Abstract:
A satellite broadcast conditional access system with key synchronization uses indexing of an authorization stream to quickly restart the decrypting process after short carrier fades and after carrier switches. The authorization stream includes cyphered seeds and index numbers which are sequentially sent to a group of receivers. The same authorization stream can also be broadcast multiple times to the group of receivers. A conditional access server selects a starting index number and increments the index number by a predefined value. The receivers have a memory to save the current index number for the authorization stream. Any receiver that loses its connection to the broadcast and thereafter reestablishes its connection can retrieve the latest index number being issued in the authorization stream and compare it with the stored index number. When the index numbers match or are within a defined threshold, the receiver will continue to decypher the seeds and decrypt the transport stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/482,235 filed Jun. 25, 2003. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to satellite broadcast systems and, more particularly, to a conditional access system for encrypting and decrypting data. 
     2. Related Art 
     A conditional access system is used to permit access to a transport stream only to subscribers who have paid for it. This is generally done by distributing the transport stream in encrypted form. Although any integrated receiver-decoder (IRD) that is connected to a satellite broadcast network can receive the encrypted transport stream, only the IRDs of those authorized subscribers are able to decrypt the encrypted transport stream. The IRD determines whether the encrypted transport stream should be decrypted and, if so, to decrypt it to produce a decrypted transport stream comprising information making up the broadcast program. 
     After a subscriber has purchased a service, a service provider sends messages to the subscriber&#39;s IRD with an authorization stream for the purchased services. The authorization stream may be sent with the transport stream or may be sent via a separate channel to an IRD. Various techniques have been used to encrypt the authorization stream. The authorization stream may include a seed as a key for a service of the service provider and an indication of what programs in the service the subscriber is entitled to receive. If the authorization stream indicates that the subscriber is entitled to receive the program of an encrypted transport stream, the IRD decrypts the encrypted transport stream using the received seed. 
     A well known problem concerning such conditional access systems is that the IRDs may suffer either carrier fades or be switched between carriers bearing the same instantiation of the service provider. It is therefore desirable for the IRDs to recover and pass a correctly decrypted transport stream to downstream processing stages as quickly as possible. However, the magnitude of time delay in the recoveries, on a typical large network (12,000 satellite IRDs) can be extremely long, such as one or two minutes in legacy systems. Other implementations of conditional access solve the problem of quick restoration of the IRD&#39;s decrypter by either risking that still-scrambled material may inadvertently be passed to the downstream processing stages, or consuming far more bandwidth in the transport stream to send cyphered seeds. 
     Hence, there is a need in the industry for an efficient and reliable technique for rapidly decrypting data after brief or extended loss of transport or authorization streams due to short carrier fades or switches. For that purpose, the conditional access system should allow the IRDs to quickly determine, after restoration of the data link following a carrier fade or switch, whether their stored copies of the decryption seeds are still current and correct. Furthermore, it is needed to greatly reduce the likelihood that the carrier fade or switch could prevent the IRD from getting at least one copy of its own messages without the need for consuming large amounts of bandwidth. 
     SUMMARY OF THE INVENTION 
     It is in view of the above problems that the present invention was developed. The present invention is a satellite broadcast conditional access system with key synchronization that allows the IRDs to quickly restart the decrypting process after short carrier fades and after carrier switches when they are within the same protected network. The invention uses an indexed authorization stream allowing the IRDs to quickly decide, after restoration of the data link following a carrier fade or switch, whether their stored copies of the decrypting seeds are still current and correct. The invention also uses multiple transmissions of the cyphered seeds during each distribution period providing the IRD with multiple opportunities to receive the current seed. 
     For the first attribute, the index numbers on all the authorization streams are assigned in a manner such that the authorization stream may be identified and that the specific time epoch of those cyphered seeds may be determined. When a conditional access server program initializes, it randomly selects the starting index number from a domain of numbers, and applies this number to each and every authorization stream bearing a cyphered seed. Then, while in operation, it increments that index by a predefined value at each new distribution period, i.e., an odd/even flavor switch according to the preferred embodiment. The IRDs, in their turn, after reestablishing connection to the carrier-borne transport stream, may quickly retrieve the index numbers being issued in the authorization stream and compare them to the same for both flavors of the cyphered seeds it keeps in volatile storage. If those numbers match, then the IRD will then immediately decypher those seed(s) and restart decrypting on the transport stream knowing it is using the correct seed. This restart may commence very quickly after the authorization stream is detected, and that the IRD need not wait until its own messages are received and decyphered. 
