DK175677B1 - Descrambler for subscriber communication system - uses available credit or authorisation signals on impulse purchase and/or prepaid basis - Google Patents

Abstract

A system in a subscriber terminal of a subscriber communication network enables descrambling of a received scrambled signal on an impulse-purchase basis. A circuit processes a received cost signal and a received credit signal to determine whether the subscriber has sufficient available credit to pay for the descrambling, and then provides a credit-available signal when it is determined that sufficient credit is available. A circuit responds to the credit-available signal for indicating that the subscriber has the option of causing the scrambled signal to be descrambled. A circuit can be enabled to accept the option and provide an accepted signal. A circuit responds to the accepted signal to descramble the signal. A separately claimed system for descrambling on a prepaid basis involves comparing a received mask signal related to the scrambled signal with a received authorisation signal. This system is compatible with the impulse-purchase descrambling system.

Description

DK 175677 B1 l

The invention relates generally to decoding systems for subscriber communication networks and in particular relates to systems which enable decoding on a selective subscription basis.

5 Selective Subscriber Subscriber Communication Networks enable decode signals on prepay and / or impulse basis.

In one type of prepayment decoding system, a decoder at the subscriber terminal is arranged to decode all communications received over a given communication channel, such as all the programs which are broadcast over a given TV channel. When the service is interrupted, a decoder must be modified to prevent decoding of signals received over the given channel. In another type of prepay decoding system, decoding is made possible by an activation signal addressed to the decoder at the subscriber terminal.

15

Typical impulse subscriber communication network decoding systems are described in US-A-2,573,349 (Miller), US-A-2,788,387 (Druz), US-A-2,864,885 (Morris), US-A-2,866,962 (Ellett ), US-A-3,531,586 (Bass), US-A-3,736,369 (Vogelman), US-A-3,916,091 (Kirk), and US-A-4,130,833 (Chomet).

20

Bass describes an impulse purchase system for a subscriber TV network in which a debit signal related to the viewer price of a given TV program is transmitted to the individual subscriber terminal and signal processed in relation to an indication of the subscriber's established credit to determine whether the subscriber is authorized to to see the given program. In Bass et 25 al system, subscriber credit is established depending on the introduction of coins into a coin mechanism in the subscriber terminal. For example, after a subscriber has accepted a given program for consideration, the program is deleted for technical reasons and a credit signal is transmitted to the subscriber terminal to restore the subscriber's established credit by an appropriate amount.

30

H DK 175677 B1 I

I I

Druz describes an impulse buying system for a subscriber TV network, in which a price tag

nal, which contains billing information related to the cost of seeing a gi- I

vet program, is transmitted to the subscribers and upon acceptance by a subscriber of the pro- I

the gram of recital and the debit information are recorded using a re- I

H 5 display device at the subscriber terminal. The registered debit information I

is collected or read from the recording device periodically and the subscriber is subsequently

the following is charged for the total cost of programs selected for recital I-

H there the earlier period. IN

Η 10 Both of these known systems require a certain type of post-reconsideration collection

component. With the Bass system, coins must be collected from each subscriber's coin machine

nism and with the Druz system, the registered debit information must be collected from I

H the registration of each subscriber. IN

15 A G Mason describes in the article "A pay-Per-View Conditional Access System for DBS I

By Means Of Secure Over-Air Credit Transmissions ”, International Conference on Se- I

cure Communication Systems, London, 22-23. February 1984, pp. 66-70, a system for I

prevent a subscriber from being able to decode a coded signal in case of fraud, such as IN

by substituting a second signal for a first control signal transmitted from an I

In 20 central station and received in a subscriber terminal in which the first control signal is I

signal processed with another control signal and must have a predetermined relation to it

second control signal in order to decode the encoded signal by subscriber termi- I

channel. The first control signal is signal processed in the central station with a given I

key signal used in the subscriber terminal for decoding the encoded signal to I

25 to provide a coded signal transmitted to the subscriber signal. It coded itself

