Patent Publication Number: US-2006018482-A1

Title: Encryption processing method and device of a voice signal

Description:
TECHNICAL FIELD  
      The present invention relates to a method and system for encrypting analog speech signals transmitted through a wired/wireless communication line.  
     BACKGROUND ART  
      In case of transmission of an audible sound such as a speech signal, in general, the audible sound is converted into an electric analog signal using a sound input device, a microphone, for instance, first. Then, the electric analog signal is encoded through PCM (Pulse Code Modulation) or ADPCM (Adaptive Differential PCM), for example, and transmitted by a communication method such as TDM (or TDMA) or CDMA.  
      However, this conventional speech signal transmission/reception system has a problem that an ill-intended third person can easily eavesdrop a speech signal transmitted through a communication line. In case of PSTN (Public Switched Telephone Network) currently widely being used, for example, a telephone that is a user terminal is connected to an exchange of a telephone office through a telephone line, and the telephone converts an audible sound inputted thereto into an electric analog signal to transmit it to the exchange. Then, the exchange encodes the received analog signal through PCM or ADPCM and sends the encoded signal to another exchange through a trunk. In this communication network, accordingly, an ill-intentioned third person can easily eavesdrop the speech signal transmitted through the telephone line only by connecting a predetermined communication terminal to the telephone line that connects the user&#39;s telephone with the exchange. This illegal eavesdropping is not limited to the above-described communication network but it can be easily carried out for all communication networks including wireless and wired communication methods.  
      Accordingly, important public institutions and facilities employ an encryption system for encrypting analog signals transmitted from user terminals to cope with the illegal eavesdropping.  
       FIG. 1  is a block diagram of a conventional analog signal encryption system. In  FIG. 1 , reference numeral  1  denotes an input part of a microphone that converts an audible sound into an analog signal, and  2  represents an encryption unit for encrypting the analog speech signal inputted through the input part  1 . This encrypting unit  2  consists of an analog/digital converter  21  for converting the analog signal into digital data, an encrypting processor  22  for encrypting the digital data outputted from the analog/digital converter  21 , and a digital/analog converter  23  for converting the digital data outputted from the encryption processor  22  into an analog signal.  
      The encrypting processor  22  rearranges the digital data outputted from the analog/digital converter  21 , that is, speech data, spatially and time-serially or executes frequency conversion for speech data of a specific time interval, to thereby encrypt the speech data. Here, spatial rearrangement means that a predetermined digital value is added to or subtracted from digital data of a predetermined section so as to change the intensity of the corresponding analog signal. The time-serial rearrangement means that the digital data is exchanged with digital data of another section or inversely arranged.  
       FIG. 2  shows an example of encryption processed by the encryption unit  2 .  FIG. 2   a  illustrates the waveform of the analog signal inputted through the input part  1  and  FIG. 2   b  shows the waveform of the analog signal outputted from the digital/analog converter  23  of the encryption unit  2 . In  FIGS. 2   a  and  2   b , the horizontal axes represent time and vertical axes indicate signal intensities. Referring to  FIGS. 2   a  and  2   b , a data value corresponding to “2” is added to the input signal so that the input signal is spatially rearranged with respect to the data of section a-b. Data of section b-c and data of section c-d are exchanged with each other and the signal of section d-e is inversely arranged such that the data of section b-e is rearranged time-serially. In addition, the conventional encryption unit  2  carries out frequency conversion for a speech signal of a predetermined time interval, which is not shown in the figure. That is, in the conventional encryption system and method, an input speech signal is rearranged spatially and time-serially and a speech signal of a predetermined section is frequency-converted so that a third person cannot recognize the speech signal.  
      However, the conventional encryption system has the following problems.  
