Protocols for Secure Communication in Wireless Sensor Networks

172 Chapter 6. Integrity-Preserving Communications
codes is straightforward. A node receiving a message also receives authentication
codes for the next (k − 1) hops, so the following hops on the path are
already determined. The current node therefore has to select only the kth hop
following the current node on the path. An authentication code for that node is
added to the message before it is forwarded along the path. Thereby, event and
query messages are being Canvas-authenticated. Establishing a random path
using Canvas requires a “lookahead” of k hops instead of one. As the paths
are set up randomly in any case, there is no fundamental difference. Thus, the
functionality of such paths is not affected by choosing randomly the kth-next
hop instead of the immediate next hop.
A second, query-driven, approach to content-based routing is to set up gradients
from a message source to interested receivers, called “directed diffusion”
[86]. First, the query is flooded. When it reaches a matching source, the
source sets up gradients to those neighbours from which the query has been
received. These nodes do the same until a path from the source to the sink is
established. Gradients are used for selecting the optimal path, and for repairing
broken paths.
Canvas authentication for flooded queries can be performed as described in
the previous paragraph. The prospective message source receives possibly multiple
copies of the request and sets up gradients in the opposite direction. Note
that a message carries the node identifiers of the last k hops in the authentication
records. The source stores the last k hops for every gradient. This allows it
to do k-Canvas authentication for the event messages as they are being sent to
interested nodes.
Note that Canvas authentication alone does not make these protocols “secure”
per se. It may still be possible for an adversary to attack these protocols
on a different level than message authentication. For example, the event paths
set up for rumor routing may be misdirected in such a way as to minimize the
chance of them hitting query messages. Or, a node may set up bogus gradients,
thereby cluttering up the memory of other nodes. Message authentication alone
cannot prevent such attacks.
Geographic Routing
Geographic routing [91, 100] uses location information for addressing nodes
instead of identifiers. It is assumed that all nodes know their locations, and
the locations of the nodes in their immediate neighbourhood. It is straightforward
to extend this knowledge to the k-hop neighbourhood, which is required
for Canvas authentication. Routing proceeds in two modes: greedy and face