(A) An individual one of two or more terminals prepares a set of a first public key and a first private key. (B) One of the two or more terminals creates a set of a second public key and a second private key and distributes the second public key and the second private key to the other terminal(s). (C) One of the two or more terminals creates a shared key by using the corresponding first private key prepared in the (A) and the second public key shared in the (B). (D) A different terminal(s) of the two or more terminals creates a shared key used to communicate with the one terminal by using the second private key shared in the (B) and the first public key of the one terminal prepared in the (A). (E) The set of terminals performs an encryption communication(s) by using the shared keys.