dezvous Specialized Microphone Defense systems are rarely able to assume support in- Biometric Sensors Sensors frastructure. Similarly, early deployments of commercial CR equipment will not be able to assume infrastructure. Consequently, it is important to have an unaided method for rendezvous. Several methods exist for systems to find Authentication each other. Applications The problem is somewhat like two men searching for each other when they are a mile apart on a moonless dark night out in the desert. Each has a flashlight the other Access to Handshaking may see, but only when it is pointed in the right di- with Remote CR CR Functions rection. They can finally signal each other, but only when each has noticed the other's flashlight, SO each must look Figure 2.4b-12 Authentication context diagram. A CR application in the proper direction at the proper time. that learns the identity of its user(s) and has an Access List In the case of CRs trying to find each other, one must Security Protocol can minimize fraud and maximize secure communications. transmit and the other must receive in the proper fre- quency "hole" at the proper time. Several methods are feasible. All involve one node transmitting "probe" sig- nals (a uniquely distinguishing waveform that can be readily correlated) until receiving a response from a net 2.4b.6.4 CR Authentication applications member. In all cases, the problem is that the frequencies the transmitter sees as being "usable holes" are different A CR can learn the identity of its user(s). Authentication from those the receiver sees. Thus, neither can be sta- applications can prevent unauthorized people from using the CR or the network functions available to the CR. tionary and just sit on one frequency hoping the other will find it: This enhanced security may be exploited by the military for classified communications or by commercial vendors Case 1: Node A transmits probes in randomly selected for fraud prevention. frequency holes, while node B listens to random Because many radios are