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.