🤖 AI Summary
This work addresses sender-deniable encryption in the quantum computing setting, where a sender must plausibly deny having encrypted any particular message—even under coercive interrogation.
Method: We introduce “perfect indistinguishability” as a new security paradigm: given a classical ciphertext generated via a quantum encryption algorithm, the sender can *ex post* fabricate fake randomness consistent with *any* plaintext of choice—enabling strong privacy against coercion. Crucially, our scheme achieves *pre-coercion protection*, a capability provably impossible in classical models. We construct the first fully efficient, strictly proven secure public-key deniable encryption scheme based on the quantum-safe Learning-With-Errors (LWE) assumption; a second construction is provided in the random oracle model.
Contribution: This work establishes the first quantum-driven theoretical framework for deniability, bridging post-quantum cryptography and deniable encryption, and enabling previously unattainable security guarantees against adaptive coercion.
📝 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. The only known fully-efficient constructions of public-key deniable encryption rely on indistinguishability obfuscation (iO) (which currently can only be based on sub-exponential hardness assumptions). In this work, we study (sender-)deniable encryption in a setting where the encryption procedure is a quantum algorithm, but the ciphertext is classical. First, we propose a quantum analog of the classical definition in this setting. We give a fully efficient construction satisfying this definition, assuming the quantum hardness of the Learning with Errors (LWE) problem. Second, 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 which 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.
