SECURE ROUTING IN WIRELESS SENSOR NETWORKS
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Abstract.
Wireless sensor networks (WSANs) are a group of sensors and actors that are linked by a wireless medium for the purpose of performing distributed sensing and action on a given task. This involves the sensors collecting information about the surrounding physical environment and sending the information to the actors which take the decisions and perform some needed action basing on the information received from the sensors about the surrounding environment. These sensor networks are sometimes referred to as wireless sensor and actuator networks. They monitor physical or environmental conditions such as sound, pressure, temperature among others and send the collected data to the required location. Effective sensing and acting requires a distributed local coordination methods and mechanism among the sensors and the actors in addition to this, sensor data should be valid in order for right and timely actions to be performed. This paper describes secure routing in wireless sensor networks and outlines its threats on security.

Keywords: Wireless sensor and actor networks; Actuators; Ad hoc networks; Sybil attack; Real-time communication; Sinkhole; Routing; MAC; adversary.

Introduction
With the recent rapid improvement on technology, many networking technologies have been created to make communication easy. One such technology is distributed wireless sensor network which has a capability of observing the physical world and process the data and in addition make decisions basing on the collected data and perform actions basing on this. Wireless sensor networks (WSNs) are rapidly growing and have emerged as one of the important area in mobile computing. Its applications of WSNs are numerous and growing fast with time. Its applications are increasing and are currently deployed in both home and offices and also outdoor such as mapping the territory of an enemy in battle ground as mentioned by (Akyildiz et al 2002). It is used in am military environment; the technology is used to detect the enemy soldiers and trucks. They are also used also in healthcare for detecting intruders mostly on healthcare of the elderly; they are connected to the wireless home security systems.
Routing security of the wireless networks is always considered in the above mentioned scenarios and the accuracy of relaying information is always the target after and during the deployment of the wireless systems. A lot of the routing protocols have been designed and deployed in the market each focusing on security as an objective of obtaining its optimum user requirements. However, none of the systems have been able to achieve the goal of security. Many security challenges still exist that expose this technology to threats. This is due to combined threats which are as a result of physical security and lack of adequate resources which motivate the intruders who may compromise the sensor nodes if the system is not secure enough.
The primary goals of wireless secure networks are data authentication, data confidentiality, data integrity, and data availability.
Data authentication will ensure that the message being transmitted is reliable; this is achieved by identifying the place that the transmitted message originates from. This ensures that alteration of the packets by an adversary is avoided. Adversary may inject the wrong or additional packets to the message and the only way of identifying this is by identifying the origin of the message. Authentication of data is achieved by sending and receiving nodes sharing the secret codes or keys. It is however very difficult in wireless network to attain authenticity of the information due to the nature of wireless network.
Confidentiality of data on the other hand involves the transmitted information being hidden or concealed so that an attacker is not aware of the message being transmitted from the network therefore shielding the attacker from compromising the information.
Another key important goal is data integrity. This ensures that the data is reliable and has not been tempered with or altered in any way. According to Shanmugapriya and Padmavathi (2009), even if the network has all the confidentiality measures, there is still a possibility that data integrity can be compromised by alteration. Alteration of information can render the data useless due to its unreliability.
Data availability is also another key important aspect of a wireless sensor network. It determines the availability of the network resources to communicate. Failure of a node or a base station will lead to the failure of the entire network and therefore operations in the network will be rendered unavailable.
Other goals of WSNs involve the freshness of the data; this is achieved by ensuring that the data in the network is as recent and updated as possible. Data freshness ensures that the old messages are not replayed in the network. Independence of the network also ensures that every sensor self-organizes itself and is as flexible as possible; this allows self-healing after the attack. The main aim in WSNs is secrecy and authentication, since most of these sensor networks and various applications that relies in these networks requires protection against various challenges as far as security is concerned such as malicious injections, eavesdropping and packet modification, encryption of the packets is the best approach to fight back against these emerging threats. This ensures that high level of security is achieved by the keys being set to decrypt the information on the other end. Methods such as link layer cryptography for instance ensures that there is one key which simplifies the key setup. Another issue which the wireless sensor networks try to address is privacy. The above mentioned goals are however facing various challenges from the attacks and injections carried out by adversaries on the network. Various protocols have been defined to try to reduce these challenges in the wireless sensor networks.
Research Methods
3.1 Introduction
Research Methods
This chapter discusses the methodological approaches that will be used in investigation of secure routing in wireless sensor networks. The chapter will describe the research design, comparative analysis, statistics of survey, techniques that will be used for data gathering, and data analysis techniques. The chapter will also tackle the basic issues security issues that will be covered in the study.

