Patent Publication Number: US-7912215-B2

Title: Data transmission apparatus, data receiving apparatus and method executed thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatuses for performing cipher communication in order to prevent illegal eavesdropping and interception by a third party, and more particularly, relates to a data transmitting apparatus, a data receiving apparatus, and a method executed thereby 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 Background Art 
     Conventionally, in order to perform 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. 11  shows a configuration of a conventional data communication apparatus based on the above-described structure. 
     In  FIG. 11 , 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 (encrypts), based on the first key information  91 , the information data  90 . The modulator section  912  converts the information data encrypted by the encoding section  911  into a modulated signal  94  in a predetermined modulation method and transmits the same to the transmission line  913 . 
     In the data receiving apparatus  9002 , the demodulator section  914  demodulates, in a predetermined demodulation method, the modulated 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 , which is shared with the encoding section  911 , is inputted. The decoding section  915  decodes (decrypts) the modulated signal  94  in accordance with the second key information  96  and outputs the original information data  98 . 
     Here, by using an eavesdropper&#39;s data receiving apparatus  9003 , eavesdropping by a third party will be described. In  FIG. 11 , the eavesdropper&#39;s data receiving apparatus  9003  includes an eavesdropper&#39;s demodulator section  916  and an eavesdropper&#39;s decoding section  917 . The eavesdropper&#39;s demodulator section  916  eavesdrops on the modulated signal  94  transmitted between the data transmitting apparatus  9001  and the data receiving apparatus  9002 , and decodes the eavesdropped modulated signal  94  in a predetermined demodulation method. The eavesdropper&#39;s decoding section  917  attempts decoding of the demodulated information data, in accordance with third key information  99 . Here, due to no key information sharing with the encoding section  911 , the third key information  99  is different in content from the first key information  91 . Therefore, the eavesdropper&#39;s decoding section  917  cannot accurately reproduce the original information data  90  inputted to the encoding section  911  even if the decoding is performed based on the third key information  99 . 
     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 as described, for example, in Japanese Laid-Open Patent Publication No. 9-205420 (hereinafter referred to as patent document 1). That is, in a PON (Passive Optical Network) structure 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 other subscribers are inputted to each of the optical receivers. Therefore, information data for each of the subscribers is encrypted by using key information which is different by the subscribers, whereby it is possible to prevent a leakage/eavesdropping of mutual information and realize safe data communication. 
     However, in the case of the conventional data communication apparatus based on the mathematical encryption technique, even if the eavesdropper does not share the key information, it is theoretically possible for the eavesdropper to succeed in decryption, with respect to a cipher text (modulated signal or encrypted information data), by means of an operations using all possible combinations of key information (an all-possible attack), or by means of a special analysis algorithm. Particularly, improvement in the processing speeds of computers has been remarkable in recent years, and thus there is a problem in that if a new computer based on a novel principle such as a quantum computer is realized in the future, it is possible to eavesdrop on the cipher text easily within finite lengths of time. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a data transmitting apparatus and a data receiving apparatus which cause an eavesdropper to take a significantly increased time to analyze a cipher text and consequently realize highly concealable data communication. 
     The present invention is directed to a data transmitting apparatus and a method for performing cipher communication. To attain the above object, the data transmitting apparatus of the present invention includes a multi-level code generation section, a combining section, an error-correction encoding section, a multi-level processing section, and a modulator section. The data transmitting method of the present invention is realized by executing respective steps included in the method. 
     The multi-level code generation section generates, by using predetermined key information, a multi-level code sequence in which a signal level changes so as to be approximately random numbers. The combining section combines accompanying data to the multi-level code sequence with predetermined frequency and generates a combined multi-level signal. The error-correction encoding section adds an error-correction code to information data, in accordance with predetermined error correction encoding processing, and outputs error-correction encoded information data. The multi-level processing section combines the combined multi-level signal and the error-correction encoded information data in accordance with predetermined processing, and generates a multi-level signal having a level uniquely corresponding to a combination of levels of the combined signals. The modulator section generates a modulated signal, in a predetermined modulation method, based on the multi-level signal. 
