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.
Pass @cite_5 formally defines the notion of deniable zero-knowledge and presents positive and negative results in the common reference string and random oracle model. In @cite_0 , establish a link between deniability and ideal authentication and further model a situation in which deniability should hold even when a corrupted party colludes with the adversary during the execution of a protocol. They show an impossibility result in the PKI model if adaptive corruptions are allowed. Cremers and Feltz introduced another variant for key exchange referred to as peer and time deniability @cite_1 , while also capturing perfect forward secrecy. More recently, Unger and Goldberg studied deniable authenticated key exchange (DAKE) in the context of secure messaging @cite_4 .
@cite_0 @cite_5 @cite_1 @cite_4
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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. 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. We revisit the definitions of zero-knowledge in the Common Reference String (CRS) model and the Random Oracle (RO) model. We argue that even though these definitions syntactically mimic the standard zero-knowledge definition, they loose some of its spirit. In particular, we show that there exist a specific natural security property that is not captured by these definitions. This is the property of deniability. We formally define the notion of deniable zero-knowledge in these models and investigate the possibility of achieving it. Our results are different for the two models: 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.