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

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.Comment: 16 pages, published in the proceedings of NordSec 201

The art of post-truth in quantum cryptography

L’établissement de clé quantique (abrégé QKD en anglais) permet à deux participants distants, Alice et Bob, d’établir une clé secrète commune (mais aléatoire) qui est connue uniquement de ces deux personnes (c’est-à-dire inconnue d’Ève et de tout autre tiers parti). La clé secrète partagée est inconditionnellement privée et peut être plus tard utilisée, par Alice et Bob, pour transmettre des messages en toute confidentialité, par exemple sous la forme d’un masque jetable. Le protocole d’établissement de clé quantique garantit la confidentialité inconditionnelle du message en présence d’un adversaire (Ève) limité uniquement par les lois de la mécanique quantique, et qui ne peut agir sur l’information que se partagent Alice et Bob que lors de son transit à travers des canaux classiques et quantiques. Mais que se passe-t-il lorsque Ève a le pouvoir supplémentaire de contraindre Alice et/ou Bob à révéler toute information, jusqu’alors gardée secrète, générée lors de l’exécution (réussie) du protocole d’établissement de clé quantique (éventuellement suite à la transmission entre Alice et Bob d’un ou plusieurs messages chiffrés classique à l’aide de cette clé), de manière à ce qu’Ève puisse reproduire l’entièreté du protocole et retrouver la clé (et donc aussi le message qu’elle a chiffré) ? Alice et Bob peuvent-ils nier la création de la clé de manière plausible en révélant des informations mensongères pour qu’Ève aboutisse sur une fausse clé ? Les protocoles d’établissement de clé quantiques peuvent-ils tels quels garantir la possibilité du doute raisonnable ? Dans cette thèse, c’est sur cette énigme que nous nous penchons. Dans le reste de ce document, nous empruntons le point de vue de la théorie de l’information pour analyser la possibilité du doute raisonnable lors de l’application de protocoles d’établissement de clé quantiques. Nous formalisons rigoureusement différents types et degrés de doute raisonnable en fonction de quel participant est contraint de révéler la clé, de ce que l’adversaire peut demander, de la taille de l’ensemble de fausses clés qu’Alice et Bob peuvent prétendre établir, de quand les parties doivent décider de la ou des clés fictives, de quelle est la tolérance d’Ève aux événements moins probables, et du recours ou non à des hypothèses de calcul. Nous définissons ensuite rigoureusement une classe générale de protocoles d’établissement de clé quantiques, basée sur un canal quantique presque parfait, et prouvons que tout protocole d’établissement de clé quantique appartenant à cette classe satisfait la définition la plus générale de doute raisonnable : à savoir, le doute raisonnable universel. Nous en fournissons quelques exemples. Ensuite, nous proposons un protocole hybride selon lequel tout protocole QKD peut être au plus existentiellement déniable. De plus, nous définissons une vaste classe de protocoles d’établissement de clé quantiques, que nous appelons préparation et mesure, et prouvons l’impossibilité d’instiller lors de ceux-ci tout degré de doute raisonnable. Ensuite, nous proposons une variante du protocole, que nous appelons préparation et mesure floues qui offre un certain niveau de doute raisonnable lorsque Ève est juste. Par la suite, nous proposons un protocole hybride en vertu duquel tout protocole d’établissement de clé quantique ne peut offrir au mieux que l’option de doute raisonnable existentiel. Finalement, nous proposons une variante du protocole, que nous appelons mono-déniable qui est seulement Alice déniable ou Bob déniable (mais pas les deux).Quantum Key Establishment (QKD) enables two distant parties Alice and Bob to establish a common random secret key known only to the two of them (i.e., unknown to Eve and anyone else). The common secret key is information-theoretically secure. Later, Alice and Bob may use this key to transmit messages securely, for example as a one-time pad. The QKD protocol guarantees the confidentiality of the key from an information-theoretic perspective against an adversary Eve who is only limited by the laws of quantum theory and can act only on the signals as they pass through the classical and quantum channels. But what if Eve has the extra power to coerce Alice and/or Bob after the successful execution of the QKD protocol forcing either both or only one of them to reveal all their private information (possibly also after one or several (classical) ciphertexts encrypted with that key have been transmitted between Alice and Bob) then Eve could go through the protocol and obtain the key (hence also the message)? Can Alice and Bob deny establishment of the key plausibly by revealing fake private information and hence also a fake key? Do QKD protocols guarantee deniability for free in this case? In this Thesis, we investigate this conundrum. In the rest of this document, we take an information-theoretic perspective on deniability in quantum key establishment protocols. We rigorously formalize different levels and flavours of deniability depending on which party is coerced, what the adversary may ask, what is the size of the fake set that surreptitious parties can pretend to be established, when the parties should decide on the fake key(s), and what is the coercer’s tolerance to less likely events and possibly also computational assumptions. We then rigorously define a general class of QKD protocols, based on an almost-perfect quantum channel, and prove that any QKD protocol that belongs to this class satisfies the most general flavour of deniability, i.e.,universal deniability. Moreover, we define a broad class of QKD protocols, which we call prepare-and-measure, and prove that these protocols are not deniable in any level or flavour. Moreover, we define a class of QKD protocols, which we refer to as fuzzy prepare-andmeasure, that provides a certain level of deniability conditioned on Eve being fair. Furthermore, we propose a hybrid protocol under which any QKD protocol can be at most existentially deniable. Finally, we define a class of QKD protocols, which we refer to as mono-deniable, which is either Alice or Bob (but not both) deniable