I nal is signal processed in the subscriber terminal with the given key signal to decode I

the first signal processing signal with the second control signal. IN

The present invention provides a system for preventing a subscriber I

I 30 is enabled to decode a coded signal through fraud by substituting another signal

nal with the first control signal transmitted from and received in an I, respectively

3 DK 175677 B1 central station and a subscriber terminal which is part of a subscriber communication network in which the terminal is arranged such that the first control signal is signal processed in the terminal with a second control signal and must have a predetermined relation to the second control signal in order to enabling a decoding of the encoded signal at the subscriber terminal, the system comprising means in the central signal processing station of the first control signal with a given key signal used in the subscriber terminal for decoding the encoded signal to provide an encoded signal and means in the the subscriber terminal for signal processing of the coded signal, characterized by means in the central station for transmitting a non-coded first control signal to the subscriber station, and means in the subscriber terminal for signal processing of the transmitted, non-coded first control signal with the given key signal or the encoded signal for delivery providing a key signal used in the subscriber terminal for decoding the encoded signal, each key signal, the first control signal, and the second control signal being separate signals.

Preferred features of the invention are set forth in the subclaims.

The invention will now be described with reference to the drawing, in which: FIG. 1 is a combined function and flow block diagram of an authorization processor for the system of the invention; FIG. 2 is a functional block diagram of a key signal encryption system in a broadcasting terminal of a subscriber communication network containing the system of the invention; FIG. 3 is a function block diagram of a key signal decryption system in a subscriber terminal containing the authorization processor of FIG. 1, and

I DK 175677 B1 I

I I

H fig. 4 is a function block diagram of an information processor providing circuitry

Hits signals transmitted to the respective subscriber terminals of the system of I

invention. IN

H 5 The preferred embodiment of the invention is included in an authorization process

sor which is described with reference to FIG. 1. The authorization processor is included: I

in a subscriber terminal in a subscriber TV network, such as are described in EP-A-0 127 I

381. The authorization processor of FIG. 1 makes it possible to decode a received coded TV-I

signal on an impulse purchase basis or on a prepayment basis. IN

I 10 I

The authorization processor receives a program price signal on line 10 and a credit signal I

on line 11. The price signal indicates the charge for decoding a given received coded I

TV signal and is applied to all the subscriber terminals of a subscriber TV network. Credit I

signals are unique to each subscriber and are individually addressed to the respective subscribers

In 15 network terminals in the network. Each credit signal indicates the accumulated credit that you

is assigned to the respective subscriber. IN

The authorization processor contains a credit register 12, an overcurrent detection device

In scheme 13, a debit register 14, an addition register 15 and a comparison unit 16. I

I 20 The credit register 12 registers the credit signal received on line 11. The debit register 14 I

You record the accumulated charges from decode signals accepted by the respective I

tive subscriber for decoding. Addition register 15 adds and records the sum of I

In debits indicated by the program price signal on line 10 and the accumulated I

In debits recorded in debit register 14. Comparison unit 16 compares

In the 25 credit signal registered in the credit register 12 with the sum of the addition registered in I

the addition register 15 to determine whether the subscriber has sufficient credit to pay I

I I

In for decoding the encoded signal. If it is established by such comparison:

I I I

In that the accumulated credit indicated by means of the credit signal recorded in the credit register I

In stern 12 no less than the sum of the addition is recorded in the addition register 15, I

In 30, a credit rating signal is provided on line 17 to a display driver and logic circuit I.

In race 19. If it is established by such comparison that such an accumulated creature

Is less than such a sum, an insufficient credit signal is provided on line 20 of the display driver and logic circuit 19.

The display driver and logic circuit 19 provide signals on line 21 for checking the contents of a display on a video display device 22, such as the television set connected to the subscriber terminal. Alternatively, the display device may be an alphanumeric display on the front panel of the decoder unit. The authorization processor also receives a program mask signal on line 23 and an authorization word signal on line 24. The program mask signal is a 56 bit signal that is unique to each coded program 10 which is broadcast over the subscriber TV network. The program mask signal is applied to all subscriber terminals in the network. The authorization word signal is a 56 bit word signal that is individually addressed to the respective subscriber terminals in the network. If the respective subscriber regularly receives all the coded programs on the given TV channel, the authorization word signal addressed to the respective subscriber terminal matches the program mask signal supplied to all the subscriber terminals in the network to the prescribed degree. If the respective subscriber is not a regular subscriber to all the coded programs on the given channel, the authorization word addressed to the respective subscriber terminal received on line 24 will not match the program mask signal received on line 23.

