A method of providing scrambled data includes providing a stream (12;22;26) of data units based on a sequence of clear data units by at least subjecting at least part of at least some of the clear data units to a cryptographic operation employing a first encryption key 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 using a first value of the first encryption key, and such that a second section includes data units including cryptograms obtained using a second value of thefirst encryption key, and by associating data units with scrambling state identifying data (17,18) for indicating a state of scrambling applicable to the associated data unit. The method further includes providing a stream of key messages, each carrying at least key information enabling an authorised decoder (33) to obtain a value of the first decryption key, the stream of key messages and streamof data units being synchronised, associating each data unit including a cryptogram obtained using any value of the first encryption key with scrambling state identifying data including an identifiervalue associated with the first decryption key, associating data units in a third section, separating the first and second sections, with scrambling state identifying data lacking an identifier valueassociated with the first decryption key, and providing 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. Subjection of at least part of the clear data units to the cryptographic operation employing the first encryption key is suspended for each data unit in the sequence included in the third section.