Method and device for scrambling and descrambling of a specific television broadcast

A receiving device comprises a pseudo-random generator (18) initialized by a control word (CW) and a memory card reader (19). In accordance with the invention, it reads control word in a table of numbers contained in a read-only memory of a memory card (17) by utilizing a mode which is chosen from among several different modes, receives from the transmitter a signal which indicates the choice made in respect of said mode at a given instant.

BACKGROUND OF THE INVENTION 
The invention relates to a method of scrambling, at the level of a 
transmitter, and descrambling, at the level of receivers, a specific 
television broadcast of limited duration, scrambling being performed by 
means of a pseudo-random generator which is initialized by a control word, 
descrambling involving the reconstruction upon reception, by means of a 
memory card device, of the control word which initializes a pseudo-random 
generator which is identical to that of the transmitter, said 
pseudo-random generator supplying the data necessary for descrambling. The 
invention also relates to a transmitter device for use in carrying out the 
method in accordance with the invention, said transmitter device 
comprising a pseudo-random generator which is initialized by a control 
word, and to a receiving device for use in carrying out the method in 
accordance with the invention, said receiving device comprising a 
pseudo-random generator which is initialized by a control word, and 
comprising a memory card reader. This method and these devices are used in 
scrambled television transmission and reception systems. 
A television scrambling method of this kind is known from GB-P-2 132 860 
(British Broadcasting Corp.). In this satellite television system, the 
control word is formed in a fully random manner and is transmitted in 
encrypted form to the receivers. The latter decrypt the control word by 
way of a calculation which is performed in an "smart" card by a process 
which is known per se, in this case being the so-called "DES" process with 
a secret key. Moreover, each card has a different identification code. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to allow to use cards which are 
intended to be used only once, that is to say cards which enable the 
descrambling of the reception of the television broadcast of a specific, 
single event. Therefore, these cards must be very inexpensive; however 
their protection may be comparatively weak because after the single use 
they will become completely unusable anyway. Prior art cards provided with 
a processor would be too expensive for such an application and the use of 
a different identifier code for each card would imply a distribution and 
control system which is too complex. 
The object of the invention is achieved by means of a method which is 
characterized in that in advance there are defined a table of numbers and 
different feasible modes of generating a control word by reading said 
table, and that this table is written into a read-only memory in 
descrambling cards issued to the users of receivers, after which at the 
instant of said specific television broadcast program the mode of reading 
a control word in said table is chosen in real time from among the various 
predefined feasible modes, which control word is read in conformity with 
said mode in order to supply it to the transmitter in real time, and 
immediately a signal is broadcast to the receivers so as to indicate the 
relevant mode of extracting the control word from the table, as a result 
of which the receivers generate this control word according to the 
indicated mode from a card comprising a simple memory in which said table 
of numbers is written. 
Thus, the card comprises only a simple memory and is very inexpensive. The 
only practical means of invading its confidentiality would be the 
fraudulent reproduction (and sale) of a large number of copies of a card, 
possibly procured in a standard manner ("cloning"). This can be 
counteracted by distributing the cards to the authorized users at the last 
moment only. 
This method, used in exceptional cases whereas normally another scrambling 
method is used, in which other method the control word is transmitted in 
encrypted form so as to enable the receivers to reconstruct it by means of 
a key, is preferably characterized in that the same control word is used 
in the two methods and continues to be transmitted in encrypted form 
during the use of the method in accordance with the invention. 
Thus, the normal customers of the transmitter concerned can consider it as 
if nothing else happens. 
Preferably, there is also transmitted a signal which indicates that the 
transmission takes place in accordance with the method of the invention. 
Thus, a receiver of a user which does not have a subscription providing 
him with a smart card, can initiate a procedure based on the use of a card 
in accordance with the invention. 
In a special embodiment said mode of extracting the control word from the 
table consists in indicating an address in this table and of a mode for 
reading as from this address. 
According to different versions, the read mode consists in reading in 
descending order of addresses, or in reading a data element every n data 
locations, of in reading with an address value progression based on a 
calculation. 
A transmitter device in accordance with the invention comprises a control 
word generator which comprises means for reading this word in a table of 
numbers by utilizing a mode from among several different feasible modes, 
means for choosing a desired mode from among said several feasible modes, 
and means for supplying the receivers with a signal indicating which 
choice has been made in respect of said mode at a given instant. 
Preferably, said means for choosing a desired mode consist essentially of a 
random number generator. Thus, until the very last instant it is 
impossible to predict the control words. 
A receiving device in accordance with the invention comprises means for 
reading the control word in a table of numbers table contained in a 
read-only memory of a memory card, utilizing a mode from among several 
different modes, means for receiving from the transmitter the signal 
indicating the choice made at a given instant in respect of said mode, and 
means for controlling the read mode of said means for reading the control 
word as a function of the signal received by said means for receiving the 
signal from the transmitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The following description is given by comparison with and with reference to 
the standard scrambling system which is referred to as "Eurocrypt" and is 
intended notably for use in association with the standard 
"D2-MAC/Packets". Nevertheless, it will be evident that the invention can 
also be used with any other system of the same kind, and also with systems 
in which the video transmission is all-digital. A system of this kind may 
be considered in respect of two aspects: scrambling and encryption. 
Generally speaking, scrambling is performed on the video (or audio) 
signals and encryption concerns the signals serving as keys for defining 
or controlling the scrambling. 
