Abstract:
A highly concealable data communication apparatus based on an astronomical complexity and causing an eavesdropper to take a significantly increased time to analyze a cipher text, is provided. In a multi-level code generation section  111   a , a random number sequence generation section  141  generates, based on predetermined key information  11,  a plurality of modulation pseudo-random number sequences. The plurality of modulation pseudo-random number sequences is inputted to a multi-level conversion section  142  as a part of an input bit sequence which is converted into a multi-level code sequence  12.  A multi-level processing section  111   b  combines the multi-level code sequence  12  and information data  10,  and generates a multi-level signal  13  having a plurality of levels corresponding to a combination of the multi-level code sequence  12  and the information data  10.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The preset invention relates to an apparatus and a method for performing secret communication in order to avoid illegal eavesdropping and interception by a third party and, more particularly, relates to a data transmitting apparatus, a data receiving apparatus and a data transmitting method for performing data communication through selecting and setting a specific encoding/decoding (modulating/demodulating) method between a legitimate transmitter and a legitimate receiver. 
     2. Description of the Related Art 
     Conventionally, in order to perform secret communication between specific parties, there has been adopted a structure for realizing secret communication by sharing key information for encoding/decoding between transmitting and receiving ends and by performing, based on the key information, an operation/inverse operation on information data (plain text) to be transmitted, in a mathematical manner.  FIG. 17  is a block diagram showing a structure of a conventional data communication apparatus based on the above-described structure. 
     In  FIG. 17 , the conventional data communication apparatus has a configuration in which a data transmitting apparatus  9001  and a data receiving apparatus  9002  are connected to each other via a transmission line  913 . The data transmitting apparatus  9001  includes an encoding section  911  and a modulator section  912 . The data receiving apparatus  9002  includes a demodulator section  914  and a decoding section  915 . 
     In the data transmitting apparatus  9001 , information data  90  and first key information  91  are inputted to the encoding section  911 . The encoding section  911  encodes (modulates), based on the first key information  91 , the information data  90 . The modulator section  912  converts, in a predetermined demodulation method, the information data  90  encoded by the encoding section  911  into a modulated (modulating) signal  94  which is then transmitted to the transmission line  913 . 
     In the data receiving apparatus  9002 , the demodulator section  914  demodulates, in a predetermined demodulation method, the modulated (modulating) signal  94  transmitted via the transmission line  913 . To the decoding section  915 , second key information  96  which has the same content as the first key information  91  is inputted. The decoding section  915  demodulates (decrypts), based on the second key information  96 , the modulated (modulating) signal  94  and then outputs information data  98 . 
     Here, eavesdropping by a third party will be described by using an eavesdropper receiving apparatus  9003 . In  FIG. 17 , eavesdropper receiving apparatus  9003  includes an eavesdropper demodulator section  916  and an eavesdropper decoding section  917 . 
     The eavesdropper demodulator section  916  demodulates, in a predetermined demodulation method, the modulated (modulating) signal  94  transmitted via the transmission line  913 . The eavesdropper decoding section  917  attempts, based on third key information  99 , decoding of a signal demodulated by the eavesdropper demodulator section  916 . Here, since the eavesdropper decoding section  917  attempts, based on the third key information  99  which is different in content from the first key information  91 , decoding of the signal demodulated by the eavesdropper demodulator section  916 , the information data  98  cannot be reproduced accurately. 
     A mathematical encryption (or also referred to as a computational encryption or a software encryption) technique based on such mathematical operation may be applicable to an access system described in Japanese Laid-Open Patent Publication No. 9-205420 (hereinafter referred to as Patent Document 1), for example. That is, in a PON (Passive Optical Network) system in which an optical signal transmitted from an optical transmitter is divided by an optical coupler and distributed to optical receivers at a plurality of optical subscribers&#39; houses, such optical signals that are not desired and aimed at another subscribers are inputted to each of the optical receivers. Therefore, the PON system encrypts information data for each of the subscribers by using key information which is different by the subscribers, thereby preventing a leakage/eavesdropping of mutual information data and realizing safe data communication. 
     Further, the mathematical encryption technique is described in “Cryptography and Network Security: Principles and Practice” translated by Keiichiro Ishibashi et al., Pearson Education, 2001 (hereinafter referred to as Non-patent Document 1) and “Applied Cryptography” translated by Mayumi Adachi et al., Softbank publishing, 2003 (hereinafter referred to as Non-patent Document 2). 
     Among the mathematical encryption, a method called a stream encryption has a simple structure in which a cipher text is generated by performing an XOR operation between a pseudo-random number sequence outputted by a pseudo-random number generator and information data (a plain text) to be encrypted, and thus is advantageous for an increase in speed. On the other hand, the method is disadvantageous in that security in the stream encryption depends only on the pseudo-random number generator. That is, if the eavesdropper can obtain a combination of the plain text and the cipher text, the pseudo-random number series can be identified accurately (this is generally called a known-plain-text attack). Further, since an initial value of the pseudo-random number generator, that is, the key information and the pseudo-random number series correspond to each other uniquely, the key information can be figured out certainly if any decryption algorithm is applied. Further, a processing speed of a computer has been improved remarkably in recent years, and thus there has been a problem in that there is an increasing danger of decryption of the cipher text within a practical time period. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a highly concealable data communication apparatus which causes the eavesdropper to take a significantly increased effort and time to analyze the cipher text, compared to a conventional stream encryption, by introducing an uncertain element into a relation among key information, a pseudo-random number sequence and a cipher text. 
     The present invention is directed to a data transmitting apparatus for encrypting information data by using predetermined key information and performing secret communication with a receiving apparatus. To attain the object mentioned above, the data receiving apparatus of the present invention includes: a multi-level code generation section for generating, based on the predetermined key information, a multi-level code sequence in which a signal level changes so as to approximately represent random numbers; a multi-level processing section for combining the multi-level code sequence and the information data and generating a multi-level signal having a plurality of levels corresponding to a combination of the multi-level code sequence and the information data; and a modulator section for treating the multi-level signal with predetermined modulation processing and outputting a modulated signal. Further, the multi-level code generation section includes: a random number sequence generation section for generating, based on the predetermined key information, a plurality of modulation pseudo-random number sequences; and a multi-level conversion section to which a plurality of bit sequences including at least a part of the plurality of modulation pseudo-random number sequences is inputted as an input bit sequence and which converts the input bit sequence into the multi-level code sequence. The input bit sequence to the multi-level conversion section is greater in a number of digits than each of the plurality of modulation pseudo-random number sequences generated by the random number sequence generation section. 
     Preferably, the multi-level processing section allocates different values of the information data to adjoining multi-levels of the multi-level signal. 
     At least one of the plurality of modulation pseudo-random number sequences is inputted to the multi-level conversion section as a lowest-order bit of the input bit sequence. 
     Preferably, the multi-level code generation section further includes a physical random number generation section for generating one or more physical random number sequences. In this case, the one or more physical random number sequences are inputted, to the multi-level conversion section, as remaining bit sequences of the input bit sequence after excluding the at least a part of the plurality of the modulation pseudo-random number sequences. 
     Further, fixed values maybe inputted, to the multi-level conversion section, as remaining bit sequences of the input bit sequence after excluding the at least a part of the plurality of the modulation pseudo-random number sequences. 
     Preferably, the multi-level code generation section further includes a physical random number generation section for generating one or more physical random number sequences. In this case, the one or more physical random number sequences are inputted to the multi-level conversion section as a part of the plurality of the bit sequences of the input bit sequence after excluding the at least a part of the plurality of the modulation pseudo-random number sequences, and fixed values are inputted, as remaining bit sequences thereof. 