Deniable Key Establishment Resistance against eKCI Attacks

In extended Key Compromise Impersonation (eKCI) attack against authenticated key establishment (AKE) protocols the adversary impersonates one party, having the long term key and the ephemeral key of the other peer party. Such an attack can be mounted against variety of AKE protocols, including 3-pass HMQV. An intuitive countermeasure, based on BLS (Boneh–Lynn–Shacham) signatures, for strengthening HMQV was proposed in literature. The original HMQV protocol fulfills the deniability property: a party can deny its participation in the protocol execution, as the peer party can create a fake protocol transcript indistinguishable from the real one. Unfortunately, the modified BLS based version of HMQV is not deniable. In this paper we propose a method for converting HMQV (and similar AKE protocols) into a protocol resistant to eKCI attacks but without losing the original deniability property. For that purpose, instead of the undeniable BLS, we use a modification of Schnorr authentication protocol, which is deniable and immune to ephemeral key leakages

An arbitrated quantum signature scheme

The general principle for a quantum signature scheme is proposed and investigated based on ideas from classical signature schemes and quantum cryptography. The suggested algorithm is implemented by a symmetrical quantum key cryptosystem and Greenberger-Horne-Zeilinger (GHZ) triplet states and relies on the availability of an arbitrator. We can guarantee the unconditional security of the algorithm, mostly due to the correlation of the GHZ triplet states and the use of quantum one-time pads.Comment: 10 pages, no figures. Phys. Rev. A 65, (In press

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

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

A publicly verifiable quantum signature scheme based on asymmetric quantum cryptography

In 2018, Shi et al. \u27s showed that Kaushik et al.\u27s quantum signature scheme is defective. It suffers from the forgery attack. They further proposed an improvement, trying to avoid the attack. However, after examining we found their improved quantum signature is deniable, because the verifier can impersonate the signer to sign a message. After that, when a dispute occurs, he can argue that the signature was not signed by him. It was from the signer. To overcome the drawback, in this paper, we raise an improvement to make it publicly verifiable and hence more suitable to be applied in real life. After cryptanalysis, we confirm that our improvement not only resist the forgery attack but also is undeniable

A publicly verifiable quantum blind signature scheme without entanglement based on asymmetric cryptography

In recent years, several cryptographic scholars have proposed quantum blind signature schemes. However, their methods require the signatories and the inspectors to share common keys in advance, which makes them not only complicated in concept, but also suffering deniable problem. Moreover, due to the fact that not everyone can verify the blind signature, it needs to have a designated verifier. In view of Laurent, et al.’s argument that other than the assumption of the pre-image being collision-free, the one-way hash function is an attractive cryptographic component in the post-quantum era when designing a cryptosystem. Inspired by this, we propose a publicly verifiable quantum blind signature scheme based on the hash function. After security analyses, we confirm that our quantum blind signature not only is secure, but also have the needed properties. It includes anonymity, unforgeability, non-repudiation, blindness, public verifiability, and traceability. Hence, we conclude that this approach is better than the state-of-the-art, and is therefore more suitable for applications in real life, such as, mobile payments, quantum voting, or quantum government

Authentication and Key Management Automation in Decentralized Secure Email and Messaging via Low-Entropy Secrets

We revisit the problem of entity authentication in decentralized end-to-end encrypted email and secure messaging to propose a practical and self-sustaining cryptographic solution based on password-authenticated key exchange (PAKE). This not only allows users to authenticate each other via shared low-entropy secrets, e.g., memorable words, without a public key infrastructure or a trusted third party, but it also paves the way for automation and a series of cryptographic enhancements; improves security by minimizing the impact of human error and potentially improves usability. First, we study a few vulnerabilities in voice-based out-of-band authentication, in particular a combinatorial attack against lazy users, which we analyze in the context of a secure email solution. Next, we propose solving the problem of secure equality test using PAKE to achieve entity authentication and to establish a shared high-entropy secret key. Our solution lends itself to offline settings, compatible with the inherently asynchronous nature of email and modern messaging systems. The suggested approach enables enhancements in key management such as automated key renewal and future key pair authentications, multi-device synchronization, secure secret storage and retrieval, and the possibility of post-quantum security as well as facilitating forward secrecy and deniability in a primarily symmetric-key setting. We also discuss the use of auditable PAKEs for mitigating a class of online guess and abort attacks in authentication protocols