     The accompanying data is a synchronous signal which is synchronizing with the information data, an N frequency-dividing clock of the information data, or a predetermined code-synchronous pattern. 
     Further, the present invention is also directed to a data receiving apparatus and a method for performing cipher communication. To attain the above object, the data receiving apparatus of the present invention includes a demodulator section, a multi-level code generation section, a multi-level identification section, and an error-correction decoding section. The data receiving method of the present invention is realized by executing respective steps included in the method. 
     The demodulator section demodulates a modulated signal, in a predetermined modulation method, generated based on error-correction encoded information data and a combined multi-level signal having accompanying data included therein, and outputs a multi-level signal obtained by the demodulation. The multi-level code generation section generates, by using predetermined key information, a multi-level code sequence in which a signal level changes so as to be approximately random numbers. The multi-level identification section identifies the multi-level signal in accordance with the multi-level code sequence, and outputs data which is identified and reproduced. The error-correction decoding section detects, from the data reproduced by the multi-level identification section, difference between the combined multi-level signal and the multi-level code sequence, in accordance with predetermined error-correction decoding processing, and outputs a result of the detection as the accompanying data, and also outputs information which is error-correction decoded as the information data. 
     Here, in the case where the multi-level code generation section generates, by using predetermined key information and a synchronous signal, a multi-level code sequence in which a signal level changes so as to be approximately random numbers, it is possible to further include a synchronous extraction section for inputting the accompanying data, extracting a synchronous signal synchronizing with the accompanying data in accordance with a predetermined procedure, and outputting the extracted synchronous signal to the multi-level code generation section. 
     It is preferable that the synchronous extraction section inputs a predetermined code-synchronizing pattern and accompanying data, extracts the synchronous signal synchronizing with the accompanying data in accordance with a predetermined procedure, and outputs the extracted synchronous signal to the multi-level code generation section. 
     According to the present invention, the information data is encoded/modulated into the multi-level signal by using the key information, and the received multi-level signal is decoded/demodulated, by using a common key information, whereby a signal-to-noise power ratio is adjusted appropriately. Accordingly, time for analyzing a cipher text is increased significantly, whereby it is possible to perform highly concealable data communication. 
     Further, the error-correction code is added to the information data to be transmitted, and the multi-level code sequences respectively in the data transmitting apparatus and the data receiving apparatus are caused to be in discord with each other by using the accompanying data, whereby the data to be transmitted is encrypted, and an error of the receiving data occurring in the receiving apparatus is corrected. Accordingly, the information data and the accompanying data is transmitted/received simultaneously, whereby it is possible to provide a highly concealable data communication apparatus. 
     Further, the synchronous signal is extracted from the accompanying data, and multi-level signal is synchronized with and identified by the multi-level code sequence generated based on the synchronous signal, whereby it is possible to realize a data communication apparatus of a simple configuration having a synchronous system. 
     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 a configuration of data communication apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating waveforms 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 waveforms 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 block diagram showing a configuration of the data communication apparatus according to the second embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating a method executed by a data transmitting apparatus  1102  of  FIG. 5 . 
         FIG. 7  is a flowchart illustrating a method executed by a data receiving apparatus  1202  of  FIG. 5 . 
         FIG. 8  is a diagram showing an action (legitimate communication) of the data communication apparatus according to the second embodiment of the present invention. 
         FIG. 9  is a diagram showing an action (eavesdropping) of the data communication apparatus according to the second embodiment of the present invention. 
         FIG. 10  is a block diagram showing a configuration of the data communication apparatus according to the third embodiment of the present invention. 
         FIG. 11  is a block diagram showing a configuration of a conventional data communication apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Herein after, embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a block diagram showing a configuration of data communication apparatus according to a first embodiment of 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 multi-level identification section  212   b . As the transmission line  110 , 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. 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  and  FIG. 3  are diagrams illustrating waveforms of a modulated signal  14  outputted from the modulator section  112 . Hereinafter, with reference to  FIG. 1  to  FIG. 4 , actions of the data communication apparatus according to the first embodiment will be described. 
