Wimax standards and Security The Wimax

FIGURE 12.7
The TEK exchange procedure.

Truncate_128(.) retrieves the first 128 bits of the hash result as the KEK and discards the rest of the bits.

The downlink HMAC key and uplink HMAC key provide data authenticity of key distribution messages from the BS to the SS and from the SS to the BS, respectively. They are both generated from the AK in a similar way as defined by Equation 12.1. The TEK exchange process relies on the downlink HMAC key and the uplink HMAC key to secure the exchanging messages.
Figure 12.7 exemplifies the TEK exchange process between an SS and the BS. The authenticated SS starts a separate TEK process for each SAID.

The TEK process periodically sends TEK key request messages to the BS, requesting a refresh of keying material. The BS responds to the TEK key request message with a TEK key reply message, containing the BS’s active keying material. The TEK is encrypted using the appropriate KEK derived from the AK. ATEK key reply message provides the requesting SS the remain- ing lifetime of the keying material, enabling the receiving SS to estimate when the BS will invalidate a particular TEK and thus when to schedule the future TEK key request. The TEK process remains active as long as the SS has a valid AK and the BS continues to provide fresh keying material when the SS requires. The TEK refreshing mechanism ensures that the SS will be able to continually exchange encrypted traffic with the BS.

4. 
   Data Encryption
   

Upon the completion of authentication and initial key exchange, data trans- mission between the BS and the SS starts by using the TEK for encryption. Figure 12.8 depicts the process, where data encryption standard with cipher

Generic MAC header

MAC SDU (service data unit)