The security aspect of wireless sensor networks has taken the attention of numerous researchers in the past several years. It has recently been proven that public keys are now feasible in wireless sensor networks but still consume a lot of processing time and memory. In this paper we propose the use of public keys based on ECC to exchange symmetric keys that will be used to encrypt communications. In addition, we propose a time segmentation approach that enables frequency hopping time slotted communications. Nodes secretly exchange frequency hopping sequences that enable them to fight against jamming and eavesdropping.