Cryptography – Key management – Having particular key generator
Patent
1991-10-17
1994-12-20
Barron, Jr., Gilberto
Cryptography
Key management
Having particular key generator
380 10, H04N 7167
Patent
active
053751680
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for scrambling and unscrambling a video signal comprising steps of, at an emitting site, scrambling the sequences of the video signal in accordance with a predetermined scrambling function which can be different for each sequence, emitting each sequence of such scrambled video signal, then, at a receiving site, unscrambling said sequences of emitted video signal.
The main application of the present invention is in pay-television (pay-TV) systems in which it is essential to scramble the video emission and unscramble or decode the program at the subscriber's location by means of a suitable decoder possessed by a subscriber who is authorized to view the emitted program.
Some methods for scrambling and a corresponding unscrambling are well known and, in such methods, it is aimed to make the emitted image and/or sound incomprehensible or their vision and/or audio uncomfortable.
A method for scrambling consisting of permuting a constant predetermined number of lines of the video frame (or video image) in accordance with a predetermined permuting function is well known. In this case, the unscrambling consists in carrying out the reverse function from the function used during the emission to permute such predetermined number of image lines. As it is not technically easy to thus permute a great number of image lines, the limit is now about 32 permuted image lines.
This well known scrambling method, sometimes called the fixed block inversion method, has a drawback which consists of requiring, at the receiving site, a memory of great size because it is necessary to have, just after sending to the TV set a sequence of 32 lines, several next memorized lines for being capable to send immediately the next image to the TV set. This method is relatively simple to carry out, at an emitting site, but there are problems of security, reliability and the cost at the reception site because of the great number of decoders. On the other hand, this method is not reliable enough relative to some kinds of pirated viewing. In fact, it is possible to recover the permutation order, even if this order frequently varies during the emitting phase.
It can be seen in the example mentioned above that the unscrambling function is almost the same as the scrambling function since both functions have the same nature and are the reciprocal. Further, this function is simple to pirate because it is unconnected with other parameters or informations.
The present invention avoids the above drawbacks.
For this purpose, the method according to the invention is characterised in that, at the transmitting site, a first video sequence is scrambled in accordance with a first scrambling function and another video signal sequence is scrambled in accordance with another scrambling function and, at the receiving site, said other video signal sequence which is scrambled in accordance with said other scrambling function is stored in place of said first video signal sequence which is scrambled in accordance with said first scrambling function, replacing it, and, during this replacement, said other video signal sequence is stored in a scrambled manner in accordance with an actual scrambling function which is equal to a predetermined combination of the first scrambling function and the other scrambling function.
According to an embodiment of the invention, each scrambling function consists in emitting a group of lines of the video image in an order different than a normal order which corresponds to the intelligible image, providing at the receiving site a storage memory comprising a predetermined number of memory lines, each memory line being able to store one received image line, the storage of the one received image line causing the sending of a said previously stored line in this memory line to the TV set, indicating for each emitted image line the address in the memory line in which this emitted image line has to be stored, and determining the emitting time of this image line in such a way that it replaces in the memory a
REFERENCES:
patent: 4405942 (1983-09-01), Block et al.
patent: 4673975 (1987-06-01), Inaba et al.
patent: 4796299 (1989-01-01), Hamilton
International Journal of Electronics, vol. 59, No. 4, Oct. 1985, Londres, GB), V. Zacharopoulos et al.: "An analogue scrambling scheme for television signals", pp. 501-509.
Barron Jr. Gilberto
Kudelski S.A. Fabrique D'Enregistreurs Nagra
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