Abstract:
An apparatus and method for allowing an analog scrambler using an internal digital cipher to generate a pseudo-random number stream to drive the scrambling of the analog signal to be cipher checked, to add strength to the generation of the pseudo-random number stream, and/or to allow either analog scrambling of analog signals or digital encryption of digital signals. Digital connections are made directly to internal digital cipher to allow the foregoing functions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to verifying the correct implementation and operation of a cipher used in secure communications, otherwise called a cipher check, and in particular, to an apparatus and method for cipher check of a digital cipher driving an analog scrambler.  
           [0003]    2. Problems in the Art  
           [0004]    Many communications systems transmit audio or video information in an analog form. The content of the audio or video is contained within an analog waveform that is modulated on another analog waveform. The receiver simply separates the carrier wave from the wave containing the audio or video content, and the content can be played or displayed on appropriate equipment.  
           [0005]    It is possible, however, for either intentional or unintentional eavesdroppers to rather easily derive the content of the communication. Methods have been developed to attempt to secure such communications.  
           [0006]    Scrambling is one such method. The analog waveform containing the content of the communication is manipulated. The content is not discernible or intelligible without knowing how the waveform was manipulated and without the wherewithal to reconstruct the original analog waveform with that knowledge.  
           [0007]    Many types of analog scramblers manipulate the analog waveform according to a pseudo random number stream (PRN). The transmitter and receiver(s) both know the pseudo random number stream and thus can synchronize to that stream so that the receiver(s) can descramble the waveform.  
           [0008]    There are different types of pseudo random number generation. Pseudo random means that there is some reproducible method of number generation (as opposed to truly random numbers). However, the more the PRN stream looks like a random number stream, the more secure the PRN stream is, and thus the more secure the scrambling would be.  
           [0009]    Some of the most secure pseudo random number generators (PRNGs) are digital ciphers. An example is the use of the Data Encryption Standard (DES) as a PRNG. It utilizes a secret key known only to the transmitter and receiver(s) and can have a very long overall cycle time, which means that it is impractical to reverse engineer the PRN stream with known processing equipment. The output of DES is a very secure pseudo random number stream. The analog information is thus essentially the “plain text” that is logically combined with the PRN stream to create the “cipher text” for the device.  
           [0010]    Therefore, there are demonstrable advantages to implementing a digital cipher, as a PRNG, in an analog Scrambler. However, there are times when a digital cipher malfunctions, or appears to malfunction. There are also times when ode simply needs to verify that the digital cipher is operating correctly. Verification that a cipher is correctly implemented is commonly referred to as a cipher check. Cryptographic systems which operate strictly in the digital realm inherently have the ability to be cipher checked. A cipher check is usually performed by inserting a predetermined key into a cipher, and then using this cipher to encrypt some predetermined plaintext. The resulting ciphertext is then compared to that from a device which is known to be working properly. If the ciphertexts match, then the cipher check has succeeded. Digital cryptographic equipment may be easily cipher checked, since the plaintext is generally binary, in which case one may simply compare ciphertext bits. However, there is no known analog cryptographic system that can be cipher checked. Since both the plaintext and ciphertext are analog, it is extraordinarily difficult to cipher check an analog scrambler. This is because analog signals are impossible to specify precisely, due to noise, quantization errors, etc.  
           [0011]    The invention solves the problem of allowing a cipher check to be performed on an analog scrambler. The Transcrypt SC20-460 analog rolling-code frequency inversion scrambler is an example of an analog scrambling device that is presently available.  
           [0012]    A prior art example of an analog scrambler which makes use of a digital cipher is the Transcrypt SC20-460, which uses a cipher as a source of cryptographically secure pseudo-random numbers. This stream of pseudo-random numbers then drives the analog scrambling algorithm which allows analog audio to be encrypted or decrypted.  
           [0013]    Therefore, there is a real need in the art for a way to cipher check a digital cipher used in an analog scrambler. It is therefore a principle object of the present invention to provide an apparatus and method for a cipher check of a digital cipher in an analog scrambler.  
           [0014]    Further objects, features and advantages of the present invention relate to providing an apparatus and method as above described which:  
           [0015]    (a) allows a cipher check to be performed on an analog scrambler;  
           [0016]    (b) allows an analog scrambler to be certified to a standard;  
           [0017]    (c) allows the ability to enhance security by adding a cipher external to the scrambler; and  
           [0018]    (d) allows the ability to use the analog scrambler alternatively as a digital encryption device if desired.  
           [0019]    These and other objects, features and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.  
         SUMMARY OF THE INVENTION  
         [0020]    The present invention includes an apparatus and method for performing a cipher check on an analog scrambler utilizing a digital block cipher to generate a pseudo-random number bit stream to drive the analog scrambler. The block cipher can be implemented in software in a digital signal processor in the scrambler. Digital input and output leads are connected from external of the apparatus to the digital signal processor. By known means, the block cipher can then be digitally tested.  
           [0021]    Optionally, the apparatus includes a second digital cipher connectable through the digital input and output leads, to allow a different or higher level of security of the pseudo-random number stream. Furthermore, optionally, the apparatus can be used as an analog scrambler and/or a digital encryptor. Also, the digital leads allow the cipher to be certified.  