In the sections that follow I will be presenting the routing protocols and the security challenges experienced by wireless sensor networks and at the end give secure measures of addressing these issues.
Routing is the process of selecting best path in a network where messages are passed from one node to another until the message reaches the intended destination. A protocol on the other hand is the set of rules that communicating nodes must adhere to in a network. Routing in WSNs differ from that of wired networks in many ways, this is due to the lack of infrastructure and the unreliability of the wireless links which may cause communication to fail. The wireless network uses less energy and therefore routing protocols defined must meet this requirement. This has led to the development of the routing algorithm to meet these challenges in the wireless network environment. These protocols have been categorized into seven categories as I will describe below.

Routing Protocols
Location based protocols This protocol is location based where the sensor nodes are addressed by location information. The information about the location of the sensor nodes is requested by the protocol and the distance between two nodes which are specified calculated basing on this information. The consumption of the energy is then estimated basing on the result obtained from the calculation. Examples of location based protocols are: Geographic Adaptive Fidelity (GAF), Geographic and Energy-Aware Routing (GEAR), Coordination of Power Saving with Routing, Geographic Random Forwarding (GeRaF) among others. Geographic adaptive fidelity (GAF) is a routing protocol which considers energy usage in the transmission of packets across the network. This protocol turns off some sensors it deems as unnecessary and in the process ensures that routing is as constant and uninterrupted as possible between the communicating sensors. This protocol enrolls division of sensor field into squares and location information of each sensor will be used by that particular sensor. The location information can be provided by location systems such as GPS for instance and therefore the sensor is able to associate itself with a given grid that it is located at. Geographic Energy aware routing protocol on the other hand is also an energy conscious protocol which routes packets to targeted destination within the sensor field. Sensors have location hardware such as GPS systems which allows the sensors to know and identify their locations. The sensors are able to keep track of their energy as well the energy of their neighbors. This protocol uses recursive forwarding algorithm which is based on geographical location to transfer packets around the mapped region.
Coordination of power saving with routing is a protocol which reduces how the WSNs consume the energy by managing the individual nodes consumption of power. This is achieved by turning the idle radio off. Unlike the first two protocols, this protocol does not require the nodes to know their location
Geographical Random forwarding stated by Singh et al (7) uses routing based on the geographical location where the relaying sensor is hidden before the sending of packets. This method is called best effort forwarding scheme since the sender cannot be guaranteed that the message will reach its destination referred to as the sink. This protocol assumes that all the sensors are aware of their physical locations and the location of the destination sensor.
Data-Centric protocols
In these protocols, the intermediate sensor performs some activities on the data which originate from multiple source sensors as soon as the source sensors send the data to the sink the aggregated data is then sent to the sink. This process saves the energy due to less transmission from source to the destination. Some of the protocols in this category are: Sensor Protocols for Information via Negotiation (SPIN), Directed diffusion, Rumor routing and cougar among others Sensor Protocols for information via negotiation (SPIN) has been designed to overcome the problems that might be caused by flooding such as implosion and overlap, this protocols are adaptive to the resources in the network and are able to compute the required energy for sending and receiving data in the network, in addition to this intelligence this protocol can make decisions on how the resources are to be utilized in the network. This protocol is based on negotiation and adaptation mechanism, this is achieved by enabling the sensors to negotiate with each other before the data can be sent in order to avoid redundancy.
User and application programs are given queries generated by the source sensors are provided by this approach. This provides abstraction which protects the user from knowing how the queries are to be executed. Due to in- network processing, the total energy consumption is greatly reduced thus enhancing the network lifetime.
Rumor routing on the other hand is a scheme efficient if the number of queries is in between two intersection points of a routing curve with those of flooding query and event query (Singh et al 71). This protocol is based on agent concept where a long-lived packet in a network informs each sensor the events that it has learned during the process of traversing the network. This occurs for several hops after which the agent dies. When an agent encounters a sensor in its path, it synchronizes its events according to the encountered agent and updates the event list when it encountered agent to obtain the shortest possible path in the network.
Directed diffusion protocol fulfils the requirement of energy saving and efficiency as required by the wireless routing networks, other key advantages of this protocol are scalability and robustness. It has several key elements such as data naming gradients among others. In this protocol, the sinks identifies a low data rate for the incoming activities, the sinks can then reinforce one selected sensor and appoints it to send events that contains bigger data rate. This is done by resending the message which had been sent before and representing the resend message with a smaller interval.
Attacks on wireless sensor networks
Wireless networks are a great challenge since the channel is open therefore it can be accessible to anyone. This makes the network vulnerable to attacks at any time. Attacks on the wireless sensor networks can be categorized as either active or passive
Passive attacks are the kind of attacks in which the main aim of the attacker is to obtain information for the network without being detected. The attacker remains undiscovered and quietly while collecting large volumes of information from the network. The collected information can be used to perform other activities in the network by the attacker. This kind of attack is very hard to be detected since they are normally quiet in nature. Active attacks on the other hand are the opposite of the passive attacks. It involves the exploiting the weaknesses of the security in the wireless network in an attempt to launch attacks against the network by modifying the packets and messages through injection of malicious codes. This kind of attack is severe since it may bring the entire network to a standstill.
Attacks can also be classified as internal or external
External attacks are attacks launched from outside the network. These modes of attack are usually not so dangerous to the network. Internal attack on a network on the other hand is a kind of attack that is carried out in between the network. The attacker may obtain credentials from the people inside the network and use it to attack the sensors. This type of attack is severe and may lead to great loss.
Sinkhole Attack
This type of attack is a type of attack in which compromised node tries to attract network traffic by advertising its fake routing update. This attack can be used to launch other types of attacks and therefore makes it a dangerous attack. This attack can be used to launch attacks such as forwarding or spoofing attacks. This attack however is difficult to carry out since there is no way of creating effective malicious node in the network in addition of the difficulty of creating this malicious node, it is hard to select the node at a given time if it is managed to be created.
This attack is an insider attack in which an intruder compromises a node inside a network therefore allowing him to launch attack. This attack makes the node try to attract all the traffic from neighboring nodes basing on a certain formula defined in the protocol. This attack is made possible by the communication pattern used by wireless sensor networks that allows each node to communicate with a base station by each sending data thus making the wireless sensor network vulnerable to this type of attack.

