines that a lower level of the selected modulation pair is “1”, and a higher level thereof is “0”, and then outputs the determined values as the information data  98 . The values of the multi-level code sequence  97  are “0 3 2 1”, that is, “even, odd, eve, odd” (even representing an even number, and odd representing an odd number). Accordingly, the decision section  916  outputs “0 1 1 1”, which is the information data  98  equal to the information data  90  (see  FIG. 13(   f )). In this manner, the decision section  916  can obtain the information data  98 , based on the multi-level signal  95  which varies the values of the information data to be allocated to the higher level and the lower level of the modulation pair, depending on whether each of the values of the multi-level code sequence  97  is even-numbered or odd-numbered. 
         [0016]    The description of the conventional transmitting/receiving apparatus does not illustrate a specific processing method for obtaining each of the values of the information data  98  depending on whether each of the values of the multi-level code sequence  97  is odd-numbered or even-numbered. However, the following processing method is generally used. First, the second multi-level code generation section  914  generates an inverted signal  99  “0 1 0 1” which is a binary signal and corresponds to the lowest bit of each of the values “0 3 2 1” of the multi-level code sequence  97 , in the case where the values are each represented in a binary form. The decision section  916  then performs an exclusive OR operation between a signal “0 0 1 0”, which represents “lower, lower, higher, lower” as a result of the above-described binary decision, and the inverted signal  99  “0 1 0 1”. From a result of the operation, the information data  98  “0 1 1 1” is obtained. 
         [0017]    As above described, in the case where the signal format is used in which the values of the information data to be allocated to the higher level and the lower level of the modulation pair vary depending on whether each of the value of the multi-level code sequence  97  is odd-numbered or even-numbered (see  FIG. 12 ), the decision section  916  uses the inverted signal  99  so as to generate the information data  98 . However, in the case where a signal format is used in which the value “1” of the information data is constantly allocated to the higher level of the modulation pair, and the value “0” of the information data is allocated to the lower level thereof, the decision section  916  does not necessarily use the inverted signal  99  so as to generated the information data  98 . 
         [0018]    Further, as above described, the multi-level signal  95  includes the noise such as the shot nose which is generated through the photo-electric conversion in the demodulator section  915 . However, by setting an interval between the levels (herein after referred to as a step width) appropriately, occurrence of erroneous binary decision may be suppressed to a negligible level. 
         [0019]    Next, possible eavesdropping (including interception) will be described. As shown in  FIG. 11 , an eavesdropper attempts decryption of the information data  90  or the first key information  91  from the modulated signal  94  by using an eavesdropping receiving section  903 , without having key information shared between a transmitting party and a receiving party. The eavesdropping receiving section  903  includes a demodulator section  921 , a multi-level decision section  922 , and a decryption processing section  923 , and is connected to the transmission line  910 . 
         [0020]    In the case where the eavesdropper performs the same binary decision as a legitimate receiving party (receiving section  902 ), the eavesdropper needs to attempt decision with respect to all possible values which the key information may take since the eavesdropper does not have the key information. However, when this method is used, the number of attempts of the decision increases exponentially in proportion to an increase in a length of the key information. Accordingly, if the length of the key information is significantly long, the method is not practical. 
         [0021]    As a further effective method, it is assumed that the eavesdropper performs multi-level decision of the multi-level signal  81  using the multi-level decision section  922 , the multi-level signal  81  having been obtained by performing the photo-electric conversion using the demodulator section  921 , decrypts the obtained received sequence  82  using the decryption processing section  923 , thereby attempting decryption of the information data  90  or the first key information  91 . In the case of using such a decryption method, if the eavesdropping receiving section  301  can receive (decide) the multi-level signal  93  as the received sequence  82  without mistake, it is possible to decrypt the first key information  91  using the received sequence  82  at a first attempt. 
         [0022]    Since the shot noise generated through the photo-electric conversion in the demodulator section  921  is overlapped on the modulated signal  94 , the shot noise is included in the multi-level signal  81 . It is known that the shot noise is inevitably generated according to the principle of quantum mechanics. Therefore, if the step width of the multi-level signal  93  is set significantly smaller than a distribution width of the shot noise, the multi-level signal  81  including the noise may be distributed over various levels other than a correct level (the level of the multi-level signal  93 ). For example, as shown in  FIG. 13(   g ), at t 1 , the multi-level signal  81  is distributed over levels  0  to  2 . Accordingly, the eavesdropper needs to perform decryption in consideration of a possibility (a possibility of erroneous decision) that the level of the received sequence  82  obtained through the decision is different from the correct level. Therefore, compared with a case without the erroneous decision, the number of the attempts, that is, computational complexity, required for the decryption is increased. As a result, security against the eavesdropping improves. 
         [0023]    However, in the above-described conventional transmitting/receiving apparatus, since the distribution width of the shot noise generated through the photo-electric conversion is small, levels resulting from erroneous multi-level decision made by the eavesdropper appear only in the vicinity of the level of the multi-level signal  93  (a correct signal). For example, at a time period t 2  shown in  FIG. 13(   g ), the level of the multi-level signal  93  is 4, whereas a level which eavesdropper may erroneously take is limited to 3 or 5. Further, since the level of the multi-level signal  93  uniquely corresponds to the multi-level code sequence  92  generated by using the pseudo random number generator, a transition pattern of the level over a plurality of symbols of the time periods do not necessarily range over all possible transition patterns, but is limited to several transition patterns which is determined by a characteristic of the pseudo random number generator used for generating the multi-level code sequence  92 . 
         [0024]    As a result, a problem is posed in that the eavesdropper extracts, among the limited transition patterns, the transition pattern which exists in the vicinity of the level of the multi-level signal  81  having been received by the eavesdropper, thereby being likely to be able to effectively identify the multi-level signal  93 . 
       SUMMARY OF THE INVENTION 
       [0025]    Therefore, an object of the present invention is to provide a data transmitting apparatus and a data receiving apparatus which use a Y-00 protocol, and are able to prevent an eavesdropper&#39;s decryption based on a transition pattern of a multi-level signal level. 
