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arXiv:1812.02245

Revisiting Deniability in Quantum Key Exchange via Covert Communication and Entanglement Distillation

Arash Atashpendar, Guru-Vamsi Policharla, Peter Browne Rønne, Peter Y. A. Ryan

Published Dec 5, 2018• 2 citations
arXivHTMLPDF

Abstract

We revisit the notion of deniability in quantum key exchange (QKE), a topic that remains largely unexplored. In the only work on this subject by Donald Beaver, it is argued that QKE is not necessarily deniable due to an eavesdropping attack that limits key equivocation. We provide more insight into the nature of this attack and how it extends to other constructions such as QKE obtained from uncloneable encryption. We then adopt the framework for quantum authenticated key exchange, developed by Mosca et al., and extend it to introduce the notion of coercer-deniable QKE, formalized in terms of the indistinguishability of real and fake coercer views. Next, we apply results from a recent work by Arrazola and Scarani on covert quantum communication to establish a connection between covert QKE and deniability. We propose DC-QKE, a simple deniable covert QKE protocol, and prove its deniability via a reduction to the security of covert QKE. Finally, we consider how entanglement distillation can be used to enable information-theoretically deniable protocols for QKE and tasks beyond key exchange.

DOI
10.48550/arxiv.1812.02245

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This poster describes ongoing work on deniability in quantum cryptography, an area of research that remains almost entirely unexplored in the quantum information processing literature. Deniability is a well-known and fundamental concept in classical cryptography and it can be defined as the ability for the sender of a message to deny the contents of a message or the very act of having participated in an exchange, e.g. having sent the said message. We discuss deniability in the context of quantum key exchange and address a particular problem, first discovered by Donald Beaver, where he claims that all QKD protocols are undeniable. The claim is that while we do get a one-time pad (OTP) using QKD, it does not provide the property of key equivocation as it is expected in the Shannon sense for a OTP. Intuitively, this difficulty lies in the quantum channel alone and it has to do with the fact that in QKD, while we generate entropy by expanding an initially short pre-shared key into an arbitrary longer secret key, we do so by exchanging information over a quantum as well as a classical channel, which could potentially leave a binding transcript of Alice's decisions to the final secret key. This is in contrast with the implicit assumption that Eve knows nothing about how two given parties have established their shared OTP in the first place. We discuss the importance of deniability in cryptography and its wide range of applications, along with cryptographic primitives other than key exchange where deniability might be a desired property. Finally, we present a series of fundamental open questions in this area of research and discuss quantum cryptographic primitives that lend themselves to devising deniable protocols.
Quantum cryptography is known the most up-to-date in domain of realistic cryptography notably the menace of quantum cryptanalysis which threatens security firmness of public key cryptography. Quantum cryptography has a famous scheme known as Quantum Key Exchange (QKE), that administrates generation and distribution of a secured random key between legitimate channel users depicted as sender and receiver. Consequently, such key could be used as a key for one-time pad hybrid cryptosystems to encrypt and authenticate messages over a quantum channel. (QKE) is based on unifying quantum physics concepts and information theory with conventional cryptographic schemes that target to produce a short secret session key between any two legitimate parties. An important phase in key creation of BB84 protocol is Privacy Amplification phase where two interconnecting parties distill highly secret shared key from a larger body of shared key, which is only partially secret. The two legitimate parties publicly exchange information to create a compressed key free from biased bits known by an eavesdropper.
Quantum cryptography is known the most up-to-date in domain of realistic cryptography notably the menace of quantum cryptanalysis which threatens security firmness of public key cryptography. Quantum cryptography has a famous scheme known as Quantum Key Exchange (QKE), that administrates generation and distribution of a secured random key between legitimate channel users depicted as sender and receiver. Consequently, such key could be used as a key for one-time pad hybrid cryptosystems to encrypt and authenticate messages over a quantum channel. (QKE) is based on unifying quantum physics concepts and information theory with conventional cryptographic schemes that target to produce a short secret session key between any two legitimate parties. An important phase in key creation of BB84 protocol is Privacy Amplification phase where two interconnecting parties distill highly secret shared key from a larger body of shared key, which is only partially secret. The two legitimate parties publicly exchange information to create a compressed key free from biased bits known by an eavesdropper.
A deniable authentication protocol enables a receiver to identify the true source of given message, but not to prove the identity of the sender to a third party.This property is very useful for providing secure negotiation over the Internet.Recently, Shi et al. proposed a quantum deniable authentication protocol based on the property of unitary transformation and quantum one-way function.A trusted center (TC) is introduced in their protocol, and a much amount of quantum resources, such as entanglement, quantum memory and quantum one-way function, are required.In this paper, a deniable authentication protocol without a trust center is proposed based on the quantum key distribution (QKD).In the presented protocol, the QKD is used firstly to share a secret key between the sender and the receiver.Then, the shared key is used to identify the true source of sender's message by the receiver.Finally, the simulation based proof shows that the receiver cannot prove the identity of the sender to any third party.
We revisit the problem of entanglement-assisted covert communication over bosonic channels and show that thebenefits\text{benefits}benefitsof entanglement can be achieved with fewer entanglement resources than previously identified. Specifically, we show thatO(nlog⁡n)\mathcal{O}(\sqrt{n}\log n)O(n​logn)covert and reliable bits can be exchanged usingω(n)∩o(n)\omega(\sqrt{n})\cap o(n)ω(n​)∩o(n)Two-Mode Squeezed-Vacuum (TMSV) pairs andω(1)∩o(n)\omega(1)\cap o(\sqrt{n})ω(1)∩o(n​)secret-key bits. The conceptual approach behind the result is to combine 1) soft-covering and secret-key resources as a coordination mechanism and 2) superposition coding in the form of a two-layer On-Off Keying (OOK) and Phase Shift Keying (PSK) to index channel uses in which TMSV pairs are encoded. This approach is related to the idea of quantum trade-off coding, specialized and extended to the covert communication setting. Our technical contribution is to develop one-shot bounds then specialized to bosonic channels.