      1. A third person can recognize that a corresponding speech signal has been encrypted even if he/she cannot eavesdrop the speech signal because the conventional encryption system rearranges the speech signal only spatially and time-serially or frequency-converts it. Accordingly, an ill-intentioned third person may record the signal to try to analyze it.  
      2. Every person has his/her own characteristic speech signal, in general, and the speech signal has continuity. Thus, an encrypted speech signal can be decoded when it is accurately analyzed on the basis of these characteristics.  
       FIG. 3  shows waveform characteristic of a speech signal with respect to time and  FIG. 4  shows spectrum characteristic of the speech signal of  FIG. 3  with respect to time. These graph the speech signal using Cool-Edit 2000 program. As shown in  FIGS. 3 and 4 , every person has his/her own characteristic speech signal having continuity. Accordingly, an encrypted speech signal that has been rearranged spatially and time-serially or frequency-converted can be easily restored to the original speech signal when the encrypted speech signal is decoded on the basis of the characteristics.  
     DISCLOSURE OF INVENTION  
      An object of the present invention is to provide an encryption method and system for securely encrypting an analog signal transmitted through a communication line.  
      To accomplish the object of the present invention, there is provided a method for encrypting a speech signal transmitted through a communication line, comprising a characteristic parameter extracting step of splitting the speech signal into predetermined frequency components and extracting a magnitude value of each of the frequency components; and a data transmission step of transmitting the parameter data extracted at the characteristic parameter extracting step through the communication line.  
      The encryption method further comprises a rearrangement step of rearranging a series of characteristic parameters obtained at the characteristic parameter extracting step.  
      To accomplish the object of the present invention, there is also provided a system for encrypting a speech signal transmitted through a communication line, comprising an analog/digital conversion means for converting an analog speech signal into digital data; a characteristic parameter extracting means for extracting a magnitude value of each of frequency components of the data; and a digital/analog conversion means for converting the data obtained by the characteristic parameter extracting means into an analog signal.  
      The encryption system further comprises a rearrangement means for rearranging a series of characteristic parameters outputted from the characteristic parameter extracting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a block diagram of a conventional analog signal encryption system;  
       FIG. 2   a  shows the waveform of the analog signal inputted through the input part  1  of the system of  FIG. 1 ;  
       FIG. 2   b  shows the waveform of the analog signal outputted from the digital/analog converter  23  of the encryption unit  2  of  FIG. 1 ;  
       FIG. 3  shows waveform characteristic of a speech signal with respect to time;  
       FIG. 4  shows spectrum characteristic of the speech signal of  FIG. 3  with respect to time;  
       FIG. 5  is a block diagram of a speech signal encryption system according to an embodiment of the present invention;  
       FIGS. 6 and 7  are waveform diagrams for explaining waveform characteristic of the encryption system of  FIG. 5 ;  
       FIG. 8  is a block diagram of an encryption system according to another embodiment of the present invention; and  
       FIG. 9  is a block diagram of a decoding system for restoring a signal transmitted through the encryption system of  FIG. 8  to the original signal.  
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings.  
      First of all, the basic concept of the present invention will be described below.  
      An analog signal can be represented by a plurality of sine and cosine functions having different number of vibrations, that is, frequencies, or by a composite function of sine and cosine. In other words, an analog signal can be divided into a plurality of frequency components having different magnitudes.  
      For instance, a periodic function f(t) can be developed as series of multiple sine functions as follows.  
         f   ⁡     (   t   )       =         a   0     2     +       a   1     ⁢   cos   ⁢           ⁢   ω   ⁢           ⁢   t     +       a   2     ⁢   cos   ⁢           ⁢   2   ⁢   ω   ⁢           ⁢   t     +   …   ⁢           +       a   n     ⁢   cos   ⁢           ⁢   n   ⁢           ⁢   ω   ⁢           ⁢   t     +       b   1     ⁢   sin   ⁢           ⁢   ω   ⁢           ⁢   t     +       b   2     ⁢   sin   ⁢           ⁢   2   ⁢   ω   ⁢           ⁢   t     +   …   ⁢           +       b   n     ⁢   sin   ⁢           ⁢   n   ⁢           ⁢   ω   ⁢           ⁢   t           
 