     Further, a signal generated based on a predetermined rule may be inputted, to the multi-level conversion section, as remaining bit sequences of the input bit sequence excluding the at least a part of the plurality of the modulation pseudo-random number sequences. The signal generated based on the predetermined rule may be generated by delaying a part or a whole of the plurality of modulation pseudo-random number sequences by a predetermined time period. 
     A condition needs to be satisfied where a ratio of an information amplitude, which corresponds to an amplitude of the information data, to a fluctuation width of the multi-level signal is greater than a signal-to-noise ratio acceptable to a legitimate receiving party. 
     Preferably, the random number sequence generation section includes: a pseudo-random number generation section for generating, based on the predetermined key information, a pseudo-random number series which is in a binary format; and a serial/parallel conversion section for performing serial/parallel conversion of the pseudo-random number series generated by the pseudo-random number generation section, and outputting the plurality of modulation pseudo-random number sequences. 
     Further, the random number sequence generation section may includes: a pseudo-random number generation section for generating, based on the predetermined key information, a pseudo-random number series which is in a binary format; a plurality of serial/parallel conversion sections for performing serial/parallel conversion of the pseudo-random number series generated by the pseudo-random number generation section and outputting the plurality of modulation pseudo-random number sequences; a first switch for switching, based on a rate selection signal, an output destination of the pseudo-random number series generated by the pseudo-random number generation section, between the plurality of serial/parallel conversion sections; and a second switch for selecting, based on the rate selection signal, and outputting the plurality of modulation pseudo-random number sequences outputted from the plurality of serial/parallel conversion sections. The plurality of serial/parallel conversion sections output respectively different numbers of the plurality of modulation pseudo-random number sequences. 
     Further, the present invention is directed to a data receiving apparatus for receiving information data encrypted by using predetermined key information and performing secret communication with a transmitting apparatus. To attain the object mentioned above, the data receiving apparatus includes: a multi-level code generation section for generating, based on the predetermined key information, a multi-level code sequence in which a signal level changes so as to approximately represent random numbers; a demodulator section for demodulating, in a predetermined demodulation method, a modulated signal received from the transmitting apparatus so as to be outputted as a multi-level signal having a plurality of levels corresponding to a combination of the information data and the multi-level code sequence; and an decision section for deciding, based on the multi-level code sequence, the information data from the multi-level signal. The multi-level code generation section includes: a random number sequence generation section for generating, based on the predetermined key information, a plurality of demodulation pseudo-random number sequences; and a multi-level conversion section to which a plurality of bit sequences including at least a part of the plurality of demodulation pseudo-random number sequences are inputted as an input bit sequence, and which converts the input bit sequence into the multi-level code sequence. The input bit sequence to the multi-level conversion section is greater in a number of digits than each of the plurality of demodulation pseudo-random number sequences generated by the random number sequence generation section. 
     Fixed values are inputted, to the multi-level conversion section, as remaining bit sequences of the input bit sequence excluding the at least a part of the plurality of demodulation pseudo-random number sequences. 
     A signal generated based on a predetermined rule may be inputted, to the multi-level conversion section, as remaining bit sequences of the input bit sequence excluding the at least a part of the plurality of demodulation pseudo-random number sequences. The signal generated based on the predetermined rule may be generated by delaying a part or a whole of the plurality of demodulation pseudo-random number sequences by a predetermined time period. 
     A condition needs to be satisfied where a ratio of an information amplitude corresponding to an amplitude of the information data to a fluctuation width of the multi-level signal corresponding to remaining bit sequences of the input bit sequence to the multi-level conversion section, after excluding the plurality of demodulation pseudo-random number sequences, is greater than a signal-to-noise ratio acceptable to a legitimate receiving party. 
     Preferably, the random number sequence generation section includes: a pseudo-random number generation section for generating, based on the predetermined key information, a pseudo-random number series which is in a binary format; and a serial/parallel conversion section for performing serial/parallel conversion of the pseudo-random number series generated by the pseudo-random number generation section, and outputting the plurality of demodulation pseudo-random number sequences. 
     Further, the random number sequence generation section may include: a pseudo-random number generation section for generating, based on the predetermined key information, a pseudo-random number series which is in a binary format; a plurality of serial/parallel conversion sections for performing serial/parallel conversion of the pseudo-random number series generated by the pseudo-random number generation section and outputting the plurality of demodulation pseudo-random number sequences; a first switch for switching, based on a rate selection signal, an output destination of the pseudo-random number series generated by the pseudo-random number generation section, between the plurality of the serial/parallel conversion sections; and a second switch for selecting, based on the rate selection signal, and outputting the plurality of demodulation pseudo-random number sequences outputted from the plurality of serial/parallel conversion sections. The plurality of serial/parallel conversion sections outputs respectively different numbers of the plurality of demodulation pseudo-random number sequences. 
     Further, the data transmission apparatus mentioned above and processing procedures performed by the modulation section maybe regarded as a data transmission method for causing a series of processing procedures to be executed. That is, the data transmission method includes: a multi-level code generation step of generating, based on the predetermined key information, a multi-level code sequence in which a signal level changes so as to approximately represent random numbers; a step of combining the multi-level code sequence and the information data and generating a multi-level signal having a plurality of levels corresponding to a combination of the multi-level code sequence and the information data; and a modulation step of treating the multi-level signal with predetermined modulation processing and outputting a modulated signal. The multi-level code generation step includes: a random number sequence generation step of generating, based on the predetermined key information, a plurality of modulation pseudo-random number sequences; and a multi-level conversion step in which a plurality of bit sequences including at least apart of the plurality of modulation pseudo-random number sequences is inputted as an input bit sequence and the input bit sequences are converted into the multi-level code sequence. The input bit sequence is greater in a number of digits than each of the plurality of modulation pseudo-random number sequences. 
     Further, respective processing procedures performed by the multi-level code generation section, the demodulation section, and the decision section which are included in the data receiving apparatus mentioned above maybe regarded as a data receiving method for causing a series of processing procedures to be executed. That is, the data receiving method includes: a multi-level code generation step of generating, based on the predetermined key information, a multi-level code sequence in which a signal level changes so as to approximately represent random numbers; a demodulation step of demodulating, in a predetermined demodulation method, a modulated signal received from the transmitting apparatus so as to be outputted as a multi-level signal having a plurality of levels corresponding to a combination of the information data and the multi-level code sequence; and an decision step of deciding, based on the multi-level code sequence, the information data from the multi-level signal. The multi-level code generation step includes: a random number sequence generation step of generating, based on the predetermined key information, a plurality of demodulation pseudo-random number sequences; and a multi-level conversion step in which a plurality of bit sequences including at least a part of the plurality of demodulation pseudo-random number sequences are inputted as an input bit sequence, and the input bit sequence is converted into the multi-level code sequence. The input bit sequence is greater in a number of digits than each of the plurality of demodulation pseudo-random number sequences. 