           [0022]    The method includes accessing the cipher directly from external to the scrambler. Its operation can be evaluated (cipher checked), digital encryption can occur in an analog scrambler, and additional levels of security can be added. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a block diagram depiction of an analog scrambler including connections to a digital signal processor which contains in software a digital block cipher which functions as a pseudo-random number generator, according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]    Overview  
         [0025]    To assist in a better understanding of the invention, a description of one embodiment or form the invention can take will now be set forth in detail. Frequent reference will be taken to the drawings. Reference numbers will sometimes be utilized to indicate certain parts or locations in the drawings. The same reference numbers will be used to indicate the same parts and locations throughout the drawings unless otherwise indicated.  
         [0026]    This description will be in the context of an analog scrambler device to scramble audio or video.  
         [0027]    Structure of Preferred Embodiment  
         [0028]    [0028]FIG. 1 illustrates an analog scrambler  10 . The basic structure of scrambler  10  (e.g. Transcrypt International SC20-460) includes a coder/decoder (codec)  12  which, as is well known in the art, operates to code an analog waveform by sampling discrete parts of the waveform and then characterizing them digitally. Conversely codec  12  can take digitally characterizations of an analog waveform and reconstruct the waveform.  
         [0029]    Codec  12  is controlled by a processor  14  that utilizes EEPROM  16 , oscillator  18 , and regulator  20  in conventional ways. Input and output to codec  12  is through lines  21 ,  22 ,  23 ,  24  and buffers  25 ,  26 ,  27 ,  28 . Processor  14  communicates with I/O ports  29  and with a digital signal processor  40  (DSP  40 ) via bus  30 . Processor  14  also communicates with codec  12  directly over line  31 , and oscillator  18  does the same over line  32  with DSP  10  and processor  14 .  
         [0030]    DSP  40  performs the function of a PRNG to provide a secure PRN stream which is used to drive the analog scrambling. All this is conventional.  
         [0031]    The invention is a modification to analog scrambler  10  which allows external access to the cipher in DSP  40  for the purposes of performing a cipher check. In order to do this, analog scrambler  10 , which includes a digital cipher internally (as a pseudo-random number generator (PRNG)), is modified so that in addition to the analog inputs  21 / 23  and outputs  22 / 24 , there is included a digital input (DATA-IN  41 ) and digital output (DATA-OUT  42 ), which allow direct access the cipher within DSP  40  of the analog scrambler  10 . In this way, digital plaintext bits can be entered, and digital ciphertext bits that have been encrypted by the cipher are received. Given that there is also a way to load a crypto-variable (i.e. secret key), analog scrambler  10  can be cipher checked in a manner very similar to that which might be used to cipher check a digital cryptographic system. The new digital connections  41 / 42  to scrambler  10  can be used for the purposes of performing a cipher check. They also can be used to enter any plaintext bit stream desired while scrambler  10  is operating. Scrambler  10  will encrypt this plaintext digital bit stream, and use the resulting digital ciphertext to drive the analog scrambling process, which will affect the way the analog plaintext is converted to analog ciphertext. In this way, the security of the scrambler is enhanced by the ability to add another digital cipher (not shown) external to analog scrambler  10 . One possible reason it may be wished to do this is if the security of the cipher being used by the analog scrambler is not trusted, and it is desired to conveniently substitute a different trusted cipher.  
         [0032]    A further benefit of the invention is it allows analog scrambler  10  to be used as a digital encryption device if need be. This is because new digital connections  41 / 42  provide a path to enter and extract digital plaintext and ciphertext, making use of the cipher embedded in the scrambler to perform the encryption or decryption. Thus analog scrambler  10  becomes analog scrambler and/or digital encryptor  10 .  
         [0033]    A still further benefit of the invention is that it will allow the cipher embedded in DSP  40  of analog scrambler  10  to be certified to a standard. Certification is a process whereby an independent test lab performs a cipher check on a cryptographic device to insure that the cipher has been properly implemented and is free from mistakes. No analog scramblers have ever been capable of being certified in the past.  
         [0034]    The preferred embodiment can be implemented in a Transcrypt SC20-DES Phoenix analog scrambler which has all the components of FIG. 1 except connections  41  and  42 . This product uses the Data Encryption Standard (DES) as the cipher, and can be implemented with a Texas Instruments TMS320F206 digital signal processor to handle the processing requirements of DES as the cipher.  
         [0035]    The primary use of the present invention is in an analog scrambler to provide a means for performing a cipher check. Other uses are obvious to those skilled in the art, based upon the description and advantages listed above.  
         [0036]    Options and Alternatives  
         [0037]    It will be appreciated that the present invention can take many forms and embodiments. The true essence and spirit of this invention are defined in the appending claims, and it is not intended that the embodiment of the invention presented herein should limit the scope thereof.  
         [0038]    For example, block cipher  10  can be DES or some other encryption method that uses a IV or its equivalent. If DES is used, it could be in a variety of modes, including CFB or OFB. Connections  41  and  42  are DATA-IN and DATA-OUT connections on a conventional DSP. A cipher implemented in software, such as well-known in the art is operatively internal of DSP.