## 1 Introduction  
Security concerns are key issues when developing applications based on sensor networks to monitor and control critical infrastructures, such as water treatment and distribution, oil and gas pipelines,electrical power transmission and distribution, or large communication systems. The use of current SCADA (Supervisory Control And Data Acquisition) systems has already come into question as they are increasingly seen as extremely vulnerable to cyberwarfare/cyberterrorism attacks [1; 2]. In particular, security researchers are concerned about: (1) the lack of concern about security and authentication in the design, deployment and operation of existing SCADA networks; (2) the mistaken belief that SCADA systems have the benefit of security through obscurity through the use of specialized protocols and proprietary interfaces; (3) the mistaken belief that SCADA networks are secure because they are purportedly physically secured; and (4) the mistaken belief that SCADA networks are secure because they are supposedly disconnected from the Internet. The security of these SCADA systems is important because compromise or destruction of these systems would impact multiple areas of society. For example, a blackout caused by a compromised electrical SCADA system would cause financial losses to all the customers that received electricity from that source. There are two distinct threats to a modern SCADA system. The first is the threat of unauthorized access to the control software, be it human access or changes induced intentionally or accidentally by virus infections and other software threats residing on the control host machine. The second is the threat of packet access to the network segments hosting SCADA devices. In many cases, there is rudimentary or no security on the actual packet control protocol, so anyone who can send packets to the SCADA device can control it. In many cases SCADA users assume that a VPN is of sufficient protection and are unaware that physical access to SCADA-related network jacks and switches provides the ability to totally bypass all security on the control software and fully control those SCADA networks. These kinds of physical access attacks bypass firewall and VPN security and are best addressed by endpoint-to-endpoint authentication and authorization commonly provided in the non-SCADA world by in-device SSL or other cryptographic techniques. But encryption is not enough for sensor devices because resource restrictions would prevent strong encryption schemes, and low-cost side-channel attacks [3] would still easily reveal the private keys. The increased interest in SCADA vulnerabilities has resulted in vulnerability researchers discovering vulnerabilities in commercial SCADA software and more general offensive SCADA techniques presented to the general security community [4; 5]. In electric and gas utility SCADA systems, the vulnerability of the large installed base of wired and wireless serial communications links is addressed in some cases by applying bump-in-the-wire devices that employ authentication and Advanced Encryption Standard encryption rather than replacing all existing nodes. But most of the risks come from the limitations of the sensor nodes: (1) many nodes in the network have very limited resources; (2) pervasiveness implies that some nodes will be in non-controlled areas and are accessible to potential intruders; (3) all these sensor nodes and controlling computers are globally interconnected, allowing attacks to be propagated step by step from the more resource-constrained devices to the more secure servers with lots of private data. Ciphers and countermeasures often imply a need for more resources (more computation requirements, more power consumption, specific integrated circuits with careful physical design, etc. ), but usually this is not affordable for this kind of applications. Even if we impose strong requirements for any individual node to be connected to our network, it is virtually impossible to update hardware and software whenever a security flaw is found.