Patent ID: 7954154

Claim:
A communication system configured to carry out communication between a first communication apparatus and a second communication apparatus using a first key and a second key, wherein: the first communication apparatus comprises: a first storing section that is configured to store an expiration time of the first key stored in the first communication apparatus; a first managing section that is configured to manage expiration of the first key based on the expiration time of the first key stored in the first storing section; a first communicating section that is configured to transmit a first message including the expiration time of the first key at predetermined intervals; and the second communication apparatus comprises: a second storing section that is configured to store an expiration time of the second key stored in the second communication apparatus; a second managing section that is configured to manage expiration of the second key based on the expiration time of the second key stored in the second storing section; a second communicating section that is configured to receive the first message; and a correcting section that is configured to correct the expiration time of the second key stored in the second communication apparatus when the expiration time of the first key and the expiration time of the second key are judged to be different based on the expiration time of the first key included in the first message, and is configured not to correct the expiration time of the second key stored in the second communication apparatus when the expiration time of the first key and the expiration time of the second key are judged to be the same based on the expiration time of the first key included in the first message; and wherein: the second communicating section is further configured to transmit a second message including the expiration time of the second key having been corrected by the correcting section.