Key distribution is an important issue in wireless sensor network (WSN) design. It is a newly developing field due to the recent improvements in wireless communications.
Wireless sensor networks are networks of small, battery-powered, memory-constraint devices named sensor nodes, which have the capability of wireless communication over a restricted area [1]. Due to memory and power constraints, they need to be well arranged to build a fully functional network.
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Key predistribution is the method of distribution of keys onto nodes before deployment. Therefore, the nodes build up the network using their secret keys after deployment, that is, when they reach their target position.
Key predistribution schemes are various methods that have been developed by academicians for a better maintenance of key management in WSNs. Basically a key predistribution scheme has 3 phases:
During these phases, secret keys are generated, placed in sensor nodes, and each sensor node searches the area in its communication range to find another node to communicate. A secure link is established when two nodes discover one or more common keys (this differs in each scheme), and communication is done on that link between those two nodes. Afterwards, paths are established connecting these links, to create a connected graph. The result is a wireless communication network functioning in its own way, according to the key predistribution scheme used in creation.
There are a number of aspects of WSNs on which key predistribution schemes are competing to achieve a better result. The most critical ones are: local and global connectivity, and resiliency.
Local connectivity means the probability that any two sensor nodes have a common key with which they can establish a secure link to communicate.
Global connectivity is the fraction of nodes that are in the largest connected graph over the number of all nodes.
Resiliency is the number of links that cannot be compromised when a number of nodes(therefore keys in them) are compromised. So it is basically the quality of resistance against the attempts to hack the network. Apart from these, two other critical issues in WSN design are computational cost and hardware cost. Computational cost is the amount of computation done during these phases. Hardware cost is generally the cost of the memory and battery in each node.
There is a most-cited key predistribution scheme which is usually called "the main scheme" that introduced the ides of random key distribution, whereby the randomness factor drastically improves resiliency [2].
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