Deniable Encryption in a Quantum World

TL;DR

This paper introduces a new form of quantum deniable encryption called perfect unexplainability, which offers stronger coercion resistance than classical methods by leveraging quantum computation's unique properties.

Contribution

The paper demonstrates that quantum algorithms enable perfect unexplainability in deniable encryption, a feat impossible with classical cryptography, and provides a secure construction based on quantum hardness assumptions.

Findings

Quantum computation enables perfect unexplainability in deniable encryption.

The proposed scheme is secure in the random oracle model assuming quantum hardness of LWE.

It offers protection against coercion both before and after encryption.

Abstract

(Sender-)Deniable encryption provides a very strong privacy guarantee: a sender who is coerced by an attacker into "opening" their ciphertext after-the-fact is able to generate "fake" local random choices that are consistent with any plaintext of their choice. In this work, we study (sender-)deniable encryption in a setting where the encryption procedure is a quantum algorithm, but the ciphertext is classical. We show that quantum computation unlocks a fundamentally stronger form of deniable encryption, which we call perfect unexplainability. The primitive at the heart of unexplainability is a quantum computation for which there is provably no efficient way, such as exhibiting the "history of the computation", to establish that the output was indeed the result of the computation. We give a construction that is secure in the random oracle model, assuming the quantum hardness of LWE. Crucially, this notion implies a form of protection against coercion "before-the-fact", a property that is impossible to achieve classically.