  In quantum key distribution, measurement-device-independent and decoy-state
techniques enable the two cooperative agents to establish a shared secret key
using imperfect measurement devices and weak Poissonian sources, respectively.
Investigations so far are not comprehensive as they restrict to less than or
equal to four decoy states. Moreover, many of them involves pure numerical
studies. Here I report a general security proof that works for any fixed number
of decoy states and any fixed raw key length. The two key ideas involved here.
The first one is the repeated application of the inversion formula for
Vandermonde matrix to obtain various bounds on certain yields and error rates.
The second one is the use of a recently proven generalization of the McDiarmid
inequality. These techniques rise the best provably secure key rate of the
measurement-device-independent version of the BB84 scheme by at least 1.25
times and increase the workable distance between the two cooperative agents
from slightly less than 60 km to slightly greater than 130 km in case there are
$10^{10}$ photon pulse pair sent without a quantum repeater.