20

The authorization processor contains another comparison unit 25 which compares the received program mask signal on line 23 with the received authorization word signal on line 24. If the compared signals do not match to the required degree, a non-prepayment signal is provided on line 24 for the display driver and the logic circuit 25 19. When the compared signals do not match, a prepayment signal is provided on line 27 of an activation port 29.

The comparison unit 25 consists of logical AND gates and the corresponding bits of the program mask signal on line 23 are compared with the authorization word signal on line 24 30 and a logical OR signal processing of the results of the logical AND operation is performed. A match in one of the corresponding bit pairs results in

I DK 175677 B1 I

I 6 I

The signal is applied to line 27. Typically, different bit positions in authorization word I

the signal corresponds to different TV channels and / or programs. An authorization word signal I

for a given decoder includes a "TRUE" bit in the bit position specified for each pro- I

grams and / or channel for which a service has been prepaid. Program I

The 5 signal includes a "TRUE" bit in each position corresponding to a given program I

and / or a given channel. - I

One of the bits of the program mask signal on conductor 23 indicates whether the received coded

In TV signal can be decoded on impulse purchase basis. If so, there is in this bit- I

In position 10 is provided an impulse acquisition signal which is supplied via conductor 30 to the display driver I

I and the logic circuit 19. I

Certain bits of the program mask signal and the authorization word signal may provide additional au

In torization functions, such as the possibility of stereo sound and selection of different languages. IN

I I

The display driver and the logic circuit 19 contain logic circuits that dictate I

In the contents of the signals supplied via the conductor 21 to the display device 22. If provided

to provide a credit rating signal on conductor 17, a non-I is provided

In prepay signal on conductor 26, and an impulse purchase signal is provided on conductor 30 and I

In the 20 display driver, and the logic circuit supplies via activation 31 an activation signal! to an I

I accept switch 32 and additionally supply a signal to conductor 21 resulting in display I

The device 22 towards the subscriber indicates that it has the opportunity to obtain it

the TV signal shown on impulse purchase basis. IN

I 25 The subscriber can indicate that he has accepted the choice by activating the switch I only

I 32 when activated by the enable signal on conductor 31. If the subscriber accepts- I

I! If the choice is made, via acceptor 33, an acceptance signal is applied to the activation port 29. It accepts

A ready signal on conductor 33 is also applied to the display driver and logic circuit 19 which I

You then respond by giving the display device a brief indication of recognition

In the 30th of the subscriber's acceptance. The decoded TV signal is then displayed on the given I

In TV channel. IN

7 DK 175677 B1

If, upon activating switch 32, the subscriber indicates that the choice is not accepted, via signal 34, a signal is applied to the display driver and logic circuit 19, which responds thereto, causing the display device 22 to briefly indicate the recognition of the subscriber lack of acceptance. Then you only see a blank screen on the given TV channel. The display driver and logic circuit 19 respond to the insufficient credit signal on conductor 20 by causing the display device 22 to briefly indicate that the subscriber's credit is insufficient to enable decoding of the encoded signal. Alternatively, the view may continuously indicate that there is not 10 permission to view the program, either because there is no prepayment or due to insufficient credit.

In addition, the accepted signal on conductor 33 is applied to the activation input of debit register 14, causing the registered sum in addition register 15 15 to be entered in debit register 14. Thus, the accumulated bills in debit register 14 are increased by the bill indicated by the program price signal received via conductor 10 .

The override detection device 13 is coupled to the addition register 15 to detect when the arithmetic limit of the debit register 14 will be exceeded by an increase of the accumulated bills in the debit register 14 with the bill indicated by the program price signal on conductor 10 and by providing an overspeed signal on conductor 18 upon such detection. The display driver and logic circuit 19 responds to the override signal 18 by preventing an activation signal on conductor 31 from relying on a credit rating signal on conductor 17. Subscribers 25 will thereby not be able to input false price and credit signals into the authorization processor to increase the debit register 14 until it is exceeded, thereby storing an amount less than the currently accumulated bills attributed to the subscriber's terminal.

