Abstract:
Apparatus for sending cryptographic key information through a turbulent medium features a radiation generator in a first enclosure for emitting radiation at a predetermined wavelength through first launching means for launching the radiation into turbulent media. A second launching means in a second enclosure is located a distance from the first enclosure for receiving the radiation launched from the first launching means after the radiation has traversed the turbulent media, and focusing the radiation onto detection means for determining a unique cryptographic key.

Description:
[0001]     The present invention generally relates to secure cryptographic key exchange, and, more specifically to use of natural ambient turbulence to generate and share cryptographic keys. This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Since perhaps the earliest use of smoke or mirrors for signaling, man has sought a truly secure method of exchanging information without third parties being privy to what information is being exchanged. Over the years many cryptographic schemes have been developed, from relatively simple alpha-numeric conversions to elaborate scrambling techniques. However, most systems devised have been subject to interception and subsequent deciphering. This was illustrated importantly by the ease with which the Allies, in World War II, broke the Japanese codes, which they intercepted, and thereby used the information obtained to seriously damage the Japanese war effort.  
         [0003]     One popular method for secure communications involves key-based cryptography, where a “key” is a sequence of random binary numbers. Key-based cryptography is a method in which a particular tool for decoding a message, the key, is relayed to the authorized recipient to allow the encoded message to be decoded. In this method, the key is used to enable the encryption and decryption of a message in such a way that an eavesdropper who has intercepted the message has no way to decipher the message without knowing the key. It is obvious with this cryptographic system that security of the key itself is of paramount importance.  
         [0004]     Recently, quantum cryptography, a process in which single photons are sent between two positions to establish a secure key based on fundamental uncertainty relations, has been developed. While very effective, it currently is uncertain how far apart the two positions can be and still have effective communication. Also, the quantum cryptography systems are very complicated, since single photon creation and detection are not simple matters.  
         [0005]     Therefore, a need exists for an equally secure system that is not as complicated and expensive as quantum cryptography. The present invention discloses such a system that uses the natural turbulence and noise between the two positions to create a virtually unbreakable key.  
       SUMMARY OF THE INVENTION  
       [0006]     In order to achieve the objects and purposes of the present invention, and in accordance with its objectives, apparatus for sending cryptographic key information through a turbulent medium comprises a radiation generator in a first enclosure for emitting radiation at a predetermined wavelength through first launching means for launching the radiation into turbulent media. A second launching means in a second enclosure located a distance from the first enclosure for receives the radiation launched from the first launching means after the radiation has traversed the turbulent media, and focusing the radiation onto detection means for determining a unique cryptographic key. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0008]      FIG. 1  is a schematic illustration of transceivers utilizing lasers according to the present invention to be used in an optical communication arrangement.  
         [0009]      FIG. 2A  is a plot of signal waveforms received at each optical transceiver in a field test, and  FIG. 2B  is a plot of auto and cross correlations versus time for the same test.  
         [0010]      FIG. 3  is a schematic illustration another embodiments of transceivers utilizing RF transmitters according to the present invention to be used in a RF communication arrangement. 
     
    
     DETAILED DESCRIPTION  
       [0011]     The present invention provides secure communication in free space. It utilizes the natural ambient turbulence to create secure keys for use in cryptographic communication. The invention can be most easily understood through reference to the drawings.  
         [0012]     Referring first to  FIG. 1 , there can be seen a schematic drawing of one embodiment of the present invention. As seen identical transceivers  10 ,  11  each has a transceiver  10 ,  11 , which have lasers  12 ,  13 , respectively, associated with it. Lasers  12 ,  13  can be battery powered diode lasers or light emitting diodes. Each laser  12 ,  13  operates at a slightly different wavelength. For example, laser  12  could operate at 651 nm and laser  13  could operate at 676 nm so that each transceiver  10 ,  11  will be sensitive only to light from the other laser.  
         [0013]     Laser  12  emits its light output through beam splitter  14 , where half of the light output is dumped to one side and the other half is focused into a parallel beam by lens  15  and launched. Lens  15  can be a commercially available f/8, 500 mm focal length, catadioptical lens. Light received at the other transceiver  11  is collected by lens  16  and directed to beam splitter  17  where half of the light is discarded and the other half is directed to narrowband interference filter  18 . Interference filter  18  admits only the wavelength of light emitted by transceiver  10  for passage to translation stage with photodetector  19 . Translation stage with photodetector  19  is a photodiode array mounted on a xyz micrometer translation stage, which is used to position light incoming from interference filter  18  at a point at the boundary of photodiode segments of the photodiode array.  
         [0014]     The signal output  19   a  of translation stage with photodetector  19  is provided to differential amplifier  20  where it is processed and provided to oscilloscope  21 , or to any appropriate output device. Oscilloscope  21  allows an operator to observe the cryptographic key sent by transceiver  10 .  
         [0015]     The same process occurs when transceiver  11  sends a key to transceiver  10 . In this case, laser  13  emits light that is collected and focused by lens  16  and is sent through the turbulent media to transceiver  10  where it is received and focused by lens  15  to beam splitter  14  and directed to interference filter  23 . The light from transceiver  11  is then passed to translation stage with photodetector  23 , where as with translation stage with photodetector  19 , the light is positioned at the boundary of photodiode segments of the photodiode array.  
         [0016]     The signal output  23   a  of translation stage with photodetector  23  is provided to differential amplifier  24 , where if is processed and provided to oscilloscope  25 , or to any other appropriate output device. As with transceiver  11 , oscilloscope  25  allows an operator to observe the cryptographic key sent by transceiver  11 .  
