A Key Agreement Algorithm for Securing Underwater Acoustic Communications

With the introduction of standards in underwater acoustic communications, protecting the content of packets sent underwater has become a pressing need. Since underwater devices can be compromised over long-term deployments, navies are reluctant to use encryption devices on the one hand; and on the other hand, they require secure communication without the use of pre-agreed secret keys for flexibility. To answer this demand, here we present a key agreement protocol to generate secret keys from the channel impulse response (CIR) between Alice and Bob. Considering the time-varying nature of the underwater acoustic CIR, our key generator is based on the parameters of the distribution of the random features that characterize the CIR rather than directly on the features themselves. Assuming CIR reciprocity, we estimate the CIR by transmitting probe signals between Alice and Bob, and synchronize the probe transmissions such that signals fly by each other while still respecting the half-duplex constraints of underwater acoustic modems. In turn, Alice and Bob's packets arriving to Eve, the attacker, expose different CIRs and possibly collide. Modeled simulation results show agreement between the keys generated by Alice and Bob, and a significant difference with respect to the keys obtained by Eve.