501 to a mobile communication switching center 150 through base station controller 130 when the registration request message 501 is received from terminal 10 the network determines whether ciphering activation should be performed when it is determined that ciphering activation should be performed the network transmits a ciphering authentication request message 502 to the terminal the terminal then transmits a ciphering authentication response message 503 to the network in response to the ciphering authentication request message 502 after the ciphering authentication response message 503 is transmitted from the terminal the network transmits a registration completion message 504 to the terminal indicating completion of the registration procedure thus an initial process for ciphering activation between the mobile communication terminal and network is finished when it is determined that ciphering activation should not be preformed the network does not transmit the ciphering authentication request message to the mobile communication terminal consequently the data is normally transferred between the terminal and network without being ciphered 505 when the ciphering activation process between the terminal and network has not been formed in the initial registration procedure and it is required to cipher the data during transferring the data form the mobile communication terminal the terminal transmits a ciphering request message during transferring data 506 to the network at that time the ciphering request message is transmitted to the network during transferring data 506 without containing a specific value rand random number when the network receives the ciphering request message during transferring data 506 the network performs the ciphering activation process when the network finishes the ciphering activation process the network transmits a ciphering authentication request message 507 to the terminal the terminal transmits a ciphering authentication response message 508 to the network in response.