  Measurement-device-independent quantum key distribution (MDI-QKD) can
eliminate all detector side-channel loopholes and has shown excellent
performance in long-distance secret keys sharing. Conventional security proofs,
however, require additional assumptions on sources and that can be compromised
through uncharacterized side channels in practice. Here, we present a general
formalism based on reference technique to prove the security of MDI-QKD against
any possible sources imperfection and/or side channels. With this formalism, we
investigate the asymptotic performance of single-photon sources without any
extra assumptions on the state preparations. Our results highlight the
importance of transmitters' security.
