Cryptographic communication between communication terminals can be
realized even when a plurality of cryptographic algorithms are present,
and secure cryptographic communication for a longer time is realized
without increasing a processing overhead at each of the communication
terminals. A key management server manages cryptographic algorithms that
can be used by each of the communication terminal, and searches for a
cryptographic algorithm common to the communication terminals, and
notifies each of the communication terminals of the cryptographic
algorithm found by the search together with plural key generation
informations, each piece containing a key to be used in the cryptographic
algorithm or a key type for generating the key. Each of the communication
terminals sequentially switches the plural key generation informations
notified from the key management server, and performs the cryptographic
communication with a communication counterpart in accordance with the
cryptographic algorithm notified from the key management server.