Protocols for Secure Communication in Wireless Sensor Networks

40 Chapter 2. Wireless Sensor Networks and Their Security
assumed to be superior. This allows it to evaluate the security of an entity (or a
set of communicating entities) facing a powerful attacker. If no vulnerabilities
can be found during a test of an entity, or even a proof of their absence can be
found, this provides strong evidence of the entitiy’s security.
2.6.3 Network-based Simulation
A network-based simulation is completely independent from implementation
details. Nodes are modelled declaratively, i.e. their behaviour is described as
a relationship between incoming and outgoing messages. Messages are never
lost and are delivered instantly. The only characteristic of the communication
medium that is carried over to the model is the limited range of a wireless connection,
which determines the connectivity graph. Fluctuations in connectivity
are disregarded, though. These simplifications lead to a static graph model of
the sensor network, where the vertices represent the nodes, and the edges of the
graph mirror the communication links between neighbouring nodes.
A simulation based on this model is executed as a message exchange between
nodes. Some nodes create messages “spontaneously”, meaning we silently
assume that there is a good reason for some node to create a message. This
very reason is unimportant for our results. A message is transmitted over the
(wireless) communication medium. In most cases, we can assume a broadcast
medium, such as electro-magnetic waves. But a directed medium is possible,
such as light. A node that receives a message will either consume the message
or transform it and then relay it to other nodes. The exact operation depends on
the current state of the node. Possible states are, abstractly, correct, malicious,
failed. The fundamental attacker model includes the attacker’s capability to
take full control over all nodes where the attack succeeds (this is what is accomplished
by a “root kit” for computers connected to the Internet). Alternatives
are possible, for example partial control over a node. Depending on its state,
a receiving node transforms the message and relays it further to one or more
of its own neighbours. This process continues until some node consumes the
message without relaying it. The state of a node influences the transformation
of messages that are going through a node. In our case, the state mainly reflects
whether a node is controlled by the adversary or not. The state remains fixed
during a simulation run.
The major advantage of network-based simulations over node-level simulations
is scalability. Network-based simulations allow the simulation of much
larger sets of nodes, since most node-internal details are disregarded. Thus,
the state information being kept for each node is minimal. Also, possible in-