| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to keep the safety of a decryption key even if a safe path to transmit the decryption key necessary to decrypt information is not used whenever the new provision object information is transmitted from one device to another.SOLUTION: A server D sends to a smart phone C an initial value Rof a common random number on a safe path such as a VPN at initial setting, and both share the same shared random number R. Whenever the smart phone C requests a new confidential file F(S14), the server D sends to the smart phone C encrypted confidential file e(K, F) to which the confidential file Fwas encrypted by means of a common key Kand a secondary key Yto which the common key Kwas encrypted by means of the shared random number R(S22). A communication path used at the time does not need to be a safe path. The smart phone C obtains the common key Kby decrypting the secondary key Yby means of the shared random number Rand decrypts the encrypted confidential file e(K, F) by means of the common key K. After this, the server D and the smart phone C update the shared random number Rwhich each of them possesses to a next value Rby use of the same unidirectional function. |