When either the prepay signal is provided on conductor 27, or the accepted signal 30 is provided on conductor 33 to the input line 35 of the activation port, the activation port 29 provides a decrypted and decoded channel key signal from conductor 36 to conductor 37 and

I DK 175677 B1 I

I I

You thereby enable a decoding of the coded TV signal. The importance of transferring

this key signal to conductor 37 is explained below with reference to FIG. 2 and 3. I

FIG. 2 and 3 show the encryption and decryption systems in the transmitter terminal and the one

In 5 separate subscriber terminal of a subscriber TV network in connection with which the systems I

according to the invention can be used. The encryption and decryption systems of FIG. IN

I 2 and 3 are particularly suitable for preventing a subscriber from being able to

encoding the coded signal for fraud by a substitution of other signals for one or I

several of the respective transmitted program mask signal, the authorization word signal, I

In the 10 program price signal and the credit signal received at the subscriber's terminal to obtain I

H conformity to said comparison and / or to give rise to,

that a credit rating signal is provided when the subscriber's credit is insufficient. IN

A key current generator 39 in the encryption system of the transmitter terminal (Fig. 2) is provided.

Η 15 produces a key current signal on conductor 40 used to encode the TV signal. An idea

In tical key current signal must be reproduced on conductor 42 using key current generator I

In clean 44 of the decryption system of the subscriber terminal (Fig. 3) to decode the subscriber I

In the down terminal it should be possible to decode the coded TV signal. IN

20 The key current generator 39 of the encryption system - see FIG. 2 - generates key current

In the signal of conductor 40, by processing a start vector (IV) frame count signal I

I on conductor 45 according to the Data Encryption Standard (DES) algorithm at I

In that the algorithm is keyed by a work key signal on conductor 46. I

I A work key generator 47 generates the work key signal on conductor 46 by signal I

In processing the frame count signal on conductor 45 in accordance with the DES algorithm at

In that the algorithm is keyed by a channel key signal on conductor 49. Channel key signal I

I on conductor 49 is provided by means of a control computer not shown at the transmitter terminal. IN

The channel key signal on conductor 49 is encrypted for transmission to the subscriber terminals. And I

In channel key generator 50, the channel key signal encrypts by generating an encrypted channel.

B1 key key signal on conductor 51 by a signal processing of a coded channel key signal on conductor 52 in accordance with the DES algorithm by keying the algorithm using a coded category key signal on conductor 54.

In connection with the scheme to prevent decoding by fraud by a substitution of the program mask signal, authorization word signal, program signal and / or credit signal, the program mask signal on conductor 55 is used to encode the channel key signal on conductor 49 by an exclusive OR signal processor of the channel key signal on conductor 49 with the program mask signal on conductor 55 to provide a channel key signal on conductor 52. In connection with the same scheme, the coded category key signal on conductor 54 is provided by the exclusive OR signal processing of a category key signal on conductor 56 by means of the control computer with a program price signal on conductor 57 using the control computer. The program price signal on conductor 57 is also provided by the control computer.

15

The symbols 59, 60, 61 and 62 of FIG. 2 represents exclusive OR elements.

The category key signal on conductor 56 is encrypted for transmission to the subscriber terminals. Before performing such encryption, the category key signal on conductor 56 encoded at 20 is an exclusive OR signal processing with a credit signal on conductor 64 by means of the control computer, thereby providing an encoded category key signal on conductor 65.

This is in the aforementioned table.

