We address the problem of selective jamming attacks in wireless networks. In these attacks, the adversary selectively targets specific packets of "high" importance by exploiting his knowledge on the implementation details of network protocols at various layers of the protocol stack. We illustrate the impact of selective jamming on the network performance by illustrating various selective attacks against the TCP protocol. We show that such attacks can be launched by performing real-time packet classification at the physical layer. We examine the combination of cryptographic primitives with physical layer attributes for preventing real-time packet classification and neutralizing the inside knowledge of the attacker.