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IEEE Journal of Oceanic Engineering

Secret Underwater Acoustic Key Generation Challenged by Eve's Simulator

Seçkin Yldrm, Konstantinos Pelekanakis, George Sklivanitis, Dimitris A. Pados, Pietro Paglierani, Roberto Petroccia, João Alves, Francesco Maria Molfese, Francesca Cuomo

Published Feb 14, 2024• 11 citations
Publisher

Abstract

Symmetric cryptography is the de-facto approach to information security for ad-hoc underwater acoustic networks. The requirement is that all network nodes possess the same cryptographic keys and key distribution that happens offline. Thus, it is hardly possible to include a new keyless node during network operation. In addition, online key replenishment is an open problem. In this article, we tackle key distribution/replenishment by formulating a new protocol that generates a key between two trusted underwater acoustic nodes (Alice and Bob) without the need to exchange the key itself. The proposed protocol initially extracts common acoustic channel features between Alice and Bob after some signal exchanges. Then, each party uses these features to generate his/her own secret bits via a random sequence generator. To correct any differences in their secret bits, we propose two reconciliation methods that employ Reed–Solomon codes. At the last step of the protocol, a 256-bit cryptographic key is generated by feeding the reconciled secret bit string to the Secure Hash Algorithm 3. We evaluate the key generation rate of the proposed protocol with real acoustic data recorded from a littoral environment, in which Alice and Bob exchanged 915 channel probes over different link geometries. To assess security performance, we invent a passive eavesdropper, Eve, who is able to simulate the acoustic link between Alice and Bob. She uses her simulated channels and the same protocol to generate her keys. Our analysis demonstrates a tradeoff between the key generation rate of Alice and Bob and Eve's ability to extract the same key. Our best result shows that Alice and Bob can independently generate 567 keys out of 915 attempts while Eve does not have access to any of the keys.