     The first multi-level code generation section  111   a  generates, by using predetermined first key information  11 , a multi-level code sequence  12  ((b) of  FIG. 2 ) in which a signal level changes so as to be approximately 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 level corresponding to a combination of the both signal levels. For example, in the case where the 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 level of 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 into the modulated signal  14  and transmits the same 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 take corresponding to levels c 1 /c 2 /c 3 /c 4 /c 5  of the multi-level code sequence  12  are respectively referred to as a first to a fifth “bases”, and a minimum interval between the 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, generates, based on the second key information  16 , a multi-level code sequence  17 . The multi-level identification section  212   b  identifies (binary determination) the multi-level signal  15  by using the multi-level code sequence  17  as a threshold, and reproduces information data  18 . Here, the modulated signal  14 , in the predetermined modulation method, which is transmitted/received between the modulator section  112  and the demodulator section  211  via the transmission line  110 , is obtained by modulating an electromagnetic wave (electromagnetic field) or an optical 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 methods, in addition to a method of generating the multi-level signal  13  by adding up 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, based on the information data  10 , the multi-level signal  13 , by modulating an amplitude of the level 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 the level of the multi-level signal  13  stored therein, the level of the multi-level signal  13 , which is corresponding to the combination of the information data  10  and the multi-level code sequence  12 . 
     Further, in  FIG. 2  and  FIG. 3 , the level of the multi-level signal  13  is represented as 8 levels, but the level of the multi-level signal  13  is 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 the 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 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 thereof may be faster (or slower) than the change rate of another, and further, the both may be in an asynchronous relation. 
     Next, eavesdropping on the modulated signal  14  by a third party will be described. 
     It is assumed that the third party, who is an eavesdropper, decodes the modulated signal  14  by using an apparatus having a configuration corresponding to the data receiving apparatus  1201  held by a legitimate receiving end or a further sophisticated data receiving apparatus (hereinafter referred to as an eavesdropper&#39;s data receiving apparatus). The eavesdropper&#39;s data receiving apparatus reproduces the multi-level signal  15  by demodulating the modulated signal  14 . However, the eavesdropper&#39;s data receiving apparatus does not share key information with the data transmitting apparatus  1101 , and thus, unlike the data receiving apparatus  1201 , the eavesdropper&#39;s data receiving apparatus cannot generate the multi-level code sequence  17 , based on the key information. Therefore, the eavesdropper&#39;s 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 may be possible under these circumstances, there is a method of simultaneously performing identification of all levels of the multi-level signal  15  (generally referred to as an all-possible attack). That is, the eavesdropper&#39;s data receiving apparatus performs simultaneous determination of the multi-level signal  15  by preparing thresholds corresponding to all possible intervals between the respective signal levels which the multi-level signal  15  may take, and attempts extraction of correct key information or information data by analyzing a result of the determination. For example, the eavesdropper&#39;s 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, and performs the multi-level determination with respect to the multi-level signal  15 , thereby attempting 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 signal  14 , whereby the respective levels of the multi-level signal  15  fluctuates temporally/instantaneously as shown in  FIG. 4 . In this case, a SN ratio (a signal-to-noise intensity ratio) of a signal (the multi-level signal  15 ), which is to be determined by the legitimate receiving end (that is, the data receiving apparatus  1201 ), is determined based on a ratio of the information amplitude of the multi-level signal  15  to the noise level. On the other hand, the SN ratio of the signal (the multi-level signal  15 ), which is to be determined by the eavesdropper&#39;s data receiving apparatus, is determined based on a ratio of the step width of the multi-level signal  15  to the noise level. 
     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&#39; data receiving apparatus is relatively smaller than that by the data receiving apparatus  1201 , and thus a transmitting feature (an error rate) of the eavesdropper&#39;s data receiving apparatus is deteriorated. That is, the data communication apparatus of the present invention utilizes this feature, and leads the all-possible attack by the third party using all the thresholds to an identification error, thereby causing the eavesdropping to be difficult. Particularly, in the case where the respective step width of the multi-level signal  15  is set 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. 