      That is, the periodic function can be divided into multiple frequency components having different magnitudes.  
      The original analog signal can be obtained by combining the multiple frequency components having different magnitudes, represented by the aforementioned equation.  
      Accordingly, if there is a predetermined agreement between transmitting and receiving systems, the transmitting system can transmit the analog signal represented by the periodic function f(t) only by delivering  
           a   0     2     ,     a   1     ,     a   2     ,   …   ⁢           ,     a   n     ,     b   1     ,     b   2     ,   …   ⁢           ,     b   n         
 
 that are magnitudes of the frequency components in the equation. 
 
      This concept can be applied even to transmission and reception of general speech signals in the same manner. Specifically, when a speech signal having frequency band of 0-4 KHz is split into thirty-two frequency components, for example, frequency components that respectively have 0, 125 Hz, 250 Hz, . . . , 4 KHz are obtained. These frequency components can be combined to restore the original speech signal. Accordingly, if speech signal transmitting and receiving systems agree on a method of splitting and combining the speech signal, the transmitting system transmits only the magnitudes of the frequency components to the receiving system for the purpose of perfect transmission and reception of the speech signal.  
      The present invention splits an input speech signal into predetermined frequency components, extracts a magnitude value of each of the frequency components, that is, characteristic parameter, converts the extracted characteristic parameter into an analog signal and transmits the analog signal through a communication line.  
       FIG. 5  is a block diagram of a speech signal encryption system according to an embodiment of the present invention. Referring to  FIG. 5 , the encryption system  5  of the present invention includes an analog/digital converter  51  for converting an analog speech signal inputted through an input part  1  into digital data, a characteristic parameter extractor  52  for extracting a characteristic parameter, that is, magnitude data of each frequency component, from the digital data, and a digital/analog converter  53  for converting parameter data outputted from the characteristic parameter extractor  52  into analog data.  
      The characteristic parameter extractor  52  is composed of a digital signal processor or a microprocessor, for example, which executes a predetermined algorithm in which inverse transform is easily performed, for instance, FFT (Fast Fourier Transform), DCT (Discrete Cosine Transform) and WAVELET transform or various subband dividing techniques using band pass filters, to extract characteristic parameters from input data.  
       FIG. 6  shows characteristic waveforms obtained when a sine wave having the frequency of 1 KHz is inputted to the encryption system of the present invention. These waveforms were acquired by using Cool Edit 2000 program.  FIG. 6   a  shows 1 KHz sine wave inputted to the encryption system of the present invention, and  FIG. 6   b  illustrates spectrum characteristic of the sine wave according to time. In addition,  FIG. 6   c  shows a variation in the magnitude of the signal outputted from the digital/analog converter  53  of the encryption system  5  with respect to time when the 1 KHz sine wave is inputted to the encryption system of the present invention.  FIG. 6   d  shows spectrum characteristic of the signal of  FIG. 6   c  according to time.  
       FIG. 7  shows characteristic waveforms obtained when an actual speech signal is inputted to the encryption system of the present invention.  FIG. 7   a  illustrates a variation in the speech signal with respect to time, and  FIG. 7   b  is a waveform diagram showing a variation in the signal outputted from the digital/analog converter  53  of the encryption system  5  with respect to time when the speech signal is inputted to the encryption system.  FIG. 7   c  illustrates spectrum characteristic of the signal outputted from the digital/analog converter  53  with respect to time.  
      Upon comparison of the waveforms shown in  FIGS. 3 and 4  according to the conventional system with the waveforms of  FIGS. 6 and 7  obtained by the present invention, the encryption system of the present invention newly generates an analog signal based on the magnitude of each of frequency components of the input analog signal. This completely destroys regularity and continuity of the original speech signal. Accordingly, in the case that a speech signal is encrypted and transmitted through the encryption system of the present invention, an ill-intentioned third person cannot confirm whether the transmitted signal is a speech signal or simple noise even if he/she eavesdrops the transmitted signal. Furthermore, even if the third person judges that the signal is a speech signal, he/she cannot recognize the transmitted signal because the signal does not have the regularity and continuity of the original speech signal.  
       FIG. 8  is a block diagram of an encryption system according to another embodiment of the present invention. This encryption system has higher degree of encryption than the encryption system of  FIG. 5 . Like reference numerals designate corresponding parts throughout  FIGS. 5 and 8 .  
      As shown in  FIG. 8 , the encryption system according to another embodiment of the invention additionally includes a rearrangement processor  80  for rearranging the digital data outputted from the characteristic parameter extractor  52  spatially or time-serially. The rearrangement processor  80  corresponds to the encryption processor  22  of the conventional encryption system, shown in  FIG. 1 , and subtracts/adds a predetermined data value from/to input data or changes the position of the data.  
      In the conventional encryption system of  FIG. 1 , the data inputted to the encryption processor  22  is magnitude data of the speech signal with respect to time. Thus, the original signal can be easily restored on the basis of continuity of the speech signal even if the input data is rearranged through the encryption processor  22  spatially and time-serially. In the encryption system shown in  FIG. 8 , however, the data extracted from the characteristic parameter extractor  52  corresponds the magnitude of each of the frequency components of the speech signal so that the data is changed into a signal completely different from the original signal when the magnitude value of the data is changed or rearranged time-serially. In the above-described embodiment, especially, magnitude data of each of the frequency components of the speech signal is set as transmission data so that regularity and continuity of the original signal are completely destroyed. Accordingly, an ill-intentioned third person cannot restore the rearranged data to the original data. As a result, the present invention can securely prevent the third person from eavesdropping the speech signal transmitted through the communication line.  
       FIG. 9  is a block diagram of a decoding system for restoring the signal transmitted through the encryption system to the original signal, which corresponds to the encryption system  8  shown in  FIG. 8 .  
      In  FIG. 9 , reference numeral  9  denotes a decoding unit for restoring the signal encrypted by the encryption unit  8  to the original signal, and  10  represents an output part for outputting the analog signal outputted from the decoding unit  9  as an audible sound, for example, a speaker.  
      The decoding unit  9  consists of an analog/digital converter  91  for converting an input analog signal into digital data, a rearrangement processor  92 , an inverse transform processor  93 , and a digital/analog converter  94  for converting digital data outputted from the inverse transform processor into an analog signal. Here, the rearrangement processor  92  inversely carries out the rearrangement performed by the rearrangement processor  80  of the encryption system of  FIG. 8 , to generate the same data as the data outputted from the characteristic parameter extractor  52 . The inverse transform processor  93  inversely transforms the transform processing performed by the characteristic parameter extractor  52 , that is, FFT, DCT and WAVELET transform, or combines the original frequency signals with magnitude data of various subbands, to restore the input signal data to the original speech data.  
      While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.  
     INDUSTRIAL APPLICABILITY  
      As described above, the present invention can securely encrypt analog speech signals transmitted through communication lines.