Sensor networks are a very specific type of wireless networks where both security and performance issues need to be solved efficiently in order to avoid manipulations of the sensed data and at the same time minimize the battery energy consumption. This paper proposes an efficient way to perform data collection by grouping the sensors in aggregation zones, allowing the aggregators to process the data generated (sensed) inside the aggregation zone in order to minimize the amount of transmissions to the sink. Moreover, the paper provides a security mechanism based on hash chains to secure data transmissions in networks with low capability sensors and without the requirements of an instantaneous source authentication.