The position of the bits of the category key signal on conductor 56 subjected to an exclusive OR signal processing with the credit signal on conductor 64 must be different from the position of the bits of the category key signal on conductor 56 subjected to an exclusive OR signal processing with the program price signal on conductor 57. of the credit signal will be described below in connection with FIG. 4. 1

The category key generator 66 encrypts the catechip key signal by generating a number of unique encrypted category key signals on conductor 67 by signal processing the encoded

I DK 175677 B1 I

I I

category key signal on conductor 65 according to the DES algorithm by algo- I

(the rhythm is keyed by a similar number of coded unique subscriber key signals on I

! leader 69. The subscriber key signals are unique in terms of the individual subscriber terminals

signals. IN

I 5 I

A number of unique subscriber key signals are provided on conductor 70 ffa a subscriber key I

register 71 by accessing register 71 by means of a number of the respective I

In tive subscriber addresses provided on conductor 72 by means of the control computer. Subscribe I

In the down-key signals on conductor 70, an exclusive OR treatment with an I is subjected

In 10 authorization word signal on conductor 74 using the control computer to provide

In the coded subscriber key signals on conductor 69. The authorization word signal on conductor 74 which I

If used for encoding the subscriber key signal on conductor 70, correspond to the program I

In the mask signal on conductor 55 depending on whether the subscriber terminal corresponds to the individual

In dual subscriber key signal during decoding, a subscriber is assigned which is a rule I

In 15 subscribers of all the programs on the given TV channel. The use of authorization- H

The word signals of conductor 77 also appear in the aforementioned diagram. IN

In the control computer, a subscriber key generation number is also supplied to conductor 75 which is I

Unique in this respect to the transmitter. IN

I 20 I

In the combination of the credit signal on conductor 64, the encrypted category key signal on lead I

In there 67, the subscriber address of conductor 72 and the authorization word signal of conductor 74, all of which I

You relate to a given subscriber terminal, transmitted together as a category I

In key message addressed to a given subscriber terminal using subscriber I

25 address in such a message. The unique category key messages will be you

Accordingly, transmitted to each of the subscriber terminals. IN

When the number of alternative authorization functions and the number of alternative programs I

and / or channels supplied to subscribers over the network exceeding 56 bits are I

In 30 positions of the authorization word signals provided and contained in separate category I

11 DK 175677 B1 rome key messages addressed to each of the respective subscriber terminals where they are stored upon receipt.

The program mask signal on conductor 55, the encrypted channel key signal on conductor 51, and the program price signal on conductor 57 all relate to the subscriber terminals and are transmitted together as a channel key message to all the subscriber terminals. The channel key message also contains a category key address signal on conductor 76 for selecting one of the stored category key messages from the storage in the subscriber terminal according to which programs and / or channels and authorization functions are indicated by the TRUE bit of the respective bit positions of the program mask signal on conductor 55. The frame count signal on conductor 45 and the subscriber key generation number on conductor 75 are also transmitted to all subscriber terminals.

The encryption system of FIG. 2, the manner in which the frame count signal, the channel dialing message, the category key messages, and the subscriber key generation number are further processed prior to transmission and the use of the key stream signal for encoding the TV signal are described in more detail in the above-mentioned United States Patent Application No. 498,800. 1 2 3 4 5 6 7 8 9 10 11

The decryption system of each subscriber terminal contains, as shown in FIG. 3 shows a 2 authorization processor 77, a memory for redirect messages 78, and active 3 ring port 29. The authorization processor 77 is described above in connection with FIG.

4 1. The redirect message register 78 stores all the various redirect messages with 5 messages addressed to the given subscriber terminal.

6 7

The decryption system of each subscriber terminal receives the frame count signal 8 of conductor 79, the program mask signal of conductor 23, the encrypted channel key signal of conductor 80, the program mask signal of conductor 10, the credit mask of conductor 11, the encrypted 10 channel key signal of conductor 81, authorization word signal of conductor 24, and authorization word signal of conductor 24. - the 11 registration number of conductor 82 and the category key address address signal of conductor 83. The credit signal of conductor 11, the encrypted category key signal of conductor 81 and the authorization key

DK 175677 B1 I

the word signal on conductor 24 is derived from the storage position of memory 78 to which I is

accessed using the address signal of conductor 83. I

The received subscriber key generation number on conductor 82 is combined with subscriber H

