A method for selecting a time length of a security association (SA) between user equipment (UE) which transmits and receives communications and a control entity in a communication system in accordance with the invention, includes transmitting a registration message (SM1) from the user equipment to the control entity (P-CSCF) requesting registration of the user equipment with the control entity; after transmission of the registration message, setting the time length of the security association between the user equipment and the control entity to be equal to a registration time length set between the user equipment and the control entity during which registration of the user equipment with the control entity is valid; and transmitting the set time length of the security association to the user equipment as part of an acknowledgment message (SM10 or SM12) to the registration message to cause the security association to have a time equal to the registration timer length.