Wireless sensor networks (WSNs) have become a hot research topic in recent years. Applications include military, rescue, environment monitoring, and smart homes. The use of WSN has foreseen big changes in data gathering, processing, routing of information and dissemination for different environments and applications. A WSN is composed of hundreds or even thousands of small, cheap sensor nodes which communicate with one another wirelessly. Sensor nodes typically do not have very much computational power, limiting the kinds of networking protocols and security mechanisms they can employ. Because WSNs are composed of so many nodes, which are frequently deployed in open, unattended and hostile environments, they are vulnerable to attacks. The open nature of the wireless medium leaves it vulnerable to intentional interference attacks, typically referred to as jamming. We address the problem of selective jamming attacks in wireless networks. We show that selective jamming attacks can be launched by performing real-time packet classification at the physical layer. To reduce these attacks, we develop three schemes that prevent real-time packet classification by combining cryptographic primitives with physical-layer attributes. We analyze the security of our methods and evaluate their computational and communication overhead.