Protocols for Secure Communication in Wireless Sensor Networks

156 Chapter 5. Multipath Communication
Threshold security schemes can cope with up to k compromised parties,
which could be represented by nodes, links, or paths, for example. A (n,t)threshold
scheme is defined by the number of shares n and the minimum number
t of shares that are necessary and sufficient to reconstruct the secret d. The
security of the scheme relies on its properties: (1) less than t parties cannot
construct d (2) any set of t distinct shares is sufficient to construct d.
As shown in [56], 2k+1 disjoint paths are required to obtain secure message
transmission when faced with a k-bounded attacker who completely controls k
paths, i.e. can read and write messages at will on these paths. Intuitively, this
result is obtained by encoding and splitting up a message such that (1) k + 1
correct parts are sufficient and necessary to reconstruct the message and (2) no
information about the message can be learned by reading k or less parts.
When using multipath routing in order to achieve a security goal, such as
confidentiality or integrity, one must not only think about encoding and routing
the messages themselves. It is also important to consider the possibility
that the adversary tries to manipulate the process by which the used paths are
constructed. If the adversary manages to convince a sender that multiple disjoint
paths exist while in fact there is only one path that is controlled by the
adversary, the sender cannot communicate securely. This issue is addressed
in [10], where an algorithm is described for constructing edge-disjoint paths
that is itself resilient against attacks.
5.6 Summary
Establishing short multiple disjoint paths in a network may be hard, thus we
devised a method that yields longer paths but makes path set-up very easy. Our
method has the additional advantage that the constructed paths are spatially
separated, which can be an advantage in certain attack scenarios.
In general, establishing disjoint paths involves a trade-off between set-up
complexity and path length. In order to minimize path lengths, a set-up procedure
is required that involves complex message exchanges and keeping local
state information. On the other hand, a simple procedure can be used if longer
path lengths are acceptable.