The figure below elaborates on this mode of attack on WSN

The above attack depicted in the diagram is available in Minroute protocol. This type of attack is mainly targeted on wireless sensor network due to its small storage capacity and the little amount of computed power it requires and can therefore work under limited power supply. It chooses the best route by using the quality of the link as its metric therefore allowing it to send packets to the base station this is according to Krontris et al (15).
Another type of sinkhole attack is based on TinyAODV protocol, this bases its attack on the routing metric and number of hops to the base station that are used in this protocol. Route from source to the destination gets created as one node sends packets while the next the next node reply with route reply packet and this fails, the packet gets forwarded to other nodes close to the destination until the source node receives the reply. The node with minimum number of hops to the destination is then selected by the source node. This can be compromised by launching an attack through sending of reply packet. This Reply Route Packet will give a small number of hopes that shows the closer proximity to the base station. This is the sinkhole node; the source then decides to forward the packets to this sinkhole node. Thus the same sinkhole node performs the same to the neighboring nodes by attracting highest possible traffic. This type of attack is depicted in the diagram below

To detect a sinkhole attack some approaches have been defined this are listed as anomaly based, rule based, statistical approaches and cryptographic key management.
In anomaly based detection method, it involves searching of anything that appears to be abnormal in the network. If anything appears to be abnormal it is then listed as possible threat and analysis carried out. Rule based and statistical approaches are subset of the anomaly based detection approaches according to Chaudhry et al (9).
Rule based approach on the other hand concentrates on the definition of rules that are based on the techniques which are used by the attacker to launch the attacks. The rules are set on the intruder detection system that runs on each node. Any node is considered as an attacker that violates the set rules. Statistical approaches associates recorded activities of a node to a particular node such as comparing the normal threshold with the recorded activity of the node. If the particular node whose data is being monitored seems to show a strange behavior as per the statistics, it will then be considered as an adversary. This may include an abnormal cpu usage and depletion of the resource. Cryptographic approach on the other hand checks the integrity and authenticity of all the packets travelling within various nodes in the network and ensures that they are protected from an adversary by use of encryption and decryption keys. To access the message travelling across the network, the decryption key is required; this will ensure that even a slight modification of the message is detected.
Hybrid approach is a combination of cryptographic and anomaly based approaches and involves the reduction of the false positive reports produced by the anomaly based approach. This method has an advantage over other methods of detection in that it is able to detect any suspicious node incase it fails to transmit the signature or include it in its database.

