| 摘要 |
PROBLEM TO BE SOLVED: To automatically detect a device which is assumed to be carried away with higher probability and judge the detected device invalid without requiring periodic communication in the system using a common key for ensuring security in the communication between a couple of devices. SOLUTION: The device 51 issues a distribution request of a session key which is used in common in the device 52. The distribution request includes an identifier of both devices. A master device 14 generates a session key in accordance with the distribution request, transmits the key to the device 51, and also records a set of the identifiers included in the distribution request to an identifier table. When the device 51 is isolated from communication service with the device 52, this device 51 cancels its own session key and transmits a cancellation message including the identifiers of own device and the device 52. Upon reception of the cancellation message, a delating unit 411 deletes a set of identifiers included in the cancellation message from the identifier table, and a judging unit 412 judges the device 52 invalid when the identifier of the device 52 is completely deleted from the identifier table. COPYRIGHT: (C)2007,JPO&INPIT
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