     The data communication apparatus of the present invention encodes/modulates, based on key information, information data into a multi-level signal which is then to be transmitted, decodes/demodulates, based on the key information, a received multi-level signal and optimizes a signal-to-noise power ratio of the multi-level signal, thereby causing a cipher text obtained by an eavesdropper to be erroneous. As a result, the eavesdropper needs to perform decoding considering that a correct cipher text is highly likely to be different from what the eavesdropper has obtained, and thus the number of attempts required for the decoding, that is the amount of computing, will be increased compared to a case of no error. Accordingly, security against eavesdropping can be improved. Further, the intervals between the levels of the multi-level signal are set appropriately, whereby an increase in a rate of the cipher text pseudo-random number generator used within the apparatus can be kept at the lowest level. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a configuration of a data communication apparatus according to the present invention; 
         FIG. 2  is a diagram illustrating a waveform of a transmission signal of the data communication apparatus according to the first embodiment of the present invention; 
         FIG. 3 . is a diagram illustrating names of the waveform of the transmission signal of the data communication apparatus according to the first embodiment of the present invention; 
         FIG. 4  is a diagram illustrating quality of the transmission signal of the data communication apparatus according to the first embodiment of the present invention; 
         FIG. 5  is a diagram illustrating the quality of another transmission signal of the data communication apparatus according to the first embodiment of the present invention; 
         FIG. 6  is a block diagram showing an example of a detailed configuration of a first multi-level code generation section  111   a  according to a second embodiment of the present invention; 
         FIG. 7  is a block diagram showing an example of a detailed configuration of a second multi-level code generation section  212   a  according to the second embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a signal format used for a data transmitting apparatus according to the second embodiment of the present invention; 
         FIG. 9  is a block diagram showing an example of a detailed configuration of a first multi-level code generation section  111   a  according to a third embodiment of the present invention; 
         FIG. 10  is a block diagram showing an example of a detailed configuration of a second multi-level code generation section  212   a  according to the third embodiment of the present invention; 
         FIG. 11  is a diagram illustrating a signal format used for a data transmitting apparatus according to the third embodiment of the present invention; 
         FIG. 12  is a block diagram showing an example of a detailed configuration of a first multi-level code generation section  111   a  according to a fourth embodiment of the present invention; 
         FIG. 13  is a diagram illustrating a signal format used for a data transmitting apparatus according to the fourth embodiment of the present invention; 
         FIG. 14A  is a block diagram showing an example of another configuration of the first multi-level code generation section  111   a  according to the fourth embodiment of the present invention; 
         FIG. 14B  is a block diagram showing an example of another configuration of the first multi-level code generation section  111   a  according to the fourth embodiment of the present invention; 
         FIG. 15  is a diagram illustrating another signal format used for the data transmitting apparatus according to the fourth embodiment of the present invention; 
         FIG. 16  is a block diagram showing an example of a detailed configuration of a first random number sequence generation section  141  according to a fifth embodiment of the present invention; and 
         FIG. 17  is a block diagram showing a configuration of a conventional data communication apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiment of the present invention will be described, with reference to drawings. 
     First Embodiment 
       FIG. 1  is a block diagram showing an example of a configuration of a data communication apparatus according to the present invention. In  FIG. 1 , the data communication apparatus according to the first embodiment has a configuration in which a data transmitting apparatus  1101  and a data receiving apparatus  1201  are connected to each other via a transmission line  110 . The data transmitting apparatus  1101  includes a multi-level encoding section  111  and a modulator section  112 . The multi-level encoding section  111  includes a first multi-level code generation section  111   a  and a multi-level processing section  111   b . The data receiving apparatus  1201  includes a demodulator section  211  and a multi-level decoding section  212 . The multi-level decoding section  212  includes a second multi-level code generation section  212   a  and a decision section  212   b . A metal line such as a LAN cable or a coaxial line, or an optical waveguide such as an optical-fiber cable can be used as the transmission line  110 . Further the transmission line  110  is not limited to a wired cable such as the LAN cable, but can be free space which enables a wireless signal to be transmitted. 
       FIG. 2  is a diagram illustrating a waveform of a transmission signal of the data communication apparatus according to the first embodiment of the present invention.  FIG. 3  is a diagram illustrating names of the waveform of the transmission signal of the data communication apparatus according to the first embodiment of the present invention.  FIG. 4  is a diagram illustrating quality of the transmission signal of the data communication apparatus according to the first embodiment of the present invention. Hereinafter, an action of the data communication apparatus according to the first embodiment of the present invention will be described, with reference to  FIGS. 1 to 4 . 
     The first multi-level code generation section  111   a  generates, based on predetermined first key information  11 , a multi-level code sequence  12  ((b) of  FIG. 2 ) in which a signal level changes so as to approximately represent random numbers. The multi-level code sequence  12  ((b) of  FIG. 2 ) and information data  10  ((a) of  FIG. 2 ) are inputted to the multi-level processing section  111   b . The multi-level processing section  111   b  combines the multi-level code sequence  12  and the information data  10  in accordance with a predetermined procedure, and generates a multi-level signal  13  ((c) of  FIG. 2 ) having a plurality of levels corresponding to a combination of the multi-level code sequence  12  and the information data  10 . For example, in the case where a level of the multi-level code sequence  12  changes to c 1 /c 5 /c 3 /c 4  with respect to time slots t 1 /t 2 /t 3 /t 4 , the multi-level processing section  111   b  regards the multi-level code sequence  12  as a bias level, adds the information data  10  to the multi-level code sequence  12 , and then generates the multi-level signal  13  in which a signal level changes to L 1 /L 8 /L 6 /L 4 . The modulator section  112  modulates the multi-level signal  13  in a predetermined modulation method, and outputs the modulated multi-level signal  13  as a modulated (modulating) signal  14  to the transmission line  110 . 
     Here, as shown in  FIG. 3 , an amplitude of the information data  10  is referred to as an “information amplitude”, a total amplitude of the multi-level signal  13  is referred to as a “multi-level signal amplitude”, pairs of levels (L 1 , L 4 )/(L 2 , L 5 )/(L 3 , L 6 )/(L 4 , L 7 )/(L 5 , L 8 ) which the multi-level signal  13  may obtain corresponding to the levels c 1 /c 2 /c 3 /c 4 /c 5  of the multi-level code sequence  12  are respectively referred to as first to fifth “bases”, and a minimum interval between signal levels of the multi-level signal  13  is referred to as a “step width”. 
     The demodulator section  211  demodulates the modulated signal  14  transmitted via the transmission line  110 , and reproduces a multi-level signal  15 . The second multi-level code generation section  212   a  previously shares second key information  16  which has the same content as the first key information  11 , and based on the second key information  16 , generates a multi-level code sequence  17 . The decision section  212   b  receives the multi-level signal  15  and reproduces information data  18  by deciding (binary determination) a value of the information data  18  using the multi-level code sequence  17  as a threshold. Here, the modulated (modulating) signal  14  which is modulated in a predetermined modulation method and is transmitted/received between the modulator section  112  and the demodulator section  211  via the transmission line  110 , is a signal obtained by modulating an electromagnetic wave (electromagnetic field) or a light wave using the multi-level signal  13 . 
     Note that, the multi-level processing section  111   b  may generate the multi-level signal  13  by using any method, in addition to a method of generating the multi-level signal  13  by adding the information data  10  and the multi-level code sequence  12  as above described. For example, the multi-level processing section  111   b  may generate the multi-level signal  13  by modulating, based on the information data  10 , an amplitude of the levels of the multi-level code sequence  12 . Alternatively, the multi-level processing section  111   b  may generate the multi-level signal  13  by reading out consecutively, from a memory having levels of the multi-level signal  13  previously stored therein, the levels of the multi-level signal  13 , which are corresponding to the combination of the information data  10  and the multi-level code sequence  12 . 
     Further, in  FIG. 2  and  FIG. 3 , the levels of the multi-level signal  13  are represented as 8 levels, but the levels of the multi-level signal  13  are not limited to the representation. Further, the information amplitude is represented as three times or integer times of the step width of the multi-level signal  13 , but the information amplitude is not limited to the representation. The information amplitude may be any integer times of the step width of the multi-level signal  13 , or is not necessarily integer times thereof. Further, in  FIG. 2  and  FIG. 3 , each of the levels of the multi-level code sequence  12  is located so as to be at an approximate center between each of the levels of the multi-level signal  13 , but each of the levels of the multi-level code sequence  12  is not limited to such a location. For example, each of the levels of the multi-level code sequence  12  is not necessarily at the approximate center between each of the levels of the multi-level signal  13 , or may coincide with each of the levels of the multi-level signal  13 . Further, the above description is based on an assumption that the multi-level code sequence  12  and the information data  10  are identical in a change rate to each other and also in a synchronous relation, but the change rate of either of the multi-level code sequence  12  or the information data  10  maybe faster (or slower) than the change rate of another, or the multi-level code sequence  12  and the information data  10  are in an asynchronous relation. 
