In this paper we propose a lightweight encryption mechanism based on rabbit stream cipher referred to as LRSA for providing confidentiality in wireless sensor networks (WSNs). LRSA fulfils both requirements of security as well as energy efficiency. Our proposed security protocol is an idea for resource constrained WSNs, and can be widely used in the applications of secure communication where the communication nodes have limited processing and storage capabilities while requiring sufficient levels of security. The results of LRSA conclude that this particular scheme is more efficient in terms of security and resource consumption for providing data confidentiality for the domain of WSNs.