Security system for SSTV encryption
BACKGROUND OF THE INVENTION 
     The present invention is related to the confidentiality of television signal transmissions, and more particularly to the protection of TV signal transmissions from unauthorized reception. The environment in which the present invention may be widely applicable, and in the context of which the invention will be described herein, is that of subscriber television and TV program distribution. 
     Subscriber television systems are becoming increasingly widespread wherein TV signals are sent out via a cable network or over the air and are intended for reception and viewing by only those subscribers who have paid a monthly fee. With the increase in subscriber television systems has also come an increase in the number of people attempting to receive and display the premium television programs without payment. Thus, there is a need for more sophisticated security techniques for preventing such unauthorized reception. 
     Many existing subscriber television systems utilize, directly or indirectly, signals transmitted via satellite, and it is becoming quite common for non-paying individuals to receive and display the premium television programs via television receive only (TVRO) antennas, thus resulting in a substantial loss of revenue for the distributors of the subscription television programs. In addition, various direct satellite broadcast television systems are currently being proposed wherein subscription television programs will be broadcast directly via satellite to individual subscriber homes. These subscription satellite television (SSTV) systems will be quite vulnerable to unauthorized reception, and an effective security technique is therefore highly desirable. 
     The purpose of a security subsystem for an SSTV system is to protect the distributor&#39;s business interest and, accordingly, the following objectives should be achieved: 
     (1) To prevent a non-subscriber from receiving intelligible video and audio signals by using a regular home television set; 
     (2) To prevent a delinquent subscriber from receiving intelligible video and audio signals by using the SSTV decoder; 
     (3) To prevent a legitimate subscriber from receiving intelligible video and audio signals of unsubscribed SSTV channels or programs; 
     (4) To discourage an average technician from building his own receiver capable of obtaining acceptable quality video and audio signals; 
     (5) To discourage a small unauthorized business concern from manufacturing and marketing devices which are capable of receiving and displaying acceptable quality video and audio signals from the SSTV channels; and 
     (6) To allow a legitimate subscriber to receive and display high quality video and audio signals from the subscribed channels or programs. 
     It would also be highly desirable to achieve the above objectives at a reasonable cost. 
     A number of security systems for CATV exist, most of which involve the suppression or removal of the horizontal sync pulses from the video signal before transmission, and the recovery of the sync pulses at the receive end. These techniques will prevent people without the sync recovery circuits from receiving and displaying the programs and may therefore achieve objectives (1) and (6) above, but those security systems do not achieve objectives (2) and (3) and, since sync recovery circuits are relatively easily designed and manufactured, also do not satisfy objectives (4) and (5). 
     More sophisticated techniques may include additional intelligence in the subscriber&#39;s decoder box, including the capability of receiving commands from a control center which are specifically addressed to an individual subscriber and are used to turn on or off some or all of the channels. These more sophisticated security techniques may succeed in achieving objectives (1)-(3) and (6), but still do not satisfy objectives (4) and (5). For example, most of these techniques involve the checking of a password, and a particular channel is turned on only if the password is matched. This could be relatively easily by-passed by modifying the subscriber&#39;s decoder box or building a separate box with all of the necessary features except the on/off switch. Further, subscribers may also be able to tamper with the decoder box to receive more programs than are actually paid for. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a security subsystem for a subscription television system wherein all of the above-mentioned objectives (1)-(6) are achieved. 
     It is a further object of this invention to provide such a security subsystem of minimal cost and complexity. 
     These and other objects are achieved according to the present invention by using a cryptographic technique for scrambling and descrambling of the video signals. The scrambling and descrambling techniques utilize a &#34;key&#34; which is changed on a regular basis and is sent only to paid subscribers, and even this &#34;key&#34; is sent in a different encrypted form to each subscriber so that delinquent subscribers cannot learn the current key from others. 