         [0026]    The present invention is directed to a data transmitting apparatus for causing information data to have multi levels by using predetermined key information and performing secret communication with a receiving apparatus. To attain the above-described objects, the data transmitting apparatus of the present invention includes: a multi-level code generation section for generating, by using the predetermined key information, a multi-level code sequence in which a value changes so as to be approximately random numbers; and a multi-level signal modulator section for generating a converted multi-level signal in accordance with information shared with the receiving apparatus, the multi-level code sequence and the information data, modulating the converted multi-level signal in a predetermined modulation method, and outputting a resultant modulated signal. The converted multi-level signal is a signal having a plurality of signal point allocations which are different from one another. The multi-level signal modulator section switches the plurality of signal point allocations of the converted multi-level signal in accordance with the information shared with the receiving apparatus. 
         [0027]    Preferably, the plurality of signal point allocations may include at least a first signal point allocation and a second signal point allocation each having a plurality of signal levels corresponding to the multi-level code sequence. The first signal point allocation and the second signal point allocation may respectively have polarities which are mutually in an inverted relation, the polarities each representing an ascending/descending order of the plurality of signal levels corresponding to the multi-level code sequence. 
         [0028]    Further preferably, the first signal point allocation may be formed based on a first signal format, and the second signal point allocation may be formed based on a second signal format. The first signal format and the second signal format may each represent a signal format which allows values of the information data and the plurality of signal levels to be allocated to the multi-level code sequence, and be mutually in a inverted relation concerning an ascending/descending order of the multi-level code sequence corresponding to the plurality of signal levels. 
         [0029]    Further, in the first signal format and the second signal format, common signal levels may be allocated to different values of the information data. 
         [0030]    Further, the multi-level signal modulator section may include: a multi-level processing section for generating a multi-level signal by using the information data and the multi-level code sequence in accordance with the first signal format; a switching random number generation section for generating a switching random number, which is constituted of binary random numbers, by using switching key information which is information shared with the receiving apparatus; a signal point allocation switching section for switching, in accordance with the switching random number, the multi-level signal to a multi-level signal based on the second signal format, and outputting a resultant converted multi-level signal; and a modulator section for modulating the converted multi-level signal, and outputting a resultant modulated signal. 
         [0031]    Further, the multi-level signal modulator section may include: a multi-level processing section for generating a multi-level signal by using the information data and the multi-level code sequence in accordance with the first signal format; a switching random number generation section for generating a switching random number, which is constituted of binary random numbers, by using switching key information which is the information shared by the receiving apparatus; and a light modulator section for switching the multi-level signal, which is an electrical signal, to a multi-level signal partially based on the second signal format in accordance with the switching random number and for modulating a resultant signal into a modulated signal which is a light signal. The light modulator section may has at least two different input level ranges respectively corresponding to output level ranges of a common level, the at least two different input level ranges showing opposite increase/decrease characteristics of the corresponding output level ranges in proportion to increases in respective inputs, and use the two input level ranges in a switched manner in accordance with the switching random number. 
         [0032]    Further, the light modulator section may include: a polarity inverted signal generation section for converting the switching random number to a polarity inverted signal having two different voltage levels; a semiconductor laser for outputting a non-modulated light; and a Mach-Zehnder light modulator for modulating the non-modulated light by using the multi-level signal and the polarity inverted signal and outputting a resultant modulated signal. A difference between the two voltage levels of the polarity inverted signal is approximately equalized with a half wavelength voltage of the Mach-Zehnder light modulator, whereby the multi-level signal may be switched to a multi-level signal based on the second signal format. 
         [0033]    Further, the multi-level signal and the polarity inverted signal may be combined together, and inputted to a single modulating electrode of the Mach-Zehnder light modulator. 
         [0034]    Further, the Mach-Zehnder light modulator may have two modulating electrodes corresponding to respective channels of an interferometer provided thereinside. The multi-level signal may be inputted to one of the two modulating electrodes, and the polarity inverted signal may be inputted to the other of the two modulating electrodes. 
         [0035]    Further, the multi-level signal modulator section may include: a switching random number generation section for generating a switching random number, which is constituted of binary random numbers, by using switching key information which is the information shared with the receiving apparatus; a code switching section for converting a code of the multi-level code sequence in accordance with the switching random number, and outputting a resultant converted multi-level code sequence; a multi-level processing section for generating, by using the information data and the converted multi-level code sequence, the converted multi-level signal, in accordance with a signal format in which values of the information data and the plurality of signal levels are allocated to the converted multi-level code sequence; and a modulator section for modulating the converted multi-level signal in a predetermined modulation method, and outputting a resultant modulated signal. When the code of the multi-level code sequence is converted, sums between respective values of the multi-level code sequence and respective values of the converted multi-level code sequence may be each constantly equal to a sum between a maximum value and a minimum value of the multi-level code sequence. 
         [0036]    Further, the multi-level code sequence may be a binary parallel signal. The code switching section may include: exclusive OR circuits whose number is equal to a number of bits of the respective values constituting the multi-level code sequence; and a D/A conversion section for collectively performing D/A conversion of output signals from the exclusive OR circuits, and outputting the converted multi-level code sequence. The exclusive OR circuits may each perform an exclusive OR operation between respective bits of the respective values constituting the multi-level code sequence and the switching random number, and output a result thereof. 
         [0037]    Further, the present invention is directed to a data receiving apparatus for reproducing, by using predetermined key information, information data from a modulated signal having been received, and performing secret communication with a transmitting apparatus. To attain the above-described object, the data receiving apparatus includes: a multi-level code generation section for generating, by using the predetermined key information, a multi-level code sequence in which a value changes so as to be approximately random numbers; a demodulator section for demodulating the modulated signal and outputting a converted multi-level signal; and a signal reproducing section for reproducing the information data in accordance with information shared with the transmitting apparatus, the multi-level code sequence and the converted multi-level signal. The converted multi-level signal is a signal having a plurality of signal point allocations which are different from one another. The signal reproducing section switches the plurality of signal point allocations of the converted multi-level signal in accordance with the information shared with the transmitting apparatus. 
         [0038]    Preferably, the plurality of signal point allocations may include at least a first signal point allocation and a second signal point allocation each having a plurality of signal levels corresponding to the multi-level code sequence. The first signal point allocation and the second signal point allocation may respectively have polarities which are mutually in an inverted relation, the polarities each representing an ascending/descending order of the plurality of signal levels corresponding to the multi-level code sequence. 
