 the WEP protocol. It adds four bytes to the WEP header to introduce an extension of the initialization vector. It adds eight bytes to the MSDU data to join the MIC (Message Integrity Code) field containing the seal calculated using the MICHAEL algorithm. The TKIP header is composed of the following fields (Figure 2.18): - the TSCO and TSC1 (TKIP Sequence Counter) fields constitute the initialization vector and are used during the second phase of the key mixing (key derivation) function; Wi-Fi Integration to the 4G Mobile Network - the TSC2 to TSC5 fields constitute an extension of the initialization vector and are used during the first phase of the key mixing (key derivation) function; - the WEPseed field is calculated from the TSC1 field; - the ExtIV field, coded on one byte, indicates the presence (bit set at ONE) of the TSC2 to TSC5 fields of the IV extension; - as for the WEP protocol, the KeyID field, coded on two bits, enables the selection of one key from among four possible ones. 4 bytes 4 bytes 8 bytes 4 bytes 4 bytes header header Ciphered part 1 byte 1 byte WEPseed 1 byte 1 byte 1 byte 1 byte b0 to b4 Reserved 1 byte Ext IV b6, b7 KeyID Figure 2.18. Format of TKIP encapsulation The processing of the transmission chain is shown in Figure 2.19. The MIC seal is calculated using the MAC addresses of the source and the destination, the priority byte and the MSDU data. The priority byte contains the priority level of the frame. The generation of the MIC or TMK is described in section 6.2.1. The set composed of MSDU and MIC data can be fragmented. In this case, the initialization vector is increased by one unit for each fragment. Conversely, the IV extension keeps the same value for all fragments of a single MSDU. MAC Layer For each MSDU, two key mixing (key derivation) phases are used to calculate the secret used for WEP processing: - the first phase operates using the transmit address (TA), the TK and the TSC2 to TSC5 vectors; - the second phase operates using the TTAK (TKIP-mixed Transmit Address and Key)