In this paper, we introduce Compact-Security (C-Sec), an energy efficient link layer encryption protocol for Wireless Sensor Networks (WSNs). The protocol minimizes energy consumption by eliminating the need for transmitting all header and trailer fields related to security, while keeping security functions and services intact. Such fields include message authentication code, freshness counter, and source address. Our work relies on merging security related data with the essential headers of the next packet. This will dramatically reduce security related communication overhead. In addition, it includes a unique security feature that does not exist in any of the current protocols: hiding the packet header information. C-Sec is implemented using Very high speed integrated circuit Hardware Description Language (VHDL). Experimental results using synthesis for Spartan-6 low-power FPGA demonstrates that the proposed protocol outperforms related work in terms of computational time and energy consumption, in addition to the large savings in communication energy and bandwidth.