Patent ID: 7602915

Claim:
A communication system comprising at least one LAN (Local Area Network), said communication system having at least three nodes connected thereto, wherein said nodes perform mutual encrypted communication by utilizing a common cipher key for encrypting and decrypting communication data, and wherein: one of said nodes functions as a main node and remaining ones of said nodes function as respective secondary nodes which receive said common cipher key from said main node, and said main node comprises means for executing a key replacement operation at successive regular or irregular intervals, said key replacement operation comprising generating a cipher key to be utilized as said common cipher key, and transmitting said generated common cipher key to each of said secondary nodes via said LAN by employing a broadcast communication mode; and each of said nodes is provided beforehand with a class cipher key, said class cipher key being a cipher key that is identical for each of said nodes and is continuously held by each of said nodes, said main node comprises means for encrypting said common cipher key by using said class cipher key, and transmitting said common cipher key in encrypted form in said key replacement operation, and each of said secondary nodes comprises means for decrypting said encrypted common cipher key when received from said main node, by using said class cipher key.