Abstract: Spectrum sensing is a premise for the realization of cognitive radio networks (CRN), but the spectrum sensing data is always falsified in hostile environment. A low overhead symmetric cryptographic mechanism utilizing message authentication code to authenticate the sensing data of secondary user (SU) is proposed to resolve this problem. Moreover, in this article, the concept of energy efficiency in cooperative spectrum sensing is first introduced, and the optimal length of message authentication code is provided to maximize the energy efficiency. Simulation results verify the efficiency of this scheme, and the solution of optimal problem is also evaluated.