A method, user equipment (UE) and system are provided for negotiating a security capability during idle state mobility of the UE from a non-long term evolution (non-LTE) network to a long term evolution (LTE) network. The UE sends UE security capabilities supported by the UE to the LTE network for a non-access stratum (NAS) security algorithm selection use. The UE then receives from the LTE network selected NAS security algorithm. The UE further generates a root key from an authentication vector-related key stored at the UE and then derives, from the generated root key, a NAS protection key for security communication with the LTE network.