B.1. Introduction

Public-key cryptography is not a solution to every security problem. Asymmetric routines are bulky and slow, and, in practice, augment symmetric cryptography by eliminating the need for prior secret establishment of keys between communicating parties. On a workstation of today’s computing technology, this is an interesting and acceptable breakthrough. A 1 GHz processor runs one public-key encryption or key-exchange primitive in tens to hundreds of milliseconds, using at least hundreds of kilobytes of memory. That is reasonable for most applications, given that the routines are invoked rather infrequently.

Now, imagine a situation, where many tiny computing nodes, called sensor nodes, are scattered in an area for the purpose of sensing some data and transmitting the data to nearby base stations for further processing. This transmission is done by short-range radio communications. The base stations are assumed to be computationally well-equipped, but the sensor nodes are resource-starved. Such networks of sensor nodes are used in many important applications including tracking of objects in an enemy’s area for military purposes and scientific, engineering and medical explorations like wildlife monitoring, distributed seismic measurement, pollution tracking, monitoring fire and nuclear power plants and tracking patients. In some cases, mostly for military and medical applications, data collected by sensor nodes need to be encrypted before transmitting to neighbouring nodes and base stations.

Evidently one has to resort to symmetric-key cryptography in order to meet the security needs in a sensor network. Appendix B provides an overview of some key exchange schemes suitable for sensor networks.