Measurement-device-independent quantum key distribution (MDI-QKD) protocolhas been demonstrated as a viable solution to detector side-channel attacks.One of the main advantages of MDI-QKD is that the security can be provedwithout making any assumptions about how the measurement device works. Theprice to pay is the relatively low secure key rate comparing with conventionalquantum key distribution (QKD), such as the decoy-state BB84 protocol. Recentlya new QKD protocol, aiming at bridging the strong security of MDI-QKD with thehigh efficiency of conventional QKD, has been proposed. In this protocol, thelegitimate receiver employs a trusted linear optics network to encodeinformation on photons received from an insecure quantum channel, and thenperforms a Bell state measurement (BSM) using untrusted detectors. One crucialassumption made in most of these studies is that the untrusted BSM locatedinside the receiver's laboratory cannot send any unwanted information to theoutside. Here, we show that if the BSM is completely untrusted, a simple schemewould allow the BSM to send information to the outside. Combined with Trojanhorse attacks, this scheme could allow Eve to gain information of the quantumkey without being detected. To prevent the above attack, either countermeasuresto Trojan horse attacks or some trustworthiness to the "untrusted" BSM deviceis required.
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