Cryptography – Key management – Having particular key generator
Patent
1991-07-08
1993-06-29
Swann, Tod R.
Cryptography
Key management
Having particular key generator
380 11, 380 20, H04N 7167
Patent
active
052241610
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of scrambling and of unscrambling composite video signals, and a device for implementation.
The methods currently implemented for scrambling video signals, in particular in the area of pay-television, have the feature of transmitting, on the one hand, access messages valid for all the receivers, simultaneously with the video signals, and, on the other hand, of periodically transmitting entitlement messages specific to each receiver through a different channel, for example through the set or through a telephone channel. Because of their mode of transmission, the periodicity of transmission of these entitlement messages cannot be other than relatively large, for example a month. Such a long period can easily be taken advantage of by "pirates" who thus have sufficient time to discover the scrambling key and to profit therefrom.
The subject of the present invention is a method of scrambling and of unscrambling composite video signals allowing modification of the scrambling key in accordance with a period sufficiently short to practically exclude any possibility of discovering it between two consecutive changes.
The subject of the present invention is also a device for implementing this method.
The method according to the invention consists in including in the unused lines of at least some of the transmitted pictures, data packets corresponding to the entitlement messages and to the access messages.
According to an advantageous aspect of the invention, the data packets also comprise access messages immediately consecutive to those in progress, used when the unscrambler is set going.
Advantageously, one data packet is included at the start of each complete picture.
According to one embodiment of the invention, there is included, at the start of each picture for each period of at least ten or so consecutive pictures, at least one access message for the period in progress, at least one access message for the immediately consecutive period, and for each one of all the other pictures, an entitlement message.
The present invention will be better understood on reading the detailed description of an embodiment, taken as non-limiting example, illustrated by the attached drawing in which:
FIG. 1 is a simplified block diagram of a transmitter of pay-television programmes scrambled according to the method of the invention and of one of the receivers receiving these programmes and comprising a corresponding unscrambling device,
FIG. 2 is a signal timing diagram showing the disposition of the various data necessary in the unscrambling in relation to certain of the first few lines of the pictures transmitted by the transmitter of FIG. 1,
FIG. 3 is a timing diagram showing the structure of the data packets transmitted by the transmitter of FIG. 1 over a period of 20 seconds, and
FIG. 4 is a timing diagram of signals produced, during one complete picture sent by the transmitter of FIG. 1.
The system for broadcasting pay-television programmes which is described below refers to a European broadcasting standard with 25 complete pictures per second, but obviously it can apply to any other standard by making therein modifications apparent to the expert. Furthermore, obviously the transmissions may be scrambled for only certain programmes and be broadcast in the clear for the other programmes.
The transmitter and the television receiver 1 are only briefly described below since the majority of their circuits are well known per se. In the transmitter, only the circuits for producing entitlement and access message packets, and the circuits for inserting these data packets into certain of the lines of the pictures to be scrambled, and in the receiver, the unscrambling circuits, form part of the invention.
The transmitter 1 comprises a picture source furnishing composite video signals. Its output is connected by way of an analog-digital convertor 3 to an intermediate video scrambling circuit 4 of a type known per se. This circuit 4 is controlled by a control unit 5 via a generator 6 of pseudo-rand
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patent: 4611242 (1986-09-01), Williams
patent: 4751428 (1988-06-01), Kamitake
patent: 4947428 (1990-08-01), Guillou et al.
Daniel Patrick
Guillon Jean-Claude
Renard Francis
Laboratoir Europeen de Recherches Electroniques Avancees, Societ
Swann Tod R.
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