A source device for receiving an input transport stream (TS_IN) comprising media content and for forwarding a scrambled version of at least part of said media content to at least one sink device (SNK1, SNK2) via an output transport stream (TSO_SRC), the input transport stream (TS_IN) further comprising at least one public information, the source device comprising: a transport stream decoder configured at least to filter the public information from the input transport stream (TS_IN); a scrambling module to scramble at least a part of the media content using a scrambling key; and a transport stream encoder to build the output transport stream (TSO_SRC) comprising the scrambled media content, said output transport stream comprising a plurality of transport stream packets each comprising a header, each header having a transport scrambling control field (TSC1, TSC2); characterised in that: the public information is an encrypted electronic control message, ECM, the encrypted ECM being changed periodically according to crypto-periods in said input transport stream; and the scrambling key is calculated by the source device using at least a private algorithm, a secret value and all or part of the public information in its encrypted form, the transport stream encoder being configured to further include the public information in the output transport stream (TSO_SRC), wherein the source device is pre-initialised by pre-storing a private algorithm and a secret value, wherein a new scrambling key is calculated by the source device using at least the private algorithm, the secret value and all or part of the public information in its encrypted form whenever a new ECM is received in said input transport stream, and wherein the transport scrambling control field (TSC1, TSC2) within the header of at least one of the transport stream packets is set to a new value at a time which is not earlier than when the new scrambling key is used to scramble the received content. a transport stream decoder configured at least to filter the public information from the input transport stream (TS_IN); a scrambling module to scramble at least a part of the media content using a scrambling key; and a transport stream encoder to build the output transport stream (TSO_SRC) comprising the scrambled media content, said output transport stream comprising a plurality of transport stream packets each comprising a header, each header having a transport scrambling control field (TSC1, TSC2); the public information is an encrypted electronic control message, ECM, the encrypted ECM being changed periodically according to crypto-periods in said input transport stream; and the scrambling key is calculated by the source device using at least a private algorithm, a secret value and all or part of the public information in its encrypted form, the transport stream encoder being configured to further include the public information in the output transport stream (TSO_SRC), wherein the source device is pre-initialised by pre-storing a private algorithm and a secret value, wherein a new scrambling key is calculated by the source device using at least the private algorithm, the secret value and all or part of the public information in its encrypted form whenever a new ECM is received in said input transport stream, and wherein the transport scrambling control field (TSC1, TSC2) within the header of at least one of the transport stream packets is set to a new value at a time which is not earlier than when the new scrambling key is used to scramble the received content.