A method for operating a base station in a wireless communication system, the method comprising: receiving a message from a terminal, wherein the message is a media access control, MAC, protocol data unit, MPDU; detecting an integrity error of the message by comparing a first integrity check value, ICV, included in the message with a second ICV generated from data in said message using a first traffic encryption key, TEK, to detect an integrity error of the message and counting a frequency of occurrence of the integrity error, when the integrity error exists in the message; and when the frequency of occurrence of the integrity error is equal to or greater than a predetermined threshold, performing a first key update procedure; the method characterised in that the method further comprises: performing a second key update procedure, after the first key update procedure, wherein performing the first key update procedure comprises: discarding the first TEK currently used for encrypting transmission data transmitted from the base station; using a second TEK used for decrypting reception data received in the base station as a downlink encryption key for encrypting transmission data; and deriving a first new TEK for decrypting the reception data, wherein the performing the second key update procedure comprises: discarding the second TEK; using the first new TEK as the downlink encryption key; and deriving a second new TEK for decrypting the reception data. receiving a message from a terminal, wherein the message is a media access control, MAC, protocol data unit, MPDU; detecting an integrity error of the message by comparing a first integrity check value, ICV, included in the message with a second ICV generated from data in said message using a first traffic encryption key, TEK, to detect an integrity error of the message and counting a frequency of occurrence of the integrity error, when the integrity error exists in the message; and when the frequency of occurrence of the integrity error is equal to or greater than a predetermined threshold, performing a first key update procedure; the method characterised in that the method further comprises: performing a second key update procedure, after the first key update procedure, wherein performing the first key update procedure comprises: discarding the first TEK currently used for encrypting transmission data transmitted from the base station; using a second TEK used for decrypting reception data received in the base station as a downlink encryption key for encrypting transmission data; and deriving a first new TEK for decrypting the reception data, wherein the performing the second key update procedure comprises: discarding the second TEK; using the first new TEK as the downlink encryption key; and deriving a second new TEK for decrypting the reception data. performing a second key update procedure, after the first key update procedure, wherein performing the first key update procedure comprises: discarding the first TEK currently used for encrypting transmission data transmitted from the base station; using a second TEK used for decrypting reception data received in the base station as a downlink encryption key for encrypting transmission data; and deriving a first new TEK for decrypting the reception data, wherein the performing the second key update procedure comprises: discarding the second TEK; using the first new TEK as the downlink encryption key; and deriving a second new TEK for decrypting the reception data. discarding the first TEK currently used for encrypting transmission data transmitted from the base station; using a second TEK used for decrypting reception data received in the base station as a downlink encryption key for encrypting transmission data; and deriving a first new TEK for decrypting the reception data, wherein the performing the second key update procedure comprises: discarding the second TEK; using the first new TEK as the downlink encryption key; and deriving a second new TEK for decrypting the reception data. discarding the second TEK; using the first new TEK as the downlink encryption key; and deriving a second new TEK for decrypting the reception data.