  Measurement-device-independent quantum key distribution (MDI-QKD) protocol
has 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 proved
without making any assumptions about how the measurement device works. The
price to pay is the relatively low secure key rate comparing with conventional
quantum key distribution (QKD), such as the decoy-state BB84 protocol. Recently
a new QKD protocol, aiming at bridging the strong security of MDI-QKD with the
high efficiency of conventional QKD, has been proposed. In this protocol, the
legitimate receiver employs a trusted linear optics network to encode
information on photons received from an insecure quantum channel, and then
performs a Bell state measurement (BSM) using untrusted detectors. One crucial
assumption made in most of these studies is that the untrusted BSM located
inside the receiver's laboratory cannot send any unwanted information to the
outside. Here, we show that if the BSM is completely untrusted, a simple scheme
would allow the BSM to send information to the outside. Combined with Trojan
horse attacks, this scheme could allow Eve to gain information of the quantum
key without being detected. To prevent the above attack, either countermeasures
to Trojan horse attacks or some trustworthiness to the "untrusted" BSM device
is required.
