The TKIP encapsulation process is shown below
FIG Courtesy – 802.11 Standard
Description of the parameters
TA – Transmitter address
TK – Temporal Key
TSC – TKIP Sequence Counter
Priority – QoS TID Priority – set to 0 if QoS control field is not present
MIC Key – MIC transmitter Key (64 bits) obtained during EAPOL handshake
TTAK – TKIP-mixed transmit address and key
The Encapsulation process is explained below (from the 802.11-2012 standard)
- TKIP MIC computation protects the MSDU Data field and corresponding SA, DA, and Priority fields. The computation of the MIC is performed on the ordered concatenation of the SA, DA, Priority, and MSDU Data fields. The MIC is appended to the MSDU Data field. TKIP discards any MIC padding prior to appending the MIC.
- If needed, IEEE Std 802.11 fragments the MSDU with MIC into one or more MPDUs. TKIP assigns a monotonically increasing TSC value to each MPDU, taking care that all the MPDUs generated from the same MSDU have the same value of extended IV.
- For each MPDU, TKIP uses the key mixing function to compute the WEP seed.
- TKIP represents the WEP seed as a WEP IV and ARC4 key and passes these with each MPDU to WEP for generation of the ICV, and for encryption of the plaintext MPDU, including all or part of the MIC, if present. WEP uses the WEP seed as a WEP default key, identified by a key identifier associated with the temporal key.