dvances in wireless communication and electronics have enabled the development of low-cost, lowpower, multifunctional sensor nodes. These tiny sensor nodes, consisting of sensing, data processing, and communication components, make it possible to deploy Wireless Sensor Networks (WSNs), which represent a significant improvement over traditional wired sensor networks. WSNs can greatly simplify system design and operation, as the environment being monitored does not require the communication or energy infrastructure associated with wired networks [1] . WSNs are expected to be solutions to many applications, such as detecting and tracking the passage of troops and tanks on a battlefield, monitoring environmental pollutants, measuring traffic flows on roads, and tracking the location of personnel in a building. Many sensor networks have mission-critical tasks and thus require that security be considered [2, 3] . Improper use of information or using forged information may cause unwanted information leakage and provide inaccurate results. While some aspects of WSNs are similar to traditional wireless ad hoc networks, important distinctions exist which greatly affect how security is achieved. The differences between sensor networks and ad hoc networks are [4]: • The number of sensor nodes in a sensor network can be several orders of magnitude higher than the nodes in an ad hoc network. • Sensor nodes are densely deployed. • Sensor nodes are prone to failures due to harsh environments and energy constraints. • The topology of a sensor network changes very frequently due to failures or mobility. • Sensor nodes are limited in computation, memory, and power resources. • Sensor nodes may not have global identification. These differences greatly affect how secure data-transfer schemes are implemented in WSNs. For example, the use of radio transmission, along with the constraints of small size, low cost, and limited energy, make WSNs more susceptible to denial-of-service attacks [5]. Advanced anti-jamming techniques such as frequency-hopping spread spectrum and physical tamper-proofing of nodes are generally impossible in a sensor network due to the requirements of greater design complexity and higher energy consumption [5]. Furthermore, the limited energy and processing power of nodes makes the use of public key cryptography nearly impossible. While the A YONG WANG, GARHAN ATTEBURY, AND BYRAV RAMAMURTHY UNIVERSITY OF NEBRASKA-LINCOLN Wireless Sensor Networks (WSNs) are used in many applications in military, ecological, and health-related areas. These applications often include the monitoring of sensitive information such as enemy movement on the battlefield or the location of personnel in a building. Security is therefore important in WSNs. However, WSNs suffer from many constraints, including low computation capability, small memory, limited energy resources, susceptibility to physical capture, and the use of insecure wireless communication channels. These constraints make security in WSNs a challenge. In this article we present a survey of security issues in WSNs. First we outline the constraints, security requirements, and attacks with their corresponding countermeasures in WSNs. We then present a holistic view of security issues. These issues are classified into five categories: cryptography, key management, secure routing, secure data aggregation, and intrusion detection. Along the way we highlight the advantages and disadvantages of various WSN security protocols and further compare and evaluate these protocols based on each of these five categories. We also point out the open research issues in each subarea and conclude with possible future research directions on security in WSNs. In addition to key distribution, secure routing protocols must be considered. These protocols are concerned with how a node sends messages to other nodes or a base station. A key challenge is that of authenticated broadcast. Existing authenticated broadcast methods often rely on public key cryptography and include high computational overhead making them infeasible in WSNs. Secure routing protocols proposed for use in WSNs, such as SPINS [8] , must consider these factors. Additionally, the constraint on energy in WSNs leads to the desire for data aggregation. This aggregation of sensor data needs to be secure in order to ensure information integrity and confidentiality [9, 10] . While this is achievable through cryptography, an aggregation scheme must take into account the constraints in WSNs and the unique characteristics of the cryptography and routing schemes. It is also desirable for secure data aggregation protocols to be flexible, allowing lower levels of security for less important data, thus saving energy, and allowing higher levels of security for more sensitive data, thus consuming more energy. As with any network, awareness of compromised nodes and attacks is desirable. Many security schemes provide assurance that data remain intact and communication unaffected as long as fewer than t nodes are compromised [11] . The ability of a node or base station to detect when other nodes are compromised enables them to take action, either ignoring the compromised data or reconfiguring the network to eliminate the threat. The remainder of this article discusses the above areas in more detail and considers how they are all required to form a complete WSN security scheme. A few existing surveys on security issues in ad hoc networks can be found in [12] [13] [14] ; however, only small sections of these surveys focus on WSNs. A recent survey article on security issues in mobile ad hoc networks also included an overview of security issues in WSNs [15] . However, the article did not discuss cryptography and intrusion detection issues. Further, it included only a small portion of the available literature on security in WSNs. The rest of this article is organized as follows. Background information on WSNs is presented, followed by a discussion of attacks in the different network layers of sensor networks. Then we focus on the selection of cryptography in WSNs, key management, secure routing schemes, secure data aggregation, and intrusion detection systems. We discuss future research directions on security in WSNs and then conclude the article.
A taxonomy of key management protocols in WSNs is described in REF .
A survey of security issues in wireless sensor networks
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