5, the address of conductor 84 from a subscriber address register 85 to provide a subscriber I

low key generation signal on conductor 86. I

Two bits in a given position of the derived subscriber key generation number on conductor 82 I

is also provided on conductor 87 for access to a given address in a "subscriber key-H

10 germ memory "88. The memory 88 stores four" subscriber key germ signals "which are H

unique to the subscriber and provides a prescribed key germ signal on H

conductor 89 according to the address of conductor 87. H

A subscriber key generator 90 reproduces the subscriber key signal on conductor 91 response-H

15 shows the subscriber key signal on conductor 70 of the system of FIG. 2 used to crawl

the encrypted category key signal in the category key message addressed to I

the given subscriber and received on manager 81. I

The subscriber key generator 90 generates the subscriber key signal on conductor 91 by signaling

20 processing the subscriber key generation signal on conductor 86 in accordance with DES H

the algorithm by keying the algorithm using the prescribed key germ signal on H

j leader 89. I

The aforementioned system for reproducing subscriber key signals using stored H

25 key germ signals appear in U.S. Patent Application No. 589,741, filed March 15, H

1984 entitled "Descrambler Subscriber Key Production Utilizin Key Seeds Stored In H

Descrambler. "H

The subscriber key signal on conductor 91 is subjected to an exclusive OR signal processing H

30 with the received authorization word signal received on conductor 24 to provide an H

13 DK 175677 B1 coded subscriber key signal on conductor 92 corresponding to the subscriber key signal for the given subscriber on conductor 69 in the system of FIG. 2nd

A category key generator 94 decrypts the encrypted category key signal on conductor 5 80 by generating a coded category key signal on conductor 95 corresponding to the signal of le 65 in the system of FIG. 2 by signal processing the encrypted category key signal on conductor 81 according to the DES algorithm by keying the algorithm by the subscriber key signal on conductor 92.

The coded category key signal on conductor 95 is subjected to an exclusive OR signal processing with the received credit signal on conductor 11 to provide a decoded category key signal on conductor 96 corresponding to the category key signal on conductor 56 of the system of FIG. 2nd

15 The symbols 97,98,99 and 100 in FIG. 3 indicates exclusive OR elements.

The decoded category key signal on conductor 96 is subjected to an exclusive OR signal processing with the received program price signal on conductor 10 to provide a coded category key signal on conductor 101 corresponding to the coded category key signal on conductor 54 of the system of FIG. 2nd

A channel key generator 102 decrypts the encrypted channel key signal received on conductor 80 by generating a coded channel key signal on conductor 104 corresponding to the signal on conductor 52 of the system of FIG. 2 by signal processing the encrypted channel key signal on conductor 80 in accordance with the DES algorithm by keying the algorithm by the coded category key signal on conductor 101.

The coded channel key signal on conductor 104 is subjected to an exclusive OR signal processing with the program mask signal on conductor 23 to provide a decoded channel key signal on conductor 36 corresponding to the channel key signal on conductor 49 of the system of FIG. 2nd

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The activation port 29 only transmits the channel key signal from conductor 36 to conductor 37 when H

the input conductor 35 of the activation gate 29 either receives a prepayment signal or H

is an acceptance signal from authorization processor 77 described in connection with H

5 FIG. I. I

A work key generator 106 generates a work key signal on conductor 107 corresponding to H

the working key signal on conductor 46 of the system of FIG. 2 by signal processing frame counter- H

the signal on conductor 79 according to the DES algorithm by keying the algorithm H

10 using the channel key signal on conductor 37. H

The key current generator 44 generates a key current signal on conductor 42 corresponding to H

the key current signal on conductor 40 of the system of FIG. 2 by signal processing frame count- H

the signal on conductor 79 according to the DES algorithm by keying the algorithm H

15 using the work key signal on conductor 107. H

, The subscriber key germ memory 88 and the debit register 14 of each subscriber H

terminal is contained in a non-volatile RAM by an integrated circuit board having an H

architecture that prevents access to the debit register outside the chip. The chip should be- H

20 are able to perform calculation, comparison and authorization functions of au-H

