Secrecy in cognitive radio networks: Turning foes to allies

We study a class of security problems in cognitive radio networks in which multiple half-duplex unlicensed (secondary) users independently eavesdrop the communications of licensed (primary) users unless granted access to communicate over the same spectrum band. The problem is to characterize the optimal rule for the primary system that grants spectrum access to selected secondary users and the optimal resource allocation for the secondary users. When granted access, the transmission of secondary users interferes with the eavesdropped primary signal at other eavesdropping users, possibly leading to improved primary secure rate. First, we formulate a simple game with only one secondary user and study its equilibria. The outcome presents optimality conditions to grant access to the secondary user as a function of channel conditions. Then, we study the game when multiple eavesdropping secondary users exist and show that it is not always optimal to grant access to the strongest eavesdropper. Interestingly, the outcome also reveals a recruiting process that turns selected eavesdroppers into helping jammers under certain conditions.