     For the second attribute, the distribution of the cyphered seeds is repeatedly sent with considerable delay between the cyphered seed messages. This greatly reduces the likelihood that a carrier switch or a short fade could prevent the IRD from getting at least one copy of its own cyphered seed message during each distribution period. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  illustrates a systematic diagram of a satellite broadcast conditional access system according to the present invention; 
         FIG. 2  illustrates a flowchart of operations that are performed at a conditional access server to generate authorization stream sent to cryptographic multiplexers; 
         FIG. 3  illustrates a diagram of how authorization stream is structured during a flavor distribution period; 
         FIG. 4  illustrates a flowchart of operations that are performed to decypher authorization stream and encrypt transport stream using an encryption seed at a cryptographic multiplexer; 
         FIG. 5  illustrates a flowchart of operations that are performed at an IRD to decypher authorization stream and maintain IRD synchronization to the conditional access system in steady state operation; 
         FIG. 6  illustrates a diagram of conditional access system timing for key synchronization when an authorization stream is distributed and a transport stream is encrypted at the cryptographic multiplexer and decrypted at the IRD; and 
         FIG. 7  illustrates a flowchart of operations that are performed at the IRD to rapidly decrypt data by key synchronization and indexing after brief or extended loss of transport stream. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings in which like reference numbers indicate like elements,  FIG. 1  illustrates a systematic diagram of a satellite broadcast conditional access system  10  according to the present invention. The conditional access system  10  provides dynamic scrambling security to an entire MPEG transport stream  12 . The conditional access system  10  generally consists of a server  14  and receivers  20 . In a preferred embodiment of the invention, the server  14  is comprised of a conditional access server  16  and cryptographic multiplexers  18 . The receivers  20  are generally referred to as integrated receiver-decoders (IRDs)  20 . 
     The encryption function  22  in the conditional access server  16  provides an authorization stream  24  bearing cyphered messages which can only be decyphered and read by authorized devices. These messages give the cryptographic multiplexers  18 , at the satellite uplink, and the authorized IRDs  20 , at the downlink sites, a sequence of cyphered encrypting seeds. The cryptographic multiplexers  18  extract their own cyphered encryption seeds using their own serial number, and their decrypter  26  decyphers the cyphered encrypting seeds to get an encryption seed. These seeds initialize scrambler  28 , in the cryptographic multiplexers  18  which appears to randomly encrypt the encrypt able portions of the MPEG transport stream  12 . The authorization stream  24  and the encrypted transport stream  30  are transmitted through an interposed satellite broadcast network  31  by the multiplexer  32  and received by the input module  34  of the IRDs  20 . Like the cryptographic multiplexers  18 , the host microprocessor  36  of IRDs extract their own cyphered encryption seeds using their own serial number, and their decrypters  38  decypher the cyphered encrypting seeds to get an original encryption seed. Since the encrypting operation is symmetric, the encrypting seed sent to the IRDs  20  allows descrambler  40  to decrypt the transport stream encrypted by the cryptographic multiplexer  18 . 
     At the uplink site, a conditional access server  16  runs the conditional access system  10 . It can retrieve database information  41  from a conditional access database  42  by a network connection to the conditional access server  16  if on separate machines. This information is used to build and edit a list of authorized IRDs  20  by serial number n  102  under local operator control.  FIG. 2  illustrates a flowchart of operations that are performed at a conditional access server  16  to generate authorization stream  24  sent to cryptographic multiplexers  18 . At initialization, or after any change to the authorized list, the conditional access server  16  accesses its encryption function  22  (operation  200 ). This function contains a secret identification number W  112  unique to the particular customer (operation  210 ). In the case where the conditional access system  10  is controlled by a service provider and one or more customers are using the system, the secret identification number is only known by each respective customer and is not known to or accessible by any person at the service provider. The serial numbers  102  are reported to the encryption function  22  (operation  200 ) and, for each one, the encryption function  22  finds the encrypted serial number S n    114  by implementing the function S n =F (W∥n); where ‘∥’ is the concatenation operator, and where “F( )” is a one-way hash function, i.e., a function that is computationally easy to perform in one direction, but extremely difficult to reverse (operation  210 ). The encryption function  22  then provides the S n &#39;s  114  to the conditional access server  16 . 