The torque processor 77 described in connection with FIG. 1, and decryption H

and decoding functions of the key signal decryption systems of FIG. 3. A chip of H

the type TMS70C40 from Texas Instruments is preferred. This chip is modified to H

use of the system according to the invention by placing the acceptance instructions outside the chip H

25 out of function. This avoids the acceptance of instructions for reading the safe housing

memory. H

A more detailed description of the decryption system of FIG. 3 and the use of key systems

the voice for decoding the received code signal is disclosed in U.S. Patent Application No. H

30 498,800. IN

In FIG. In, a debit signal on conductor 109 indicating the contents of the debit register is supplied to the display driver and logic circuit 19 which, at the request of the operator, supplies a signal on conductor 21 to the display 22, thereby indicating the subscriber terminal summed bills. The subscriber is thereby able to record information on accumulated bills and transmit the same to the radio station on demand, for example when the service is interrupted, when the subscriber's terminal equipment is transferred to another subscriber or on payment of the subscriber's periodic bill.

This information about accumulated bills is used by the sending station to resolve any discrepancies regarding the notes and could occur with the subscriber or to facilitate the calculation of the credit when the operation is interrupted or the equipment is transferred.

In order to verify the accumulated bill information supplied to the subscriber station, the authorization information is also shown on the copy 22 display and transfer to the subscriber station. The authorization information is displayed depending on an authorization signal provided on conductor 110 of the display driver and logic circuit 19 by means of a legalization generator 111. The legalization generator 111 generates a legalization signal on conductor 110 by processing the debit signal on conductor 109 in accordance with the algorithm algorithm. 20 is read by the subscriber key signal on conductor 91.

In a subscriber TV network with a bidirectional communication link between the transmitting terminal and the subscriber terminal, the debit signal on conductor 109 and / or the legalization signal on conductor 110 can be stored in a memory not shown in the subscriber terminal and access can be obtained by an order. to transmit to the transmitter terminal.

Reference is again made to the system in which the subscriber records and transmits the displayed information regarding accumulated bills and the legalization information upon receipt of the subscriber's claim regarding accumulated bills and authentication information, the transmitting station introducing the same into the control computer. The control computer converts the entered information into debit and legalization signals with the same pre-

DK 175677 B1 I

mat such as the signals supplied to the display driver and logic circuit 19 on conductors 109 I

and 110. I

The converted debit and legalization signals in the control computer - see fig. 4 - added to I

5 to conductors 114 and 115. The debit signal is entered in a debit register 116, and legalization, - I

the signal is entered in a legalization register 117. The debit signal from the debit register 116. IN

signal processing by a debit generator 118 in accordance with DES al-I

the algorithm by keying the algorithm using the subscriber key signal on conductor 70 which I

is related to the subscriber terminal of the subscriber who transmitted legalization

10 information. The debit generator 118 thereby generates a legalization debit signal H

on conductor 119, which signal is compared to the legalization signal in the legalization register.

streamed 117 by means of a comparator 120. Accordingly, information is provided

the subscription from the subscriber is verified and a signal is applied to conductor 121 enabling H

inserting the contents of the debit register 116 into a debit memory 122. H

FIG. 4 illustrates how a credit signal for a given subscriber is provided on conductor H

64 - see FIG. 2 - for transmission to the subscriber terminal.

An indication of the amount of the individual payment made by a given subscriber is entered

20 in the control computer for supplying a payment signal on conductor 122 to an addition register H

124. Addition register 124 adds and records the sum of the amount indicated by

using the installment signal on conductor 124 and an amount stored in a payment memory I

125, which is increased by the amount of the individual payment. When the service of the subscriber on- I

beginning, the initial amount in the payment memory 125 is the amount that is then layer I

25 in the debit memory 122 for the corresponding subscriber terminal. When the amount of each I

new payment is entered in the control computer, the sum is registered in the addition register I

124 in the payment memory 125, thereby increasing the stored payment record I

with the amount of the new payment. I 1

A signal indicating the amount of credit of a given subscriber is provided on le I

there 126 and stored in a credit memory 127. An addition register 128 summarizes and I

B1 records the value of the credit from the credit memory 124, and the amount stored in the payment memory 125 to provide the credit signal to manager 64, indicating the accumulated credit for the given subscriber.

