The importance of digital audio content protection has increased significantly in recent years, particularly in the personal computing domain. For example, distributors of audio compact discs (CDs), artists creating the audio content, and software companies are concerned about the unauthorized copying of copyrighted digital audio content. Personal computer users wishing to capture and distribute copyrighted digital audio content can use a software application to capture raw digital audio data as it travels through the audio layers of an operating system to the hardware associated with the playback of the audio content.
One known method of preventing the copying of the copyrighted digital audio content encrypts a stream of digital audio data (i.e., an audio stream) at the source (e.g., an audio player application such as Windows Media Player™) and decrypts the audio stream at the destination (e.g., the hardware used to playback the audio and/or a software driver). This method is suitable if the encrypted audio content is not manipulated or mixed with any other audio source(s) as it travels from the source to the destination. However, if another sound (e.g., a system sound or any other unencrypted media source) is played back simultaneously, the operating system audio mixer will attempt to mix the encrypted audio stream with the unencrypted audio stream, rendering the resulting audio stream unintelligible at the destination.
Another known method of preventing copying of digital audio content encrypts the audio content at the source and decrypts the audio content at the operating system audio mixer before mixing in the second audio source. This method requires a significant amount of computational power because the audio content must be decrypted before any processing is performed on the encrypted audio content and, as a result, may cause noticeable delays in audio playback. Also, this method is not secure because each software component in the operating system audio layer is required to be aware of the encryption and the encryption key.