A content transmission apparatus that encrypts and transmits contents of digital broadcasting, comprising: a storage unit configured to store a scrambling key (Ks) that changes as time passes, a content key (Kc) provided to each content, a plurality of different work keys (Kw) retained for a time period longer than a duration of the contents, and a master key (Km) provided commonly to a receiver side; a content encryption unit configured to encrypt the contents using the scrambling key into encrypted contents; a related information encryption unit configured to encrypt first information related to the contents, the first information including at least the scrambling key, using the content key into encrypted related information; a content key related information encryption unit configured to encrypt second information related to the content key, the second information including at least the content key, using a selected one of said work keys into encrypted content key related information; a work key related information encryption unit configured to encrypt third information related to the selected work key, the third information including at least the selected work key, using the master key into encrypted work key related information; a multiplexer unit configured to multiplex the encrypted contents, the encrypted related information, the encrypted content key related information, and the encrypted work key related information into multiplexed encrypted contents; and a multiplexed encrypted content transmission unit configured to transmit the multiplexed encrypted contents multiplexed by said multiplexer unit, characterized in that if a plurality of security modules are correspondingly provided in reception apparatuses at the receiver side, each security module storing decrypted content key related information and being unreadable from exterior, the plurality of security modules being grouped into a plurality of groups, said different work keys are respectively used for respective different groups of the plurality of security modules. a storage unit configured to store a scrambling key (Ks) that changes as time passes, a content key (Kc) provided to each content, a plurality of different work keys (Kw) retained for a time period longer than a duration of the contents, and a master key (Km) provided commonly to a receiver side; a content encryption unit configured to encrypt the contents using the scrambling key into encrypted contents; a related information encryption unit configured to encrypt first information related to the contents, the first information including at least the scrambling key, using the content key into encrypted related information; a content key related information encryption unit configured to encrypt second information related to the content key, the second information including at least the content key, using a selected one of said work keys into encrypted content key related information; a work key related information encryption unit configured to encrypt third information related to the selected work key, the third information including at least the selected work key, using the master key into encrypted work key related information; a multiplexer unit configured to multiplex the encrypted contents, the encrypted related information, the encrypted content key related information, and the encrypted work key related information into multiplexed encrypted contents; and a multiplexed encrypted content transmission unit configured to transmit the multiplexed encrypted contents multiplexed by said multiplexer unit, characterized in that if a plurality of security modules are correspondingly provided in reception apparatuses at the receiver side, each security module storing decrypted content key related information and being unreadable from exterior, the plurality of security modules being grouped into a plurality of groups, said different work keys are respectively used for respective different groups of the plurality of security modules.