A Wireless Sensor Network (WSN) is a collection of spatially distributed autonomous sensor devices. Because of design and cost constraints, the sensor nodes are not equipped with tamper-resistant hardware. Due to the unattended nature, they are more exposed to physical captures by adversaries. One of the most vulnerable attacks in a sensor network is node replication attack where one or few replicas or clones are fabricated and are deployed into the network by the adversary. These replicas which bear the same identity as genuine nodes are employed in launching various kinds of insidious attacks which bring the entire network under the adversary's control.