QFactory: Classically-Instructed Remote Secret Qubits Preparation

Abstract

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.

Security Limitations of Classical-Client Delegated Quantum Computing

“…5, we show that the protocol family RSP CC contains protocols with reasonably restricted leakage that can be used as subroutines in specific applications resulting in combined protocols that offer a decent level of security. Specifically, we prove the blindness property of QF-UBQC, a concrete UBQC CC protocol that consists of the universal blind quantum computation (UBQC) protocol of [BFK09] and the specific LWE-based remote state preparation (RSP CC ) protocol from [CCKW19]. This yields the first provably secure UBQC CC protocol from standard assumptions with a classical RSP protocol as a subroutine.…”

“…Theorem 4 (Game-Based Security of QF-UBQC). The universal blind quantum computation protocol with a classical client UBQC CC that combines the RSP CC protocol of [CCKW19] and the UBQC protocol of [BFK09] is blind in the gamebased setting. We call this protocol QF-UBQC.…”

“…While RSP CC was first introduced in [CCKW18] (under a different terminology), (game-based) security was only proven against weak (honest-but-curious) adversaries. Security against malicious adversaries was proven for a modified protocol in [CCKW19], where a verifiable version of RSP CC was also given, but security was not proven in full generality. This protocol, called QFactory, is the basis of the positive results in this work.…”

“…The UBQC protocol from [BFK09], where the quantum interaction is replaced by a RSP 8−states CC protocol, is shown in Protocol 1. In this section, we replace the RSP 8−states CC protocol with a concrete protocol proposed in [CCKW19]. This protocol, known by the name of 8-states QFactory (we consider the case where abort occurs with negligible probability) exactly emulates the capability of RSP 8−states CC .…”

On the Possibility of Classical Client Blind Quantum Computing

“…Following the introduction of this family of primitives, known as classical client remote state preparation (CC − RSP), in our earlier work [22] (version 2), more works have arose that present constructions of CC − RSP achieving security against arbitrarily deviating, computationally bounded, adversaries. In [23], it is provided a CC − RSP protocol, whose security holds against any malicious server. Concurrently in [24], the authors provide a verifiable version of this CC − RSP primitive with composable security, but under some assumption known as "measurement buffer".…”