         [0039]    Further, the first signal point allocation may be formed based on a first signal format, and the second signal point allocation may be formed based on a second signal format. The first signal format and the second signal format may each represent a signal format which allows values of the information data and the plurality of signal levels to be allocated to the multi-level code sequence, and be mutually in a inverted relation concerning an ascending/descending order of the multi-level code sequence corresponding to the plurality of signal levels. 
         [0040]    Further, in the first signal format and the second signal format, common signal levels may be allocated to different values of the information data. 
         [0041]    Further, the signal reproducing section may include: a switching random number generation section for generating a switching random number, which is constituted of binary random numbers, by using switching key information which is the information shared with the transmitting apparatus; a signal point allocation switching section for switching the converted multi-level signal to a signal based on the first signal format in accordance with the switching random number, and outputting a resultant multi-level signal; and a decision section for performing binary decision of the multi-level signal in accordance with the multi-level code sequence, and outputting a resultant signal as the information data. 
         [0042]    Further, the signal reproducing section may include: a switching random number generation section for generating a switching random number, which is constituted of binary random numbers, by using switching key information which is the information shared with the transmitting apparatus; a code switching section for converting a code of the multi-level code sequence in accordance with the switching random number, and outputting a resultant converted multi-level code sequence; and a decision section for performing, by using the converted multi-level code sequence, the binary decision of the converted multi-level signal in accordance with a signal format in which values of the information data and the plurality of signal levels are allocated to the converted multi-level code sequence. When the code of the multi-level code sequence is converted, sums between respective values constituting the multi-level code sequence and respective values constituting the converted multi-level code sequence may be each constantly equal to a sum between a maximum value and a minimum value of the multi-level code sequence. 
         [0043]    Further, the multi-level code sequence is a binary parallel signal. The code switching section may include: exclusive OR circuits whose number is equal to a number of bits of the respective values constituting the multi-level code sequence; and a D/A conversion section for collectively performing D/A conversion of output signals from the exclusive OR circuits, and outputting the converted multi-level code sequence. The exclusive OR circuits may each perform an exclusive OR operation between respective bits of the respective values constituting the multi-level code sequence and the switching random number, and output a result thereof. 
         [0044]    Further, the present invention is directed to a light modulator apparatus for modulating a multi-level signal, which is an electric signal having a plurality of levels, to a modulated signal, which is an optical signal, in accordance with a switching random number which is constituted of binary random numbers. To attain the above-described object, the light modulator apparatus of the present invention includes at least two different input level ranges respectively corresponding to output level ranges of a common level. The at least two input level ranges show opposite increase/decrease characteristics of the corresponding output level ranges in proportion to increases in respective inputs, and are used in a switched manner in accordance with the switching random number. 
         [0045]    Further, the light modulator apparatus may include: a polarity inverted signal generation section for converting the switching random number to a polarity inverted signal having two different voltage levels; a semiconductor laser for outputting a non-modulated light; and a Mach-Zehnder light modulator for modulating the non-modulated light by using the multi-level signal and the polarity inverted signal, and outputting a resultant modulated signal. A difference between the two voltage levels of the polarity inverted signal is approximately equalized with a half wavelength voltage of the Mach-Zehnder light modulator, whereby signal point allocation of the multi-level signal may be switched. 
         [0046]    Further, the multi-level signal and the polarity inverted signal may be combined together, and inputted to a single modulating electrode of the Mach-Zehnder light modulator. 
         [0047]    Further, the Mach-Zehnder light modulator may have two modulating electrodes corresponding to respective channels of an interferometer provided thereinside. The multi-level signal may be inputted to one of the two modulating electrodes, and the polarity inverted signal may be inputted to the other of the two modulating electrodes. 
         [0048]    Further, the present invention is directed to a data transmitting method for causing information data to have multi levels by using predetermined key information and performing secret communication with a receiving apparatus. To attain the above-described object, the data transmitting method of the present invention includes the steps of: generating, by using the predetermined key information, a multi-level code sequence in which a value changes so as to be approximately random numbers; and generating a converted multi-level signal in accordance with information shared with the receiving apparatus, the multi-level code sequence and the information data, modulating the converted multi-level signal in a predetermined modulation method, and outputting a resultant modulated signal. The converted multi-level signal is a signal having a plurality of signal point allocations which are different from one another. The plurality of signal point allocations of the converted multi-level signal are switched in accordance with the information shared with the receiving apparatus. 
         [0049]    Further, the present invention is directed to a data receiving method for reproducing, by using predetermined key information, information data from a modulated signal having been received and performing secret communication with a transmitting apparatus. To attain the above-described object, the data receiving method of the present invention includes the steps of: generating, by using the predetermined key information, a multi-level code sequence in which a value changes so as to be approximately random numbers; demodulating the modulated signal and outputting a converted multi-level signal; and reproducing the information data in accordance with the information shared with the transmitting apparatus, the multi-level code sequence and the converted multi-level signal. The converted multi-level signal is a signal having a plurality of signal point allocations which are different from one another. The plurality of signal point allocations of the converted multi-level signal are switched in accordance with the information shared with the transmitting apparatus. 
         [0050]    As above described, according to the data transmitting apparatus and the data receiving apparatus (data communication apparatus) of the present invention, it is possible to significantly displace a signal intensity level of the multi-level signal by randomly using a plurality of signal formats. Therefore, it is possible to complicate narrowing down of the key information by using the transition pattern of the multi-level signal level, and to improve security against the eavesdropping. 