     Next, an action of eavesdropping by a third party will be described. It is assumed that the third party, who is an eavesdropper, decodes the modulated (modulating) signal  14  by using a configuration corresponding to the data receiving apparatus  1201  held by a legitimate receiving party or a further sophisticated data receiving apparatus (hereinafter referred to as an eavesdropper data receiving apparatus). The eavesdropper data receiving apparatus reproduces the multi-level signal  15  by demodulating the modulated (modulating) signal  14 . However, the eavesdropper data receiving apparatus does not share the key information with the data transmitting apparatus  1101 , and thus, unlike the data receiving apparatus  1201 , the eavesdropper data receiving apparatus cannot generate, based on the key information, the multi-level code sequence  17 . Therefore, the eavesdropper data receiving apparatus cannot perform binary determination of the multi-level signal  15  by using the multi-level code sequence  17  as a reference. 
     As an action of the eavesdropping which maybe possible under these circumstances, there is a method of identifying all the levels of the multi-level signal  15  (generally referred to as “all-possible attacks”). That is, the eavesdropper data receiving apparatus performs a determination of the multi-level signal  15  by preparing thresholds corresponding to all possible intervals between the signal levels which the multi-level signal  15  may obtain, and attempts an extraction of correct key information or information data by analyzing a result of the determination. For example, the eavesdropper data receiving apparatus sets all the levels c 0 /c 1 /c 2 /c 3 /c 4 /c 5 /c 6  of the multi-level code sequence  12  shown in  FIG. 2  as the thresholds, performs the multi-level determination of the multi-level signal  15 , and then attempts the extraction of the correct key information or the information data. 
     However, in an actual transmission system, a noise occurs due to various factors, and the noise is overlapped on the modulated (modulating) signal  14 , whereby the levels of the multi-level signal  15  fluctuates temporally/instantaneously as shown in  FIG. 4 . In this case, an SN ratio (a signal-to-noise intensity ratio) of a signal to be determined (the multi-level signal  15 ) by the legitimate receiving party (the data receiving apparatus  1201 ) is determined based on a ratio of the information amplitude to a noise level of the multi-level signal  15 . On the other hand, the SN ratio of the signal to be determined (the multi-level signal  15 ) by the eavesdropper data receiving apparatus is determined based on a ratio of the step width to the noise level of the multi-level signal  15 . 
     Therefore, in the case where a condition of the noise level contained in the signal to be determined is fixed, the SN ratio of the signal to be determined by the eavesdropper data receiving apparatus is relatively smaller than that by the data receiving apparatus  1201 , and thus a transmission feature (an error rate) of the eavesdropper data receiving apparatus deteriorates. The data communication apparatus of the present invention utilizes this feature so as to induce an identification error in the all-possible attacks by the third party using all the thresholds, thereby causing the eavesdropping to be difficult. Particularly, in the case where the step width of the multi-level signal  15  is set at an order equal to or smaller than a noise amplitude (spread of a noise intensity distribution), the data communication apparatus substantially disables the multi-level determination by the third party, thereby realizing an ideal eavesdropping prevention. 
     As the noise to be overlapped on the signal to be determined (the multi-level signal  15  or the modulated (modulating) signal  14 ), a thermal noise (Gaussian noise) included in a space field or an electronic device, etc. maybe used, in the case where an electromagnetic wave such as a wireless signal is used as the modulated (modulating) signal  14 , and a photon number distribution (quantum noise) may be used in addition to the thermal noise, in the case where the light wave is used. Particularly, signal processing such as recording and replication is not applicable to a signal using the quantum noise, and thus the step width of the multi-level signal  15  is set by using the quantum noise level as a reference, whereby the eavesdropping by the third party is disabled and an absolute security of the data communication is secured. 
     As above described, according to the data communication apparatus based on the first embodiment of the present invention, when the information data to be transmitted is encoded as the multi-level signal, the interval between the signal levels of the multi-level signal  13  is set with respect to the noise level so as to disable eavesdropping by the third party. Accordingly, quality of the receiving signal at the time of the eavesdropping by the third party is crucially deteriorated, and it is possible to provide a further safe data communication apparatus which causes decryption/decoding of the multi-level signal by the third party to be difficult. 
     Note that the multi-level encoding section  111  may fluctuate the step width (S 1  to S 7 ) of the multi-level signal  13 , as shown in  FIG. 5 , depending on a fluctuation level of each of the levels, that is, the noise intensity distribution overlapped on each of the levels. Specifically, the interval between the signal levels of the multi-level signal  13  is distributed such that respective SN ratios determined based on respective adjoining two signal levels of the signal to be determined which are inputted to the decision section  212   b  become approximately uniform. Further, the step width of each of the levels of the multi-level signal  13  is set in a uniform manner, in the case where the noise level to be overlapped on each of the levels is constant. 
     Generally, in the case where a light intensity modulated signal whose light source is a diode laser (LD) is assumed as the modulated signal  14  outputted from the modulator section  112 , a fluctuation width (the noise level) of the modulated (modulating) signal  14  will vary depending on the levels of the multi-level signal  13  inputted to the diode laser. This results from the fact that the diode laser emits light based on the principle of stimulated emission which uses a spontaneous emission light as a “master light”, and the noise level contained in the modulated signal outputted from the diode laser is defined based on a relative ratio of a stimulated emission light level to a spontaneous emission light level. That is, the higher an excitation rate of the diode laser (the excitation rate of the diode laser corresponds to a bias current to be injected) is, the larger a ratio of the stimulated emission light level becomes, and consequently the noise level becomes small. On the other hand, the lower the excitation rate of the diode laser is, the larger a ratio of the natural emission light level becomes, and consequently the noise level becomes large. Accordingly, as shown in  FIG. 5 , the multi-level encoding section  111  sets the step width to be large in a range where the level of the multi-level signal  13  is small, and sets the step width to be small in a range where the level of the multi-level signal is large, in a non-linear manner, whereby it is possible to set, in an approximately uniform manner, the respective SN ratios of the intervals between the respective adjoining signal levels of the signal to be determined. 
     Further, in the case where a light modulated signal is used as the modulated (modulating) signal  14 , a SN ratio of a receiving signal will be determined mainly based on a shot noise as long as a noise caused by the spontaneous emission light or the thermal noise to be used for an optical receiver is sufficiently small. Under such condition, the larger the level of the multi-level signal is, the larger the noise level included in the multi-level signal becomes. Therefore, contrary to the case of  FIG. 5 , the multi-level encoding section  111  sets the step width to be small in the range where the level of the multi-level signal is small, and sets the step widths to be large in the range where the level of the multi-level signal is large, whereby it is possible to set, in an approximately uniformmanner, the respective SN ratios of the intervals between the respective adjoining signal levels of the signal to be determined. Accordingly, the quality of the receiving signal at the time of the eavesdropping by the third party is crucially deteriorated in a uniform manner, and it is possible to cause decryption/decoding of the multi-level signal by the third party to be difficult. 