     When the conditional access server&#39;s encryption engine is activated, it generates a sequence of random numbers K i    122  and associated index numbers i  124  (operation  220 ). While each K i  in the sequence is independently random, the i values preferably begin with a randomly selected number, i.e., the initial index number is randomly generated. In a preferred embodiment of the invention, the i index then increments by a given value, preferably one, for each new (K i ,i) pair  122 ,  124  that is generated. For each pair  122 ,  124  in the sequence, the conditional access server  16  creates a cyphered message for every authorized IRD  20  plus all encrypting cryptographic multiplexers  18 . It does this using the list of secret serial numbers S n    114 . Each cyphered message (CM) contains a value C ni    126 , the index i,  124  the destination unit serial number n  102 , and an even/odd flavor indicator  128 . The value C ni  is calculated (operation  220 ): C ni =K i  xor F(S n ∥i) and it is called the cyphered seed  126 . After the entire set of cyphered messages is distributed, the conditional access server  16  sends either an encryption ON or OFF message  130 , addressed to all. The aggregate of all these messages (C ni    126 , i  124 , n  102 , an even/odd flavor indicator  128 , an encryption ON or OFF message  130 ) is generally called the authorization stream  24 . This stream then feeds the cryptographic multiplexers  18  (operation  230 ). 
     The authorization stream  24  is preferably structured as shown in FIG.  3 . The time interval over which cyphered messages are used to distribute a (K i ,i) pair  122 ,  124  to the universe of IRDs  20  and cryptographic multiplexers  18  is the odd/even flavor distribution period  142 . Within this period, all the cyphered messages  144  intended for the downlink IRDs  20  are sent first as an ordered group. The ordered group is a set of cyphered messages (CM 1 , CM 2 , . . . , CM m ) corresponding with the group of IRDs (IRD 1 , IRD 2 , . . . , IRD m ), respectively. For each distribution period, the cyphered messages will all contain the same index number and even/odd flavor indicator, but will vary according to the IRD n  serial numbers (S n1 , S n2 , . . . , Sn nm ). Of course, the cyphered seed  126  will also vary according to the different serial numbers based on operation  220 . Then that whole set of messages  146  is repeated in the same order. Following this, there is a delay period  148  where no messages are transmitted. Then cyphered messages  150  addressed to all the cryptographic multiplexers  18  listed in the conditional access database  42  are sent, in order, just once. This is followed preferably, without delay, by some number of encryption ON or OFF commands  130 . After this, there is another delay  154  before transmission of the next (K i ,i) pair  122 ,  124  begins, which preferably has the opposing odd/even flavor  156 . 
     In a preferred embodiment of the invention, the conditional access system  10  may be in one of three states. They are (1) encryption off; (2) encryption on and starting up; (3) encryption on static. In the first state, the engine continues to create the (K i ,i) pairs  122 ,  124 , but only a single encryption off authorization message is sent at the end of each distribution period. In the second state, the engine begins distribution of the encrypting seeds. At the end of the first two distribution periods, the conditional access server  16  sends an encryption off message  130  to all devices. After the second state, the conditional access system  10  enters the third state. Here, after the seeds have been distributed to the IRDs  20  and cryptographic multiplexers  18 , an encryption on message  130  is sent to all devices. Note that there is no similar transition from the encryption on state to the off state. As soon as the user orders encryption to stop, distribution of new seeds ceases immediately and the very next authorization message sent is an encryption off message  130 . 
     In the preferred embodiment of the invention, the list of all cryptographic multiplexers  18  which may do encryption is found in the associated conditional access database  42 . The presence or absence of the cryptographic multiplexer  18  from conditional access system&#39;s authorized list does not mean the same thing as the presence or absence of an IRD  20 , as shall be seen. If a cryptographic multiplexer  18  is in the conditional access database  42 , then, when the conditional access state is encryption on, the cryptographic multiplexer  18  will always be receiving addressed authorization messages from the conditional access system  10 . However, the cryptographic multiplexer behavior is then affected by the conditional access mode in use while encryption is on. In the preferred embodiment of the invention, only the authorized cryptographic multiplexers  18  receive addressed encryption on commands, while the unauthorized cryptographic multiplexers (in the conditional access database but not authorized in conditional access) receive addressed encryption off commands. For all networks logically connected to those unauthorized cryptographic multiplexers  18 , this has the effect of leaving them completely in the clear (unencrypted). 