         [0051]    These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]      FIG. 1  is a block diagram showing an exemplary configuration of a data communication apparatus  1  according to a first embodiment; 
           [0053]      FIG. 2  is a diagram showing exemplary signal formats used by a transmitting section  101  and a receiving section  201 ; 
           [0054]      FIG. 3  is a diagram specifically illustrating an operation of the transmitting section  101  provided in the data communication apparatus  1 ; 
           [0055]      FIG. 4  is a diagram specifically illustrating an operation of the receiving section  201  provided in the data communication apparatus  1 ; 
           [0056]      FIG. 5  is a diagram showing another exemplary signal format used by the transmitting section  101  and the receiving section  201 ; 
           [0057]      FIG. 6  is a diagram showing a multi-level signal modulator section  125  in which a first signal point allocation switching section  115  and a modulator section  116  provided in the multi-level signal modulator section  112  according to the first embodiment are exemplified by specific apparatuses; 
           [0058]      FIG. 7  is a diagram showing a general input/output characteristic of a Mach-Zehnder light modulator; 
           [0059]      FIG. 8  is a diagram showing another configuration of the multi-level signal modulator section  125 ; 
           [0060]      FIG. 9  is a block diagram showing an exemplary configuration of a data communication apparatus  3  according to a third embodiment; 
           [0061]      FIG. 10  is a diagram showing a configuration of a first code switching section  131 ; 
           [0062]      FIG. 11  is a diagram showing an example of a conventional transmitting/receiving apparatus using a Y-00 protocol which is disclosed in Japanese Laid-Open Patent Publication No. 2005-57313; 
           [0063]      FIG. 12  is a diagram showing an exemplary signal format used by a multi-level processing section  912 ; and 
           [0064]      FIG. 13  is a diagram specifically showing an operation of the conventional transmitting/receiving apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0065]      FIG. 1  is a block diagram showing an exemplary configuration of a data communication apparatus  1  according to a first embodiment of the present invention. As shown in  FIG. 1 , the data communication apparatus  1  has a configuration in which a data transmitting apparatus (herein after referred to as a transmitting section)  101  and a data receiving apparatus (herein after referred to as a receiving section)  201  are connected to each other via a transmission line  110 . The transmitting section  101  includes a first multi-level code generation section  111 , a multi-level processing section  113 , a first switching random number generation section  114 , a first signal point allocation switching section  115 , and a modulator section  116 . The receiving section  201  includes a demodulator section  211 , a second multi-level code generation section  212 , a second switching random number generation section  214 , and second signal point allocation switching section  215 , and a decision section  216 . As the transmission line  110 , a metal line such as a LAN cable and a coaxial cable, or a light waveguide such as an optical-fiber cable may be used. Further, without limiting to these wired cables, free space which enables a wireless signal to be transmitted may be used. Further, the eavesdropping receiving section  301  is an apparatus used by an eavesdropper, and is not included in the data communication apparatus  1 . 
         [0066]    First, the transmitting section  101  and the receiving section  201  previously retain first key information  11  and second key information  16 , respectively, which are key information identical in content to each other. The transmitting section  101  and the receiving section  201  also previously retain first switching key information  21  and second switching key information  31 , respectively, which are key information identical in content to each other. The transmitting section  101  and the receiving section  201  also retain signal formats, respectively, which are described hereinbelow by using  FIGS. 2 and 5  as examples. Hereinafter, an operation of the transmitting section  101  will be described. In the same manner as a conventional first multi-level code generation section  911  (see  FIG. 11 ), the first multi-level code generation section  111  generates a multi-level code sequence  12 , which is a multi-level pseudo random number series having M digits of values from “0” to “M−1” (M is an integer of 2 or more), in accordance with the first key information  11  and by using a pseudo random number generator. Regarding a signal mode, the multi-level code sequence  12  may be a multi-level serial signal, or may be a binary parallel signal. 
         [0067]    Here, the signal format retained and used by each of the transmitting section  101  and the receiving section  201  will be described.  FIG. 2  is a diagram showing exemplary signal formats used by the transmitting section  101  and the receiving section  201 , respectively. As shown in  FIG. 2 , the signal format A is the same as a signal format (see  FIG. 12 ) described with respect to the conventional transmitting/receiving apparatus, where as the signal format B is a signal format which is obtained by inverting an order of values of the multi-level code sequence, with respect to the signal format A, from an ascending order to a descending order. That is, in the signal format A, levels and the values of the multi-level code sequence are both arranged in the ascending order, and in the signal format B, the levels are arranged in the ascending order, whereas the values of the multi-level code sequence are arranged in the descending order. 
         [0068]    The signal format A and the signal format Bare not limited to those shown in the drawing. One of the signal formats may be such that the levels and the values of the multi-level code sequence are arranged in a common ascending/descending order, whereas the other signal format may be such that the levels and the values of the multi-level code sequence are arranged in mutually opposite ascending/descending orders. Here, a signal format in which the levels and the values of the multi-level code sequence are arranged in the common ascending/descending order, as with the signal format A, and a signal format in which the levels and the values of the multi-level code sequence are arranged in mutually opposite ascending/descending orders, as with the signal format B, are herein after referred to as being opposite to each other with respect to polarity of the signal formats. 
         [0069]    The multi-level processing section  113  performs a processing, which is similar to that of the multi-level processing section  912  of the conventional transmitting/receiving apparatus (see  FIG. 11 , (a), (b), (c) of  FIG. 13  and descriptions thereof), by using the signal format A shown in  FIG. 2 . That is, the multi-level processing section  113  selects a modulation pair corresponding to inputted values of the multi-level code sequence  12 , select one level of the modulation pair which corresponds to a value of information data  10  having been inputted, and outputs the multi-level signal  13  having the selected level. 
         [0070]    The first switching random number generation section  114  generates, based on the first switching key information  21 , a switching random number  22  which is a binary pseudo random number series. In the case where the value of the inputted switching random number  22  is “1”, the first signal point allocation switching section  115  switches a signal point allocation by switching the multi-level signal  13 , which is obtained by using the signal format A, to a multi-level signal, which is to be obtained by using the signal format B which is opposite in the polarity to the signal format A, and then outputs a resultant signal as a converted multi-level signal  23 . In this manner, to switch a multi-level signal obtained by using a certain signal format to another multi-level signal obtained by using another certain signal format which is opposite in the polarity to the former certain signal format is herein after referred to as “to invert the polarity”. This inversion of the polarity is performed, in the first signal point allocation switching section  115 , by setting an average level of the multi-level signal  13  as 0, multiplying the multi-level signal  13  by +1 or −1 in the case where a value of the switching random number  22  is “0” or “1”, respectively, adding an appropriate bias to a resultant multi-level signal  13 , and then outputting a resultant signal as a converted multi-level signal  23 . Further, in the case where the value of the inputted switching random number  22  is “0”, the first signal point allocation switching section  115  outputs the multi-level signal  13  as the converted multi-level signal  23  without inverting the polarity thereof. The modulator section  116  modulates the inputted converted multi-level signal  23  in a predetermined modulation method, and transmits a resultant signal as a modulated signal  14  to the transmission line  110 . 
