Abstract: An apparatus and a method for processing encoded digital audio data, wherein the digital audio data is encoded using one of a plurality of encoding formats. The present invention prevents encoded digital audio data that has been copied onto a particular type of data storage medium having a unique identifier, such as a compactflash memory card, from another one of the particular type of data storage medium from being playable in an audio playback device according to the present invention. In accordance with the present invention, an encoded audio data file is encrypted using a unique identifier associated with the data storage device and a second key, an associated decoder file is encrypted using a first key, and the encrypted data and decoder files are stored onto the data storage device. During playback, a digital signal processor decrypt the audio data file in response to the second key and decrypts the decoder file in response to the first key.