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 , 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.
In a framework based on the simulation paradigm, introduced the notion of deniable authentication @cite_10 , followed by the work of Di on the formalization of deniable key exchange @cite_24 . Both works rely on the formalism of zero-knowledge (ZK) proofs, with definitions formalized in terms of a simulator that can produce a simulated view that is indistinguishable from the real one. In a subsequent work, Di Raimondo and Gennaro gave a formal definition of forward deniability @cite_31 , requiring that indistinguishability remain intact even when a (corrupted) party reveals real coins after a session. Among other things, they showed that statistical ZK protocols are forward deniable.
@cite_24 @cite_31 @cite_10
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We extend the definitional work of Dwork,Naor and Sahai from deniable authentication to deniable key-exchange protocols. We then use these definitions to prove the deniability features of SKEME and SIGMA, two natural and efficient protocols which serve as basis for the Internet Key Exchange (IKE)protocol.SKEME is an encryption-based protocol for which we prove full deniability based on the plaintext awareness of the underlying encryption scheme. Interestingly SKEME's deniability is possibly the first "natural" application which essentially requires plaintext awareness (until now this notion has been mainly used as a tool for proving chosen-ciphertext security).SIGMA, on the other hand,uses non-repudiable signatures for authentication and hence cannot be proven to be fully deniable. Yet we are able to prove a weaker, but meaningful, "partial deniability" property: a party may not be able to deny that it was "alive" at some point in time but can fully deny the contents of its communications and the identity of its interlocutors.We remark that the deniability of SKEME and SIGMA holds in a concurrent setting and does not essentially rely on the random oracle model. Deniable Authentication protocols allow a Sender to authenticate a message for a Receiver, in a way that the Receiver cannot convince a third party that such authentication (or any authentication) ever took place. We present two new approaches to the problem of deniable authentication. The novelty of our schemes is that they do not require the use of CCA-secure encryption (all previous known solutions did), thus showing a different generic approach to the problem of deniable authentication. These new approaches are practically relevant as they lead to more efficient protocols. In the process we point out a subtle definitional issue for deniability. In particular, we propose the notion of forward deniability, which requires that the authentications remain deniable even if the Sender wants to later prove that she authenticated a message. We show that a simulation-based definition of deniability, where the simulation can be computationally indistinguishable from the real protocol does not imply forward deniability. Thus, for deniability one needs to restrict the simulation to be perfect (or statistically close). Our new protocols satisfy this stricter requirement. Traditionally, secure one-round key exchange protocols in the PKI setting have either achieved perfect forward secrecy, or forms of deniability, but not both. On the one hand, achieving perfect forward secrecy against active attackers seems to require some form of authentication of the messages, as in signed Diffie-Hellman style protocols, that subsequently sacrifice deniability. On the other hand, using implicit authentication along the lines of MQV and descendants sacrifices perfect forward secrecy in one round and achieves only weak perfect forward secrecy instead. We show that by reintroducing signatures, it is possible to satisfy both a very strong key-exchange security notion, which we call eCK-PFS, as well as a strong form of deniability, in one-round key exchange protocols. Our security notion for key exchange is stronger than, e.g., the extended-CK model, and captures perfect forward secrecy. Our notion of deniability, which we call peer-and-time deniability, is stronger than that offered by, e.g., the SIGMA protocol. We propose a concrete protocol and prove that it satisfies our definition of key-exchange security in the random oracle model as well as peer-and-time deniability. The protocol combines a signed-Diffie-Hellman message exchange with an MQV-style key computation, and offers a remarkable combination of advanced security properties. Zero knowledge plays a central role in cryptography and complexity. The seminal work of Ben- (STOC 1988) shows that zero knowledge can be achieved unconditionally for any language in NEXP, as long as one is willing to make a suitable *physical assumption*: if the provers are spatially isolated, then they can be assumed to be playing independent strategies. Quantum mechanics, however, tells us that this assumption is unrealistic, because spatially-isolated provers could share a quantum entangled state and realize a non-local correlated strategy. The MIP^* model captures this setting. In this work we study the following question: does spatial isolation still suffice to unconditionally achieve zero knowledge even in the presence of quantum entanglement? We answer this question in the affirmative: we prove that every language in NEXP has a 2-prover *zero knowledge* interactive proof that is sound against entangled provers; that is, NEXP ⊆ ZK-MIP^*. Our proof consists of constructing a zero knowledge interactive PCP with a strong algebraic structure, and then lifting it to the MIP^* model. This lifting relies on a new framework that builds on recent advances in low-degree testing against entangled strategies, and clearly separates classical and quantum tools. Our main technical contribution is the development of algebraic techniques for obtaining unconditional zero knowledge; this includes a zero knowledge variant of the celebrated sumcheck protocol, a key building block in many probabilistic proof systems. A core component of our sumcheck protocol is a new algebraic commitment scheme, whose analysis relies on algebraic complexity theory.