Chun-Wei Yang

This study focuses on semi-quantum private comparison (SQPC) protocols, where participants with limited quantum capabilities compare private information with the help of a quantum third party (TP). Yan et al. introduced an SQPC protocol in 2021, but Li et al. later found it vulnerable to double controlled NOT (CNOT) operation attacks and proposed improvements. However, this study identifies a different vulnerability in Yan et al.’s protocol — a reflecting attack that allows information leakage. Bob can intercept and reflect quantum states, avoiding detection and then use Alice’s measurement outcomes and pre-shared key to access her confidential information. The study also proposes an enhanced method to secure the protocol.

This study tackles critical issues in semi‐quantum signature (SQS) protocols to enhance security and efficiency. It revisits Xia et al.’s SQS protocol, which uses EPR steering for security but introduces inconsistencies in a semi‐quantum environment. Xia et al. assume that both the signatory and arbitrator have full quantum capabilities, while the verifier only has partial capabilities, making them a classical participant. However, their protocol requires the verifier to perform eavesdropping checks, necessitating quantum storage, which is not aligned with a semi‐quantum environment. The study proposes a novel SQS protocol using EPR steering and single photons, with pre‐shared keys to determine photon arrangements to address this. This ensures that only the communicating parties know the inspection and message transmission locations, solving the quantum storage issue and enabling efficient eavesdropper detection and identity authentication. This new protocol adheres to semi‐quantum principles and sets a foundation for future advancements in quantum cryptography and secure communication.

This study identifies a participant attack vulnerability in Li et al.’s SQPC protocol. The participant attack allows a malicious participant, Bob, to obtain the participant Alice’s secret information by intercepting and measuring photons sent to Alice, and later decrypting Alice’s encrypted comparison result. An improved SQPC protocol is proposed to address this problem. The key enhancement is introducing a mutual eavesdropping check requiring the third party and participants to verify they obtain the same photon measurement results. This enables detecting participant attacks like the one identified. The SQPC protocol proposed in this study not only withstands attacks from internal participants but also achieves quantum efficiency almost equivalent to Li et al.’s SQPC protocol. Thus, this study presents a more secure SQPC protocol without compromising quantum efficiency.

A previous study [Entropy 25.4 (2023): 590] proposed a quantum secure multi-party summation protocol wherein n participants could obtain the modulo-2 summation result using single photons and single-particle operations. Owing to the absence of an entanglement state, this protocol was more practical than other quantum summation protocols that used entanglement properties. Thus, this study investigated the claim that this proposed protocol prevented the participants’ secret inputs from being disclosed. It was found that the previously proposed protocol was vulnerable to a quantum Trojan horse attack owing to the circular qubit transmission mode. Using a security loophole, this study proposed an attack strategy that allowed an outside attacker to steal any participant’s secret input without being detected by a third party and participants. In addition, a simulation experiment was conducted to demonstrate the feasibility of the proposed attack and evaluate the number of quantum Trojan horse probes that the attacker must inject into regular qubit sequences. Finally, a solution was proposed by modifying the protocol to securely prevent quantum Trojan horse attacks.