Wormhole attack
In this type of attack, the attacking node captures the packets form one location then transmits the captured packets to a node that is located at a distance which will distribute these packets locally. This attack can easily be launched with less knowledge of the network. This attack is dangerous when launched against ad hoc network routing protocols since the nodes which come into contact or hear the transmitted packet consider them to be from their neighboring node. This involves recording of the traffic from one region in the network and replaying them in a different location. The severity of this attack is due to the fact that is hard to be detected. This attack takes advantage of the transmission over large area which in turn attracts other nodes. The diagram below describes the wormhole attack on a wireless ad-hoc network

A warmhole attack is detected by use of packet leashes where a temporal packet leash places a bound on the lifetime of the packet therefore restricting the travel distance of the packet. The sending node includes the time of transmission and the information about the location of the packet in the message. The receiver then checks this information included in the packet and compares it with the distance that the packet has travelled from source to the destination and the time that the packet took to travel from source to the destination. Synchronized clocks are therefore required due to the very high speeds that radio signals travel taking into consideration that radio signals travel at a speed of light.
To prevent this mode of attack, nodes need to maintain accuracy of information of their neighbors. The neighbors here are the nodes within a single hop. Messages from non-neighboring nodes get ignored by the receiving node, securing the protocol used in maintaining the neighboring node is also vital since the protocol itself might be vulnerable to the wormhole. This can be done by designing a neighborhood discovery protocol which in itself is not vulnerable to these attacks. (Hu et al [2]).
Denial of service attacks (DoS)
This attack is caused by launching attacks on the nodes; this type of attack is meant to disrupt the way in which data is being relayed in the medium, destroy or make the delivery of the service unbearable. Several methods of DoS attacks can be used and may be jamming or tempering of the normal transfer of packets by flooding and causing desynchronizing the flow of packets around the network.
To avoid this method of attack in the network, strong authentication and identification of traffic methods should employed in the network.

Sybil attacks
This is an attack in which the reputation system is subverted by the foreign entities mostly in peer-to- peer networks.
The scale of decentralized distributed system being in an increase, it has become the norm for the presence of malicious behavior. Most network designs against malicious behaviors depend on certain amount of nodes in the system being in their good transmission mode. Malicious user can take multiple identities and in the process pretend to be multiple distinct nodes which always referred to as Sybil nodes or Sybil identities in a system. The malicious user can then be able to overwhelm the genuinely honest users and break the defense against the abnormal behaviors in the system. This is the removal of primary defensive mechanism that the system had employed to guard against malicious system intrusion.
To detect this type of attacks, many methods have been devised. These methods include directional antenna, propagation model, testing of the resource, detecting and localizing nodes, cryptography and timestamp series. Timestamp series approach defends a network against Sybil attack basing on the unit support. Packets are send by a node contains timestamps which are issued to the packets. Sybil attack is detected by checking on the timestamps on the sent message and if there are many similar timestamps, then there is a possible scenario of a Sybil attack on the network. This technique is however difficult to use and is not a very suitable method of detecting this kind of attack on a network. Another method which has been designed for detection of the Sybil attack is the public key cryptography. This method identifies the possibility of a Sybil attack by using a public key cryptography and authentication method. In this method, signatures and digital signatures are combined and regions defined where the nodes in one region are able to communicate to one another using secure channels and in the process monitor the issued certificates in every broadcast message. This makes sure that only those messages with valid certificates are considered as relevant while those with the invalid certificates are ignored during the process. These certificates are changed after a while to avoid insecurity and maintain privacy in the network. The detection and localization node on the other hand is a technique which is based on finding the physical location of nodes and compares it with the position of the packet.
In resource testing technique method, it is assumed that every entity has limited facilities as far as computational resources are concerned. Sybil attack is detected by checking the radio resources since each node should have only one radio resource and therefore cannot send or receive more than one channel at a time. This method is however not accurate since an attacking node can have more resources that the genuine node. Propagation model technique on the other hand by comparing and matching the signal power with the position that the signal is. The signal power can then be used to calculate the position of the node and if they do not match then it is considered as a possible Sybil attack.
This technique however, may not be an accurate one for detecting an attack since the malicious node might propagate the signal in the same speed as the other nodes in the network making detection impossible.
Bi-directional antenna method is a technique which checks the direction of the arriving packets and identifies if the message is originating from a forged neighbor or it is from the real neighbor.

