  ZigBee is a fairly new but promising wireless sensor network standard that
offers the advantages of simple and low resource communication. Nevertheless,
security is of great concern to ZigBee, and enhancements are prescribed in the
latest ZigBee specication: ZigBee-2007. In this technical report, we identify
an important gap in the specification on key updates, and present a methodology
for determining optimal key update policies and security parameters. We exploit
the stochastic model checking approach using the probabilistic model checker
PRISM, and assess the security needs for realistic application scenarios.
