  To establish secure (point-to-point and/or broadcast) communication channels
among the nodes of a wireless sensor network is a fundamental task. To this
end, a plethora of (socalled) key pre-distribution schemes have been proposed
in the past. All these schemes, however, rely on shared secret(s), which are
assumed to be somehow pre-loaded onto the sensor nodes. In this paper, we
propose a novel method for secure initialization of sensor nodes based on a
visual out-of-band channel. Using the proposed method, the administrator of a
sensor network can distribute keys onto the sensor nodes, necessary to
bootstrap key pre-distribution. Our secure initialization method requires only
a little extra cost, is efficient and scalable with respect to the number of
sensor nodes. Moreover, based on a usability study that we conducted, the
method turns out to be quite user-friendly and easy to use by naive human
users.
