## 3 PPDA Protocol Designing Principles, Issues and Challenges  
WSNs can be deployed in different private data generating fields, as we mentioned in the previous section. PPDA protocols to design supporting sensitive data aggregation face multiple issues and challenges because of bandwidth limited WSNs with resource-constrained sensor nodes. So, a PPDA protocol designed for WSNs must address the features mentioned below in order to be acceptable in the WSN domain. _(a) Eavesdropping and privacy preservation:_ Eavesdropping is an attack in which an attacker or a curious individual attempts to obtain private information by overhearing transmissions over its neighboring wireless links. Eavesdropping threatens the privacy of data held by an individual sensor node. In opposition, privacy preservation ensures data privacy against both trusted sensor nodes and adversaries. There are some techniques which can prevent revealing the actual data of a sensor node to other sensor nodes and adversaries even though they overhear the sensor data. For example, the PDA proposed two new trends of data transmission to hide actual sensor data [16]. They are CPDA and SMART protocols. To maintain data privacy, in CPDA, seeds (real numbers) are mixed with actual sensor data before they are sent to a parent node. In SMART, the data of a sensor node is divided into small pieces which are sent to a neighboring node. However, such techniques generate unnecessary data traffics which cause a high cost for resource-constrained sensor nodes. _(b) Data pollution and data integrity:_ Another type of attack is data pollution, in which an attacker tampers with the intermediate result of sensor data during data aggregation at an aggregator. The purpose of the attack is to make a base station receive a wrong aggregation result, thus resulting in improper or wrong decisions. Because the result of data aggregation is used to make critical decisions, a base station or a user needs to attest to the integrity of the aggregated result before accepting it. Ensuring the correctness of the received aggregated data is highly desirable in civilian applications. Therefore, it is necessary to protect the aggregation results from being polluted by attackers. Moreover, if data pollution can be detected as soon as possible, its impact in the final aggregated result can be avoided. To resolve the problem, a commitment-and-attestation technique is presented in the SDAP and the SIA for secure data aggregation [23, 24]. But, the commitment-and-attestation technique delays the decision making process because it needs the searching cost of _O(log N)_ to detect the spot of data pollution and ensure data integrity, where \(N\) is the total number of sensor nodes in a network. In addition, it needs a significant amount of resources for resource-limited sensor nodes. _(c) Efficiency:_ In WSNs, data aggregation should achieve both bandwidth and energy efficiency through in-network processing. In private data aggregation protocols, an additional communication overhead cannot be avoided when additional features are realized. However, additional overheads due to the resource-constrained nature of sensor nodes, such as communication cost, computation cost, memory and payload size, should be kept to a minimum. The privacy homomorphism technique performs privacy-preserved data aggregation by using the minimal amount of sensor resources [33, 35]. However, the privacy homomorphism technique is not flexible enough to include other features, like supporting a variety of aggregation functions and data integrity within a system. Because most existing PDDA protocols [17, 19, 31] are not seriously concerned about the efficient usage of the resources, it is a challenging task to devise an elastic PPDA protocol for WSNs by considering the resource limitations of sensor nodes. _(d) Accuracy:_ Because WSNs are not always reliable, it cannot be expected that all sensor nodes will reply to all requests.