A Unified Approach to Quantum Key Leasing with a Classical Lessor
Fuyuki Kitagawa, Jiahui Liu, Shota Yamada, Takashi Yamakawa
TL;DR
This work presents a modular framework for secure key leasing with a classical lessor, enabling revocation of quantum keys through purely classical communication and verifiable deletion. By embedding cryptographic keys into BB84-like quantum states and dequantizing the lessor via dual-mode secure function evaluation and watermarking, the authors construct SKL for public-key encryption, pseudorandom functions, and digital signatures under VRA security relying on LWE. The framework yields a non-interactive key-generation variant and introduces dual-mode SFE, along with watermarkable WPKE, WUPF, and WDS primitives that support parallel extraction and robust security reductions. The results unify several SKL constructions under a single modular blueprint, improving practicality for near-term quantum deployments and enabling secure key leasing across multiple cryptographic functionalities with a classical client. The work thus advances secure delegation and revocation in quantum-enabled cryptography and provides foundational tools for future quantum software leasing and certified-deletion-inspired protocols.
Abstract
Secure key leasing allows a cryptographic key to be leased as a quantum state in such a way that the key can later be revoked in a verifiable manner. In this work, we propose a modular framework for constructing secure key leasing with a classical-lessor, where the lessor is entirely classical and, in particular, the quantum secret key can be both leased and revoked using only classical communication. Based on this framework, we obtain classical-lessor secure key leasing schemes for public-key encryption (PKE), pseudorandom function (PRF), and digital signature. We adopt the strong security notion known as security against verification key revealing attacks (VRA security) proposed by Kitagawa et al. (Eurocrypt 2025) into the classical-lessor setting, and we prove that all three of our schemes satisfy this notion under the learning with errors assumption. Our PKE scheme improves upon the previous construction by Goyal et al. (Eurocrypt 2025), and our PRF and digital signature schemes are respectively the first PRF and digital signature with classical-lessor secure key leasing property.