Degree:Master of Engineering (Computer Engineering)Keywords: wireless sensor networks, homomorphic encryption, network security, aggregation Wireless sensor networks have become increasingly popular in many applications such as environment monitoring and law enforcement. Data aggregation is a method used to reduce network traffic but cannot be used together with conventional encryption schemes because it is not secure and introduces extra overhead. Homomorphic encryption is an encryption scheme that allows data processing on encrypted data as opposed to plaintext. It has the benefit that each intermediate node does not have to decrypt each packet, but the resulting cyphertext is usually much larger than the original plaintext. This could negatively affect system performance because the energy consumption of each node is directly proportional to the amount of data it transmits.This study investigates the benefits and drawback of using homomorphic encryption in the aggregation process particularly in the context of scalable networks. It was found that conventional encryption outperforms the homomorphic encryption for smaller networks, but as the network size grows, homomorphic encryption starts outperforming conventional encryption. It was also found that the homomorphic encryption scheme does significantly reduce the performance of plaintext aggregation. This performance reduction will however be acceptable for most applications where security is a concern. ............................................................................ . The sensor changes depending on the specific application the sensor node is used in.The processor is usually a simple processor with low computational power. The power source is usually a limited power supply such as commercial battery.These resource constraints mean that the efficiency of data transfer is paramount in these applications. This is because the energy consumption of the nodes is directly proportional to the amount of data transferred [2]. One of the methods that can be used to reduce traffic in the network is called data aggregation. This process involves combining data coming from different sources enroute [3].Aggregation however becomes a problem when security is an issue in the system [4]. This is because each node would have to decrypt each packet, aggregate the data, and then encrypt the result before sending it to the next hop. Secure information aggregation in WSNs is a growing field and one of the proposed solutions to achieving this is called homomorphic encryption. Homomorphic encryption is an encryption scheme that allows data processing on encrypted data as opposed to plaintext [5].Homomorphic encryption allows the data to be aggregated without having to decrypt each incoming packet. In homomorphic encryption schemes, for a given encryption key, each plaintext can be encrypted into a number of different cyphertexts [6]. This means the resulting cyphertext is larger than the original plaintext. As already mentioned, the energy consumption of the nodes i...