     The cryptographic multiplexer  18  has three functions within the conditional access system  10 : (1) to receive and decypher the next encrypting seed, (2) to encrypt the required program IDs (PIDs) in the MPEG transport stream  12  using that seed, and to (3) inject the authorization stream into a ghost PID of the transport stream for use by the authorized IRDs. In support of these functions, the cryptographic multiplexer  18  accepts the authorization stream  24  from the conditional access server  16 . In addition, it accepts an MPEG transport stream  12 , provides the encrypting processing, and then outputs it, preferably for ultimate distribution to a network of downlink IRDs  20 . 
       FIG. 4  illustrates a flowchart of operations that are performed to decypher authorization stream  24  and encrypt transport stream  12  using an encryption seed  122  at a cryptographic multiplexer  18 . Near the end of the flavor distribution period  142  of a particular odd/even flavor  128 , there is sequence of authorization streams  24  directed to cryptographic multiplexers  18 . If the host processor in a cryptographic multiplexer receiving the stream detects it&#39;s own unit serial number n  102  in an authorization stream  24  (operations  400  and  410 ), then that stream is passed to a decrypter  26 . This decrypter, when it was programmed at the factory, had been given the unit&#39;s pre-calculated, encrypted serial number S n    114 . This is the same S n  also calculated by the encryption function  22  in the conditional access server  16 . So the decrypter  26  then takes the incoming (C ni ,i) pair and computes the corresponding K i    122  from the equation (operation  420 ): K i =C ni  xor F(S n ∥i). This is the same K i  value which originated in the conditional access server  16 . It is an encryption seed value  122 , which is then loaded into the encrypting hardware, scrambler  28 . 
     In a preferred embodiment of the invention, once the new encryption seed value is available, the host processor immediately sets the scrambler  28  to begin encrypting using that value if (1) the conditional access server  16  has previously sent an encryption ON command  130  more recently than an encryption OFF command, and (2) the cryptographic multiplexer  18  has been set to accept those commands. The encryption seed value used for encrypting is the starting state of a linear feedback shift register (LFSR) generator of the scrambler  28  (operation  430 ), a device which creates a pseudo-random bit sequence. This sequence of bits is XOR&#39;d with several of the low-order bits in nearly every byte of the payload of the eligible MPEG packets  12 , not including the authorization stream-carrying packets. The encryption bit on those packets is then set to indicate to IRD descrambler  40  that those packets are encrypted. In addition, the even-odd bit is set to show which flavor of seed was used to do that encrypting. When the next encryption seed is received by the cryptographic multiplexer  18 , it will have the opposing flavor, and when transport streams are encrypted using that new encryption seed, the odd-even bit in the transport streams is toggled to that new opposing state. 
     While the cryptographic multiplexer  18  is decyphering new encryption seeds and using them to encrypt the transport stream  12 , it is also injecting the authorization stream  24  into the transport (operation  440 ). This operates as a simple logical pipe from the cryptographic multiplexer host processor to all the IRD host processors  36 . The authorization stream  24  is inserted as the payload into MPEG packets. As these packets are built, they are queued within the cryptographic multiplexer  18 . Each authorized IRD  20  in the receiving network has three tasks to perform within this conditional access system  10 : (1) extract and decypher its own authorization streams to get new encryption seeds, (2) decrypt the encrypted transport stream packets  30  and pass the new clear packets to the payload processing portion of the IRD  20 , and (3) achieve and maintain synchronization to the timing of the cryptographic multiplexer scrambler  28 , to ensure that decrypting is done with the correct seed. 
