Covert Adversaries

Yuchen Zhang, Sa Xiao, Jianquan Wang +2 more

In this paper, we investigate covert communications under multiple-antenna eavesdropping, and explore the impact of the channel state information (CSI) at the adversary on the covertness of the system with noise uncertainty. We first analyze the detection performance of the adversary without the CSI of the eavesdropping channel. Specially, the generalized likelihood ratio (GLR) detection is adopted to handle this scenario. Then, a more practical scenario with the uncertain noise power at the adversary is considered, and the covertness of the system is analyzed. In addition, the optimal transmit power at the legitimate transmitter is derived theoretically to maximize the covert throughput. Finally, we present numerical results to compare the covert throughputs when the adversary knows or doesn't know the CSI. The results demonstrate the significance of unknown CSI at the adversary when considering the covert throughput of the system.

Carmit Hazay, Yehuda Lindell

Peter Schöll, Mark Simkin, Luisa Siniscalchi

Multiparty computation protocols (MPC) are said to be secure against covert adversaries if the honest parties are guaranteed to detect any misbehavior by the malicious parties with a constant probability. Protocols that, upon detecting a cheating attempt, additionally allow the honest parties to compute certificates, which enable third parties to be convinced of the malicious behavior of the accused parties, are called publicly verifiable. In this work, we make several contributions to the domain of MPC with security against covert adversaries. We identify a subtle flaw in a protocol of Goyal, Mohassel, and Smith (Eurocrypt 2008), meaning that their protocol does not allow to identify a cheating party, and show how to fix their original construction to obtain security against covert adversaries. We present generic compilers that transform arbitrary passively secure preprocessing protocols, i.e. protocols where the parties have no private inputs, into protocols that are secure against covert adversaries and publicly verifiable. Using our compiler, we construct the first efficient variants of the BMR and the SPDZ protocols that are secure and publicly verifiable against a covert adversary that corrupts all but one party, and also construct variants with covert security and identifiable abort. We observe that an existing impossibility result by Ishai, Ostrovsky, and Seyalioglu (TCC 2012) can be used to show that there exist certain functionalities that cannot be realized by parties, that have oracle-access to broadcast and arbitrary two-party functionalities, with information-theoretic security against a covert adversary.

Carmit Hazay, Yehuda Lindell

Ramin Soltani, Dennis Goeckel, Don Towsley +2 more

Covert communication conceals the transmission of the message from an attentive adversary. Recent work on the limits of covert communication in additive white Gaussian noise channels has demonstrated that a covert transmitter (Alice) can reliably transmit a maximum of O(√n) bits to a covert receiver (Bob) without being detected by an adversary (Warden Willie) in n channel uses. This paper focuses on the scenario where other “friendly” nodes distributed according to a two-dimensional Poisson point process with density m are present. We propose a strategy where the friendly node closest to the adversary, without close coordination with Alice, produces artificial noise. We show that this method allows Alice to reliably and covertly send O(min{n, mγ/2√n}) bits to Bob in n channel uses, where γ is the path-loss exponent. We also consider a setting where there are Nw collaborating adversaries uniformly and randomly located in the environment and show that in n channel uses, Alice can reliably and covertly send O(min{n, (mγ/2√n/Nwγ)}) bits to Bob when γ>2, and O(min{n, (m√n/Nw2log2Nw)}) when γ=2. Conversely, we demonstrate that no higher covert throughput is possible for γ>2.

Lin Zhou, Lin Bai