Security risk during the transient in a key exchange protocol using random signals and feedback

Random signals and feedback may facilitate the exchange of shared keys in secure communications systems. In this Letter, the security risk during the initial turn-on is examined. Results indicate that causality plays a critical role. If signals are continuous, the eavesdropper can use extrapolation to breach the security. In digital signaling, however, two parties in communication are in control. They use sampling and set the quantization accuracy to limit the information available to eavesdropper. They can gradually increase the feedback coefficient. By using these countermeasures, they can prevent the eavesdropper from gathering useful information during the transient.