A method and apparatus is described that prevents viewing or other type of playback of unauthorized copies of media files, whether received through a network or retrieved from a magnetic or optical disc. The apparatus uses data in a metadata field that forms a part of the media files. Viewing or playback is allowed only if the data meets certain predetermined criteria. An apparatus is also provided that can be used to generate a new metadata field if a media file has a metadata field with unacceptable data.