Protocols for Secure Communication in Wireless Sensor Networks

220 Chapter 7. Conclusion
ments are overwhelming and such a system would be very much constrained in
its flexibility to accomodate extensions to the network.
Our approach to secure communication relies on the availability of shared
keys only between nodes that are in close proximity to each other. This limits
the overhead for key storage considerably. The Canvas scheme proposed in
this work requires that each node shares a key with each of its one- to k-hop
neighbours (k ≥ 2). In a typical deployment setting with k = 2, this would
require each node to store about 10 to 20 keys, which sensor nodes are well
capable of. A message that is about to be transmitted to a remote node is authenticated
by the source using at least two keys, which are shared with the
following nodes on the communication path. This requires only minor adjustments
on the routing layer, namely a look-ahead of k nodes on the routing path.
The same authentication pattern is repeatedly applied by all nodes on the path.
The protection provided by such a scheme is able to render single compromised
nodes ineffective. A message is authenticated by at least two authentication
codes, but only one of them can be manipulated by the compromised node,
thus any change in the message’s content would be discovered by the next node
on the path. At that point, the network would become aware of the attack –
something an attacker wants to avoid as this degrades the trust in the network,
which also degrades the value of the attack.
This interleaving of message authentication codes corresponds to creating
multiple independent authentication paths, i.e. paths on which authentication
information is passed. With k = 2, Canvas creates two such paths. Thus it is
able to accomodate compromised nodes on both paths that are not adjacent to
each other on the communication path. Canvas is similar to having two physically
disjoint communication paths in the regard that a single compromised
path cannot break the authentication. Additionally, Canvas gives the advantage
that both paths are interlinked and breaking both of them requires a certain
configuration of compromised nodes.
The Canvas scheme has a limited reach in that only isolated compromised
nodes can be countered. As soon as the attacker manages to subvert clusters
of nodes, the scheme becomes partially ineffective. Any message that passes
through a pair of compromised nodes would be subject to manipulation. In
order to counter certain attack patterns, it is therefore necessary to introduce
long-distance authentication relationships.