Enhanced security design for cryptography in mobile communication systems

The fundamental view of the present invention is strengthening or updating a basic code security algorithm by adding a change peculiar to an algorithm to the security key information generated in the usual key share procedure of a mobile communications system. When performing communication with a moving terminal, the network side chooses the one strengthening version in two or more basic code security algorithms which a movable body usually supports, and transmits the information showing the selected algorithm to a moving terminal. Next, the basic security key obtained as a result of the key share procedure (AKA10) between a moving terminal and a network is changed based on a selection algorithm, and a security key peculiar to (22) and an algorithm is generated. Next, the security of the communication protected in a mobile communications network is strengthened by applying a basic security algorithm (24) to a security key peculiar to this algorithm as a keystroke.