Patent ID: 8165293
Filing Date: 2012-04-24
Classification: H04N

Abstract:
1. A method in a head-end system of providing scrambled data, comprising: providing a stream of data units based on a sequence of clear data units originating from a storage system by at least subjecting at least part of at least some of the clear data units to a cryptographic operation in a scrambling system employing a first encryption key from a control word generator which forms a key pair with a corresponding first decryption key, such that a first section of the stream of data units includes data units including cryptograms obtained by the scrambling system using a first value of the first encryption key, and such that a second section includes data units including cryptograms obtained by the scrambling system using a second value of the first encryption key, associating, by the scrambling system, data units with a plurality of scrambling control bits from a program specific information generator for indicating a first, second, third, or fourth state of scrambling applicable to the associated data unit, providing a stream of key messages from an ECM generator, each carrying at least key information enabling an authorised decoder to obtain a value of the first decryption key, the stream of key messages and stream of data units being synchronised by a synchronisation system, associating in the scrambling system each data unit including a cryptogram obtained using any value of the first encryption key with scrambling control bits indicating a first state of scrambling, said first state associated with the first decryption key, associating in the scrambling system data units in a third section, separating the first and second sections, with scrambling control bits indicating one of: a second, third, and fourth states of scrambling, providing from the scrambling system in at least one key message coinciding with one of the first and third sections key information enabling an authorised decoder to obtain a value of the first decryption key corresponding with the second value of the first encryption key, and suspending subjection of at least part of the clear data units to the cryptographic operation in the scrambling system employing the first encryption key for each data unit in the sequence included in the third section; subjecting at least part of at least some of the clear data units in the third section to a cryptographic operation in the scrambling system employing a second encryption key from a control word generator, which forms a key pair with a corresponding second decryption key; wherein key information enabling an authorised decoder to obtain a value of the second decryption key corresponding to a value of the second encryption key used for a particular control word period is provided in one of the stream of key messages coinciding with a section preceding the section of the stream of data units corresponding to the particular control word period; wherein, for each two consecutive control word periods: