An MME negotiates security in case of idle state mobility for a UE from a first network to a LTE network. The UE sends its security capabilities including non-access stratum (NAS) security capabilities supported by the UE to the LTE network. The MME selects a NAS security algorithm, in accordance with the NAS security capabilities of the UE, and sends the selected NAS security algorithm to the UE, sharing the NAS security algorithm between the UE and the LTE network when the UE moves from the first network to the LTE network. The MME also derives, in accordance with the selected NAS security algorithm, a NAS protection key from an authentication vector-related key so as to security communication between the UE and the LTE network.