Wireless Sensor Networks are a rapid grown technology trying to adopt several new applications. Such applications have brought new challenges to the table and in consequence there are services that became to be critical factors, just like authenticity in WSN. Node authenticity in WSN is a commodity service that allows verifying that the message source is a node belonging to the sensor network, providing a countermeasure against node impersonation and messagepsilas injection. However, such service is considered too expensive when public key cryptography is involved. In this paper an authentication protocol for WSN is described with an estimation of costs on operations over the base field.