Wimax standards and Security The Wimax

FIGURE 12.5
The authentication process.

PKMv1 mandates the use of X.509 digital certificates together with the RSA public-key encryption algorithm [8] to conduct authentication; readers are referred to Appendix III for details of the RSA.

3. 
   Key Exchange
   

There are five kinds of keys used to secure WiMAX communications: AK, key encryption key (KEK), downlink hash function-based message authentication code (HMAC) key, uplink HMAC key, and traffic encryption key (TEK). AK is activated by a BS during the authentication process. As the shared secret between the SS and the BS, AK is used to secure subsequent key exchanges in PKMv1.
As shown in Figure 12.6, a 128-bit AK is used to generate the 128-bit KEK by the BS. KEK is used for TEK encryption and distribution. The KEK is derived from the AK by the following formula:

KEK = Truncate_128{SHA1[(AK | 0⁴⁴) ⊕ 53⁶⁴]} (12.1)

| ⊕
In Equation 12.1, AK concatenates with 0⁴⁴ and XORs 53⁶⁴ as denoted by (AK 0⁴⁴) 53⁶⁴. The result is hashed by the secure hash algorithm SHA1, the most commonly used hash function defined by the secure hash standard.









HMAC for downlink — 160 bits




Key encryption key (KEK) — 128 bits

HMAC for uplink — 160 bits
FIGURE 12.6
The key derivation process.

	[Encry
TEK

SS _BS
TEK key request
[AK sequence number, SAID, HMAC-SHA1]


TEK key reply
pted TEK, TEK key lifetime, ber, CBC-IV, HMAC-SHA1]