Method of partially scrambling a data stream

A method of partially scrambling a data stream including transport stream packets, each transport stream packet having a header and a payload, wherein a sequence of transport stream packets has payloads carrying encoded data elements, arranged in units, includes: selecting transport stream packets forming a subsequence of the sequence, and scrambling at least part of the payloads of each transport stream packet in the subsequence. The method further includes monitoring the payloads of at least some of the transport stream packets in the sequence for the presence of data indicating a boundary between two subsequent units, and, for selected units, including at least one of the transport stream packets carrying data forming part of the selected unit in the sub-sequence.