In a cognitive radio network there are mainly two schemes for secondary users to share the spectrum resources. In one
scheme the secondary users reuse the spectrum that is not used by the primary users. The other scheme, called spectrum
sharing, allows the secondary users to transmit concurrently with the primary users as long as they do not harm the transmission of the primary users. While cognitive radio is an efficient technique to relieve the pressure of wireless spectrum
scarcity, at the same time the characteristics of cognitive radios have introduced entirely new types of security threats
and challenges in networks. Since the primary users and the secondary users coexist in the same network, both of them
need to be protected, and they are more vulnerable to security attacks compared to the traditional wireless networks without
using cognitive radios. Therefore, providing strong security protections is one of the most important requirements for
cognitive radio networks.