Security was not considered when current wireless sensor nodes were designed. As a result providing high level of security on current WSNs platforms is unattainable, especially against attacks based on key resolving and node compromise. In this paper we scrutinize the security holes in current WSNs platforms, and compare the main approaches to implementing the cryptographic primitives used to provide security services for these platforms, in terms of security, energy, and time efficiency. To secure these holes and provide more efficiency we propose a custom hardware platform for WSNs. The choice of cryptographic primitives for our suggested platform is based on their compatibility with the constrained nature of WSNs and their security status. We also discuss the most efficient configurations and implementation methodologies of these primitives, and review their specialized implementations for WSNs in recent literature. Based on that, we provide a hardware implementation of a crypto-processor using Very high speed integrated circuit Hardware Description Language (VHDL). Experimental results using synthesis for Spartan-6 low-power FPGA shows that the proposed protocol outperforms related work in terms of computation time and energy consumption.