       FIG. 5  illustrates a flowchart of operations that are performed at an IRD  20  to decypher authorization stream  24  and maintain IRD synchronization to the conditional access system  10  in steady state operation. In each IRD  20  receiving the encrypted transport stream  30 , the authorization stream  24  is demultiplexed out by the transport demux chip  44  (operation  500 ). This stream  24  is passed to the local host microprocessor  36  and it extracts the secret (C ni ,i)  126 ,  124  message addressed to that particular unit by serial number  102  (operation  510 ). In a preferred embodiment of the invention, every IRD&#39;s (C ni ,i) message is sent twice (refer to FIG.  3 ), which greatly reduces the likelihood that a carrier switch or a short fade could prevent the IRD  20  from getting at least one copy of its own cyphered seed message during each flavor distribution period. As received, cyphered messages are passed to the decrypter  38 . This decrypter  38  is preferably identical to the decrypter  26  installed in cryptographic multiplexers  18 . It proceeds to decypher the new K i  seed values  122  in the same manner as the decrypter  26  within the cryptographic multiplexer  18  (operation  520 ). Those new seeds are then loaded to the odd/even flavor register in the descrambler  40  corresponding to that seed&#39;s flavor (operation  530 ). When this is done, a flag is set in the descrambler  40  to signal that a new valid seed of a particular odd/even flavor is available. 
     As described above, the IRD  20  detects authorization streams  24  addressed to itself and routes the enclosed (C ni ,i) pair  126 ,  124  to the decrypter  38 . In addition, it maintains a circular buffer in volatile memory where the last messages received of each odd/even flavor are stored. When new messages are received, they overwrite the previous message of the same flavor. The purpose of this, which shall be discussed in more detail below, is to provide a way for IRDs  20  to recover from brief losses of transport stream input and, of course, loss of the authorization stream as well. 
     The IRD  20  accepts an incoming MPEG transport stream  12 , either from a satellite carrier or from a terrestrial interface. It applies a process of decrypting the transport stream which is essentially identical to the encrypting operation. The payload of the transport stream packets are XOR&#39;d by the same pseudo-random bit sequence which encrypted them jin the cryptographic multiplexer  18 . This process restores the payloads of those transport stream packets back to the clear or normal state. Those packets are then routed to the downstream processing circuitry  46  within the IRD  20 . 
     IRD synchronization to the conditional access system  10  differs depending on the state of the system. Steady state operation of an authorized IRD  20  and the several transient states are discussed in detail below: (1) authorization by conditional access system, (2) de-authorization by conditional access system, (3) brief transport stream loss, and (4) extended transport stream loss. 
     In steady state operation of the system, authorization streams bearing the cyphered seeds of a particular flavor are distributed to the cryptographic multiplexers  18  and IRDs  20  while those same units are encrypting and decrypting with the previously distributed seed of the opposing odd/even flavor. Within the IRDs themselves, the synchronization is maintained as follows. When a seed of a particular flavor is received, decyphered, and loaded to the IRD  20 , an X_SEED_WRITTEN flag is SET within the IRD  20  (where X designates the seed&#39;s odd/even flavor). When the IRD detects that the odd/even flavor bit in the incoming encrypted transport streams changes (operation  540 , referring to FIG.  5 ), it looks to see if the X_SEED_WRITTEN flag corresponding to the new flavor is set (test  550 ). If so, it knows it has a valid seed for that new flavor, and it begins decrypting immediately (operation  560 ). If not, it blocks all incoming encrypted transport streams  30  from entering the IRD demux chip  44  and clears the X_SEED_WRITTEN flag (operation  570 ). When the very next flavor change occurs in the incoming encrypted transport packet stream  30 , that same flag clears in anticipation of the distribution of the next seed of that flavor. 
       FIG. 6  illustrates a diagram of conditional access system timing for key synchronization when an authorization stream is distributed and a transport stream is encrypted at the cryptographic multiplexer  18  and decrypted at the IRD  20 . The new odd seed is written to odd seed register  158 , setting the ODD_SEED_WRITTEN flag. At that moment, the incoming transport stream is still being encrypted with the previous even seed  160  at the cryptographic multiplexer  18  during an even flavor period  162 . Later, the transport stream flavor  128  switches from even to odd. The odd seed then begins being used to decrypt at the IRD  20  during an odd flavor period  164 . At the next flavor switch within the transport stream, from odd back to even, the ODD_SEED_WRITTEN flag will be cleared. But the authorization stream distribution period  164  for odd seeds is just beginning, and soon a new odd seed will be received, setting the flag once again. At that time, new even seed is written to even seed register  166 , setting the EVEN_SEED_WRITTEN flag. 
