## 2 Related Work  
### Privacy Schemes  
A number of a privacy schemes such as [1; 3; 4; 5; 6; 7] have been proposed for WSNs that are discussed below.  
[PERSON] _et al_. [3] proposed a phantom routing scheme for WSNs, which helps to prevent the location of a source from the attacker. In this scheme, each message reaches the destination in two phases: 1) a walking phase, in which the message is unicasted in a random fashion within first \(h_{walk}\) hops, 2) after that, the message is flooded using the baseline flooding technique. The major advantage of their scheme is the source location privacy protection, which improves as the network size and intensity increase because of high path diversity. But on the other hand, if the network size increases, the flooding phase will consume more energy. This scheme does not provide identity privacy. Also, it is unable to provide data secrecy in the presence of identity privacy.  
[PERSON] _et al_. [4] proposed a phantom single-path routing scheme that works in a similar fashion as the original phantom routing scheme [3]. The major difference between these two schemes is that after the walking phase, a packet will be forwarded to the destination via a single path routing strategy such as the shortest path routing mechanism. This scheme consumes less energy and requires slightly higher memory as compared to first one. This scheme also does not provide identity privacy. Also, it is unable to provide data secrecy in the presence of identity privacy.  
[PERSON] and [PERSON] [5] proposed two schemes: Simple Anonymity Scheme (SAS) and Cryptographic Anonymity Scheme (CAS) for establishing anonymity in clustered WSNs. The SAS scheme use dynamic pseudonyms instead of true identity during communications. Each sensor node needs to store a given range of pseudonyms that are non-contiguous. Therefore, the SAS scheme is not memory efficient. On the other hand, the CAS scheme uses keyed hash functions to generate pseudonyms. This scheme is memory efficient as compare to the SAS but it requires more computation power. The authors do not propose any routing scheme. Sender node may always send packets to the destination via shortest path. In that case, for an adversary who is capable of performing hop-by-hop trace back (with the help of direction information) can find out the location of the source node.  
[PERSON] _et al_. [1] proposed a Greedy Random Walk (GROW) scheme to protect the location of the source node. This scheme works in two phases. In a first phase, the sink node will set up a path through random walk with a node as a receptor. Then the source node will forward the packets towards the receptor in a random walk manner. Once the packet reaches at the receptor, it will forward the packet to the sink node through the pre-established path. Here receptor is acting a central point between the sink and the source node for every communication session. A criterion of selecting a trustworthy receptor is essential, however not defined in the author's work.  
[PERSON] _et al_. [PERSON] _et al_. (2007) proposed a Cyclic Entrapment Method (CEM) to minimize the chance of an adversary in finding out the location of the source node. In the CEM, when the message is sent by the source node to the base station, it will activate the predefined loop(s) along the path. An activation node will generate the fake message and forwarded it towards the loop, and original message is forwarded to the base station via specific routing protocol such as shortest path. Energy consumption in the CEM scheme is mainly dependent on the number of existing loops in the path and their size.