An incomplete/partly corrupted media file is delivered ‘in the clear’; in addition, a delta file is delivered to users meeting access control criteria which, when combined with the incomplete/partly corrupted parts allows a complete and uncorrupted version of the media file to be re-constructed. The method allows, in one implementation, a secure music purchase system to operate over digital radio: for example, the start of a song when played over the radio is usually deliberately talked over and the end cut short to prevent listeners being able to record a complete copy. With the present system, this practice can continue, but listeners can also purchase the missing or corrupted sections to enable them to possess a complete and uncorrupted version for playback.