  How to remove detector side channel attacks has been a notoriously hard
problem in quantum cryptography. Here, we propose a simple solution to this
problem---*measurement* device independent quantum key distribution. It not
only removes all detector side channels, but also doubles the secure distance
with conventional lasers. Our proposal can be implemented with standard optical
components with low detection efficiency and highly lossy channels. In contrast
to the previous solution of full device independent QKD, the realization of our
idea does not require detectors of near unity detection efficiency in
combination with a qubit amplifier (based on teleportation) or a quantum
non-demolition measurement of the number of photons in a pulse. Furthermore,
its key generation rate is many orders of magnitude higher than that based on
full device independent QKD. The results show that long-distance quantum
cryptography over say 200km will remain secure even with seriously flawed
detectors.