     Second Embodiment 
     An overall configuration of a data communication apparatus according to a second embodiment of the present invention is the same as that of the data communication apparatus as shown in  FIG. 1 , and thus description thereof will be omitted. The data communication apparatus according to the second embodiment is different, only with regard to configurations of a first multi-level code generation section  111   a  and a second multi-level code generation section  212   a , from the first embodiment.  FIG. 6  is a block diagram showing an example of a detailed configuration of the first multi-level code generation section  111   a  according to the second embodiment of the present invention. In  FIG. 6 , the first multi-level code generation section  111   a  has a first random number sequence generation section  141  and a first multi-level conversion section  142 . The first random number sequence generation section  141  includes a pseudo-random number generation section  1411  and a serial/parallel conversion section  1412 . Here, an example of a case where the number of bits of the multi-level code sequence  12  is 8 bits (m=8) is shown. 
     The pseudo-random number generation section  1411  generates, based on inputted first key information  11 , a binary pseudo-random number series  31 . The serial/parallel conversion section  1412  performs serial/parallel conversion of the pseudo-random number series  31 , and outputs first to eighth modulation pseudo-random number sequences  32   a  to  32   h . The first to eighth modulation pseudo-random number sequences  32   a  to  32   h  are inputted to the first multi-level conversion section  142 . Further, the first modulation pseudo-random number sequence  32   a  is inputted to the multi-level processing section  111   b . The first multi-level conversion section  142  converts the first to eighth modulation pseudo-random number sequences  32   a  to  32   h  into the multi-level code sequence  12  having 2 m  multi-levels, and then outputs the same to the multi-level processing section  111   b.    
       FIG. 7  is a block diagram showing an example of a detailed configuration of the second multi-level code generation section  212   a  according to the second embodiment of the present invention. In  FIG. 7 , a configuration of the second multi-level code generation section  212   a  is basically the same as that of the first multi-level code generation section  111   a . Note that, in the second multi-level code generation section  212   a , outputs from a serial/parallel conversion section  2412  are referred to as first to eighth demodulation pseudo-random number sequences  42   a  to  42   h . The second multi-level code generation section  212   a  outputs a multi-level code sequence  17  and the first demodulation pseudo-random number sequence  42   a  to the decision section  212   b.    
       FIG. 8  is a diagram illustrating a signal format used for the data transmitting apparatus according to the second embodiment of the present invention. With reference to  FIG. 8 , a value of the multi-level code sequence  12  used in the present embodiment is determined based on the first to eighth modulation pseudo-random number sequences  32   a  to  32   h . Further, a level of a multi-level signal is determined based on the value of the multi-level code sequence  12  and a value of the information data  10 . Further, a step width of the multi-level signal is set to be equal to or smaller than a noise level. 
     The multi-level processing section  111   b  allocates respectively adjoining levels of the multi-level signal to different values of the information data  10  (“0” or “1”) in an alternate manner. For example, in the levels of the multi-level signal included in an upper half side of  FIG. 8 , the multi-level processing section  111   b  allocates the information data “0” in the case where the multi-level code sequence  12  is odd-numbered, and the information data “1” in the case where the multi-level code sequence  12  is even-numbered. Further, in the levels of the multi-level signal included in a lower half side of the  FIG. 8 , the multi-level processing section  111   b  allocates the information data “1” in the case where the multi-level code sequence  12  is odd-numbered, and the information data “0” in the case where multi-level code sequence  12  is even-numbered. In other words, a manner of the multi-level processing section  111   b  relating each of the levels of the multi-level signal to either “0” or “1” is determined based on a value of the first modulation pseudo-random number sequence  32   a  which corresponds to a lowest-order bit of the multi-level code sequence  12 . Accordingly, it becomes impossible for an eavesdropper who does not have key information to identify data directly, and consequently the eavesdropper is forced to try to identify the key information so as to execute eavesdropping by first performing a multi-level determination of all the levels of the multi-level signal. 
     On the other hand, in the data receiving apparatus, an identification level of a received multi-level signal is determined based on values of the first to eighth demodulation pseudo-random number sequences  42   a  to  42   h . The decision section  212   b  decides the value of the information data in accordance with a level of the received multi-level signal, the identification level of the multi-level signal, and a value of the first demodulation pseudo-random number sequence  42   a.    
     Specifically, the decision section  212   b  decides the value of the information data as “1” in the case where the level of the received multi-level signal is larger than the identification level, and the value of the first demodulation pseudo-random number sequence  42   a  is “0”, also in the case where the level of the received multi-level signal is smaller than the identification level, and the value of the first demodulation pseudo-random number sequence  42   a  is “1”. Contrary to this, the decision section  212   b  decides the value of the information data as “0” in the case where the level of the received multi-level signal is larger than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “1”, and also in the case where the level of the received multi-level signal is smaller than the identification level, and the value of the first demodulation pseudo-random number sequence  42   a  is Note that, the examples of  FIG. 6  and  FIG. 7  illustrate cases where the number of the modulation pseudo-random number sequences is 8, however, the number of the modulation pseudo-random number sequences is not limited thereto, and can be set arbitrarily. 
     As above described, according to the present embodiment, in the case where the eavesdropper attempts the multi-level determination of the multi-level signal so as to identify the key information, an error in identification of the multi-level signal will occur, as with a case of the first embodiment, since the step-width of the multi-level signal is set to be equal to or smaller than the noise level. Accordingly, the data communication apparatus according to the second embodiment can crucially deteriorates quality of a receiving signal at the time of eavesdropping by a third party, whereby it is possible to provide a safe data communication apparatus which causes decryption/decoding of the receiving signal to be difficult. 
     Third Embodiment 
     In the data communication apparatus according to the second embodiment (see  FIG. 6  and  FIG. 7 ), it is necessary to change the first to eighth modulation pseudo-random number sequences  32   a  to  32   h  and the value of the multi-level code sequence  12  at the same rate as a bit rate of the information data  10 . Here, a rate of a pseudo-random number series  31  (that is, a random number generation rate of a pseudo-random number generation section  1411 ) is obtained from a product of the bit rate of the information data  10  and the number of the bits of the multi-level code sequence  12 . Therefore, the random number generation rate of the pseudo-random number generation section  1411  increases as the number of multi-levels of the multi-level code sequence  12  increases. On the other hand, a receiving SN ratio of an eavesdropper deteriorates as the number of the multi-levels increases, and thus the more the number of the multi-levels increases, the more significant identification error the eavesdropper will incur. Accordingly, the more the number of the multi-levels are increased for the sake of security, the more the random number generation rate required to the pseudo-random number generation section  1411  is increased, which lead to a problem in that it is difficult to realize such pseudo-random number generation section  1411 . The present embodiment aims to solve such problem. 
     An overall configuration of a data communication apparatus according to a third embodiment of the present invention is the same as that of the data communication apparatus as shown in  FIG. 1 , and thus description thereof will be omitted. The data communication apparatus according to the third embodiment is different, only with regard to configurations of a first multi-level code generation section  111   a  and a second multi-level code generation section  212   a , from the second embodiment. Hereinafter, component parts which are the same as those of the second embodiment are omitted by providing common reference characters, and the data communication apparatus according to the third embodiment will be described by mainly focusing such components parts that are different from those of the second embodiment. 
       FIG. 9  is a block diagram showing an example of a detailed configuration of the first multi-level code generation section  111   a  according to the third embodiment of the present invention. In  FIG. 9 , the first multi-level code generation section  111   a  has a first random number sequence generation section  141  and a first multi-level conversion section  142 . The first random number sequence generation section  141  includes a pseudo-random number generation section  1411  and a serial/parallel conversion section  1412 . Here, an example of a case where the number of bits of the multi-level code sequence  12  is 8 bits (m=8) is shown. 
