Wireless body area networks (WBANs) are receiving a significant interest as an emerging technology in next generation wireless networks. Security challenges as well as this interest are also beginning to rise to the surface. We derived specific security requirements from WBANs, and proposed a new hybrid security protocol for WBANs which satisfies these requirements. This proposed protocol dealt with the overall security including up to the in-, on-, and out-body, and utilized two heterogeneous cryptosystems (the symmetric and asymmetric) for diverse environments of WBANs. Our protocol shows that it satisfies more security requirements than existing security protocols including the overall coverage of WBANs.