Protocols for Secure Communication in Wireless Sensor Networks

6.3. Performance Evaluation 175
A
B
C
D
Figure 6.6: Aggregation tree
nodes C, D, and E, and forwards it to A. Usually, the message from B to A only
contains the aggregated data calculated by B, based on the input from B, C, D,
and E. This leaves A with the only choice of trusting that B sends correct data.
However, from time to time, A checks whether the calculation done by B is
correct. For doing so, it requests the following additional data:
• The original inputs from B, C, D, and E.
• Authentication codes from C, D, and E for their original inputs to B. Since
these nodes are 2-hop neighbours of A, creating such authentication codes
is possible.
This data allows A to check whether the calculation of B is correct by performing
the same calculation itself. If the result differs from what B has sent, this
should be interpreted as a sign for a potential ongoing attack. Additionally, B
may be considered for exclusion from further operation in the network, and the
aggregation tree rebuilt.
The times when A performs such checks must be chosen randomly to avoid
that B being able to predict them, which would obviously allow B to render
these checks ineffective. Also, the additional data that is required for checking
amounts to a significant transmission load, so the check should only be
done rarely. This constitutes, of course, a trade-off between efficiency and the
likelihood of catching a malicious node.
6.3 Performance Evaluation
The evaluation of Canvas is based on a comparison with a generic end-to-end
signature scheme. We compare the increase in time to delivery, which is due
to additional, e.g. cryptographic, operations, and the transmission delay for
additional data. We first look at the overhead for a single message. Then, we
consider the overhead for a sequence of messages, e.g. a sensor data stream.
E