         [0017]     Referring now to  FIG. 2A , where the waveforms received by each transceiver  10 ,  11  in a test of this embodiment of the present invention is shown. In this outdoor test, transceivers  10 ,  11  were separated by 100 m at a time when the wind speed was 4.3 m/s, the temperature was 21.8° C., the humidity was 16%, and the solar radiation was 313 W/m 2 . The period of transmission was 400 ms. Use of the waveforms to develop a unique cryptographic key is straightforward, particularly to those with skill in this art. Initially, appropriate care is employed to determine the mean of the sampled waveforms, and remove the dc component. It has been determined that sampling the waveform at approximately 10 ms intervals aids in avoiding cross-correlation problems.  FIG. 2B  illustrates the plot of auto and cross correlations versus time shift for this same test.  
         [0018]     Another embodiment of the present invention is illustrated in  FIG. 3 . Here, the source of radiation emits a radio-frequency (RF) signal. As seen, transmitter  32  in transceiver  31  emits a signal  31   a , preferably at a frequency in the range of megahertz, through antenna  33  toward transceiver  34 . At transceiver  34 , antenna  35  and/or antenna  36  receive signal  31   a  after it has traversed a distance through ionospheric turbulence. The reason that antenna  36  may or may not receive signal  31   a  is that there exist two primary detection means for this embodiment. The first is to compare the phase of signal  31   a  with a reference signal provided by a local oscillator or other external reference at the site of transceiver  33 . In this case, there is no need for antenna  36  to be in use. The second means utilizes two separate propagation paths, and the difference in phases between the two paths received by both antennas  35 ,  36  is used as the random signal.  
         [0019]     As shown, antenna  35  is connected to phase detector  37  whose output is provided to differential amplifier  38 . Antenna  36  is connected to phase detector or reference oscillator  38  whose output also is connected to differential amplifier  39 . It is interesting with this embodiment of the present invention that the transmissions, in addition to earthbound operation, can be used in earth to satellite transmissions, and in satellite-to-satellite transmissions.  
         [0020]     Those with skill in this art will understand that when electromagnetic radiation propagates through a random medium, such as the atmosphere, the surface of uniform phase, called the wavefronts, are distorted. A random medium is a medium whose properties, such as the number density, vary in space and time from their average values by amounts that cannot be described by any prior information, but only by their statistical distributions. For statistical distributions created by turbulence, it is generally observed that the spatial variations are described by a specific mathematical distribution, known as “gaussian,” and that the temporal variations are correlated only for observations within a finite time interval. Data sampled for longer time intervals are uncorrelated, and represent independent measurements. Although it is usually assumed that the wavefronts are initially uniformly spaced parallel planes, it is also possible to create wavefronts that have initial variations in time that are only known to the operators to transceivers  10 ,  11  or  31 ,  34 .  
         [0021]     This initial variation can provide an additional layer of security for this invention. If both operators already have shared key material, a sequence of encrypted initial phase tilts could be incorporated into both transceivers  10 ,  11  or  31 ,  34 . The initial tilts do not have to be the same for each transceiver  10 ,  11  or  31 ,  34  as long as each operator knows the initial tilt used by the other operator. As the phase tilted light propagates through the turbulent media, it is further perturbed by the random propagation path. Therefore the received bit string is the logical product of the initial tilted string and the random string that is produced by the propagation path. In this situation, an eavesdropper who may know the received tilt string at one end, would have no way of inferring the actual key string produced by the propagation path.  
         [0022]     In the propagation path, time delays, tilts and/or curvatures of the wavefronts are induced by the variations of the speed of the wavefronts in the medium. These variations are described by the index of refraction, the factor by which the speed in a vacuum is reduced due to the effect of the medium. When the index of refraction of a medium differs only slightly from one, which is the case in a gaseous medium of sufficiently low density, its value differs from one by an amount proportional to the number of particles per unit volume (air molecules for optical transmission, or electrons for ionospheric plasmas) multiplied by a quantity called the polarizability. This quantity is a characteristic of the medium whose dimensions are those of volume. The propagation of optical signals in the atmosphere or radio waves in the ionosphere differ only by the values of their particle density and ploarizability, and the statistical parameters of their random spatial and temporal random variations.  
         [0023]     Although the propagation of optical signals in the atmosphere, or radio waves in the ionosphere obey the same statistical distributions and similar physical mechanisms, the manner in which the variations are detected are detected at the receiving transceiver  10 ,  11  or  31 ,  34  is different.  
         [0024]     In the case of optical signals, it is known that optical wavelengths generally are small compared to the diameter of the receiving transceiver  10 ,  11 , which allow the detection of tilt in the phase wavefronts. An optical system represents a transformation from the direction of the input signal to a position in the focal plane of the receiving transceiver  10 ,  11 . An optical signal whose phase wavefronts have been tilted mimic a signal arriving from a different direction. Variations in the position of a focal spot can be measured by the difference in the signals from two closely spaced electronic radiation detectors, such as the silicon photodiodes of translation stage with photodetectors  19 ,  23 . Curvatures of the wavefronts create focal spots in front of or behind the focal plane, and can be ignored for receiving apertures smaller than a known value, determined by the properties of the random medium. In any event, the lateral changes in position are measurable.  
         [0025]     Alternatively, radio signals have wavelengths that are now small large compared to the receiving transceivers  31 ,  34  apertures. Instead of spatial variation caused by wavefront tilts, the random phase errors created by propagation variations are used. This requires a time reference for detection, which can be provided by a stable local signal reference, or by a separate signal received by both transceivers  31 ,  34 . Actually, optical phase errors can be detected easily in optical systems. To accomplish this requires only an interferometer and a coherent reference optical beam. The waveforms obtained in this embodiment are used, as described above, to obtain a random cryptographic key.  
         [0026]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.