The temporal key is then used for the encryption and MIC temporal keys
(described in Section 7.3.3). The transient key provides the key material for the TKIP perpacket
RC4 encryption and the TKIP MIC function.
In TKIP, it is critical that for any encryption key, a given IV be used to encrypt one
and only one frame. Proper use of IVs is ensured by the application of IV sequencing rules.
First, the notion of a sequence counter is introduced. The sequence counter is incremented
by the transmitter on a per-frame basis. As part of the per-packet mixing function, the
Wireless Local Area Network Security 149
sequence counter is mapped to the WEP IV. Then, the receiver must verify that the
received frames increment sequentially, per Quality of Service traffic class. If the IV of a
frame is less than or equal to that of a previously received frame, it is discarded by the
receiver.
80-bit Array
Temporal Key
Sequence Ctr
128-bit Array Phase 2
Hash 16 bits
Figure 7.2: Phase 2 hash.
Calculate Transient Key
Secret Key
Use Temporal Key Calculate Temporal Key
Figure 7.3: Temporal key derivation concept.
7.3.3 TKIP Message Integrity Code
A Message Integrity Code (MIC)2 is needed to verify the authenticity of a transferred data
packet.
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