prevent it from receiving any further broadcast or multicast messages from the access point. The current GTK is shared with an associating client station in the third EAPoL exchange of the 4-way handshake, but a further handshake, the group key handshake, is used when the GTK needs to be updated. A new GTK will be derived by the access point using a pseudo-random function of the GMK together with its MAC address and a nonce (GNonce). The new GTK is then distributed via the group key handshake, as follows; Step (1) The access point sends the new GTK in encrypted unicast messages to each station in the BSS. The new GTK is encrypted using each station's unique KEK and protects the data from being tampered using a MIC. Wireless LAN Security Step (2) Each station replies to inform the access point that the new GTK is installed. All stations will then use the new GTK to decrypt future broadcast or multicast messages. Advanced Encryption Standard (AES) The Advanced Encryption Standard (AES) is a cipher that was developed by Belgian cryptographers Joan Daemen and Vincent Rijmen, and was adopted as an encryption standard in November 2001 by the US National Institute of Standards and Technology (NIST) after a four year selection process. In June 2003, the US Government authorised the use of AES to protect classified information, including "Top Secret" information, provided that either 192 or 256 bit key lengths were used. Unlike RC4, which is a stream cipher and can encrypt a message of arbitrary length, AES is a block cipher and uses a fixed message block size of 128 bits together with an encryption key of 128, 192 or 256 bits. This is a specific instance of Daemen and Rijmen's original cipher, also known as the Rijndael cipher, which can use block and key sizes of 128 to 256 bits, in steps of 32 bits. The cipher operates on 4 X 4 arrays of bytes (i.e. 128-bits), and each round of the cipher consists of four steps; Step (1) A SubByte step - where each byte in the array is substituted with its entry in a fixed 8-bit lookup