



02441nas a2200181   4500008004100000245007300041210006900114260001300183520187800196100002702074700002402101700001902125700001902144700002202163700001702185700002002202856003702222       2020                        eng d00aSecurity Limitations of Classical-Client Delegated Quantum Computing0 aSecurity Limitations of ClassicalClient Delegated Quantum Comput  c7/3/20203 a<p>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 (RSPCC), where a client remotely prepares a quantum state using a classical channel. However, the privacy loss incurred by employing RSPCC as a sub-module is unclear.<br />In this work, we investigate this question using the Constructive Cryptography framework by Maurer and Renner (ICS&#39;11). We first identify the goal of RSPCC as the construction of ideal RSP resources from classical channels and then reveal the security limitations of using RSPCC. 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 RSPCC protocol can replace the quantum channel at least in some contexts, such as the Universal Blind Quantum Computing (UBQC) protocol of Broadbent et al. (FOCS &#39;09). However, we show that the resulting UBQC protocol cannot maintain its proven composable security as soon as RSPCC is used as a subroutine. Third, we show that replacing the quantum channel of the above UBQC protocol by the RSPCC protocol QFactory of Cojocaru et al. (Asiacrypt &#39;19), preserves the weaker, game-based, security of UBQC.</p>1 aBadertscher, Christian1 aCojocaru, Alexandru1 aColisson, Léo1 aKashefi, Elham1 aLeichtle, Dominik1 aMantri, Atul1 aWallden, Petros  uhttps://arxiv.org/abs/2007.01668