Method of data operation facilitating utilization of legitimate users without
violating the copyright and literary property of digital content. A part of digital
content (11) is duplicated to create discrete data unit (43), and
is encrypted with content key (45). Content key (45) and image-compositing
information (42) are encrypted with secret key (47) to create authorization
information (48). Content information (41) is visibly embedded in
digital content (11). Authorization information-added data unit (50)
in which authorization information (48) is embedded as invisible information
and encrypted discrete data unit information (46) are composited into composite
data (60), which is then distributed.