In the device whose diagram is shown in FIG. 1, the elements 8 to 12 are 
elements provided for the "EUROCRYPT" standard, and they are not modified 
for the method of the invention. The device comprises a video signal 
source 9 (for example, a camera or video recorder), which applies said 
video signals in the baseband to a scrambling circuit 10. In this context 
it suffices to note that the latter operates on the basis of a signal 
which is referred to as a "scrambling sequence" which fully determines how 
scrambling is to be realised. The signals thus scrambled are applied to a 
modulator/transmitter 8 for transmission in conformity with any known 
method. 
Said scrambling sequence is generated and applied to the scrambler 10 by a 
scrambling sequence generator 11 which comprises notably a pseudo-random 
generator which is initialized by a control word CW and which also takes 
into account the number of the current frame which is applied thereto by 
the video source 9. The generator 11 is also of a non-modified, known 
type. 
An access control module 15 applies said control word to the pseudo-random 
generator of the scrambling sequence generator 11. It also generates data 
which are transmitted to enable each receiver to proceed with the 
verifications necessary in respect of the right to receive the current 
transmission. 
In the known systems the control word is formed in a fully random manner by 
a circuit 13 which forms random digits in known manner, for example, on 
the basis of the noise generated by a diode. 
The control word is applied from the access control module 15 to the 
scrambling sequence generator circuit 11 and also to the 
modulator/transmitter 8 so as to be dispatched, via the transmission 
channel, in the form of digital packets according to the D2-MAC standard. 
It is to be noted that the control word CW is not transmitted in clear 
form but is encrypted by means of a key K in an encryption element 12 
before being applied to the modulator/transmitter 8. 
According to the method of the invention, the circuit 13 is not used 
directly and it is a circuit 14 which supplies the control words. The 
circuit 14 is a deterministic control word generator in a sense that the 
words generated thereby are not completely random but can be predicted 
subject to given conditions. Means 2 enable the reading of this word in a 
table of numbers 7, utilizing a mode from among several different 
predetermined modes. Means 1 enable the selection of a desired mode from 
among said several feasible modes. Said mode of extracting the control 
word from the table consists, for example in indicating an address in the 
table, and of a read mode starting from this address. The read modes may 
notably be: backwards reading, reading of a byte from every n bytes (if 
reading takes place in bytes), reading with a progression of address value 
based on a calculation or on the reading of an address table. Means 3 
enable immediate transmission to the receivers of a signal, indicating 
which mode of extracting the control word from the table has been chosen, 
in the form of digital indications which are transmitted to the modulator 
8 so as to be dispatched via the transmission channel. 
The means 1 are formed essentially by a circuit for generating two numbers 
at random, a first number which is between, for example 1 and 5 if there 
are 5 read modes, and a second number which is between 1 and 1024 if, for 
example, the table of numbers comprises 1024 numbers. The first number 
indicates the read mode and the second number indicates the starting 
address in the table. The means 1 may utilize the circuit 13 for 
generating the numbers in question. If reading goes beyond the end of the 
table, it continues with the start of the table. 
The means 2 are formed essentially by a wired processor which generates the 
signals necessary for reading the table in the memory on the basis of the 
two numbers generated by the means 1. 
The means 3 are connected to the general clock 24 of the device and supply, 
at the correct instant, serial-type data representing the relevant numbers 
to the modulator 8 so that they are transmitted, in the form of digital 
packets of the D2-MAC standard, by being added, for example to one of the 
messages which are referred to as "ECM" and which are provided for the 
transmission of data indicating the transmission characteristics. In these 
ECM messages the definition of given blocks, referred to as "PI" 
(parameter identifier), is still open so that it is readily possible to 
add such information thereto and still remain compatible with the 
standard. 
FIG. 2 shows a receiving device. It comprises a receiving circuit 16 which 
supplies baseband video signals from an antenna 23 for satellite reception 
(or an antenna for terrestrial reception which is not shown). The video 
signals are applied to a descrambling circuit 20. The circuit 20 is also 
of a known type and does not require a detailed description. It 
corresponds to the scrambling circuit 10. It operates on the basis of a 
signal which is referred to as a "descrambling sequence" and which 
completely determines how descrambling is to be performed. The signals 
thus descrambled are applied to a known display device 4. 
Said descrambling sequence is generated and applied to the descrambling 
circuit 20 by a descrambling sequence generator 18 which comprises notably 
a pseudo-random generator which is initialized by a control word CW. The 
generator 18 is also of a known type. 
An access control module 22 applies said control word to the pseudo-random 
generator of the descrambling sequence generator 18. It also performs all 
verifications necessary in view of the right of the receiver to receive 
the current transmission. 
In accordance with the invention, the control word is read by means of a 
"chip card" reader 19 in a "chip card" 17. This card contains simply a 
read-only memory in which there is stored a table of numbers which is 
identical to the table 7 and it need not comprise an integrated 
microprocessor. The memory in question has a capacity of, for example one 
Kilobyte. Means 5 enable control of the reading of the control word in the 
table of numbers of the card 17 by utilizing an appropriate mode, 
indicated by the transmitter, from among several different predetermined 
modes. This mode is indicated in the form of a number contained in the 
data packets received by the receiver and applied to the means 6 by the 
receiving circuit 16. The means 6 generate an instruction which is adapted 
to the nature of the means 5 so as to obtain the desired mode of reading 
the data from among the various modes feasible. 
The means 1, 2, 3, 5, 6 with their described functions readily can be 
implemented by those skilled in the art on the basis of their functions, 
each of which is known per se. 
The scrambling of sound is realised with bit sequences generated on the 
basis of the same control word CW as the video signal. The control word CW 
formed in accordance with the invention is thus also used for scrambling 
the sound.