When the service of the given subscriber is complete, the amount of any reimbursements is determined by means of a difference register 129 which registers and feeds on manager 130 a signal indicating the difference in the amounts stored in the payment memory 125 and the debit memory 122.

Claims (5)

A system for preventing a subscriber from being able to decode an encoded signal H 5 by fraud, such as substitution of a second signal with a first control signal I transmitted from and received at a central station and a subscriber terminal, respectively, IH which is part of a subscriber communication network in which the terminal is arranged HI such that the first control signal is signal processed in the terminal with a second control II signal and must have a predetermined relation to the second control signal to enable II 10 to decode the encoded signal by the subscriber terminal, the system H1 comprising means in the central processing station for processing the first control signal With a given key signal used in the subscriber terminal for decoding the HI encoded signal to provide an encoded signal and means in the subscriber terminal for HI signal processing of the encoded signal, characterized by II means in the central station for transmitting a -coded first control signal to the II subscriber station, II and o means in the subscriber terminal for signal processing of the transmitted encoded HI first control signal at the given key signal or encoded signal to provide In a key signal used in the subscriber terminal for decoding the encoded signal, HI 20, each key signal, the first control signal and the other control signal is separate II signals. H System according to claim 1, characterized in that the first control signal is a mask II signal {PM} uniquely related to the coded signal and the second signal is an authorization signal {AW} indicating whether the subscriber is authorized to decode the encoded signal, that the mask signal {PM} is signal processed in the central station with the given key signal {CH} to provide the encoded signal (CH (PM)}, that the - The second masked signal {PM} and the encoded signal {CH (PM)} are transmitted to the subscriber II terminal, and the non-encoded mask signal {PM} is signal processed with the encoded signal {CH (PM) } in the subscriber terminal to provide the given key signal II {CH}. I 19 DK 175677 B1 System according to claim 1, characterized in that the first control signal is an authorization signal (AW) indicating whether the subscriber is authorized to decode the encoded signal and the second control signal is a mask signal {PM} which is uniquely related. to the encoded signal, that the authorization signal {AW} is signal processed with the given key-in signal {SU} in the central station to provide the encoded signal {SU (AW)}, and that the encoded signal {SU (AW) } is used in the central station for encrypting a second key signal {CA (CR)} for transmission to the subscriber terminal, that the unencrypted authorization channel {AW} and the encrypted second key signal are transmitted to the subscriber terminal and encoded authorization signal {AW} is signal processed in the subscriber terminal with the given key signal {SU} to provide the encoded signal {SU (AW)} used in the subscriber terminal to decrypt the second key signal {CA (CR)} used in the subscriber terminal for decoding the encoded signal. 15 System according to claim 1, characterized in that the first signal is a price signal {PC} indicating the charge for decoding the encoded signal and the second control signal is a credit signal {CR} indicating the credit that is due to available to the subscriber that the price signal {PC} is signal processed by the given key signal {CA} at the central station to provide the encoded signal {CA (PC)}, that the non-encoded signal {CA (PC)} is used in the central station for encrypting another key signal {CH (PM)} for transmission to the subscriber terminal, that the encoded price signal {PC} and the encrypted other key signals are transmitted to subscriber terminal, and that the unencrypted price signal {PC} is signal processed in the subscriber terminal with the given key signal to provide the encoded signal {CA (PC)} used in the subscriber terminal to decrypt the second key signal {CH (PM)} used in the subscriber terminal to decode it coded signa l. System according to claim 1, characterized in that the first control signal is a credit signal {CR} indicating the credit available to the subscriber and the second control signal is a price signal {PC} indicating the charge. for decoding the encoded signal that the credit signal {CR} is signal processed in the central station with the given II key signal {CA} to provide the encoded signal {CA (CR)} II encoded credit signal {CR.} And encoded signal (CA (CR)} are transmitted to the subscriber terminal) and that non-encoded credit signal {CR} is signal processed with a coded signal (CA (CR)). } in the subscriber terminal to provide the given key signal II {CA} .. II 10 II! H