     In the first multi-level code generation section  111   a , the pseudo-random number generation section  1411  generates, in a similar manner to the second embodiment (see  FIG. 6 ), a binary pseudo-random number series  31  in accordance with the first key information  11 . The serial/parallel conversion section  1412  performs serial/parallel conversion of the pseudo-random number series  31  and outputs first to fourth modulation pseudo-random number sequences  32   a  to  32   d . Here, the number of the modulation pseudo-random number sequences outputted from the serial/parallel conversion section  1412  is smaller than the number of bits of a bit sequence to be inputted to the first multi-level conversion section  142  (that is, an input bit sequence). The first to fourth modulation pseudo-random number sequences  32   a  to  32   d  are inputted to the first multi-level conversion section  142  as a part of the input bit sequence. For example, as shown in  FIG. 9 , the modulation pseudo-random number sequences  32   a  and  32   b , and the modulation pseudo-random number sequences  32   c  and  32   d  are inputted to low-order 2 bits and to high-order 2 bits, respectively, of an 8-bit input bit sequence. Fixed values are inputted to remaining parts of the input bit sequence. The first multi-level conversion section  142  converts the inputted bit sequences into the multi-level code sequence  12  having 2 m multi-levels and then outputs the same to the multi-level processing section  111   b.    
       FIG. 10  is a block diagram showing an example of a detailed configuration of the second multi-level code generation section  212   a  according to the third embodiment of the present invention. In  FIG. 10 , the second multi-level code generation section  212   a  has a second random number sequence generation section  241  and a second multi-level conversion section  242 . The second random number sequence generation section  241  includes a pseudo-random number generation section  2411  and a serial/parallel conversion section  2412 . 
     In the second multi-level code generation section  212   a , the pseudo-random number generation section  2411  generates and outputs, based on the second key information  21 , a binary pseudo-random number series  41 . The serial/parallel conversion section  2412  performs serial/parallel conversion of the pseudo-random number series  41 , and outputs first to fourth demodulation pseudo-random number sequences  42   a  to  42   d . Here, the number of the demodulation pseudo-random number sequences outputted from the serial/parallel conversion section  2412  is smaller than the number of bits of a bit sequence to be inputted to the second multi-level conversion section  242  (that is, the input bit sequence). A part of the demodulation pseudo-random number sequences outputted from the serial/parallel conversion section  2412  is inputted to the second multi-level conversion section  242  as a part of the input bit sequence. 
     For example, as shown in  FIG. 10 , the third and the fourth demodulation pseudo-random number sequences  42   c  and  42   d  are inputted to the second multi-level conversion section  242  as high-order bits of the input bit sequence. It is preferable that a position in the input bit sequence to the second multi-level conversion section  242  to which the demodulation pseudo-random number sequences are to be inputted is the same as that of a high-order bit in the input bit to the first multi-level conversion section  142  to which the modulation pseudo-random number sequences are inputted. Fixed values are inputted to remaining bit sequence positions of the input bit sequence to the second multi-level conversion section  242  to which the demodulation pseudo-random number sequences are not inputted. The second multi-level conversion section  242  converts the input bit sequence into the multi-level code sequence  22  having 2 m multi-levels and then outputs the same. 
       FIG. 11  is a diagram illustrating a signal format used for a data transmitting apparatus according to the third embodiment of the present invention. With reference to  FIG. 11 , in the case where four bits of the input bit sequence to the first multi-level conversion section  142  are fixed values, the number of the levels which the multi-level code sequence  12  may actually obtain is 16. The level of the multi-level signal is determined based the multi-level code sequence  12  and a value of the information data  10  (“0” or “1”), and thus the number of the levels which the multi-level signal  13  may obtain is 32. These levels are divided into 8 groups respectively having four levels respectively including values which are close to one another. A step width of the multi-level signal in each of the groups is set to be equal to or smaller than a noise level. Further, it is preferable that a difference between a highest-order level and a lowest-order level in each of the groups is equal to or smaller than the noise level. 
     Further, the multi-level processing section  111   b  allocates, in each of the groups, respectively adjoining levels of the multi-level signal to different values of the information data  10  (“0” or “1”) in an alternate manner. For example, in the levels of the multi-level signal included in an upper-half side as shown in  FIG. 11 , the multi-level processing section  111   b , allocates the information data “0” in the case where the multi-level code sequence  12  is odd-numbered, and allocates the information data “1” in the case where the multi-level code sequence  12  is even-numbered. Further, in the levels of the multi-level signal included in a lower-half side as shown in  FIG. 11 , the multi-level processing section  111   b  allocates the information data “1” in the case where the multi-level code sequence  12  is odd-numbered, and allocates the information data “0” in the case where the multi-level code sequence  12  is even-numbered. In other words, a manner in which the multi-level processing section  111   b  relates each of the levels of the multi-level signal to either of “0” or “1” is determined based on a value of the first modulation pseudo-random number sequence  32   a  which corresponds to a lowest-order bit of the multi-level code sequence  12 . 
     On the other hand, in a data receiving apparatus, an identification level of a received multi-level signal is determined based on values of the third and the fourth demodulation pseudo-random number sequences  42   c  and  42   d . The data receiving apparatus may also use values of the first and the second demodulation pseudo-random number sequences  42   a  and  42   b  when determining the identification level, however, since fluctuation of the identification level corresponding to the values is small, an error rate after identification will not deteriorate even if the identification level is determined with the fluctuation being ignored. The decision section  212   b  decides the value of the information data in accordance with the level of the received multi-level signal, the identification level of the multi-level signal, and the value of the first demodulation pseudo-random number sequence  42   a.    
     Specifically, the decision section  212   b  decides the value of the information data as “1” in the case where the level of the received multi-level signal is greater than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “0”, and also in the case where the level of the received multi-level signal is smaller than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “1”. On the other hand, the decision section  212   b  decides the value of the information data as “0” in the case where the level of the received multi-level signal is greater than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “1”, and also in the case where the level of the received multi-level signal is smaller than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “0”. 
     The random number generation rate required to the pseudo-random number generation section  1411  in the configuration of  FIG. 9  is four times of the bit rate of the information data  10 , since the number of output bits (the number of the modulation pseudo-random number sequences) of the serial/parallel conversion section  1412  is four, and compared to the case of the configuration of  FIG. 6  (8 times of the bit rate of the information data  10 ), the random number generation rate of the pseudo-random number generation section  1411  can be halved. 
     Note that the fluctuation of the levels of the multi-level signal corresponding to the first and the second demodulation pseudo-random number sequences  42   a  and  42   b  which are not used for generating the identification level leads to a deterioration of a signal level, that is, an deterioration of an SN ratio, at the time of identification. However, if such deteriorated SN ratio is set so as to satisfy a required value of the data receiving apparatus  1201 , a legitimate receiving party can identify the multi-level signal without an error. That is, a ratio of a information amplitude to a fluctuation width of the multi-level signal corresponding to the low-order bits of the demodulation pseudo-random number sequences is set so as to satisfy a condition of being greater than the SN ratio acceptable to the legitimate receiving party. The SN ratio acceptable to the legitimate receiver is determined based on a bit error rate of data required by the legitimate receiving party. For example, in optical communications, a value equal to or smaller than 10 −12  are generally used, as an acceptable bit error rate, and for this case, acceptable SN rate is equal to or more than 23 dB. 
     Further, in the example of  FIG. 9 , the number of input bits to the first multi-level conversion section  142  is 8 bits, and the number of the modulation pseudo-random number sequences is four, and the example shows that the modulation pseudo-random number sequences are inputted to the high-order 2 bits and low-order 2 bits of the input bit sequence to the first multi-level conversion section  142 , but is merely one example. The number of input bits to the first multi-level conversion section  142  is arbitrary, and the numbers of the modulation pseudo-random number sequences and the demodulation pseudo-random number sequences can be set arbitrarily in accordance with a ratio of a feasible random number generation rate to a required bit rate. Further, the number of the modulation pseudo-random number sequences to be allocated to the high-order bits and low-order bits of the input bit to the first multi-level conversion section  142  can be set arbitrarily if it satisfies a condition where any of the modulation pseudo-random number sequences is definitely inputted to the lowest-order bit of the input bit sequence. 
