According to one embodiment, a server includes a message communicator, a key information storage, and a key controller. The message communicator is configured to communicate a message to a client. The key information storage is configured to store key information including at least a key value being value of key, a validity term of key, and assignment information of key. The key controller is configured to generate a key notification message including a key value of the client and a key update time when the message communicator receives a key request message from an authenticated client. The key value is corresponding with the assignment of the key information. The key update time decides from the validity term of the key and a validity term of authentication succeeded state of the client. The message communicator transmits the key notification message to the client.