     As the noise to be overlapped on the signal to be determined (the multi-level signal  15  or the modulated signal  14 ), a thermal noise (Gaussian noise) contained in a space field or an electronic device, etc. may be used, in the case where an electromagnetic wave such as a wireless signal is used as the modulated signal  14 , and a photon number distribution (quantum noise) at the time of a photon being generated may be used, in addition to the thermal noise, in the case where the optical wave is used. Particularly, signal processing such as recording and replication is not applicable to a signal including 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 it is possible to cause the eavesdropping by the third party to be difficult and to secure safety of the data communication. 
     As above described, according to the data communication apparatus according to the first embodiment of the present invention, when the information data to be transmitted is encoded as a multi-level signal, the interval between the signal levels of the multi-level signal is set appropriately with respect to a noise level so as to cause the eavesdropping by the third party to be difficult. With such setting, 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. 
     Second Embodiment 
       FIG. 5 . is a block diagram showing a configuration of a data communication apparatus according to a second embodiment of the present invention. In  FIG. 5 , the data communication apparatus according to the second embodiment of the present invention has a configuration in which a data transmitting apparatus  1102  and a data receiving apparatus  1202  are connected to each other via a transmission line  110 . The data transmitting apparatus  1102  includes a multi-level encoding section  121  and a modulator section  112 . The multi-level encoding section  121  includes a first multi-level code generation section  111   a , a multi-level processing section  111   b , a combining section  111   c , and an error-correction encoding section  111   d . The data receiving apparatus  1202  includes a demodulator section  211 , and a multi-level decoding section  222 . The multi-level decoding section  222  includes a second multi-level code generation section  212   a , a multi-level identification section  212   b , and an error correction decoding section  212   c.    
     As shown in  FIG. 5 , the configuration of the data communication apparatus according to the second embodiment illustrates in detail configurations of the first multi-level code generation section  111   a  and the second multi-level code generation section  212   a , and is different, compared to the data communication apparatus according to the above-described first embodiment, in that the configuration includes the combining section  111   c , the error-correction encoding section  111   d , and the error-correction decoding section  212   c . Hereinafter, components which are the same as those of the above-described first embodiment will be provided with common reference characters, and explanation thereof is omitted, and the data communication apparatus according to the second embodiment will be described by mainly focusing on different components. 
     The first multi-level code generation section  111   a  includes a first random number sequence generation section  121   a  and a first multi-level conversion section  121   b . The first random number sequence generation section  121   a  inputs predetermined first key information  11  (step S 601  in  FIG. 6 ), and generates a first binary random number sequence  31  ((a) of  FIG. 9 ) by using the first key information  11 . The first multi-level conversion section  121   b  converts, in accordance with a predetermined encoding method, the first binary random number sequence  31  in a multi-level manner, and generates a multi-level code sequence  12  ((c) of  FIG. 8  and (b) of  FIG. 9 ) in which a signal level changes so as to be approximately random numbers (step S 602  in  FIG. 6 ). 
     Accompanying data  20  ((b) of  FIG. 8 ) is inputted to the combining section  111   c  (step S 603  in  FIG. 6 ). The accompanying data  20  is data to be used to intentionally cause the information data  10  to be misinterpreted, and, for example, a signal synchronizing with the information data  10  (an N frequency-dividing clock of information data or a predetermined code synchronizing pattern, etc.) may be used. The combining section  111   c  inputs the multi-level code sequence  12  and the accompanying data  20 , and in accordance with logic of the accompanying data  20 , generates a multi-level code sequence (a combined multi-level signal)  21  ((d) of  FIG. 8  and (c) of  FIG. 9 ) in which a signal level of the multi-level code sequence  12  is converted with predetermined frequency (step S 604  in  FIG. 6 ). 
