On Deniability in Quantum Key Exchange

Arash Atashpendar, Guru-Vamsi Policharla, Peter Browne Rønne +1 more

Arash Atashpendar, Guru-Vamsi Policharla, Peter Browne Rønne +1 more

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.

Nik Unger, Ian Goldberg

Abstract A deniable authenticated key exchange (DAKE) protocol establishes a secure channel without producing cryptographic evidence of communication. A DAKE offersstrong deniabilityif transcripts provide no evidence even if long-term key material is compromised (offline deniability) and no outsider can obtain evidence even when interactively colluding with an insider (online deniability). Unfortunately, existing strongly deniable DAKEs have not been adopted by secure messaging tools due to security and deployability weaknesses. In this work, we propose three new strongly deniable key exchange protocols—DAKEZ, ZDH, and XZDH—that are designed to be used in modern secure messaging applications while eliminating the weaknesses of previous approaches. DAKEZ offers strong deniability in synchronous network environments, while ZDH and XZDH can be used to construct asynchronous secure messaging systems with offline and partial online deniability. DAKEZ and XZDH provide forward secrecy against active adversaries, and all three protocols can provide forward secrecy against future quantum adversaries while remaining classically secure if attacks against quantum-resistant cryptosystems are found. We seek to reduce barriers to adoption by describing our protocols from a practitioner’s perspective, including complete algebraic specifications, cryptographic primitive recommendations, and prototype implementations. We evaluate concrete instantiations of our DAKEs and show that they are the most efficient strongly deniable schemes; with all of our classical security guarantees, our exchanges require only 1 ms of CPU time on a typical desktop computer and at most 464 bytes of data transmission. Our constructions are nearly as efficient as key exchanges with weaker deniability, such as the ones used by the popular OTR and Signal protocols.

Kiyoshi Tamaki, Go Kato

Donald Beaver, Syntechnica Llc

Abstract. We show that claims of “perfect security ” for keys produced by quantum key exchange (QKE) are limited to “privacy ” and “integrity.” Unlike a one-time pad, QKE does not necessarily enable Sender and Receiver to pretend later to have established a different key. This result is puzzling in light of Mayers ’ “No-Go ” theorem showing the impossibility of quantum bit commitment. But even though a simple and intuitive application of Mayers ’ protocol transformation appears sufficient to provide deniability (else QBC would be possible), we show several reasons why such conclusions are ill-founded. Mayers ’ transformation arguments, while sound for QBC, are insufficient to establish deniability in QKE. Having shed light on several unadvertised pitfalls, we then provide a candidate deniable QKE protocol. This itself indicates further shortfalls in current proof techniques, including reductions that preserve privacy but fail to preserve deniability. In sum, purchasing undeniability with an off-the-shelf QKE protocol is significantly more expensive and dangerous than the mere optic fiber for which “perfect security ” is advertised. 1

雅一 吉田