Patent ID: 7796762

Claim:
A communication system comprising plural general communication apparatuses each holding an old common key before an update and a common key control apparatus that is connected to each general communication apparatus via a certain network and updates the old common key to a new common key, said common key control apparatus comprising: data transmission processing means for transmitting a first status transition request to all general communication apparatuses that have made a transition to a distribution completed status, and transmitting a second status transition request to all the general communication apparatuses when the new common key has been distributed to all the general communication apparatuses, and each general communication apparatus comprising: transition means for causing a transition to take place from an update completed status to the distribution completed status upon receipt of the new common key from the common key control apparatus, causing a transition to take place from the distribution completed status to an in-transit status upon receipt of the first status transition request, and restoring the status to the update completed status from the in-transit status upon receipt of the second status transition request; common key holding means comprising a memory for holding a most recent common key alone during the update completed status, and holding both the old common key and the new common key during the distribution completed status and the in-transit status; and encryption means for encrypting transmission data using the most recent common key during the update completed status, encrypting transmission data using the old common key during the distribution completed status, and encrypting the transmission data using the new common key during the in-transit status.