Cross-Layer Security in Wireless Sensor Networks - Security for Wireless Sensor Networks Using Identity-Based Cryptography - page 168

costs because of the reduced MAC computation. In this case, three different methods
for the selection of links to be secured have been proposed—proactive, reactive, and
straight. With the proactive selection, attempts are made to secure the links with all the
physical neighbors in the shared key and path key discovery phases. With the reactive
selection, attempts are made to secure only the links with floating neighbors in shared
key discovery. If all of them cannot be secured, then the path key discovery process is
applied for the remaining floating neighbors, just one time. Straight selection only con-
siders the neighbors having shared keys discovered in the shared key discovery phase,
and path key discovery does not run in this mode.
8.5.2 Pairwise Key Setup with ECDH
The cross-layer implementation of ECDH key exchange with the clustering protocol
does not require additional messaging for the clustering protocol. The implementation
is simple since for a pair of sensor nodes the transaction only requires the exchange of
public keys Pu (identity in case of ID-based cryptography) to calculate the shared key
for that link. A first broadcasts the polling message for neighbor discovery, which also
includes its public key Pu A . Then all the receiving neighbors n i ( i = 1, 2, . . . , d ) include
their public keys Pu i in their reply messages. The bulk of the cost of this operation is the
point multiplication of the neighbor public key with the private key to calculate the link
key for each neighbor sensor node.
We assume that the public keys are calculated offline and loaded to sensor nodes
before deployment. This operation saves the one-point multiplication operation in each
sensor node needed for calculating the public key from the private key. Because the links
to be secured are controlled by the clustering protocol, there may be no need for A to
establish a secure link with all of its neighbors for acceptable network connectivity per-
formance. Here, we can apply reactive and proactive link selection methods for ECDH,
as well. For the proactive selection, attempt is made to secure all the possible links among
the neighbors, which will provide the same connectivity performance of the clustering
protocol without security implementation. With the reactive selection, attempt is only
made to secure the links with floating neighbors. Because the link key probability is 1 for
the ECDH, if the network density is selected as low as possible then the computational
cost could be reduced accordingly.
8.6 Open Challenges
In recent times, we have seen the emergence of cross-layer proposals in WSN. However,
open challenges exist in the smooth functioning of a few cross-layer design schemes.
Since cross-layer schemes are designed by individuals with specialized skills, the coexis-
tence of different cross-layer proposals with one another in a real-time environment is
considered to be an open challenge. In addition, care should be taken that a given cross-
layer design scheme won't stifle innovation in the future. In the following sections, we
discuss some of the challenges in detail.