We present a novel lightweight hardware security primitive for wireless body sensor networks (WBSNs). Security of WBSNs is crucial and the security solution must be lightweight due to resource constraints in the body senor nodes. The presented security primitive is based on digital implementation of bidirectional bijective function. The one-to-one input-output mapping of the function is realized using a network of lookup tables (LUTs). The bidirectionality of the function enables implementation of security protocols with lower overheads. The configuration of the interstage interconnection between the LUTs serves as the shared secret key. Authentication, encryption/decryption and message integrity protocols are formulated using the proposed security primitive. NIST randomness benchmark suite is applied to this security primitive and it passes all the tests. It also achieves higher throughput and requires less area than AES-CCM.