     When an IRD  20  is unauthorized in the conditional access system  10 , it does not receive the cyphered authorization streams, addressed to itself, bearing its own (C ni ,i) value pair. Without the (C ni ,i) pair  126 ,  124 , seeds cannot be decyphered, so the X_SEED_WRITTEN flags remain continuously clear, and the IRD removes all incoming encrypted transport streams and substitutes null streams. When the IRD  20  is first authorized in the conditional access system  10 , authorization streams addressed to it begin to be received. In the flavor distribution period corresponding to the first addressed stream received by the IRD  20 , the IRD basically performs the following steps: (1) a seed of a particular flavor is later received, decyphered, and loaded to the descrambler  40 , setting that respective X_SEED_WRITTEN flag; (2) the odd/even flavor bit in the incoming encrypted transport stream packets later changes over to that flavor; and (3) the seed is used to decrypt the encrypted transport streams. Starting with the steady state described earlier, when an RD  20  is de-authorized by conditional access system  10 , it stops receiving authorization streams. 
     Since the IRDs  20  may suffer either short carrier fades or deliberate carrier switches between carriers bearing the same instantiation of a conditional access system  10 , transport streams could be briefly lost.  FIG. 7  illustrates a flowchart of operations that are performed at the IRD  20  to rapidly decrypt data by key synchronization and indexing after brief or extended loss of transport stream. When the transport stream is first lost (operation  700 ), the IRD host  36  resets the descrambler  40  (operation  710 ). This clears the X_SEED_WRITTEN flags and will block encrypted transport packets from entering the IRD demux chip  44 . But, authorization stream will not be blocked. Later, when the host  36  detects the restored transport stream (operation  720 ), it will begin monitoring the authorization stream channel (if available). The first authorization stream  24  detected, even if not addressed to itself, will be examined for its i index  124  and its odd/even flavor  128  (operation  730 ). The IRD host  36  will then exploit the simple knowledge that if the currently distributed encryption seed has an index of i 0 , then the current encrypting is being done using the seed associated with index i 0 −1. If either of the stored authorization streams has an i index value equal to either i 0  or i 0 −1 l (test  740 ), then the assumption is made that (1) the new transport stream bears the same authorization stream as before and (2) the IRD  20  already has the stored authorization streams corresponding at least to the current seed being used to encrypt. In this case, the IRD  20  then progresses through the following sequence: (1) the stored authorization streams whose i index values equal i 0  or i 0 −1 are sent by the IRD host  36 , in order of increasing magnitude, to the decrypter  38 ; (2) the decrypter  38  decyphers one or two authorization streams and the K i    122  results are loaded to the respective odd/even flavor encrypting register(s) (operation  750 ); (3) the X_SEED_WRITTEN flags corresponding to whichever flavor seed(s) was/were loaded are set; (4) the next arriving encrypted transport stream is treated as if it was logically an odd/even flavor change and, if the X_SEED_WRITTEN flag for the new incoming encrypting flavor is set; (5) the IRD descrambler  40  commences to decrypt all the incoming encrypted transport streams (operation  760 ). The IRD then functions as described in the steady state operation. 
     For all losses of transport streams, the X_SEED_WRITTEN flags are cleared, the IRD host  36  resets the descrambler  40 . As just described, when the transport stream is restored, the IRD host  36  examines the first authorization streams received. In the case where the first incoming authorization stream&#39;s i index value is not exactly equal to, or is not equal to one more than either of the i index values in the stored authorization streams, then the IRD host  36  assumes that the stored cyphered seeds are unusable. From then on, it behaves as if it had just boot up. The IRD  20  remains unauthorized until the IRD first gets an addressed cyphered seed through authorization stream and, thence until the succeeding transport encrypting flavor switch. Note that this holds true if the IRD  20  switched to an encrypted transport stream with a different authorization stream, or if the IRD  20  has been disconnected from the original authorization stream for an extended period. In a preferred embodiment of the invention, an extended period would be any outage exceeding half of the difference between flavor distribution period  142  and the total delays  148 ,  154  where double-sending of the cyphered seeds is employed (referring to FIG.  3 ). Failing to use double-sending of the seeds could cause an IRD  20  to miss its current seed distribution on even the shortest outages. In this case, the IRD  20  will appear to initially recover after an outage, but revert to unauthorized at the next flavor switch and remain that way through that next flavor distribution period. 
     In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. 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. 
     As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.