     A record is kept of unique user ID codes corresponding to each subscriber, and in a transmitter according to the preferred embodiment of this invention, the key is ciphered with each subscriber&#39;s unique ID code prior to sending the key to that subscriber. A random number generator in the transmitter generates a new random number at regular intervals, for example, every second, and this number is combined with the key, and the combined number is then used as a seed to reset a PN sequence generator every second. This PN sequence generator will thus generate a PN sequence with a random seed in one-second segments, and the segmented PN sequence is supplied to a signal processor where it is used to scramble the audio and video program signals. The random number generator is also ciphered with the key and the enciphered random number is continually transmitted with the scrambled video signal. 
     At the receiver, the enciphered key, which has been sent either via satellite or mail, is deciphered in the receiver utilizing the particular subscriber&#39;s unique ID code, which ID code is internal to the receiver and is unknown to the subscriber. The deciphered key is then in turn used to decipher the enciphered random number received with the scrambled program signal. The deciphered key and random number are then combined as in the transmitter, and the combined signal is used to continually reset a PN sequence generator identical to that in the transmitter so that a segmented PN sequence will be generated in the receiver which is identical to that generated in the transmitter, and this segmented PN sequence can then be used to descramble the received signal. The descrambled signal will then be supplied to the subscriber television set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more clearly understood with reference to the following description in conjunction with the accompanying drawing wherein the single FIGURE is a block diagram of the essential components of the SSTV security system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing illustrates a functional block diagram of the SSTV security system according to this invention. The SSTV transmitter will typically include or have access to a billing system computer 10 which will store subscriber information including a list of paid subscribers and their corresponding unique user ID codes. This information may typically be stored in a user data base 12 within the computer. Also within the computer will be a register 14 or the like containing a key which will be changed on a regular basis, e.g. monthly. In preparation for sending this &#34;key of the month&#34; to each current subscriber, the key is enciphered in an encipherer 16 with the user ID code unique to that particular current subscriber, and the enciphered key is then sent to the subscriber. 
     The transmitter includes a pseudo-random number (PN) sequence generator 18 and a random number generator 20. The random number generator 20 periodically generates a new random number, e.g. once every second, and the outputs of the random number generator 20 and key register 14 are combined and loaded into the PN sequence generator 18 to periodically reset or &#34;seed&#34; the PN sequence generator 18 in a manner well known in the art. Each seeding of the sequence generator 18 will begin a new segment of the PN sequence. The program signal from source 22 is supplied to a signal processor 24 where it is encrypted with the segmented PN sequence from generator 18. The encryption technique used may be any one of a variety of well known techniques and need not be discussed in detail herein. The encrypted, or scrambled, signal is then provided to a transmitter 26 for transmission over link 100 to the various subscriber receivers. 
     The random number from generator 20 is enciphered with the key of the month in an encipherer 28, and the enciphered random number is transmitted with the scrambled video signal over the link 100. 
     At the receiver, a register 30 or the like internal to the subscriber TV receiver contains a subscriber-specific secret user ID code which is set prior to installation and is stored in the user data base 12 of the billing computer at the transmitter. Thus, when the subscriber receiver receives the enciphered key or when the user receives the enciphered key by mail and enters the enciphered key into the receiver, a decipherer 32 in the receiver deciphers the enciphered key with the secret user ID code specific to that particular subscriber, and the deciphered key is provided to a decipherer 34. A receiver 36 separates the scrambled signal from the enciphered random number received over link 100 and provides the enciphered random number to the decipherer 34 where it is deciphered with the key received from the decipherer 32. The deciphered random number and key are then combined and loaded into the PN sequence generator 38 to reset or &#34;seed&#34; the sequence generator in the same manner as in the transmitter, to thereby result in the same segmented PN sequence as was used for scrambling in the SSTV transmitter signal processor 24. This segmented PN sequence is then provided to signal processor 40 where it is used to descramble the received program signal. The descrambled signal is then provided to the subscriber television set 42. 
