International Association for Cryptologic Research

International Association
for Cryptologic Research


Alexandru Cojocaru


Security Limitations of Classical-Client Delegated Quantum Computing 📺
Secure delegated quantum computing allows a computationally weak client to outsource an arbitrary quantum computation to an untrusted quantum server in a privacy-preserving manner. One of the promising candidates to achieve classical delegation of quantum computation is classical-client remote state preparation ($\sf{RSP}_{CC}$), where a client remotely prepares a quantum state using a classical channel. However, the privacy loss incurred by employing $\sf{RSP}_{CC}$ as a sub-module is unclear. In this work, we investigate this question using the Constructive Cryptography framework by Maurer and Renner (ICS'11). We first identify the goal of $\sf{RSP}_{CC}$ as the construction of ideal \RSP resources from classical channels and then reveal the security limitations of using $\sf{RSP}_{CC}$. First, we uncover a fundamental relationship between constructing ideal \RSP resources (from classical channels) and the task of cloning quantum states. Any classically constructed ideal \RSP resource must leak to the server the full classical description (possibly in an encoded form) of the generated quantum state, even if we target computational security only. As a consequence, we find that the realization of common \RSP resources, without weakening their guarantees drastically, is impossible due to the no-cloning theorem. Second, the above result does not rule out that a specific $\sf{RSP}_{CC}$ protocol can replace the quantum channel at least in some contexts, such as the Universal Blind Quantum Computing ($\sf{UBQC}$) protocol of Broadbent et al. (FOCS '09). However, we show that the resulting UBQC protocol cannot maintain its proven composable security as soon as $\sf{RSP}_{CC}$ is used as a subroutine. Third, we show that replacing the quantum channel of the above $\sf{UBQC}$ protocol by the $\sf{RSP}_{CC}$ protocol QFactory of Cojocaru et al. (Asiacrypt '19) preserves the weaker, game-based, security of $\sf{UBQC}$.
QFactory: Classically-Instructed Remote Secret Qubits Preparation
The functionality of classically-instructed remotely prepared random secret qubits was introduced in (Cojocaru et al. 2018) as a way to enable classical parties to participate in secure quantum computation and communications protocols. The idea is that a classical party (client) instructs a quantum party (server) to generate a qubit to the server’s side that is random, unknown to the server but known to the client. Such task is only possible under computational assumptions. In this contribution we define a simpler (basic) primitive consisting of only BB84 states, and give a protocol that realizes this primitive and that is secure against the strongest possible adversary (an arbitrarily deviating malicious server). The specific functions used, were constructed based on known trapdoor one-way functions, resulting to the security of our basic primitive being reduced to the hardness of the Learning With Errors problem. We then give a number of extensions, building on this basic module: extension to larger set of states (that includes non-Clifford states); proper consideration of the abort case; and verifiablity on the module level. The latter is based on “blind self-testing”, a notion we introduced, proved in a limited setting and conjectured its validity for the most general case.