Biblio

We propose a method to let a source and a destination agree on a key that remains secret to a potential eavesdropper in an underwater acoustic network (UWAN). We generate the key from the propagation delay measured over a set of multihop routes: this harvests the randomness in the UWAN topology and turns the slow sound propagation in the water into an advantage for the key agreement protocol. Our scheme relies on a route discovery handshake. During this process, all intermediate relays accumulate message processing delays, so that both the source and the destination can compute the actual propagation delays along each route, and map this information to a string of bits. Finally, via a secret key agreement from the information-theoretic security framework, we obtain an equal set of bits at the source and destination, which is provably secret to a potential eavesdropper located away from both nodes. Our simulation results show that, even for small UWANs of 4 nodes, we obtain 11 secret bits per explored topology, and that the protocol is insensitive to an average node speed of up to 0.5 m/s.

Underwater acoustic communications experiments often involve custom implementations of schemes and protocols for the physical and data link layers. However, most commercial modems focus on providing reliable or optimized communication links, rather than on allowing low-level reconfiguration or reprogramming of modulation and coding schemes. As a result, the physical layer is typically provided as a closed, non-reprogrammable black box, accessible by the user only through a specific interface. While software-defined modems would be the ultimate solution to overcome this issue, having access to the symbols transmitted by the modems using a proprietary modulation format already opens up a number of research opportunities, e.g., aimed at the cross-layer design and optimization of channel coding schemes and communication protocols. In this paper, we take the latter approach. We consider the commercial EvoLogics modem, driven by a custom firmware version that bypasses the channel coding methods applied by the modem, and allows the user to set the transmit bit rate to any desired value within a given set. This makes it possible to evaluate different coding schemes in the presence of different bit rates. Our results show that the custom firmware offers sufficient flexibility to test different configurations of the coding schemes and bit rates, by providing direct access both to correctly decoded and to corrupted symbols, which can be separated at the receiver for further processing. In addition, we show that the DESERT Underwater framework can also leverage the same flexibility by employing low-level physical layer access in more complex networking experiments.