A method for performing integrity protection in a receiver of a wireless communications system includes receiving a concatenated message including a Non-Access Stratum message, a first message authentication code of the Non-Access Stratum message, a Radio Resource Control message and a second message authentication code of the concatenated message, performing an integrity protection procedure for the concatenated message, and discarding the concatenated message and not delivering the Non-Access Stratum message to an upper layer when the second message authentication code does not pass the integrity protection procedure.