Patent ID: 7668311

Claim:
A multi-channel transmission system of encrypted information for Pay-TV, comprising a management centre and at least one subscriber unit, the management centre transmitting mono-channel authorization messages encrypted for each channel, said management center also transmitting encrypted multi-channel authorization messages common to a group of channels, said multi-channel authorization messages being transmitted to a subscriber unit at a lower frequency of transmission than that of said mono-channel authorization messages, said subscriber unit including a security cryptographic unit for decrypting said mono-channel authorization messages and said multi-channel authorization messages, said cryptographic unit using a first encryption algorithm for decrypting said mono channel authorization messages to obtain a first information and using a second encryption algorithm for decrypting the multi-channel authorization messages received after switching on of the subscriber unit to obtain a second information, said first and second algorithms being different, said cryptographic unit combining the decrypted multi-channel authorization messages with the decrypted mono-channel authorization messages for the retrieval of at least one control word, said at least one control word being formed by a combination of first information contained in at least one of said mono-channel authorization messages and second information contained in at least one of said multi-channel authorization messages received after the switching on of said subscriber unit.