         [0071]    Next, an operation of the receiving section  201  will be described. The demodulator section  211  performs photo-electric conversion of the modulated signal  14  transmitted via the transmission line  110 , and outputs a resultant signal as a converted multi-level signal  33 . In the same manner as the first switching random number generation section  114 , the second switching random number generation section  214  generates a switching random number  32 , which is a binary pseudo random number series, in accordance with the second switching key information  31 . In the same manner as the first signal point allocation switching section  115 , the second signal point allocation switching section  215  inverts the polarity of the converted multi-level signal  33  in the case where a value of the switching random number  32  is “1”, and does not invert the polarity of the converted multi-level signal  33  in the case where the value of the switching random number  32  is “0”, and then outputs a resultant signal as a multi-level signal  15 . 
         [0072]    In the same manner as the first multi-level code generation section  111  of the transmitting section  101 , the second multi-level code generation section  212  generates a multi-level code sequence  17 , which is a multi-level pseudo random number series having M digits of values from “0” to “M−1” (M is an integer of 2 or more) in accordance with the second key information  16 , and also generates an inverted signal  35  which is a binary signal. When each of the values of the multi-level code sequence  17  is represented in a binary form, the inverted signal  35  corresponds to the lowest bit of each of the values. The decision section  216  determines, by using the signal format A shown in  FIG. 2 , a modulation pair corresponding to respective values constituting the multi-level code sequence  17  inputted from the second multi-level code generation section  212 . The decision section  216  then performs binary decision in accordance with the determined modulation pair (a pair of levels) and the multi-level signal  15  inputted from the second signal point allocation switching section  215 , performs the exclusive OR between a binary signal obtained by the binary decision and the inverted signal  35 , and then outputs a result of the operation as information data  18  which is equal to the information data  10 . 
         [0073]    In the transmitting section  101 , the multi-level processing section  113 , the first switching random number generation section  114 , the first signal point allocation switching section  115 , and the modulator section  116  may be collectively regarded as a multi-level signal modulator section  112  which converts a multi-level signal obtained from the information data  10 . In the receiving section  201 , the second switching random number generation section  214 , the second signal point allocation switching section  215 , and the decision section  216  may be collectively regarded as a signal reproduction section  213  which obtains the information data  18  from the multi-level signal. 
         [0074]      FIG. 3  is a diagram specifically illustrating the operation of the transmitting section  101  provided in the data communication apparatus  1 . Hereinafter, by using an exemplary case where the modulated signal  14  is a light signal, and with reference to  FIG. 3 , a case where a value of the information data  10  changes “0 1 1 1” and a value of the multi-level code sequence  12  changes “0 3 2 1”, as with the description of the operation of the conventional transmitting/receiving apparatus shown  FIG. 13 , will be described. Here, the multi-level processing section  113  and the conventional multi-level processing section  912  perform identical processing to each other. Accordingly, the multi-level signal  13  (see  FIG. 3(   c )) and the conventional multi-level signal  93  (see  FIG. 13(   c )) are identical to each other, and thus description of the multi-level signal  13  will be omitted. 
         [0075]    First, in the case where the values of the switching random number  22  are “1 0 0 1” (see  FIG. 3(   d )), the signal format used for generating the converted multi-level signal  23  is, as already described, “B A A B” (see  FIG. 2  and  FIG. 3(   e )). Accordingly, as shown in  FIG. 3(   f ), at each of the time periods t 1  and t 4  in which the signal format B is used, the converted multi-level signal  23  has the polarity inverted with respect to the multi-level signal  13  and the signal point allocation thereof is switched. As a result, the converted multi-level signal  23  has the signal level switched from 1 to 8, at the time period t 1 , and the signal level switched from 2 to 7, at the time period t 4 . The converted multi-level signal  23  is, as already described, converted by the modulator section  116  from an electric signal to the light signal (herein after referred to as an electric-photo conversion), and transmitted as a modulated signal  14 . 
         [0076]      FIG. 4  is a diagram specifically illustrating the operation of the receiving section  201  provided in the data communication apparatus  1 . The demodulator section  211  performs photo-electric conversion of the modulated signal  14  transmitted via the transmission line  110 , and outputs a resultant signal as a modulated multi-level signal  33  including a noise such as a shot noise (see  FIG. 4(   g )). In accordance with the values “1 0 0 1” (see  FIG. 4(   h )) of the switching random number  32  which is equal to the switching random number  22 , the second signal point allocation switching section  215  appropriately inverts the polarity of the converted multi-level signal  33  having been inputted, and then outputs a resultant signal as a multi-level signal  15  (see  FIG. 4(   k )). Specifically, the second signal point allocation switching section  215  inverts the polarity of the converted multi-level signal  33  at the time periods t 1  and t 4 , and does not invert the polarity of the multi-level signal  33  at the time periods t 2  and t 3 , and outputs a resultant signal as the multi-level signal  15 . In the same manner as the conventional second multi-level code generation section  914 , the second multi-level code generation section  212  generates, by using the second key information  16 , the multi-level code sequence  17  “0 3 2 1” and the inverted signal  35  “0 1 0 1”, the multi-level code sequence  17  being a multi-level pseudo random number series equal to the multi-level code sequence  12 . As with the processing performed by the conventional decision section  916  (see (d), (e), (f) of  FIG. 13  and description thereof), the decision section  216  uses the multi-level code sequence  17  “0 3 2 1” inputted from the second multi-level code generation section  212 , thereby performing binary decision (see (j) and (k) of  FIG. 4)  with respect to the multi-level signal  15  inputted from the second signal point allocation switching section  215 , and also uses inverted signal  35  “0 1 0 1” inputted from the second multi-level code generation section  212 , thereby obtaining information data  18  (see  FIG. 4(   l )), which is equal to the information data  10 , from a binary signal “0 0 1 0” which indicates “low, low, high, low” and is obtained by the binary decision. 
