Data player, digital contents player, playback system, data embedding apparatus, and embedded data detection apparatus

A data player for reading contents encrypted by a decoding key from a digital medium, and playing the encrypted contents by using the decoding key which is stored in a key storage unit, said data player comprising:

• key obtaining means for performing mutual authentication with the key storage unit to obtain the decoding key stored in the key storage unit;
• key holding means for holding the decoding key;
• playback state obtaining means for monitoring the playback state of the digital medium;
• and
• contents decoding means for decoding the encrypted contents by using the decoding key;
• wherein the decoding key is obtained by the key obtaining means and stored in the key holding means,
• wherein the encrypted contents read from the digital medium is decoded with the decoding key by the contents decoding means to play the contents,
• wherein the playback state obtaining means obtains the playback state of the digital medium, and
• wherein, if it is determined that the playback state of the digital recording medium has changed, then the decoding key stored in the key holding means is discarded.