Protocols for Secure Communication in Wireless Sensor Networks

88 Chapter 3. A Security Model for Wireless Sensor Networks
large parts of the memory are used for storing application data such as sensor
readings and aggregated data. In principle though, deploying more memory
would be possible.
Full pairwise key distribution therefore seems to be a viable option for sensor
networks. There are, however, limiting factors:
• A static distribution of keys is inflexible as it does not allow to add more
nodes to the network later.
• Although a certain network size can be supported, the approach is nevertheless
not scalable to larger networks, say in the order of magnitude of
10 6 or larger
• Sensor nodes should be as small as possible, thus the amount of memory
that can be added is limited. Also, the operator of a sensor network probably
prefers using the available memory for application purposes and is
not willing to reserve a large part of the available resources for security
purposes.
• Although keys are available for every pair of nodes, most of them will
never be used, since a sensor node will interact with only a tiny fraction
of the nodes in the network during its lifetime. Thus, most of the memory
used for storing the keys is never used productively.
We conclude that up to a certain network size, full pairwise key distribution
is feasible if one is willing to dedicate a significant amount of memory to storing
keys. It is, however, not a generally applicable solution to the problem of
secure communication. The deployment of additional nodes during the lifetime
of the network is not well-supported. Most importantly, the approach does not
make efficient use of the available resources. For the most attractive use cases
of wireless sensor networks, where nodes are very small yet there is a large
number of them, this approach does not work.
3.5 Approximating End-to-End Security
As elaborated in the previous section, End-to-end security mechanisms achieve
two goals: (1) Secure matchmaking of communication partners and (2) secure
message exchange. Essentially, they guarantee that dishonest parties cannot
interefere with the communication of honest parties in any way.
One could relaxe these criteria to a certain degree and demand, for example,
that, say, 90% of all matchmakings occur between legitimate parties; or, that