  In the context of wireless sensor networks, the pairwise key distribution
scheme of Chan et al. has several advantages over other key distribution
schemes including the original scheme of Eschenauer and Gligor. However, this
offline pairwise key distribution mechanism requires that the network size be
set in advance, and involves all sensor nodes simultaneously. Here, we address
this issue by describing an implementation of the pairwise scheme that supports
the gradual deployment of sensor nodes in several consecutive phases. We
discuss the key ring size needed to maintain the secure connectivity throughout
all the deployment phases. In particular we show that the number of keys at
each sensor node can be taken to be $O(\log n)$ in order to achieve secure
connectivity (with high probability).
