Wireless sensor networks (WSN) grows in size and gain new applications in our lives ranging from military applications to civilian ones. However security in WSN was not carefully carried out, since only some symmetric encryption based protocols are proposed in literature, under the assumption that the nature of sensor nodes does not support public key encryption due to the limitation in battery and CPU power. However the new development of sensors technologies may allow more computational power and gives us the possibility to use public key encryption in WSN if the used algorithm is energy efficient such as ECC. Therefore in this paper we propose a lightweight implementation of Public Key Infrastructure (PKI). Our proposed protocol called ¹PKI uses public key encryption only for some speci¯c tasks as session key setup between the base station and sensors giving the network an acceptable threshold of confidentiality and authentication