Patent ID: 7995763
Filing Date: 2011-08-09
Classification: H04L

Abstract:
1. A user apparatus communicating with a content distribution apparatus, comprising: a storage device storing a threshold value k, sub-content item SC 1 , an encryption key KSC, n-l encrypted sub-content items E(KscI, KI//SC 2 ), . . . , E(Ksc, Kn_I//SCn) and an encrypted content item E(KM, Cm), when the content distribution apparatus encrypts a content to view, with a content encryption key KM, thereby generating an encrypted content item E(KM, C), performs the (k, n) threshold secret sharing scheme on the content decryption key KM, using the threshold value k that is equal to or smaller than the number n of sub-content items SCI, . . . , SCn able to be played back before the content item C is played back, thereby generating n distribution keys KI, . . . , Kn_I, Kn (where 3˜n) and a sub-content encryption key KSC, concatenates the n-i distribution keys KI, . . . , Kn_ 1 included in the n distribution keys to the n-i sub-content items SC 2 , . . . , SCn included in the n sub-content items, respectively, thereby generating n-l concatenated data items KI//SC 2 , . . . , Kn_I//SCn, encrypts the n-l concatenated data items KI//SC 2 , . . . , Kn_I//SCn, one by one, based on the sub-content encryption key KSC, thereby generating n-l encrypted sub-content items E(Ksc, KI//SC 2 ), . . . , E(Ksc, Kn_I//SCn), and distributes one sub-content not included in the concatenated data items, the sub-content encryption key KSC, the n-i encrypted sub-content items E(Ksc, KI//SC 2 ), . . . , E(Ksc, Kn_l//SCn) and the encrypted content item E(KM, C), one after another; a first decryption device decrypting the encrypted sub-content items E(Ksc, KI//SC 2 ), . . . , one after another, based on the sub-content encryption key KSC stored in the storage device, thereby generating the concatenated data items KI//SC 2 , . . . ; a first playback device playing back, one after another, the first sub-content item SC 1 stored in the storage device and stored sub-content items SC 2 , . . . included in the concatenated data items generated by the first decryption device; a distribution key storage device storing, one after another, the distribution keys KI, . . . included in the concatenated data items generated by the first decryption device; a content encryption key restoration device performing (k, n) threshold secret sharing scheme, based on the decryption key KI, . . . stored in the distribution key storage device, thereby restoring the content decryption key KM; a second decryption device decrypting the encrypted content item E(KM, C) stored in the storage device, based on the content decryption key KM restored; a second playback device playing back the content item C decrypted; a comparing device comparing the threshold value k with the number i of distribution keys stored in the distribution key storage device (i˜i˜n-l); a warning device monitoring the playback state in the first playback device and the result of comparison performed by the comparing device, thereby outputting a warning and first inquiry information when the number i of distribution keys is smaller than the threshold value k (i k) and when the playback state is either fast forwarding or skipping, the warning indicating that the content item C cannot be played back unless the playback state returns to one before the fast forwarding or skipping, and the first inquiry information asking whether the playback state should be returned to one before the fast forwarding or 20 skipping; a playback position correction device correcting the position where the first playback device plays back the sub-content, to a position where the first playback device played back a sub-content before the fast forwarding or the skip is performed, when first response information requesting that the position should be so corrected is input in reply to the warning and the first inquiry information; and a distribution key deletion device erasing the distribution key acquired in the fast-forwarding state or skipping state from the distribution key storage device, when the first response information requesting that the position should not be so corrected is input in reply to the warning and the first inquiry information.