     The error correction encoding section  111   d  inputs the information data  10  (step S 605  in  FIG. 6 ), and outputs the information data  10  to the multi-level processing section  111   b  after adding an error-correction code to the information data  10  (step S 606  in  FIG. 6 ). The multi-level processing section  111   b  inputs the multi-level code sequence  21  and the information data  10  to which the error-correction code is added by the error correction encoding section  111   d  ((a) of  FIG. 8 ), combines the both signals in accordance with a predetermined procedure, and generates a multi-level signal  13  having a level uniquely corresponding to a combination of levels of the combined both signals (step S 607  in  FIG. 6 ). The modulator section  112  generates a modulated signal  14  in a predetermined modulation method, by using the multi-level signal  13  as original data, and transmits the generated modulated signal  14  to the transmission line  110  (step S 608  in  FIG. 6 ). 
     The demodulator section  211  inputs the modulated signal  14  (which is in the predetermined modulation method and generated based on the information data  10  error encoded and the multi-level code sequence  21  including the accompanying data  20 ) transmitted by the data transmitting apparatus  1102  via the transmission line  110  (step S 701  in  FIG. 7 ). The demodulator section  211  demodulates the modulated signal  14  and reproduces and outputs the multi-level signal  15  (step S 702  in  FIG. 7 ). The second multi-level code generation section  212   a  includes a second random number sequence generation section  222   a  and a second multi-level conversion section  222   b . The second random number sequence generation section  222   a  inputs second key information  16  which has the same content as the first key information  11  (step S 703  in  FIG. 7 ), and generates a second binary random number sequence  32  in accordance with the second key information  16 . The second multi-level conversion section  222   b  converts, in accordance with a predetermined encoding method, the second binary random number sequence  32 , in a multi-level manner, and generates a multi-level code sequence  17  ((e) of  FIG. 8 ) in which a signal level changes so as to be approximately random numbers (step S 704  in  FIG. 7 ). 
     The multi-level identification section  212   b  identifies the multi-level signal  15  (binary determination) by using the multi-level code sequence  17  as a threshold, and reproduces information data  22  ((f) of  FIG. 8 ) (step S 705  in  FIG. 7 ). The error correction decoding section  212   c  inputs the information data  22 , and reproduces and outputs information data  18  ((g) of  FIG. 8 ) by detecting error bits of the information data  22  (or positions of the error bits: x marked positions in (f) of  FIG. 8 ) and correcting errors of the error bits (step S 706  in  FIG. 7 ). 
     Here, the bit errors detected by the error correction decoding section  212   c  corresponds to difference between the multi-level code sequence  21  and the multi-level code sequence  17 , that is, a determination error based on logic of the accompanying data  20 . Therefore, by detecting the positions of the error bits, accompanying data  23  ((h) of  FIG. 8 ) can be reproduced (step S 707  in  FIG. 7 ). 
     Next, eavesdropping on a modulated signal by a third party will be described. The third party does not share the first key information  11  with the multi-level encoding section  121  of the data transmitting apparatus  1102 , and thus performs simultaneous determination by means of the all-possible attack by preparing thresholds with respect to all intervals between respective signal levels possibly taken by a demodulated multi-level signal, and analyzing a result of the determination, thereby extracting correct key information or information data. Here, as a analyzing method of the key information by the third party, there may be, for example, a method of decrypting the first key information  11  by generating, based on a result of the determination, a binary random number sequence corresponding to the first binary random number sequence  31  ((a) of  FIG. 9 ) generated by the data transmitting apparatus  1102 , and by obtaining, from the binary random number sequence, consecutive bits (2 kbits) whose length is twice as long as key length k of the first key information  11 . On the other hand, in the present invention, the signal level of the multi-level code sequence  12  is converted depending on the accompanying data  20  in the data transmitting apparatus  1102 , whereby difference between the first binary random number sequence  31  ((a) of  FIG. 9 ) generated in the data transmitting apparatus  1102  and the binary random number sequence ((e) of  FIG. 9 ) generated based on the result of the determination are provided. Accordingly, it is possible to prevent obtainment by the third party of correct consecutive bits ( 2   k ), and cause decryption of the first key information  11  to be difficult. 
