ute the key to the station and access point and to set up key management for the session. TKIP combines the temporal key with each station's MAC address, plus the TKIP sequence counter, and adds a 48-bit initialisation vector to produce the initial keys for data encryption. With this approach each station will use different keys to encrypt transmitted data. TKIP then manages the update and distribution of these encryption keys across all stations after a configurable key lifetime that might be from once every packet to once every 10,000 packets, depending on security requirements. Although the same RC4 cipher is used to generate an encryption key stream, TKIP's key mixing and distribution method significantly improves WLAN security, replacing the single static key used in WEP with a dynamically changing choice from 280 trillion possible keys. WPA supplements TKIP with a message integrity checking (MIC) that determines whether an attacker has captured, altered and resent data packets. Integrity is checked by the transmitting and receiving stations computing a mathematical function on each data packet. While the simple CRC-32, when used to compute the ICV in WEP, is adequate for error detection during transmission, it is not sufficiently strong to assure message integrity and prevent attacks based on packet forgery. This is because it is relatively easy to modify a message and re-compute the ICV to conceal the changes. In contrast, MIC is a strong cryptographic hash function, which is calculated using source and destination MAC addresses, input data stream, the MIC key and the TKIP sequence counter (TSC). If the MIC value computed by the receiving station does not match the MIC value received in the decrypted data packet, the packet is discarded Chapter Eight Temporal key WEP IV Phase 1 Transmitter key mixing Per packet key MAC address sequence Encrypted counter encapsulation MPDUs MIC key Source address Fragmentation Destination address MSDU plaintext Figure 8-4: TKIP Key Mixing and Encryption Process and counterm