         [0077]    As with the description of the conventional receiving section  902  shown in  FIG. 11 , for example, in the case where a signal format, in which the value “1” of the information data is constantly allocated to the higher level of the modulation pair, and the value “0” of the information data is constantly allocated to the lower level of the modulation pair, is to be used, the decision section  216  does not need to use the inverted signal  35  so as to generate the information data  18 . 
         [0078]    Hereinafter, a case where eavesdropping (including interception) is to be performed will be described, with reference to  FIG. 1  and  FIG. 4(   m ). As described relating to the eavesdropping of the conventional transmitting/receiving apparatus, it is assumed that the eavesdropper uses the eavesdropping receiving section  301 , reproduces the multi-level signal  13  from the modulated signal  14  without having the key information or the switching key information, and attempts decryption of the information data  10 . The eavesdropping receiving section  301  is constituted of a demodulator section  311 , a multi-level decision section  312 , and a decryption processing section  313 , and is connected to the transmission line  110 . 
         [0079]    In this case, as shown in  FIG. 4(   m ), signal levels of a multi-level signal  41 , which are obtained through the photo-electric conversion of the received modulated signal  14  performed by the demodulator section  311 , distribute over several levels in the vicinity of a legitimate signal (the converted multi-level signal  23 ) due to an effect of the noise caused by quantum fluctuation. 
         [0080]    Here, a case will be considered where the eavesdropper narrows down transition patterns of the multi-level signal which is determined depending on a characteristic of the pseudo random number generator used by the first multi-level code generation section  111  provided in the transmitting section  101 , and extracts transition patterns, which exist in the vicinity of the level of the multi-level signal  41 , among the narrowed down transition patterns, and then attempts identification of the first key information  11 . 
         [0081]    First, a case will be considered where the eavesdropper assumes that the signal format A is used for the multi-level signal  41 . The signal format B used for the multi-level signal  41  at the time periods t 1  and t 4  is in an inverted relation (see  FIG. 2 ), in terms of the polarity, with the signal format A which is used for the multi-level signal  41  at the time periods t 2  and t 3 . Therefore, the polarity of the multi-level signal  41  at the time periods t 1  and t 4  is inverted with respect to the polarity of the multi-level signal  41  at the time periods t 2  and t 3 . Accordingly, at each of the time periods t 1  and t 4 , the multi-level signal  41  has a level which is significantly displaced from the multi-level signal  13 , which is the legitimate signal. Therefore, at each of the time periods t 1  and t 4 , the multi-level signal  41  takes a level which cannot be obtained from the correct first key information  11 . As a result, the eavesdropper fails in narrowing down of the first key information  11 , and thus decryption of the information data  10  is impossible. 
         [0082]    Next, a case will be considered where the eavesdropper assumes that the signal format B is used for the multi-level signal  41 . The multi-level signal  41  at the time periods t 1  and t 4  is in an inverted relation, in terms of the polarity, with the multi-level signal  41  at the time periods t 2  and t 3 , in a similar manner. Accordingly, the multi-level signal  41  at each of the time periods t 2  and t 3  has a level significantly displaced from the multi-level signal  13 , which is the legitimate signal. Therefore, the multi-level signal  41  takes a level which cannot be obtained from the correct first key information  11  at each of the time periods t 2  and t 3 . As a result, in the same manner as the case where the signal format A is assumed to be used for the multi-level signal  41 , the eavesdropper fails in the narrowing down of the first key information  11 , and thus the decryption of the information data  10  is impossible. 
         [0083]    Here, with reference to  FIG. 5 , another exemplary signal format in the first embodiment will be described. A signal format A 1  is the same as the signal format A shown in  FIG. 2 . In the same manner as the signal format B shown in  FIG. 2 , a polarity of a signal format B 1  is opposite to that of the signal format A 1 , and in addition, correspondence between a level and information data in the signal format B 1  is displaced by one step width compared with the signal format B. Accordingly, with respect to common levels, a value of the information data corresponding thereto in the signal format B 1  is different from a value of the information data corresponding thereto in the signal format A 1 . By using the signal format A 1  and the signal format B 1 , the above-described narrowing down of the first key information  11  becomes further complicated and it also becomes impossible to attempt identification of the value of the information data  10  directly from the level. The inversion of the polarity, in the case where the signal format A 1  and the signal format B 1  are used, may be realized when the first signal point allocation switching section  115  adds a minute change, which is as minute as the step width, to the level in the case where the value of the switching random number  22  is “1”, in addition to the above-described multiplication processing. 
         [0084]    The signal formats described with reference to  FIGS. 2 and 5  are merely examples, and may be replaced with a signal format whose polarity can be inverted with respect to the multi-level signal  13 . Further, the number of the signal formats to be used is not limited to two, but a configuration may be adopted in which three or more signal formats are used for switching the level. In this case, the first switching random number generation section  114  and the second switching random number generation section  214  generate a multi-level switching random number instead of the binary switching random number. Further, in  FIGS. 3 and 4 , a case where the multi-level number of the multi-level signal is eight is exemplified, however, the multi-level number is not limited to this, but may be replaced with any even number equal to or more than four. The key information and the switching key information retained by each of the transmitting section  101  and the receiving section  201  may be replaced with one piece of common key information. In this case, the common key information is inputted to the multi-level code generation section and the switching random number generation section which are both provided to the transmitting section and the receiving section. 
         [0085]    As above described, in the data communication apparatus according to the first embodiment, a plurality of signal formats are used randomly, and the signal intensity level of the multi-level signal is displaced significantly. Accordingly, it becomes difficult to narrow down the key information by using the transition patterns of the level of the multi-level signal, and consequently security against the eavesdropping can be improved. 
       Second Embodiment 
       [0086]    In a second embodiment, an example will be described in which the first signal point allocation switching section  115  and the modulator section  116 , which are both provided to the multi-level signal modulator section  112  described in the first embodiment (see  FIG. 1 ), are each replaced with a specific device. The other configurations excluding the multi-level signal modulator section  112  are the same as those described in the first embodiment, and thus description thereof will be omitted.  FIG. 6  is a diagram showing an exemplary configuration of a multi-level signal modulator section  125  according to the second embodiment of the present invention. As shown in  FIG. 6 , the multi-level signal modulator section  125  includes the multi-level processing section  113 , the switching random number generation section  114 , and a light modulator section  121 . The light modulator section  121  is constituted of a polarity inverted signal generation section  122 , a semiconductor laser  123 , a Mach-Zehnder light modulator  124 , and an adder  126 . 