     As above described, according to the present embodiment, in the case where the eavesdropper attempts a multi-level determination of the multi-level signal so as to identify the key information, an identification error of the multi-level signal occurs in the similar manner to the first embodiment since the step width of the multi-level signal in a single group is set to be equal to or smaller than the noise level. Further, the signal levels of the multi-level signal is allocated appropriately, whereby it is possible to keep, at a low level, an increase in the random number generation rate required to the pseudo-random number generator, thereby improving the security. Therefore, the data communication apparatus according to the third embodiment can crucially deteriorates quality of a receiving signal at the time of eavesdropping by a third party, whereby it is possible to provide a safe data communication apparatus which causes decryption/decoding of the receiving signal to be difficult. 
     Fourth Embodiment 
     An overall configuration of a data communication apparatus according to a fourth embodiment of the present invention is the same as that of the data communication apparatus as shown in  FIG. 1 , and thus description thereof will be omitted. The data communication apparatus according to the fourth embodiment is different, only with regard to a configuration of a first multi-level code generation section  111   a , from the third embodiment. Hereinafter, component parts which are the same as those of the third embodiment are omitted by providing common reference characters, and the data communication apparatus according to the fourth embodiment will be described by mainly focusing such components parts that are different from those of the third embodiment. 
       FIG. 12  is a block diagram showing an example of a detail configuration of the first multi-level code generation section  111   a  according to the fourth embodiment of the present invention. In  FIG. 12 , the first multi-level code generation section  111   a  has a first random number sequence generation section  141 , first multi-level conversion section  142 , and a physical random number generation section  143 . The first random number sequence generation section  141  includes a pseudo-random number generation section  1411  and a serial/parallel conversion section  1412 . Here, an example of a case where the number of bits of the multi-level code sequence  12  is 8 bits (m=8) is shown. A second multi-level code generation section  212   a  in the present embodiment has a configuration as shown in  FIG. 7 , as with the second embodiment. 
     Next, an action of the data communication apparatus according to the present embodiment will be described. Actions of the pseudo-random number generation section  1411  and the serial/parallel conversion section  1412  are the same as those of the second embodiment. The physical random number generation section  143  generates and outputs one or a plurality of physical random number sequences. In the example of  FIG. 12 , the physical random number generation section  143  outputs first to fourth physical random number sequences  33   a  to  33   d . Here, the number of modulation pseudo-random number sequences  32   a  to  32   d  outputted from the serial/parallel conversion section  1412  is set so as to be smaller than the number of bits of the input bit sequence to the first multi-level conversion section  142 . The first to fourth modulation pseudo-random number sequences  32   a  to  32   d  are inputted as a part of the input bit sequence to the first multi-level conversion section  142 . The first to the fourth physical random number sequences  33   a  to  33   d  are inputted to a remaining part of the input bit sequence. The first multi-level conversion section  142  converts the input bit sequence into a multi-level code sequence  12  having 2 m  multi-levels and outputs the same. 
       FIG. 13  is a diagram illustrating a signal format used for the data transmitting apparatus according to the fourth embodiment of the present invention. The signal format as shown in  FIG. 13  corresponds to the configuration of the first multi-level code generation section  111   a  as shown in  FIG. 12 . With reference to  FIG. 13 , the first multi-level code generation section  111   a  determines high-order 2 bits and low-order 2 bits of 8 bits of the multi-level code sequence  12 , in accordance with the modulation pseudo-random number sequences  32   a  to  32   d , and also determines intermediate 4 bits in accordance with the physical random number sequences  33   a  to  33   d . Therefore, the number of levels of the multi-level code sequence corresponding to the first to fourth modulation pseudo-random number sequences  32   a  to  32   d  is  16 . A step width of the multi-level signal is set to be equal to or smaller than a noise level. Further, respectively adjoining levels of the multi-level signal are allocated to different values of the information data. 
     On the other hand, in a data receiving apparatus, an identification level of a received multi-level signal is determined, in a similar manner to the second embodiment, based on values of the third and the fourth demodulation pseudo-random number sequences  42   c  and  42   d . In the decision section  212   b , a value of the information data is decided based on the level of the multi-level signal, the identification level of the multi-level signal, and the value of the first demodulation pseudo-random number sequence  42   a.    
     Specifically, the decision section  212   b  decides the value of the information data as “1” in the case where the level of the received multi-level signal is greater than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “0”, and also in the case where the level of the received multi-level signal is smaller than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “1”. On the other hand, the decision section  212   b  decides the value of the information data as “0” in the case where the level of the received multi-level signal is greater than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “1”, and also in the case where the level of the received multi-level signal is smaller than the identification level and the value of the first demodulation pseudo-random number sequence  42   a  is “0”. 
     Note that fluctuation of the levels of the multi-level signal corresponding to the first to the fourth physical random number sequences which are not used for generating the identification level leads to a deterioration of a signal level, that is, a deterioration of an SN ratio, at the time of identification. However, if such deteriorated SN ratio is set so as to satisfy a required value of the data receiving apparatus  1201 , a legitimate receiving party can identify the multi-level signal without error. That is, a ratio of a information amplitude to a fluctuation width of the multi-level signal corresponding to the physical random number sequence is required to be set so as to satisfy a condition of being greater than the SN ratio acceptable to the legitimate receiving party. 
     As a configuration which can obtain the same effect as the first multi-level code generation section  111  as shown in  FIG. 12 , a configuration as shown in  FIG. 14A  may be considered.  FIG. 14A  is a block diagram showing an example of another configuration of the first multi-level code generation section  111   a  according to the fourth embodiment of the present invention.  FIG. 14A  is the same, with regard to functional blocks and actions thereof contained in the configuration, as  FIG. 12 , but is different from  FIG. 12  in that  FIG. 14A  includes a bit sequence, as the input bit sequence to the first multi-level conversion section  142 , to which not only the modulation pseudo-random number sequences  32   a  to  32   c  and the physical random number sequences  33   a  to  33   b  but also fixed values are inputted.  FIG. 15  illustrates a multi-level signal format in this exemplary configuration. In this case, fixed values are allocated to 2 bits of the input bit sequence to the first multi-level conversion section  142 , the number of levels which the multi-level code sequence  12  may obtain is  64 . Since the level of the multi-level signal corresponds to the multi-level code sequence  12  and the value of the information data  10  (“0” or “1”), the number of the level to be obtained is  128 . These levels are divided into 8 groups respectively having 16 levels respectively including values which are close to one another. The step width of the multi-level signal in each of the groups is set to be equal to or smaller than the noise level. Further, in each of the groups, respectively adjoining levels of the multi-level signal are allocated to different values of the information data. On the other hand, the identification level is determined, in a similar manner to a case of  FIG. 9 , based on the values of the third and the fourth demodulation pseudo-random number sequences  42   c  and  42   d.    
     Further, as a configuration which can obtain the same effect as the first multi-level code generation section  111   a  as shown in  FIG. 12 , a configuration as shown in  FIG. 14A  may be considered.  FIG. 14B  is a block diagram showing an example of another configuration of the first multi-level code generation section  111   a  according to the fourth embodiment of the present invention.  FIG. 14B  is basically the same, with regard to functional blocks and actions thereof contained in the configuration, as  FIG. 12 , but is different from  FIG. 12  in that, as a part of the input bit sequence to the first multi-level conversion section  142 , signals generated based on a predetermined rule are inputted instead of the physical random number sequences  33   a  to  33   d . In the example as shown in  FIG. 14B , signals, which are generated by providing predetermined delay time to the modulation pseudo-random number sequences  32   a  to  32   c , are inputted to the first multi-level conversion section  142  as the signals generated based on the predetermined rule. 