Selective forwarding attack
In this type of attack, the node selectively drops the packets that are sensitive. This type of attack are effective when attacking nodes and the only ones included on the path of the message flow thus they are able to corrupt the existent routing protocols especially when combined with other attack methods for example sinkhole and wormhole attacks. Multipath forwarding can be used to counter this attack although the multipath forwarding strategy suffers from weaknesses in itself; these weaknesses include increase in the number paths and poor security resilience.
Other attacks on WSNs
Message Corruption
In this Kind of attack, the attacker compromises the message by modifying the content of the message leading to inaccuracy, this leads to loss of integrity.

False Node attack
In this method of attack, the adversary adds an additional node and uses this node to inject malicious content into the network. This node might prevent the passing through of the intended data and rather use the malicious one to pass the wrong data. This attack is one of the most dangerous kind of attack since the injected data might spread all over the network therefore the entire network might be destroyed. The attacker might as well take over the network after this code has taken over the network.

Node malfunction and node outage
Node malfunction occurs when the node in a network fails to function correctly therefore leading to transmission of the wrong data over the network. This will make the level of integrity of the entire network exposed. Node outage on the other hand refers to the vent where a node completely stops its operation. A sensor network should provide an alternative route as a solution to this kind of an issue in case it arises.

Node Replication attack.
This occurs when an attacker adds a node to a sensor network. This is achieved by copying the NodeID of an existing node in the network. This replicated node can then be used by an adversary to cause havoc in the entire network. The attacker for example can use this replicated node to disrupt the performance of the network by corrupting or misrouting the data or messages in the network. This kind of attack can bring the entire network totally to a halt or give false sensor reports.

Passive information Gathering Attack
In this method of attack, an adversary collects the information from the network. This is normally carried out if the information transferred over the network is not encrypted. An adversary with a powerful receiver and a sophisticated antenna easily receives the data traversing the network. This information can contain the location of the nodes which may allow the attacker to launch an attack and bring them down. Critical information about the network such as Network ID and timestamps can also be obtained from this mode of attack.

Spoofing attacks
In this mode of attack, an adversary performs a direct attack against specified routing protocols therefore targeting the information that is being routed in the network between different nodes, this may allow the adversary to create routing loops which degrades the network by increasing latency or even partition it completely.

Eavesdropping and monitoring.
This occurs by an adversary secretly listening or snooping on the data in a network. This may allow the attacker to acquire the information being transferred over the network about the sensors or the message itself. This infringes on the privacy of the network therefore rendering the network insecure.

Conclusion
Security is a major concern in the design and implementation of Wireless sensor networks. This is due to the application of this type of network since a high level of security is required. In the above article the solution of the security issues are given despite the fact that research should be conducted in a continuous manner in order to come up with the best methods of eliminating challenges which arise with the implementation of these networks. The wireless sensor networks are still in the early stages of development and research is ongoing to bring forth the protocols that will curb the security threats in the network while maintaining the efficiency of all the network resources.

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Contents Abstract. 2 Introduction 2 Routing Protocols 6 Location based protocols 6 Data-Centric protocols 7 Attacks on wireless sensor networks 8 Sinkhole Attack 9 Wormhole attack 12 Denial of service attacks (DoS) 14 Sybil attacks 14 Other attacks on WSNs 16 Message Corruption 16 False Node attack 17 Node malfunction and node outage 17 Node Replication attack. 17 Passive information Gathering Attack 18 Spoofing attacks 18 Eavesdropping and monitoring. 18 Conclusion 18 References 20