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.
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.
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.
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].
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.
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.
Two legitimate parties, referred to as Alice and Bob, wish to generate secret keys from the wireless channel in the presence of an eavesdropper, referred to as Eve, in order to use such keys for encryption and decryption. In general, the secret key rate highly depends on the coherence time of the channel. In particular, a straightforward method of generating secret keys in static environments results in ultra-low rates. In order to resolve this problem, we introduce a low-complexity method called induced randomness. In this method, Alice and Bob independently generate local randomness to be used together with the uniqueness of the wireless channel coefficients in order to enable high-rate secret key generation. In this work, two scenarios are considered: first, when Alice and Bob share a direct communication channel, and second, when Alice and Bob do not have a direct link and communicate through an untrusted relay. After exchanging the induced randomness, post-processing is done by Alice and Bob to generate highly-correlated samples that are used for the key generation. Such samples are then converted into bits, disparities between the sequences generated by Alice and Bob are mitigated, and the resulting sequences are then hashed to compensate for the information leakage to the eavesdropper and to allow consistency checking of the generated key bit sequences. We utilize semantic security measures and information-theoretic inequalities to upper bound the probability of successful eavesdropping attack in terms of the mutual information measures that can be numerically computed. Given certain reasonable system parameters this bound is numerically evaluated to be 2-31and 2-10.57in the first and the second scenario, respectively.
Preestablished secret keys are often used to encrypt and decrypt data in communication systems, which are not secure once the keys are compromised. One desirable method is to generate secret keys dynamically using correlated channel measurements. We explore this concept in underwater acoustic (UWA) channels, and present a protocol that can generate secret keys dynamically based on the channel frequency response (CFR) in orthogonal frequency-division multiplexing systems. The multi-bit quantization is carried out on the amplitude of each tone, and the Bose-Chaudhuri-Hocquenghem codes are used for information reconciliation. Part of the protocol was implemented in lake tests, and multiple data sets were collected. The lake test results verify that the amplitude of CFR can be used as a randomness source for key generation in UWA channels, and we also find the low correlation between the mutual channels of the legitimate users. Based on the lake test results, we incorporate two modules into the protocol for performance improvement. The first module employs the adaptively weighted probing signaling to increase the channel correlation, and the second module of block-sliced key verification is used to deal with channel dynamics and increase the key agreement probability. The simulation results demonstrate the improved performance of the enhanced protocol.