Protocols for Secure Communication in Wireless Sensor Networks

50 Chapter 2. Wireless Sensor Networks and Their Security
of nodes and eventually in the inoperability of the overall network. Such disturbances
may not be easily identified as malicious interferences, and therefore
no effective countermeasures exist. An attentive observer may notice a higher
failure rate than expected, but still the cause may remain obscure.
While battery exhaustion attacks are effective, see [170], they require some
sophistication on behalf of the adversary and are time-consuming. For example,
the attacker has to match the activity cycle of the victim device. For an
immediate effect, more radical approaches are required, such as the physical
destruction of nodes or jamming the communication channels. However, most
of these attacks can be easily detected by those nodes being unaffected.
For example, if the attacker is jamming a region of the network, nodes within
this region cannot receive messages anymore. However, they will notice that
they are being jammed and may be able to issue messages reporting the attack.
Nodes at the border of the jammed region pick these messages up. They can
then further report the attack to the operator, and they can set up paths for
routing messages around the jammed region such that the operation of the rest
of the network is not affected. If jammed nodes are not able to send anything,
the nodes at the border will have to assume that some of their neighbours have
failed. A geographic routing mechanism will then automatically start to route
messages around that dead area [201, 198].
Jamming and physical destruction of nodes are simple forms of denial-ofservice
attacks that can be compensated for if they appear only on a small scale,
i.e. if only a small area is affected. Sometimes, the effects of a denial-of-service
attack can be mitigated if nodes are able to extend their sleep cycle when they
notice that an attack is going on, such that they conserve as much power as
possible. This would make the sensor network inoperable during the attack,
but at least it can continue operation once the attack has ceased. Of course, if
an exhaustion attack is successfully executed on a large scale, affecting large
areas, there is no possiblity to recover other than by deploying new nodes after
destruction.
2.8 Cryptography for Sensor Networks
Cryptography plays an important role in securing networked computer systems.
It provides the basic functionality for protecting the confidentiality, integrity,
and authenticity of messages and data. Here, we present the cryptographic
building blocks that will be important in later chapters. Their main applications
will be key agreement and message authentication. We include a separate
section where general issues in key management are discussed, which will not