A data transfer method which eliminates erroneous operation of
conventional devices not supporting encryption when copy-protected AV
information is encrypted and sent on an IEEE 1394 bus. Synchronous data
transferred through isochronous communication contains i) encryption
identification information for indicating encryption of actual data and
ii) actual data. Only the actual data is encrypted. Encryption
identification information indicating encryption of actual data in
synchronous data is sent together with actual data from the sending
device. A receiving device detecting encryption of actual data from this
encryption identification information requests decrypting information
from the sending device. The receiving device decrypts the actual data
using decrypting information received from the sending device according
to this request.