Patent ID: 8713315

Claim:
A content distribution system comprising: a management device that manages encrypted content information; and a viewing device that acquires the encrypted content information from the management device, decodes the encrypted content information, and allows the decoded encrypted content information to be viewed, the management device including a view control information issuing unit that, upon receipt of a request from the viewing device to issue view control information including an ordinal number (i), issues the view control information including decryption key information corresponding to an (i)-th random number corresponding to the ordinal number (i) among a plurality of random numbers and period-of-validity information about a period of validity, the plurality of random numbers being shared between the management device and the viewing device, the viewing device including a storage unit that stores the encrypted content information acquired from the management device, a counting unit that counts the ordinal number (i), a holding unit that holds encrypted common key information, the encrypted common key information being encrypted using the (i)-th random number corresponding to the ordinal number (i) among the plurality of random numbers, a request unit that sends a request to the management device to issue the view control information including the ordinal number (i), a calculation unit that calculates common key information by receiving the view control information transmitted from the management device in response to the request sent from the request unit and by decoding the encrypted common key information held in the holding unit using the decryption key information included in the view control information, a display controller that decodes the encrypted content information stored in the storage unit using the common key information calculated by the calculation unit to produce viewable content information, and that displays the viewable content information, an update unit that updates the encrypted common key held in the holding unit when the period of validity indicated by the period-of-validity information included in the received view control information expires, by re-encrypting the common key information calculated by the calculation unit using an (i+1)-th random number corresponding to an ordinal number (i+1) counted by the counting unit, and a deletion unit that deletes the common key information subjected to re-encryption, the (i+1)-th random number used for re-encrypting the common key information among the plurality of shared random numbers being shared between the management device and the viewing device, and the view control information after the update unit has updated the encrypted common key information.