An MPEG2-PS format digital stream is composed of VOBUs and each VOBU except an RDI-Pack at the top is encrypted using a content key Kc. The content key Kc is an encryption key obtained by adding a title key Kt to CCI, and then converting a time varying coefficient using the sum. Accordingly, the resulting content key Kc differs for each VOBU. Since each content key Kc is all different for such a small unit of VOBU, copyright of the digital stream is firmly protected. CCI contained in RDI-Pack represents conditional Copy Freely status which allows the digital stream to be copied on condition that the digital stream is recorded only in encrypted form.