     Note that the examples of  FIG. 9  and  FIG. 10  shows that the number of input bits to the first multi-level conversion section  142  is 8 bit, and the numbers of the modulation pseudo-random number sequences and the demodulation pseudo-random number sequences are respectively four, and the modulation pseudo-random number sequences are inputted to the high-order 2 bits and the low-order 2 bits of the input bit sequence to the first multi-level conversion section  142 , but these are merely one examples, respectively. The number of the input bits to the first multi-level conversion section  142  is arbitrary, and the numbers of the modulation pseudo-random number sequences and the demodulation pseudo-random number sequences can be set arbitrarily in accordance with a ratio of a feasible random number generation rate to a required bit rate. Further, the number of the physical random number sequences can be set arbitrarily if the number of the same is equal to or smaller than a difference between the number of the input bits to the first multi-level conversion section  142  and the number of the modulation pseudo-random number sequences. Further, selection of whether either of the modulation pseudo-random number sequence or the physical random number sequence, or the fixed value is to be inputted to respective positions of the input bit sequence can be set arbitrarily if it satisfies a condition where the modulation pseudo-random number sequence is definitely inputted to the lowest-order bit of the input bit sequence. 
     As above described, according to the present embodiment, the number of the levels which the multi-level signal may obtain is greater than the third embodiment, and thus the number of the levels of the multi-level signal which is likely to be identified erroneously at the time of the multi-level determination by the eavesdropper also increases, whereby eavesdropping will become difficult. Further, it is possible to keep, at a low level, an increase in the random number generation rate required to the pseudo-random number generator, thereby improving the security. Therefore, the data communication apparatus according to the fourth embodiment can crucially deteriorates quality of a receiving signal at the time of eavesdropping by a third party, whereby it is possible to provide a safe data communication apparatus which causes decryption/decoding of the receiving signal to be difficult. 
     Fifth Embodiment 
     The fifth embodiment of the present invention aims to keep a pseudo-random number generation rate constant and to transmit information data  10  at different bit rates. An overall configuration of a data communication apparatus according to the fifth embodiment of the present invention is the same as that of the data communication apparatus as shown in  FIG. 1 , and thus description thereof will be omitted. The data communication apparatus according to the fifth embodiment is different, only with regard to configurations of a first random number sequence generation section and a second random number sequence generation section  241 , from the third embodiment. Hereinafter, component parts which are the same as those of the third embodiment are omitted by providing common reference characters, and the data communication apparatus according to the third embodiment will be described by mainly focusing such components parts that are different from those of the third embodiment. 
       FIG. 16  is a block diagram showing an example of a detail configuration of the first random number sequence generation section  141  according to the fifth embodiment of the present invention. In  FIG. 16 , the first random number sequence generation section  141  has a pseudo-random number generation section  1411 , a first switch  1413 , a first serial/parallel conversion section  1414 , a second serial/parallel conversion section  1415 , and a second switch  1416 . 
     Next, an action of the data communication apparatus according to the present embodiment will be described. In a similar manner to the second embodiment, the pseudo-random number generation section  1411  generates a binary pseudo-random number series  31  in accordance with the first key information  11 . The first switch  1413  switches, based on a rate selection signal  36  to be inputted, an output destination of the pseudo-random number series  31  between the first serial/parallel conversion section  1414  and the second serial/parallel conversion section  1415 . The first serial/parallel conversion section  1414  performs serial/parallel conversion of the pseudo-random number series  31 , and outputs first to eighth modulation pseudo-random number sequences  34   a  to  34   h . The number of the modulation pseudo-random number sequences outputted from the first serial/parallel conversion section  1414  is the same as the number of the input bits to the first multi-level conversion section  142 . The second serial/parallel conversion section  1415  performs serial/parallel conversion of the pseudo-random number series  31  and outputs a first to a fourth modulation pseudo-random number sequences  35   a  to  35   d . The number of the modulation pseudo-random number sequences outputted from the second serial/parallel conversion section  1415  is set to be smaller than the number of the input bits to the first multi-level conversion section  142 . 
     The first to eighth modulation pseudo-random number sequences  34   a  to  34   h  outputted from the first serial/parallel conversion section  1414  and the first to fourth modulation pseudo-random number sequences  35   a  to  35   d  outputted from the second serial/parallel conversion section  1415  are inputted to the second switch  1416 . The second switch  1416  selects, based on the rate selection signal  36 , either of the inputs from the first serial/parallel conversion section  1414  or the second serial/parallel conversion section  1415 , to be outputted to the first multi-level conversion section  142 . Here, to the second serial/parallel conversion section  1415 , the first to the fourth modulation pseudo-random number sequences  35   a  to  35   d  are inputted, and fixed values are also inputted as remaining bit sequences. The configuration and an action of the second random number sequence generation section  241  are not shown, but are the same as those of the first random number sequence generation section  141 . 
     In the case where the first switch  1413  and the second switch  1416  are switched to the first serial/parallel conversion section  1414  side, the data communication apparatus according to the present embodiment performs the same action as that according to the second embodiment. A bit rate of such case is ⅛ of the random number generation rate in the pseudo-random number generation section  1411 . On the other hand, the first switch  1413  and the second switch  1416  are switched to the second serial/parallel conversion section  1415  side, the data communication apparatus according to the present embodiment performs the same action as that according to the third embodiment. The bit rate of such case is ¼ of the random number generation rate in the pseudo-random number generation section  1411 . In this manner, a plurality of serial/parallel conversion sections, which respectively output different numbers of modulation pseudo-random number sequences, is prepared and used by switching therebetween, whereby it is possible to correspond to different bit rates in spite of being a single pseudo-random number generation rate. That is, since a product of the number of the modulation pseudo-random number sequence and the bit rate is equal to the pseudo-random number generation rate, and thus it is possible to vary the bit rate by switching the number of the modulation pseudo-random number sequences, which is limited to a case where remaining configuration blocks which are not shown in  FIG. 16  can be adapted to any transmittable bit rates. 
     An exemplary configuration of  FIG. 16  is merely an example, and any configuration may be possible if the bit rate can be switched by switching the number of the modulation pseudo-random number sequences while the pseudo-random number generation rate is kept constant. Further, the value of the bit rate to be switched is not limited to two, and can be set arbitrarily as necessary. 
     As above described, according to the present embodiment, it is possible to respond to a plurality of bit rates while the random number generation rate of the pseudo-random number generation section is kept constant. 
     Note that each of the data communication apparatuses according to the first to the fifth embodiments may have a configuration which combines features of the remaining embodiments. Further, processing performed by each of the data transmitting apparatuses, the data receiving apparatuses, and the data communication apparatuses according to the above-described first to fifth embodiments may be respectively regarded as a data transmitting method, a data receiving method, and a data communication method, each of which cause a series of processing procedure to be executed. 
     Further, the above-described data transmitting method, the data receiving method, and the data communication method may be realized by causing a CPU to interpret and execute predetermined program data which is capable of executing the above-described processing procedure stored in a storage device (such as a ROM, a RAM, and a hard disk). In such case, the program data may be executed after being stored in the storage device via a storage medium, or may be executed directly from the storage medium. Note that the storage medium includes a ROM, a RAM, a semiconductor memory such as a flash memory, a magnetic disk memory such as a flexible disk and a hard disk, an optical disk such as a CD-ROM, a DVD, and a BD, a memory card, or the like. Further, the storage medium is a notion including a communication medium such as a telephone line and a carrier line. 
     The data communication apparatus according to the present invention is useful as a safe secret communication apparatus which is unsusceptible to eavesdropping/interception. 
     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.