Protocols for Secure Communication in Wireless Sensor Networks

128 Chapter 4. Key Establishment
The security of these key agreement schemes is based on the fact that no
central entity within the sensor network has complete knowledge of the basic
key pool. With a small set of captured nodes, an attacker has only a very
small probability of acquiring knowledge about the key material of other nodes.
However, with an increasing number of captured nodes, this probability rises
and allows the attacker to leverage the key material from her captured nodes
for breaking into the communication of other nodes, i.e. eavesdrop on links or
impersonate nodes.
We have discussed two basic key agreement schemes for wireless sensor
networks. For the q-composite scheme, a set of keys is distributed to each
node before deployment. Since all keys are drawn from a large, common pool
of keys, it is likely that any pair of nodes has a certain number of keys in
common. However, it is unlikely that any other pair of nodes shares the same
keys. Whenever two nodes want to establish a common key, they can use their
common keys to set up a link key, which is unique with a high probability.
The second scheme is based on hash chains. A set of hash chains is created in
advance. Each node is assigned a position on each of these chains. The oneway
property of hash functions makes it easy to compute the chain values at
positions in the chain that are located below a given position. Out of this, a key
agreement scheme can be constructed that provides comparable resilience as
the q-composite scheme, though at a higher memory complexity. Finally, we
have devised a novel approach that combines both key agreement schemes and
provides higher resilience than any one of them alone.
Key agreement is a prerequisite for secure communication between nodes,
both for ensuring the confidentiality and the authenticity of messages. As we
have discussed in Section 3.4, key agreement between remote nodes in a WSN
incurs a significant overhead that is often not justified given the dominant transient
communication patterns. Therefore, our goal is to leverage the existence
of shared keys on a local level, i.e. within a limited neighbourhood of a node, in
order to enable secure communication between remote nodes without incurring
the usual overhead. In the next chapter, we show how to achieve secure communication
over long distances by interleaved local message authentication. The
basic scheme will be extended by using few long-range security relationships
per node, which provides additional protection against certain attack patterns.
The presented schemes protect the integrity of messages and thus are effective
countermeasures against manipulation attacks.