Security key synchronization method and associated communications apparatus

A security key synchronization method for synchronizing security key(s) derived by a communications apparatus with security key(s) derived by a network device, wherein the security key synchronization method is performed by a processor of the communications apparatus and comprises:

• performing a Non Access Stratum (NAS) security mode command procedure to derive a new set of NAS key(s) in response to a NAS security mode command message received from the network device;
• transmitting a NAS security mode complete message to the network device for informing the network device that the NAS security mode command procedure is complete;
• and
• performing a NAS procedure to communicate with the network device by using the new set of NAS key(s) instead of any sets of NAS key(s) derived before deriving the new set of NAS key(s) in response to reception of an RRCReconfiguration message indicating derivation of a set of Access Stratum (AS) keys from the network device when an acknowledge message associated with the NAS security mode complete message has not been received from the network device after transmitting the NAS security mode complete message to the network device.