     The above-described security system provides a novel technique for generating and synchronizing a segmented pseudo-random number (PN) sequence, and a secure key distribution method. The segmented PN sequence generated is used to control the video and audio signal processors that scramble and descramble the program signals. Since a different segmented PN sequence will be generated by each distinct key, the scrambling sequence is different for each key, and by periodically changing the key the scrambling and descrambling sequences will change. Thus, it is not possible for anyone without exact knowledge of the current key to descramble the received program signal with or without a descrambling device. 
     For each given duration of time, a particular channel is scrambled by a PN sequence that is generated by a randomly selected number and the key of the month. To prevent subscribers of different channels from exchanging the keys among themselves, it is essential that the key for a given channel distributed to each subscriber look different, and this accomplished by enciphering the key with each subscriber&#39;s unique user ID code. In this way, although a single key is provided by the register 14 at any one time, a different key is required by each subscriber. It is only when the subscriber-specific key is entered into the receiver that the true key of the month contained in register 14 can be provided to the decipherer 34 and sequence generator 38, and the deciphering of this true key of the month in the decipherer 32 is performed internally of the subscriber receiver and without the subscriber&#39;s knowledge. 
     An important feature of any security system is that a legitimate subscriber must be capable of obtaining synchronization within a short period of time. In the system according to the present invention, the PN sequence used for scrambling and descrambling the signal in signal processors 24 and 40, respectively, is separated into short segments each of which is seeded by the combination of the key of the month and a random number which changes, for example, once every second. Thus, assuming that a legitimate subscriber does have his appropriate key, the time required to acquire synchronization will be substantially equal to the duration of each random number so that synchronization can be acquired rapidly in case of loss of sync due to power outages, rainstorms, changing of channels, etc. 
     The individual components in the security system according to the present invention are known in the art and need not be described in detail herein since the internal details of these components do not constitute a part of the present invention. The encipherers used to encipher the key of the month and the random number can be two different encipherers, but for the sake of hardware simplicity at the receive side, and consequent cost savings in mass production of the subscriber receivers, it is preferable that the same encipherers be used. The encipherer may employ any enciphering method as long as it has a sufficiently high level of security. 
     The PN sequence generator can be any general PN sequence generator as long as it also has sufficient security strength, e.g. a properly selected non-linear feedback shift register may suffice. 
     The random number generator in the transmitter may be a well known thermal noise generator which generates &#34;true&#34; random numbers, or it may be a pseudo-random number generator similar to the sequence generator 18, implemented in a well known manner with digital electronics or computer software. Similarly, the technique for combining the key of the month and the random number generator to produce the &#34;seed&#34; for the PN sequence generators 18 and 38 is not critical, with the simplest technique being a bit-by-bit modulo-2 addition of the two numbers. 
     In general, each of the functional blocks in the drawing can be implemented with existing techniques, with system complexity and cost and security strength depending on the particular implementation of each of the functional blocks. 
     The transformation of the simple cipherer is specified by a variable which is different for each channel or special program, and is changed every month. 
     The user ID code 30 within each subscriber set may be a set of binary switches or a bit pattern programmed into a read-only memory in a sealed box to prevent the subscriber from seeing or changing the number. 
     The use of a simple cipherer in addition to the non-linear feedback shift-register may seem to increase the system complexity unnecesarily. However, since only a small amount of data, namely the &#34;seed&#34;, need be handled each time, and since the statistical properties of the cipherer do not impact to the output of the PN sequence generator, the cipherer can be very simple. One possible approach, for example, is a ROM table of random bits with or without cipher feedback. The use of this simple cipherer greatly simplifies the problem of cryptosynchronization and key distribution, and therefore reduces the overall system complexity. 
     Suitable alternatives for the scrambling of the program signals include conventional scrambling techniques such as on-off switching, randomly inverting lines, fields or frames, and delaying horizontal lines or fields by certain randomly fixed steps. In any case, the technique used will require the generation of a PN sequence which must be synchronized at both the transmit and receive sides.