Certainly it's possible to generate a unique and valid key for one person only. For example, the input parameters will be the list of songs to unlock, some identifying information about the customer, and the bit necessary to validate the unlock. This information is then encrypted and can be unlocked using some public key on the CD itself. So generating these could only be possible from a private key at the sales source itself. This way every person gets their own unique key.

But sure, it might well be cracked, but then when you shift away from a music as a product to a distribution method for music there will be ways around everything. The advantages outway the piracy that will inevitably occur anyway.

Calvin