     As above described, the data communication apparatus according to the second embodiment of the present invention adds the error-correction code to the information data  10  to be transmitted, and utilizes the accompanying data  20  so as to cause the multi-level code sequence  21  in the data transmitting apparatus  1102  to be in discord with the multi-level code sequence  17  in the data receiving apparatus  1202 , thereby encrypting transmitting data and correcting errors in receiving data occurring in the data receiving apparatus  1202 . Accordingly, it is possible to transmit/receive the information data  18  and the accompanying data  23  simultaneously and realize a highly concealable data communication apparatus. 
     Third Embodiment 
       FIG. 10  is a block diagram showing a configuration of a data communication apparatus according to a third embodiment of the present invention. In  FIG. 10 , the data communication apparatus according to the third embodiment has a configuration in which a data transmitting apparatus  1102  and a data receiving apparatus  1203  are connected to each other via a transmission line  110 . The data transmitting apparatus  1102  includes a multi-level encoding section  121  and a modulator section  112 . The data receiving apparatus  1203  includes a demodulator section  211  and a multi-level decoding section  232 . The multi-level decoding section  232  includes a second multi-level code generation section  212   a , a multi-level identification section  212   b , an error correction decoding section  212   c , and a synchronous extraction section  212   d.    
     As shown in  FIG. 10 , the configuration of the data communication apparatus according to the third embodiment is different, compared to the data communication apparatus according to the above-described second embodiment, in that the configuration includes the synchronous extraction section  212   d . Hereinafter, with respect to such components that are the same as those of the above-described second embodiment, description thereof will be omitted by providing common reference characters, and the data communication apparatus according to the third embodiment will be described by mainly focusing on different components. 
     The second random number sequence generation section  222   a  generates a second binary random number sequence  32  in accordance with second key information  16  which has the same content as first key information  11 . The second multi-level conversion section  222   b  inputs a synchronous signal  33 , converts the second binary random number sequence  32 , in a multi-level manner, into a multi-level code sequence  17 , which is then synchronized with the synchronous signal  33  and generated. The multi-level identification section  212   b  identifies the multi-level signal  15  (binary determination) by using the multi-level code sequence  17  as a threshold, and reproduces information data  22 . The error-correction decoding section  212   c  inputs the information data  22 , and simultaneously reproduces information data  18  and accompanying data  23  by detecting error bits of the information data  22  and by correcting errors of error bits. The synchronous extraction section  212   d  inputs the accompanying data  23 , extracts the synchronous signal  33 , which is in synchronization with the accompanying data  23 , and outputs to the second multi-level conversion section  222   b.    
     In accordance with this action, in the case where, in the data transmitting apparatus  1102 , the information data  10  and the accompanying data  20  are in synchronization with each other, it is possible to obtain synchronization between the multi-level code sequence  17  generated, based on the synchronous signal  33 , by the second multi-level code generation section  212   a  and the multi-level signal  15  outputted from the demodulator section  211 . Accordingly, the multi-level signal  15  can be synchronized with and identified by the multi-level code sequence  17 . 
     As above described, the data communication apparatus according to the third embodiment of the present invention adds error-correction code to the information data  10 , and causes the multi-level code sequences  21  and  17 , which are respectively in the data transmitting apparatus  1102  and the data receiving apparatus  1203 , to be in discord with each other by using the accompanying data  20  synchronizing with the information data  10 , and thereby encrypting transmitting data and correcting errors in receiving data occurring in the data receiving apparatus  1202 . Accordingly, it is possible to transmit/receive the information data  18  and the accompanying data  23  simultaneously and realize a highly concealable data communication apparatus. Further, it is possible to realize a data communication apparatus of a simple configuration having a synchronous system, by extracting the synchronous signal  33  from the accompanying data  20  and by synchronizing and identifying the multi-level signal  15  using the multi-level code sequence  17  generated based on the synchronous signal  33 . 
     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.