Sensor networks are expected to be used at anywhere in the near future and recently their security problems have been rising. Due to the limited computing power of senor nodes, it is impossible to use asymmetric cryptography approaches for the session key establishment or to apply advanced encryption method such as AES and DES for the data encryption. In this paper, therefore, we propose a key establishment and a data encryption scheme for secure and energy-efficient data transmission in wireless sensor networks (WSNs). The proposed schemes just treat simple operations as a message authentication code (MAC), exclusive-OR (XOR) and time-spacing key derivation function (TSDF). Additionally, the security of the proposed protocols is analyzed.