 defines the opera- tions in each state. The security keys, such as PMK, AK, and CMAC, are locally derived by using the shared master key and other parameters during the key agreement procedure, as shown in Figure 8-3. 8.3.3 Key Usage Each SA maintains two security keys for downlink and uplink encryption. The TEK DL key is used for encryption of DL data by the BS and the TEKUL key is used for encryption of UL data by the MS, the decryption is carried out according to the Encryption Key Sequence (EKS). Note that the EKS field in the legacy generic MAC header carries a 2-bit key sequence of associated TEK [1]. In certain instances where the BS derives a new TEKUL and sets the TEK DL to the old TEKU the BS TEK DL and MS TEKUI are the same TEK with the same EKS, and both can transfer data securely until the TEK update happens from the MS side and the MS is re-syncronized with the new TEKUL- The security key update is triggered either by exhausting packet numbers" for the TEKDL or TEKUL, or by reauthentication [1]. 'In security schemes, a nonce is an abbreviation for number used once. It is often a random or pseudo-random number that is issued in an authentication protocol to ensure old communications cannot be reused in replay attacks. For example, the nonces that are used in HTTP digest access authentication to calculate an MD5 digest of the password. Message-Digest algorithm 5 (MD5) is a widely used cryptographic hash function with a 128-bit hash value specified in IETF RFC 1321 [11]. The nonces are different each time the authentication challenge response code is presented, and each client request has a unique sequence number, thus making replay attacks and dictionary attacks virtually impossible. The initialization vectors are also referred to as nonces for the above reasons. To ensure that a nonce is used only once, a time-variant number such as a time stamp or a random number with a large number of bits is utilized, resulting in a statistically insignificant chance of repeating previously generated values [10].