DOI
10.1109/joe.2023.3281978

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Symmetric cryptography is the de-facto approach to information security for ad-hoc underwater acoustic networks. The requirement is that all network nodes possess the same cryptographic keys and key distribution that happens offline. Thus, it is hardly possible to include a new keyless node during network operation. In addition, online key replenishment is an open problem. In this article, we tackle key distribution/replenishment by formulating a new protocol that generates a key between two trusted underwater acoustic nodes (Alice and Bob) without the need to exchange the key itself. The proposed protocol initially extracts common acoustic channel features between Alice and Bob after some signal exchanges. Then, each party uses these features to generate his/her own secret bits via a random sequence generator. To correct any differences in their secret bits, we propose two reconciliation methods that employ Reed–Solomon codes. At the last step of the protocol, a 256-bit cryptographic key is generated by feeding the reconciled secret bit string to the Secure Hash Algorithm 3. We evaluate the key generation rate of the proposed protocol with real acoustic data recorded from a littoral environment, in which Alice and Bob exchanged 915 channel probes over different link geometries. To assess security performance, we invent a passive eavesdropper, Eve, who is able to simulate the acoustic link between Alice and Bob. She uses her simulated channels and the same protocol to generate her keys. Our analysis demonstrates a tradeoff between the key generation rate of Alice and Bob and Eve's ability to extract the same key. Our best result shows that Alice and Bob can independently generate 567 keys out of 915 attempts while Eve does not have access to any of the keys.
We propose a strategy for generating the same crypto-key between two trusted underwater acoustic nodes (Alice and Bob) without revealing it to an eavesdropper (Eve). Our work builds upon the results of [1] where a methodology for generating a string of bits for Alice, Bob and Eve based on channel feature extraction and quantization is discussed. In this paper, we aim to reconcile the respective bits of Alice and Bob while minimizing the information leaked to Eve. To this end, we examine various Reed Solomon (RS) codes and measure the reconciliation rate of Alice, Bob and Eve. Additionally, we propose the Secure Hash Algorithm-3 (SHA-3) as means to eliminate any information that Eve acquires during reconciliation. We evaluate our reconciliation and privacy amplification strategies with bits generated from real underwater acoustic channel probe exchanges between Alice and Bob and Bellhop-simulated channels for Eve. Our analysis confirms that appropriate combinations of channel features and RS codes lead to a computationally secure generation of a 256-bit crypto-key according to the principles of the National Institute of Standards and Technology (NIST), even if Eve is informed about the RS encoder and the SHA-3 function.
With the growing use of underwater acoustic communications and the recent adoption of standards in this field, it is becoming increasingly important to secure messages against eavesdroppers. In this paper, we focus on a physical-layer security solution to generate sequences of random bits (keys) between two devices (Alice and Bob) belonging to an underwater acoustic network (UWAN); the key must remain secret to a passive eavesdropper (Eve) not belonging to the UWAN. Our method is based on measuring the propagation delay of the underwater acoustic channel over multiple hops of the UWAN: this harvests the randomness in the UWAN topology and turns the slow sound propagation in water into an advantage against eavesdropping. Our key generation protocol includes a route discovery handshake, whereby all UWAN devices at intermediate hops accumulate their message processing delays. This enables Alice and Bob to compute the actual propagation delays along each route and to map such information to a sequence of bits. Finally, from these bit sequences, Alice and Bob obtain a secret key. We analyze the performance of the protocol theoretically and assess it via extensive simulations and field experiments.
During the Rapid Environmental Picture 2018 (REP18) sea trial, two underwater acoustic nodes (Alice and Bob) exchanged 897 channel probes over different ranges and environmental conditions. In this short paper, Alice and Bob independently process their received probes offline with the aim to generate a cryptographic key based on Physical Layer Security (PLS). Using their estimated Channel Impulse Responses (CIRs), they compute and quantize four pre-agreed channel features. Eve is a simulated eavesdropper who is aware of the PLS algorithm, the 3D positions of Alice and Bob and the acoustic properties of the environment. Eve uses the de facto standard Bellhop acoustic simulator to predict the bi-directional CIRs between Alice and Bob and compute her own quantized features. We calculate the Bit Disagreement Ratio (BDR), which is a function of the number of disagreeing bits between a pair of nodes. Our results confirm that the proposed features are robust enough to yield a lower BDR between Alice and Bob than that for Eve. The BDR impact on reconciliation and secret key generation is studied in a subsequent paper [1].
Physical layer security (PLS) can be used to pro-vide keyless and innately secure communications by exploiting the propagation characteristics of the wireless channel at the physical layer (PHY) layer to co-generate cipher keys for symmetric encryption. While existing methods use channel state information as input for secret key generation, we consider a novel approach that derives features from frequency-time domain representation of in-phase and quadrature (IQ) samples to derive initial keys with high agreement rate. A recurrent neural network (RNN) with a quadruplet loss function is trained based on spectrograms collected from bi-directional time-division duplex channel probing between the two parties and on spectrograms collected from a passive eavesdropper. Information reconciliation is achieved using Reed Solomon codes. At the last step of the protocol, a 512-bit cryptographic key is generated by feeding the reconciled bit sequence to the Secure Hash Algorithm 3. Our experiments with software-defined radios in both indoor and outdoor environments evaluate the proposed key generation system under different channel propagation environments and hardware inaccuracies. We demonstrate that the proposed RNN-assisted channel feature extractor trained on indoor channel probe data operates successfully in two outdoor setups and produces up to 70 amplified keys out of 100 probes exchanged between two trusted parties i.e., Alice and Bob, while the passive eavesdropper fails to directly predict any of the generated keys.
Many military and civil applications using underwater acoustic channels (UWACs) are paired with security services, providing, for instance, secrecy and authenticity to the communication.These mechanisms however often require periodically renewed symmetric keys among two communicating devices, namely Alice and Bob.To this end, secret keys can be agreed upon using the physicallayer communication channel, leveraging, in particular, the reciprocity and randomness of the UWAC to extract a common key that remains secret to an attacker (Eve).In this paper, we propose a novel solution for the advantagedistillation part of the secret key generation procedure.After channel probing, the advantage of distillation lets Alice and Bob extract a sequence of bits from their own (analog) measurements.We propose an asymmetric advantage-distillation protocol with two novel features: i) the quantizers used by Alice and Bob are derived maximizing the secret key capacity cooperating through a public channel and ii) Alice transmits a correction over a public authenticated side channel.Numerical results prove the effectiveness of our approach, showing that exchanging a few bits of information before information reconciliation allows extracting more secret bits from the channel.
This work presents a novel method to generate secret keys shared between a legitimate node pair (Alice and Bob) to safeguard the communication between them from an unauthorized node (Eve). To this end, we exploit the reciprocal carrier frequency offset (CFO) between the legitimate node pair to extract common randomness out of it to generate shared secret keys. The proposed key generation algorithm involves standard steps: the legitimate nodes exchange binary phase-shift keying (BPSK) signals to perform blind CFO estimation on the received signals, and do equi-probable quantization of the noisy CFO estimates followed by information reconciliation-to distil a shared secret key. Furthermore, guided by the Allan deviation curve, we distinguish between the two frequency-stability regimes-when the randomly time-varying CFO process i) has memory, ii) is memoryless; thereafter, we compute the key generation rate for both regimes. Simulation results show that the key disagreement rate decreases exponentially with increase in the signal to noise ratio of the link between Alice and Bob. Additionally, the decipher probability of Eve decreases as soon as either of the two links observed by the Eve becomes more degraded compared to the link between Alice and Bob.