System for determining successful reception of a message

A method for determining that all of a scrambled message has been successfully received, the scrambled message being scrambled in accordance with a scrambling key and being capable of being descrambled in accordance with a descrambling key, descrambling key generating information for generating the descrambling key being included within a control message (ECM) associated with the scrambled message. The method includes providing a scrambled message scrambled in accordance with a scrambling key, generating an unmodified ECM including descrambling key generating information for generating a descrambling key for descrambling the scrambled message, generating a first derived value based on the scrambled message using a first value derivation function, applying a first modification function to the unmodified ECM, the first modification function being based, at least in part, on the first derived value, thereby producing a modified ECM, transmitting the scrambled message and the modified ECM to a receiver, and performing, at the receiver, the following steps: generating a second derived value based on the received scrambled message using a second value derivation function, applying a second modification function to the modified ECM, the second modification function being based, at least in part, on the second derived value and being an inverse of the first modification function, thereby producing the unmodified ECM, and employing the unmodified ECM to determine that all of the message has been successfully received. Related apparatus and methods are also provided.