         [0087]    Hereinafter, with reference to  FIG. 6 , operations of respective units constituting the light modulator section  121  will be described in detail. Description of the multi-level processing section  113  and the first switching random number generation section  114  is performed in the first embodiment, and thus will be omitted here. The polarity inverted signal generation section  122  outputs a polarity inverted signal  24  having two predetermined voltage levels corresponding to values of the switching random number  22  inputted from the first switching random number generation section  114 . The semiconductor laser  123  outputs a non-modulated light  25 . The adder  126  adds the multi-level signal  13  inputted from the multi-level processing section  113  and the polarity inverted signal  24  inputted from the polarity inverted signal generation section  122 , and then outputs an added signal  45 . The Mach-Zehnder light modulator  124  modulates the non-modulated light  25  inputted from the semiconductor laser  123  by using the added signal  45  inputted from the adder  126 , and outputs a resultant modulated signal  14 . 
         [0088]    Here, the Mach-Zehnder light modulator  124  generally has a periodic input/output characteristic as shown in  FIG. 7 . Specifically, output light intensity changes in a sinusoidal manner in proportion to an increase in an input voltage. Accordingly, the input/output characteristic is such that the output light intensity increases in a certain range in proportion to the increase in the input voltage, and the output light intensity decreases in another certain range in proportion to the increase in the input voltage. Therefore, a bias voltage to be applied to the Mach-Zehnder light modulator  124  is switched in an appropriate manner in accordance with the switching random number  22 , whereby a polarity of the multi-level signal  13  is inverted appropriately, and electric-photo conversion is performed with respect to the multi-level signal obtained by inverting the polarity thereof. Accordingly, a resultant modulated signal  14  is outputted. 
         [0089]    Specifically, light modulator section  121  selects two operation ranges of the Mach-Zehnder light modulator  124  (see A and B in  FIG. 7 ). In the two operation ranges, the output light intensity changes substantially linearly with respect to the input voltage, and increase/decrease in the output light intensity in proportion to the increase in the input voltage shows an opposite relation. Further, levels of the output light intensity in the two operation ranges are identical to each other. The light modulator section  121  sets a voltage amplitude of the multi-level signal  13  to the same voltage width as these operation ranges, and also sets two voltages of the polarity inverted signal, the voltages corresponding to the bias voltage, to V b  and V b +Vπ, respectively, which are lower limits of input voltages to the two operation ranges. Here, Vπ is a half wavelength voltage of the Mach-Zehnder light modulator  124 . Accordingly, the light modulator section  121  is capable of generating the modulated signal  14  constituted of a modulated signal which is obtained by performing the electric-photo conversion of the multi-level signal based on the signal format A and a modulated signal which is obtained by performing electric-photo conversion of the multi-level signal based on the signal format B (see  FIG. 2 ). When a difference between the two voltage levels of the polarity inverted signal  24  is set smaller than the half wavelength voltage Vπ by a voltage level corresponding to the step-width, the signal formats A 1  and B 1  shown in  FIG. 5  may be also used for the transmitting section  101  and the receiving section  201 . 
         [0090]    A signal mode and an effect on the eavesdropping in the second embodiment are the same as those described in the first embodiment with reference to  FIGS. 3 and 4 , and thus description thereof will be omitted. 
         [0091]    There is a type of the Mach-Zehnder light modulator which is capable of performing modulation individually in two channels of an internal interferometer provided therein. In the case where this type of the Mach-Zehnder light modulator  127  is used, it is possible to configure the light modulator section  121  as shown in  FIG. 8 . In other words, Mach-Zehnder light modulator  127  has two electrodes corresponding to the two channels of the internal interferometer. The multi-level signal  13  is inputted to one of the electrodes, and the polarity inverted signal  24  is inputted to the other of the electrodes. Accordingly, the adder  126  for adding the multi-level signal  13  and the polarity inverted signal  24  becomes unnecessary. 
         [0092]    In the above-described configuration, the two electrodes of the Mach-Zehnder light modulator  127  are in the opposite relation to each other with respect to the increase/decrease in the output light intensity (output signal intensity) in proportion to the increase in the input voltage. Therefore, the level V b  Of the polarity inverted signal  24  corresponds to an operation range B shown in  FIG. 7 , and the level V b +Vπ thereof corresponds to an operation range A shown in  FIG. 7 . Other relations relating to the input/output characteristic are the same as those already described with reference to  FIG. 7 . 
         [0093]    According to the input/output characteristic shown in  FIG. 7 , a case is described where the output light intensity becomes “0” when the input voltage is “0”. However, the input voltage, in the case where the output light intensity is actually “0”, varies depending on the light modulator. Therefore, the fixed bias level V b  needs to be set appropriately in accordance with the light modulator to be used. With reference to  FIGS. 6 and 8 , a case is described where the fixed bias level V b  is included in the polarity inverted signal  24 . However, the fixed bias level V b  may be added to the multi-level signal  13 , and the level of the polarity inverted signal  24  may be set to 0 and Vπ. Further,  FIGS. 6 and 8  each shows the configuration in which the Mach-Zehnder light modulator is used. However, the light modulator section  121  may be configured with an element whose input/output characteristic satisfies the following conditions. 
         [0000]    1. The element has at least two different input level ranges which respectively correspond to outputs of a common level.
 
2. The at least two input level ranges show opposite increase/decrease characteristics of the corresponding outputs in proportion to the increases in the inputs.
 
         [0094]    As above described, in the data transmitting apparatus and the data receiving apparatus (the data communication apparatus) according to the second embodiment, the light modulator for modulating a light signal is used, whereby the first signal point allocation switching section  115  and the modulator section  116  of the first embodiment may be collectively replaced with the light modulator section  121 . As a result, particularly in the case where the light signal is modulated by using the light modulator which is an external component part, the number of component parts to be added may be minimized, and an effect in improving security against eavesdropping can be obtained in the same manner as the first embodiment. 
