Wimax standards and Security The Wimax

FIGURE 10.6
Creation of a data association.

verifying the source and integrity of future messages. Message source and integrity are verified with message authentication (HMAC) keys, e.g., HMAC(1) proves the integrity of the first message from the BS to the SS. Two separate HMAC keys are derived from the AK, for the BS-to-SS (downlink) and SS-to-BS (uplink) directions. AKEK is also derived from the AK. The KEK is used for key exchange messages to obtain the transmission encryption keys used when transmitting data.

In WiMAX, the SS and BS exchange management messages for authentica- tion as shown and then proceed to key management (as shown in Figure 10.6) before transmitting data. The old TEK is the one currently used; it will be replaced by the new TEK at its expiration. Chapters 11 and 12 will discuss known issues in the different standards that govern keys and authentication. Several flaws have been found in this standard. Refs. 1,8,11 discuss some of them. An improved scheme for key management, based on extensible authentication protocol (EAP), is presented in Ref. 12. Other improvements
and issues are discussed in Chapters 11 and 12 in this book.



10.4 Conclusions
We have distilled the fundamental aspects of the conceptual architecture of WiMAX, in particular its security in the form of UML models. We have separated the conceptual architecture from implementation details, aspects

that are intermingled in the standards. This separation is very important for evolving standards like this one, where the implementation is expected to change relatively frequently, but the conceptual architecture should remain stable. These models can be used to understand the more complex aspects of the standard and to analyze weaknesses and improvements to the protocol. We are now converting these models into security patterns in the style of Ref. 10. In the form of patterns, these models can be used to guide the design of wireless systems and to compare standards.