  Measurement-device-independent quantum key distribution (MDIQKD) is proposed
to be secure against any possible detection attacks. The security of the
original proposal relies on the assumption that the legitimate users can fully
characterize the encoding systems including sources. Here, we propose a MDIQKD
protocol where we allow uncharacterized encoding systems as long as qubit
sources are used. A security proof of the MDIQKD protocol is presented that
does not need the knowledge of the encoding states. Simulation results show
that the scheme is practical.
