Recent advancements in computing and communication technologies have made the secure transmission of confidential information with conventional cryptographic schemes more challenging. Security at the physical layer has gained attention to mitigate the limitations of cryptographic methods. In this work, we propose a cyclic prefix based secure side-channel (CP-SSC), which can be used to share crucial information over long-term evolution device-to-device (LTE D2D) sidelink even in the vulnerable primary channel. The search space using an exhaustive key-search is analyzed to show the robustness of the security of the proposed side-channel. The bit error rate (BER) results show that the insertion of the proposed side-channel does not have a noticeable degradation on the performance of the primary channel. At the same time, the proposed side-channel provides a high error rate at the eavesdropper. © 2021 Elsevier B.V.