  Quantum key distribution (QKD) networks hold promise for sharing secure
randomness over multi-partities. Most existing QKD network schemes and
demonstrations are based on trusted relays or limited to point-to-point
scenario. Here, we propose a flexible and extensible scheme named as
open-destination measurement-device-independent QKD network. The scheme enjoys
security against untrusted relays and all detector side-channel attacks.
Particularly, any users can accomplish key distribution under assistance of
others in the network. As an illustration, we show in detail a four-user
network where two users establish secure communication and present realistic
simulations by taking into account imperfections of both sources and detectors.
