Hiding Data in Plain Sight: Undetectable Wireless Communications Through\n Pseudo-Noise Asymmetric Shift Keying

Undetectable wireless transmissions are fundamental to avoid eavesdroppers.\nTo address this issue, wireless steganography hides covert information inside\nprimary information by slightly modifying the transmitted waveform such that\nprimary information will still be decodable, while covert information will be\nseen as noise by agnostic receivers. Since the addition of covert information\ninevitably decreases the SNR of the primary transmission, key challenges in\nwireless steganography are: i) to assess the impact of the covert channel on\nthe primary channel as a function of different channel conditions; and ii) to\nmake sure that the covert channel is undetectable. Existing approaches are\nprotocol-specific, also we notice that existing wireless technologies rely on\nphase-keying modulations that in most cases do not use the channel up to its\nShannon capacity. Therefore, the residual capacity can be leveraged to\nimplement a wireless system based on a pseudo-noise asymmetric shift keying\n(PN-ASK) modulation, where covert symbols are mapped by shifting the amplitude\nof primary symbols. This way, covert information will be undetectable, since a\nreceiver expecting phase-modulated symbols will see their shift in amplitude as\nan effect of channel/path loss degradation. We first investigate the SER of\nPN-ASK as a function of the channel; then, we find the optimal PN-ASK\nparameters that optimize primary and covert throughput under different channel\ncondition. We evaluate the throughput performance and undetectability of PN-ASK\nthrough extensive simulations and on an experimental testbed based on USRP N210\nsoftware-defined radios. We show that PN-ASK improves the throughput by more\nthan 8x with respect to prior art. Finally, we demonstrate through experiments\nthat PN-ASK is able to transmit covert data on top of IEEE 802.11g frames,\nwhich are correctly decoded by an off-the-shelf laptop WiFi.\n