  In this article we present a new prepare and measure quantum key distribution
protocol that decouples the necessary quantum channel error estimation from its
dependency on sifting, or otherwise post-selecting, the detection outcomes.
Rather than estimating Eve's coupling to the quantum channel from the
statistics of the sifted key, we infer this information from weak measurements
made equally on all of the received photons immediately prior to post-selection
by the photon detectors. We prove that the accuracy of the weak measurement
parameter estimation is robust to reasonable device imperfections, even in an
adversarial environment, and hence the asymptotic security of this protocol can
be inferred from the security analysis of BB84. In addition to eliminating
detector basis-dependent attacks, such as detector blinding, we demonstrate
that this new prepared and measure QKD protocol is immune to a very powerful
class of measurement-side device attacks that also allow an adversary control
of the weak measurement outcomes given two modest requirements placed on the
measurement-side devices. Finally, we compare the asymptotically achievable
secure key rate of a decoy state version of the weak measurement protocol and
show it is essentially equal to that of BB84 with decoy states and
significantly higher than MDI-QKD for realistic system parameters.
