Wireless networks rely on the uninterrupted availability of the wireless medium to interconnect participating nodes. The open nature of this medium leaves it vulnerable to multiple security threats. Anyone with a transceiver can eavesdrop on wireless transmissions, inject spurious messages, or jam legitimate ones. While eaves- dropping and message injection can be prevented using cryptographic methods, jamming attacks are much harder to counter . They have been shown to actualize severe Denial-of-Service (DoS) attacks against wireless networks. In the simplest form of jamming, the adversary interferes with the reception of messages by transmitting a continuous jamming signal, or several short jamming pulses. We present a packet-hiding scheme based on cryptographic puzzles. The main idea behind such puzzles is to force the recipient of a puzzle execute a predefined set of computations before he is able to extract a secret of interest. The time required for obtaining the solution of a puzzle depends on its hardness and the computational ability of the solver. The advantage of the puzzle-based scheme is that its security does not rely on the PHY-layer parameters. In this paper the problem of selective jamming attacks in wireless networks has been addressed and considered an internal adversary model in which the jammer is part of the network under attack, thus being aware of the protocol specifications and shared network secrets. Showed that the jammer can classify transmit ted packets in real time b y decoding the first few symbols of an ongoing transmission.