       Third Embodiment 
       [0095]      FIG. 9  is a block diagram showing an exemplary configuration of a data communication apparatus  3  according to a third embodiment of the present invention. Here, the data communication apparatus  1  of the first embodiment switches a signal point allocation of the multi-level signal  13  outputted by the multi-level processing section  113 , thereby generating the converted multi-level signal  23  in which the signal point allocation is switched. On the other hand, a data communication apparatus  3  converts the multi-level code sequence  12  and inputs the resultant signal to the multi-level processing section  113 , thereby generating the converted multi-level signal  23  in which the signal point allocation is switched. As shown in  FIG. 9 , the data communication apparatus  3  has a configuration in which a data transmitting apparatus (herein after referred to as a transmitting section)  103  and a data receiving apparatus (herein after referred to as a receiving section)  203  are connected to each other via the transmission line  110 . The transmitting section  103  includes the first multi-level code generation section  111 , the multi-level processing section  113 , the first switching random number generation section  114 , a first code switching section  131 , and the modulator section  116 . The receiving section  203  includes the demodulator section  211 , the second multi-level code generation section  212 , the second switching random number generation section  214 , a second code switching section  231 , and the decision section  216 . In the third embodiment, components parts described in the first embodiment will be each provided a common reference character, and description thereof will be omitted. 
         [0096]    First, an operation of the transmitting section  103  will be described. As shown in  FIG. 9 , to the first code switching section  131 , a multi-level code sequence  12  is inputted from the first multi-level code generation section  111 , and in the case where the value of a switching random number  22  inputted from the first switching random number generation section  114  is “0”, a code of the multi-level code sequence  12  is not switched, whereas in the case where the value of the switching random number  22  is “1”, the code of the multi-level code sequence  12  is switched as described hereinbelow (by switching a coding rule), and then outputs a resultant converted multi-level code sequence  26 . 
         [0097]    An operation of the first code switching section  131  will be described in detail in the case where the number of multi levels of the multi-level code sequence  12  is M (the multi-level code sequence takes 0 to M−1 values). In the case where the value of the inputted switching random number  22  is “1”, the first code switching section  131  determines a value of the converted multi-level code sequence  26  such that a sum between the value of the multi-level code sequence  12  and the value of the converted multi-level code sequence  26  is M−1. In the case where the value of the inputted switching random number  22  is “0”, the first code switching section  131  uses the value of the multi-level code sequence  12  as the value of the converted multi-level code sequence  26 . In other words, in the case where the value of the switching random number  22  is “1”, the first code switching section  131  sets the converted multi-level code sequence  26  such that the sum between the value of the multi-level code sequence  12  and the value of the converted multi-level code sequence  26  is constantly equal to a sum between a maximum value and a minimum value of the multi-level code sequence  12 . Accordingly, in the same manner as the first signal point allocation switching section  115  of the first embodiment, the multi-level processing section  113  of the third embodiment is capable of generating the converted multi-level signal  23  in which the signal point allocation is switched in accordance with the switching random number  22 . For example, in the case where the multi-level code sequence  12  is constituted of four values of “0 3 2 1”, and the switching random number  22  is constituted of “1 0 0 1” (see (b) and (d) of  FIG. 3 ), the converted multi-level code sequence  26  comes to “3 3 2 2”. The multi-level processing section  113  interrelates the values “0 1 1 1” of the information data  10  with the values “3 3 2 2” of the converted multi-level code sequence  26  in accordance with a predetermined procedure described in the first embodiment, by using a signal format A shown in  FIG. 2 , and then generates the converted multi-level signal  23  constituted of values of “8 4 7 7” (see (a) and (f) of  FIG. 3 ). 
         [0098]    Next, an operation of the receiving section  203  will be described. As shown in  FIG. 9 , the second code switching section  231  performs code conversion of the inputted multi-level code sequence  17  by using the value of the switching random number  32 , in accordance with the same procedure as the first code switching section  131 , and then outputs a converted multi-level code sequence  36  which is equal to the converted multi-level code sequence  26 . By using the inputted converted multi-level code sequence  36 , the decision section  216  performs decision (binary decision) of the converted multi-level signal  33  in accordance with a predetermined procedure described in the first embodiment, and obtains information data  18  in accordance with a result of the decision and the inverted signal  35  having been inputted. 
         [0099]    As with the description of the conventional receiving section  902  shown in  FIG. 11 , for example, when a signal format is used, in which the value “1” of the information data is constantly allocated to a higher level of a modulation pair, and the value “0” of the information data is constantly allocated to a lower level thereof, then the decision section  216  does not need to use the inverted signal  35  for generating the information data  18 . 
         [0100]    The operations (configurations) of the first code switching section  131  and the second code switching section  231  vary depending on the signal mode of the multi-level code sequence  12  (or the multi-level code sequence  17 ). In the case where the multi-level code sequence  12  is a multi-level serial signal, the first code switching section  131  regards an average level of the multi-level code sequence  12  as 0, multiplies the value of the multi-level code sequence  12  by +1 or −1 in the case where the value of the switching random number  22  is “0” or “1”, respectively, adds an appropriate bias thereto, and then outputs a resultant converted multi-level code sequence  26 . The second code switching section  231  also performs a similar operation. On the other hand, in the case where the multi-level code sequence  12  is a binary parallel signal, the first code switching section  131  is configured as shown in  FIG. 10 . In this case, the first code switching section  131  is configured with exclusive OR circuits  1321  to  132 N, the number of which corresponds to the number of bits of respective values constituting the multi-level code sequence  12 , and a D/A conversion section  133 . To each of the exclusive OR circuits  1321  to  132 N, each of the bits of the respective values constituting the multi-level code sequence  12  and the switching random number  22  are inputted, and a result of an exclusive OR operation is outputted therefrom. The D/A conversion section  133  has the result of the exclusive OR operation inputted thereto, performs D/A conversion of the result, and outputs a converted multi-level code sequence. The second code switching section  231  also has a similar configuration. 
         [0101]    As above described, the data communication apparatus according to the third embodiment has a configuration different from the data communication apparatus according to the first embodiment, but is capable of exerting the same effect as the data communication apparatus according to the first embodiment. 
         [0102]    While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.