# Source Anonymity against Global Adversary in WSNs Using Dummy Packet Injections: A Survey
## 1 Introduction  
Wireless Sensors Networks consist of small, independent, and low-cost devices. They are limited in resources such as CPU, memory, bandwidth, transmission range, and power [1; 2; 3]. The sensor node consists of four components: sensing unit, processing unit, transceiver unit, and power unit [4]. Sensor nodes sense mostly phenomena such as temperature and humidity [5] and, in several situations, they are used for tracking and monitoring assets [6; 7; 8; 9]. Further, sensors in WSNs convert analog signals into digital signals using Analog-Digital-Converter (ADC). Then, the signals are transmitted to the sink node that has no memory or power issue compared to ordinary sensor nodes. The transmission is made through intermediate sensor nodes, which act as forwarders. Security in WSNs is challenging since the network can be utilized in battlefields, border surveillance, building monitoring, hospitals, airports, etc. The information exchange between two sensor nodes or between a sensor node and the sink needs to be secure and confidential. Moreover, the communications between nodes are vulnerable to adversaries who can overhear and analyze the traffic [10]. Therefore, each sensor node should have the ability to confirm that the message is received from a legitimate node which, is known as integrity. WSNs should be able to avoid spoofing and altering the transmitted packets. Energy is another challenge due to the limitations of the WSNs. They run on batteries and, in many cases, are out of reach [11]. Accordingly, the security algorithms must be designed in a way to minimize the power consumption while achieving the maximum possible anonymity. Therefore, algorithms should perform reasonable processing and operations not to drain the battery of the sensor node. Since thecommunications among sensor nodes consume the majority of the power compared to the other sensor components, the overhead should be reduced to minimal. A WSN needs to operate itself after deployment and handle a variety of operations such as the self-network configuration and connectivity maintenance [12]. Security of WSNs is categorized into content [13; 14] and context attacks [15; 16; 17]. Content attacks can be countered by using the confidentiality and integrity in the existing routing protocols [18; 19; 20] since the adversary focuses only on disclosing the content of packets. Context attacks such as _SLP_ cannot be protected only by confidentiality and integrity. It requires more convoluted techniques to secure the location of the source as the adversary attempts to locate the source by eavesdropping on the network traffic. _SLP_ keeps the originator sensor node untraceable and unlinkable. Untraceability means that the adversary is unable to trace back the origin node, whereas unlinkability means that the adversary cannot gain the identity of the source [21]. In _SLP_, the real event (of an asset) mostly has three parameters: event type, event time and, event location [22]. These parameters are continually targeted by the adversary to gain information about them (Figure 1). There are various schemes to overcome context threats from the global adversary model [1]. For instance, separate path routing allows each packet to travel towards the sink using a different route [23]; network location anonymization uses a fake identity for the source [24; 25]; network coding breaks down each packet into smaller pieces that follow different routes to the sink [26; 27]; and dummy data sources create dummy sources that generate fake traffic, which confuses the adversary about the identity and location of the source [28; 29]. Using fake traffic is by far the most effective method against global adversaries because it uses different types of packets that appear similar from the outside. In this paper, dummy and fake are used interchangeably. Real packets carry information related to the real event such as its location. Dummy packets do not carry any information about the real event. They are often utilized to mislead the adversary [1].