Patent ID: 8553879

Claim:
A content transmission device that encrypts and transmits each of portions that constitute content to a content reception device, the content transmission device comprising: a transmission control unit operable to receive a request for transmitting a portion of the content from the content reception device, and to transmit to the content reception device (i) an encrypted portion, which is generated in response to the request, and (ii) a first key parameter, based on which a content key is generated, the content key being used for encrypting the portion; a key management unit operable to (i) upon receiving the request, generate the first key parameter when the first key parameter has not yet been generated, and update the first key parameter when the first key parameter has already been generated, and (ii) store therein the generated or updated first key parameter; and an encryption unit operable to generate the content key based on the first key parameter, and to encrypt the portion corresponding to the request by using the generated content key so as to generate the encrypted portion, wherein the key management unit receives a second key parameter from the content reception device, and calculates a difference between the first key parameter and the received second key parameter, wherein the transmission control unit judges, by using the difference, whether or not to change a transmission speed at which the encrypted portion is to be transmitted, selects one of a plurality of transmission speeds based on a result of the judgment, and transmits the encrypted portion at the selected transmission speed, wherein when the difference is greater than a first threshold value, the transmission control unit selects one of the transmission speeds that is slower than a current transmission speed, wherein when the difference is smaller than or equal to a second threshold, the transmission control unit selects one of the transmission speeds that is faster than the current transmission speed, and wherein the second threshold value is smaller than the first threshold value.